U.S. patent application number 13/478026 was filed with the patent office on 2012-09-13 for insulating layer for rigid printed circuit boards.
This patent application is currently assigned to INTEGRAL TECHNOLOGY, INC.. Invention is credited to Christopher A. Hunrath.
Application Number | 20120227258 13/478026 |
Document ID | / |
Family ID | 40418872 |
Filed Date | 2012-09-13 |
United States Patent
Application |
20120227258 |
Kind Code |
A1 |
Hunrath; Christopher A. |
September 13, 2012 |
INSULATING LAYER FOR RIGID PRINTED CIRCUIT BOARDS
Abstract
One or more embodiments contained herein disclose rigid printed
circuit boards (PCBs) and methods for manufacturing the same
comprising strain resistant layers configured to, among others,
minimize defects from occurring in cap layers of the PCBs.
Inventors: |
Hunrath; Christopher A.;
(San Juan Capistrano, CA) |
Assignee: |
INTEGRAL TECHNOLOGY, INC.
LAKE FOREST
CA
|
Family ID: |
40418872 |
Appl. No.: |
13/478026 |
Filed: |
May 22, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12329488 |
Dec 5, 2008 |
8188373 |
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13478026 |
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61012392 |
Dec 7, 2007 |
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61016292 |
Dec 21, 2007 |
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61078315 |
Jul 3, 2008 |
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Current U.S.
Class: |
29/832 ;
156/182 |
Current CPC
Class: |
H05K 1/0271 20130101;
H05K 3/025 20130101; Y10T 428/31678 20150401; Y10T 156/10 20150115;
H05K 3/4652 20130101; H05K 2203/0152 20130101; H05K 2201/0195
20130101; H05K 3/4655 20130101; H05K 2203/1536 20130101; H05K
2201/0355 20130101; H05K 3/4626 20130101; Y10T 29/4913 20150115;
H05K 3/4688 20130101; H05K 3/4682 20130101; H05K 1/09 20130101;
H05K 2201/0154 20130101; Y10T 428/31721 20150401 |
Class at
Publication: |
29/832 ;
156/182 |
International
Class: |
H05K 3/30 20060101
H05K003/30; B32B 37/00 20060101 B32B037/00 |
Claims
1. A method of manufacturing printed circuit boards, the method
comprising: providing a component comprising a first surface of a
strain resistant cap layer engaging a first surface of a conductive
layer, wherein the strain resistant cap layer comprises polyimide;
and attaching the component a stack of laminates by attaching a
second surface of the strain resistant cap layer to a top surface
of a first layer of the stack of laminates, wherein the stack of
laminates comprises at least one rigid insulating layer extending
throughout the entire length of the printed circuit board to define
the printed circuit board comprising entirely rigid portions.
2. The method of claim 1, wherein providing the component further
comprises providing a discardable layer engaging a second surface
of the conductive layer.
3. The method of claim 2, further comprising releasing the
discardable layer from the component, wherein the second surface of
the conductive layer becomes a surface conductive layer.
4. The method of claim 1, further comprising mounting an electronic
device on the rigid printed circuit board to form a rigid printed
circuit board assembly.
5. The method of claim 1, wherein the conductive layer comprises
rolled-annealed copper.
6. The method of claim 1, wherein the conductive layer comprises
electrodeposited copper.
7. The method of claim 1, wherein providing the component comprises
providing the strain resistant cap layer comprising at least two of
the following characteristics: ductility in the range of about
20-80%; Tg in range of about 220-420.degree. C.; and tensile
strength in the range of about 10,000-50,000 psi.
8. The method of claim 1, wherein the strain resistant cap layer is
substantially free of materials selected from the group of:
halogen, fiberglass, and lead.
9. The method of claim 1, wherein the component is fully cured.
10. The method of claim 1, wherein the component comprises
pre-formed layers of the conductive layer and the strain resistant
cap layer.
11. The method of claim 1, wherein providing the component
comprises attaching the conductive layer with the strain resistant
cap layer using one of the following processes: cast to foil, vapor
deposition, sputtering, and plating.
12. A component for manufacturing rigid printed circuit boards, the
component comprising: a conductive layer comprising a first surface
and a second surface; a discardable layer comprising a first
surface, wherein the first surface of the discardable layer is
attached to the first surface of the conductive layer; and a strain
resistant layer comprising a first surface, wherein the first
surface of the strain resistant layer is attached to the second
surface of the conductive layer, and wherein the strain resistant
layer comprises at least two characteristics selected from a group
of: ductility of at least about 15%, Tg of at least about
220.degree. C., and tensile strength of at least about 10,000
psi.
13. The component of claim 12, wherein the discardable layer
comprises aluminum.
14. The component of claim 12, wherein the strain resistant layer
comprises polyimide.
15. The component of claim 12, wherein the strain resistant layer
comprises at least one characteristic selected from the group of:
fully cured, substantially halogen free, non-glass reinforced,
substantially lead free, and substantially fiberglass free.
16. The component of claim 12, wherein the conductive layer
comprises copper.
17. The component of claim 15, wherein the conductive layer
comprises rolled-annealed copper.
18. The component of claim 15, wherein the conductive layer
comprises electrodeposited copper.
19. The component of claim 12, wherein the discardable layer
comprises a second surface, and wherein the component further
comprises: a second conductive layer comprising a first surface and
a second surface, wherein the first surface of the second
conductive layer is attached to the second surface of the
discardable layer; and a second strain resistant layer comprising a
first surface, wherein the first surface of the second strain
resistant layer is attached to the second surface of the second
conductive layer.
20. The component of claim 19, wherein the second strain resistant
comprises at least two of the following characteristics: ductility
in the range of about 20-80%; Tg in the range of about
220-420.degree. C.; tensile strength in the range of about
10,000-50,000 psi; and CTE X,Y of about 40 ppm/.degree. C. or less.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/329,488, filed Dec. 5, 2008, now U.S. Pat.
No. 8,188,373, which claims the priority benefit of U.S.
Provisional Application No. 61/012,392, filed on Dec. 7, 2007, and
also claims the priority benefit of U.S. Provisional Application
No. 61/016,292, filed on Dec. 21, 2007, and U.S. Provisional
Application No. 61/078,315, filed on Jul. 3, 2008, the disclosures
of all of these applications are hereby expressly incorporated by
reference in their entirety.
BACKGROUND
[0002] 1. Field
[0003] The disclosure herein relates to printed circuit boards, and
more particularly, insulating layers for use with rigid printed
circuit boards.
[0004] 2. Description of the Related Art
[0005] Printed circuit boards (PCB) comprise one or more layers of
electrically conductive material such as copper and one or more
electrically insulating layers such as dielectrics. Multilayer PCBs
typically comprise two or more inner and/or surface conductive
layers formed over and separated by a plurality of insulating
layers with holes, vias, and through holes providing electrical
connection between the various inner conductive layers and other
inner conductive layers and/or the surface conductive layers.
[0006] Several aspects of the PCB manufacturing and assembly
processes subject PCB components to strain or stress (e.g.,
mechanical, thermal, physical, chemical, and the like). For
example, manufacturing exposes PCBs to a range of temperatures,
including high soldering temperatures which have increased even
more in response to the industry's recent adoption of lead-free
processes. Strain can cause defects in components, resulting in
electrical and/or mechanical failure. For example, thermal strain
arising from increasing temperatures can cause cracks in the PCB
components, including pad cratering, a type of crack typically
occurring in insulating layers that engage surface conductive
layers. Various embodiments disclosed herein contemplate certain
more stable and damage-resistant PCB components for use with rigid
PCBs that may substantially increase the yield of rigid PCBs while
possibly reducing defects such as voids and cracks and increasing
the structural integrity of the rigid PCBs and portions of rigid
PCBs such as junctions between insulating layers and surface
conductive layers.
SUMMARY
[0007] In an embodiment, a device for mounting electrical
components comprises: a printed circuit board comprising: a surface
conductive layer configured to interface with the electrical
components; a strain resistant cap layer configured to engage the
surface conductive layer, wherein the strain resistant cap layer
comprises polyimide; and one or more rigid insulating layers,
wherein at least one of the one or more rigid insulating layers
extends throughout the entire length of the printed circuit board
such that the entire printed circuit board defines a rigid printed
circuit board.
[0008] In accordance with some embodiments, a method of
manufacturing printed circuit boards comprises: providing a
component comprising a first surface of a strain resistant cap
layer engaging a first surface of a conductive layer, wherein the
strain resistant cap layer comprises polyimide; and attaching the
component a stack of laminates by attaching a second surface of the
strain resistant cap layer to a top surface of a first layer of the
stack of laminates, wherein the stack of laminates comprises at
least one rigid insulating layer extending throughout the entire
length of the printed circuit board to define the printed circuit
board comprising entirely rigid portions.
[0009] In certain embodiments, a component for manufacturing rigid
printed circuit boards comprises a conductive layer comprising a
first surface and a second surface; a discardable layer comprising
a first surface, wherein the first surface of the discardable layer
is attached to the first surface of the conductive layer; and a
strain resistant layer comprising a first surface, wherein the
first surface of the strain resistant layer is attached to the
second surface of the conductive layer, and wherein the strain
resistant layer comprises at least two characteristics selected
from a group of: ductility of at least about 15%, Tg of at least
about 220.degree. C., and tensile strength of at least about 10,000
psi.
[0010] In some embodiments, a printed circuit board comprises: a
surface conductive layer configured to interface with the
electrical components, wherein the surface conductive layer
comprises rolled-annealed copper; a strain resistant cap layer
configured to engage the surface conductive layer, wherein the
strain resistant cap layer comprises polyimide; and one or more
rigid insulating layers, wherein at least one of the one or more
rigid insulating layers extends throughout the entire length of the
printed circuit board such that the printed circuit board defines a
rigid printed circuit board.
[0011] In some embodiments, a rigid circuit board comprises a
surface conductive layer engaging a strain resistant cap layer. In
an embodiment, a component for manufacturing printed circuit boards
such as rigid printed circuit boards comprises a surface copper
layer and a strain resistant layer, wherein the strain resistant
layer comprises polyimide. In a certain embodiment, a rigid PCB
comprises one or more strain resistant layers. Further still, the
printed circuit board in one embodiment comprises: a surface
conductive layer configured to interface with the electrical
components, wherein the surface conductive layer comprises
rolled-annealed copper; a strain resistant cap layer configured to
engage the surface conductive layer, wherein the strain resistant
cap layer comprises ductility of at least 15% and tensile strength
of at least 10,000 psi; and one or more rigid insulating layers,
wherein at least one of the one or more rigid insulating layers
extends throughout the entire length of the printed circuit board
such that the printed circuit board defines a rigid printed circuit
board.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] These and other features will now be described with
reference to the drawings summarized below. These drawings and the
associated description are provided to illustrate one or more
embodiments described in the present patent application and not to
limit the scope of the disclosed embodiments.
[0013] FIG. 1 depicts an embodiment of a multilayer printed circuit
board.
[0014] FIG. 2 illustrates another embodiment of a multilayer
printed circuit board comprising via holes and conductive
plates.
[0015] FIG. 3A illustrates a printed circuit board comprising ball
grid array (BGA) packaging.
[0016] FIG. 3B is a cross sectional view of an electrical component
mounted on a printed circuit board to form a printed circuit board
assembly.
[0017] FIG. 3C illustrates an embodiment of a printed circuit board
showing a cap layer engaging a surface conductive layer.
[0018] FIG. 3D illustrates an embodiment of a printed circuit board
showing a cap layer comprising a defect engaging a surface
conductive layer.
[0019] FIG. 3E illustrates an embodiment of a printed circuit board
showing a cap layer comprising another defect engaging a surface
conductive layer.
[0020] FIG. 4 illustrates an embodiment of a printed circuit board
comprising strain resistant layers.
[0021] FIG. 5 illustrates an embodiment of a printed circuit board
comprising strain resistant layers.
[0022] FIG. 6 illustrates an embodiment of a method of
manufacturing multilayer printed circuit board stacks using
components comprising discardable material.
[0023] FIG. 7A illustrates an embodiment of a component for
manufacturing printed circuit boards comprising a discardable layer
and strain resistant layers.
[0024] FIG. 7B illustrates another embodiment of a component for
manufacturing printed circuit boards comprising a discardable layer
and a strain resistant layer.
[0025] FIG. 8 is a table listing material characteristics for
example strain resistant materials and other insulating
materials.
[0026] FIG. 9 is a table comparing expansion characteristics of
example strain resistant materials and other dielectric
materials.
[0027] FIG. 10A illustrates an embodiment of a rigid printed
circuit board.
[0028] FIG. 10B illustrates an embodiment of a rigid printed
circuit board comprising a strain resistant cap layer.
DETAILED DESCRIPTION
[0029] In describing various embodiments in the present
application, reference will be made herein to FIGS. 1-10B of the
drawings, in which like numerals refer to like features unless
indicated otherwise.
A. General Description of Non-Limiting Embodiments
[0030] The terms "printed circuit boards," "PCBs," and "electrical
interconnect systems," as used in the present patent application,
are interchangeable and are broadly defined and comprise, without
limitation, any and all systems that provide, among others,
mechanical support to electrical components, electrical connection
to and between these electrical components, combinations thereof,
and the like. PCBs comprise systems that generally include a base
platform to support the electrical components (for example, a thin
board of insulating material) and conductors such as conductive
pathways, surfaces, solderable attachments, and the like to provide
an electrical interconnection between the electrical components.
PCBs can employ a broad range of technologies to support the
electrical components (for example, through-hole, surface-mount,
mixed-technology, components mounted on one or both sides, etc.)
and can comprise a wide range of single or multilayer constructions
(for example, single-sided, double-sided, multilayer, flexible,
rigid-flex, stripline, etc). The various embodiments herein can
apply to PCBs existing at any stage of the PCB manufacturing
process, including, by way of non-limiting examples, partially
incomplete PCBs lacking one or more PCB components typically
present in more complete PCBs such as, for example, insulating
layers, conductive circuit patterns, conductive plates, via holes,
and the like. As used throughout this application, the term "rigid
PCB" includes its standard meaning in the industry and also defines
PCBs having no bendable portions. As defined herein, "partially
rigid PCBs" broadly refers to interconnect systems comprising at
least some rigid and non-bendable portions. Layers belonging to the
rigid and non-bendable portions of rigid or partially rigid PCBs
can be at least substantially coplanar and may lie in the same
plane (e.g., horizontal plane, vertical planes, planes
therebetween, etc.) and can be configured to maintain a coplanar
structure in operation. PCBs and rigid PCBs can comprise one or
more rigid insulating layers. "PCB assembly" broadly refers to
printed circuit board systems on which electrical components are
partially, substantially, or fully mounted (e.g., electrically
attached or connected).
[0031] The terms "insulating layer," "dielectric layer," and
"dielectric substrate" are broadly interpreted herein, are
interchangeable, include their standard meaning in the industry,
and describe nonconductive PCB layers generally configured to
resist or substantially resist the flow of electricity and to
provide physical support for, among others, conductive layers and
electrical components. The term "rigid" as used in connection with
PCB insulating layers (e.g., rigid insulating layers, rigid
dielectrics, etc.) is broadly defined and describes insulating
layers comprising ordinary "rigid materials" including, without
limitation, materials that are typically non-bendable and
reinforced with fiberglass, papers, cotton fabric, asbestos sheet,
glass in various forms such as cloth and continuous filament mat,
ceramic material, molybdenum, various types of plastics, etc.
Several other rigid materials or mixes of rigid materials can be
used to produce rigid insulating layers, including prepregs (short
for preimpregnated) such as, for example, flame retardant (FR) 2
(cellulose paper impregnated with phenolic resin), FR-3 (cotton
paper impregnated with epoxy), FR-4 (epoxy-resin impregnated woven
glass cloth), FR-5 (woven glass impregnated with epoxy), etc. Rigid
layers comprise rigid materials generally used to manufacture rigid
PCBs or rigid portions of partially rigid PCBs.
[0032] As used herein, "strain resistant layers" is defined broadly
and refers to insulating layers for manufacturing printed circuit
boards, including rigid and partially rigid PCBs, comprising, among
others, one or more characteristics that can endure more strain
(e.g., mechanical, thermal, physical, chemical, and the like)
and/or can be more stable (e.g., thermally, chemically, physically,
etc) than ordinary insulating layers, including rigid insulating
layers as described herein. Strain resistant layers comprise a
broadly defined array of "strain resistant materials," including,
without limitation, thermosetting and/or thermoplastic plastics,
such as, for example, polyimide, polyester, fluorinated
hydrocarbon, polymers, polyacrylate, liquid crystal polymer,
synthetic fibers, aramids, fluorocarbons, etc. Strain resistant
layers can also comprise a mixture of one or more of the
thermosetting and/or thermoplastic plastic materials or a mixture
of one or more of the plastic materials with other materials (e.g.,
fillers, hardeners, etc.).
[0033] "Cap layer," as used herein, is broadly defined and
describes dielectric substrates and insulating layers that
interface with or engage the outermost conductive layers, also
referred to herein as "surface conductive layers," such as, for
example, surface copper pads. "Surface conductive layer," "outer
conductive layer," or "surface layer" used in connection with PCB
conductive layers broadly refer to the outermost conductive layers
of PCBs, such as, for example, surface copper layers and etched
surface conductive pads generally configured to engage electrical
devices mounted on the PCBs, such as, for example, electrical
components. "Electronic" or "electrical" components broadly
describe any PCB-mountable device capable of handling electricity
for which PCBs are designed to provide, among others, physical
support and/or electrical connection and without limitation include
electrical devices, electronic devices, electronic circuits,
electrical elements, integrated circuits, hybrid systems, and the
like.
[0034] The term "layer" as used in this application implies a
position in the cross section (profile) of PCBs or components of
PCBs. A layer in a PCB may be continuous or discontinuous, and may
or may not be planar or substantially planar. For example, a PCB
may comprise an inner or outer conductive discontinuous layer such
as an etched printed circuit layer. As used in relation to one PCB
layer in connection with another PCB layer, the terms "engage" or
"attach" or "over" (e.g., as in one layer over another layer) are
broadly defined to describe a layer or portions of the layer
directly or indirectly connected or attached to another layer or
portions of the other layer. Non-limiting examples of an indirect
connection include, for example, a layer in a PCB connected or
attached to another layer through an intermediate layer, such as,
for example, a mask, a coating layer, a thin film, soldering
material, and the like. Similarly, "forming," "depositing,"
"positioning," or "providing," as used herein in connection with
creating or positioning one layer over or on another layer,
generally disclose arranging or creating PCB layers such that at
least portions of one layer are directly or indirectly engaging at
least portions of the other layer. A rigid layer "extending"
throughout the entire length of the PCB defines a layer generally
provided over the length of the PCB (e.g., may or may not be
continuous, may or may not have same boundaries with the PCB, etc.)
such that the PCB is a rigid PCB.
[0035] As used herein, the terms "pre-form" or "pre-forming" PCB
layer components or layers to be used with PCBs define a
discontinuity between manufacturing the components and
manufacturing PCBs using the pre-formed components such that the
component manufacturing and the PCB manufacturing qualify as
"independent manufacturing processes." A non-limiting example of
independent manufacturing processes includes manufacturing PCBs
using a component manufactured by an entity different from the
entity manufacturing the PCBs, such as, without limitation,
3.sup.rd parties (e.g., original equipment manufacturers,
distributors, wholesalers, discount sellers, suppliers, retailers,
etc.), affiliates, subsidiaries, parent entities,
licensors/licensees, other legally different entities, combinations
thereof, and the like. PCB "manufacturing" is broadly defined
herein and includes all stages of the PCB manufacturing and
assembly process, including, for example, preparing or obtaining
materials to make PCB layers, providing at least a first PCB layer,
processing one or more PCB layers to form circuit patterns
separated by insulating layers, assembling a PCB by mounting an
electrical component onto a partially, substantially or fully
completed PCB, testing a PCB assembly package comprising electric
devices mounted thereon, etc. Various embodiments herein describing
manufacturing rigid PCBs are also applicable to manufacturing rigid
portions of partially rigid PCBs.
[0036] Referring now to FIG. 1, an embodiment of a multilayer
printed circuit board (PCB) 100 is illustrated. The PCB 100
comprises first and second conductive outer or surface layers 120,
120a, first and second insulating layers 125, 125a, first, second,
and third conductive inner layers 130, 130a, 130b, and first and
second insulating inner layers 135, 135a. The first and second
insulating layers 125, 125a of FIG. 1 engage the first and second
surface conductive layers 120, 120a and, therefore, are cap layers.
The conductive inner layers 130, 130a, 130b can be etched to form
first, second, and third circuit patterns 123, 123a, 123b.
[0037] As illustrated in FIG. 1, the first surface conductive layer
120 is over the first insulating layer 125 and the first insulating
layer 125 is provided over the top surface of the first circuit
pattern 123. The first circuit pattern 123a is over the top surface
of the first insulating inner layer 135, the latter of which is
positioned over the second circuit pattern 123b, which in turn is
over the top surface of the second insulating inner layer 135a. As
shown in FIG. 1, the third circuit pattern 123b is over the top
surface of the second insulating layer 125a and the second
insulating layer 125a is provided over the second surface
conductive layer 120a.
[0038] The surface conductive layers 120, 120a and/or the circuit
patterns 123, 123a, 123b can comprise any suitable conductive
metals, such as, for example, copper, gold, aluminum, nickel,
kovar, steel, resistance alloys, etc. PCB conductive layers are
typically made of thin copper foil. As shown in FIG. 1, at least
one of the insulating layers 125, 125a and/or the insulating inner
layers 135, 135a comprises an ordinary insulating layer comprising
a wide array of rigid materials such as epoxy resin, FR-3, FR-4,
etc. In certain embodiments, at least one of the insulating layers
125, 125a and/or the insulating inner layers 135, 135a comprising
rigid material is substantially coplanar with at least one of the
surface conductive layers 120, 120a such that the PCB 100 is a
rigid PCB. In certain embodiments, the at least one of the
insulating layers 125, 125a and/or the insulating inner layers 135,
135a comprising rigid material is substantially coextensive (e.g.,
cross sectional length) with at least one of the surface conductive
layers 120, 120a. In various embodiments, the PCB 100 comprises at
least one rigid insulating layer (e.g., the first and second
insulating layers 125, 125a, the first and second insulating inner
layers 135, 135a, or another insulating layer not shown) extending
through the entire length of the PCB 100 such that the PCB 100 is a
rigid PCB. Dielectric materials, including the insulating layers
125, 125a and the insulating inner layers 135, 135a can be selected
based on properties such as, for example, thermal stability,
dielectric constant, flexibility, tensile strength, and dimensional
stability.
[0039] FIG. 2 illustrates another embodiment of a multilayer PCB
200 disclosing the PCB 100 of FIG. 1 further comprising first,
second, third, and fourth levels of via holes 250, 250a, 250b, 250c
and conductive plates 265, 265a, 265b, 265c. The via holes 250,
250a, 250b, 250c and the conductive plates 265, 265a, 265b, 265c,
the latter being at least partially over some portions of the via
holes 250, 250a, 250b, 250c, the surface conductive layers 120,
120a, and the circuit patterns 123, 123a, 123b, generally are
configured to electrically connect various conductive layers of the
PCB 200, as will be discussed below.
[0040] As illustrated in FIG. 2, the first level via holes 250 are
shown penetrating the first surface conductive layer 120 and the
first insulating layer 125. In FIG. 2, the first layer via holes
250 and some portions of the first surface conductive layer 120 are
coated with the conductive plate 265. The first level via holes 250
and the conductive plate 265 connect some portions of the surface
conductive layer 120 with some portions of the first conductive
inner layer 130. The second layer of via holes 250a are shown
penetrating the first conductive inner layer 130 and the first
insulating inner layer 135. The second layer via holes 250a and
some portions of the first conductive inner layer 130 are coated
with the conductive plate 265a. The second layer via holes 250a and
the conductive plate 265a connect some portions of the first
conductive inner layer 130 with some portions of the second
conductive inner layer 130a.
[0041] Still with reference to FIG. 2, the third layer of via holes
250b are shown penetrating the second conductive inner layer 130a
and the second insulating inner layer 135b. The third layer via
holes 250b and some portions of the second conductive inner layer
130a are coated with the conductive plate 265b and connect some
portions of the second conductive inner layer 130a with some
portions of the third conductive inner layer 130b. The fourth layer
via holes 250c penetrate the third conductive inner layer 130b and
the second insulating layer 125a. The fourth layer via holes 250c
and portions of the third conductive inner layer 130b are coated
with the conductive plate 265c and connect some portions of the
third conductive inner layer 130b with some portions of the second
surface conductive layer 120a. In some embodiments, the first
surface conductive layer 120, the second surface conductive layer
120a, or both the first and second surface conductive layers 120,
120a are etched to create pads 299, for example, to electrically
connect an electrical component such as a semiconductor chip (not
shown) with the PCB 200.
[0042] As shown in FIGS. 1 and 2, the PCB 100 and the PCB 200 are
provided as non-limiting illustrative embodiments and although the
figures show PCBs comprising four insulating layers (the first and
second insulating layers 125, 125a and the first and second
insulating inner layers 135, 135a) and five conductive layers (the
first and second surface conductive layers 120, 120a and the first,
second, and third circuit patterns 123, 123a, 123b) arranged in the
configurations disclosed therein, the various embodiments and
features disclosed throughout this application can be used in
connection with PCBs comprising a different number (for example,
more or fewer than five conductive layers and/or four insulating
layers) and a different arrangement of conductive and/or insulating
layers. For example, although the via holes 250, 250a, 250b, 250c
of PCB 200 penetrate only a single layer of electrically conductive
layer and a single layer of insulating layer, the via holes 250,
250a, 250b, 250c in other embodiments can be configured to comprise
varying lengths penetrating more or fewer layers. In one
embodiment, a solder resist layer can be further deposited on the
top surface of one or both of the outer most conductive layers 120,
120a. In some embodiments, as least some of the via holes 250,
250a, 250b, 250c can be at least partially filled with solder
resist material. In certain embodiments, the PCB 200 can comprise
one or more through-holes penetrating one or more of the layers of
the PCB 200 to accommodate insertion of electrical component leads.
In some embodiments, at least one of the first and second
insulating layers 125, 125a or the first and second insulating
inner layers 135, 135a comprises rigid material. In certain
embodiments, portions of the at least one of the first and second
insulating layers 125, 125a or the first and second insulating
inner layers 135, 135a comprising rigid material are coplanar with
some portions of the at least one of the surface conductive layers
120, 120a such that the PCB 200 comprises at least some rigid
portions comprising the rigid portions of a partially rigid PCB. In
certain embodiments, substantial portions of the at least one of
the first and second insulating layers 125, 125a or the first and
second insulating inner layers 135, 135a comprising rigid material
are coplanar with the at least one of the surface conductive layers
120, 120a such that the PCB 400 is a rigid PCB. In various
embodiments, the PCB 400 comprises at least one rigid insulating
layer (e.g., the first and second insulating layers 125, 125a, the
first and second insulating inner layers 135, 135a, or another
insulating layer not shown) extending through the entire length of
the PCB 400 such that the PCB 400 is a rigid PCB. In some
embodiments, the PCB 400 comprises a plurality of rigid insulating
layers, some of which extend substantially less than the entire
length of the PCB 400 (e.g., half, a third, etc.), arranged in a
manner such that the combination of the plurality of rigid
insulating layers makes the PCB 400 a rigid PCB (e.g., a rigid
insulating layer extending roughly through half the length of the
PCB 400, another rigid layer extending roughly through the
remaining half, etc.).
[0043] FIGS. 3A-3E include depictions of various embodiments
illustrating an example defect that can be caused by, among others,
the strain (e.g., thermal strain) put on PCBs during the
manufacturing process. FIG. 3A is a top plan view of the top
surface 325 of a PCB 300 comprising, for example, one or more of
the PCBs 200 of FIG. 2. The top surface 325 has thereon a
simplified dog bone design comprising conductive pads 320, plated
via holes 340, and connectors 330. The PCB 300 comprises a ball
grid array (BGA) mounting technology wherein the array of
conductive pads 320 are configured to connect to corresponding
conductive pads of surface mountable electrical components (not
shown) to electrically attach or mount the electrical components
with the PCB 300. In some embodiments, the PCB 300 can be
configured to comprise different electrical component packaging
technologies such as, without limitation, Dual In-line Packaging
(DIP), Pin Grid Array (PGA), Leadless Chip Carrier (LCC), Flip-chip
BGA (FCBGA), Plastic Quad Flat Pack (PQFP), Small-Outline
Integrated Circuit (SOIC), Plastic Leaded Chip Carrier (PLCC),
System in Package (SIS), combinations thereof, and the like.
[0044] FIG. 3B shows a cross-section view of portions of an
electrical component 308, soldering material 307, and a portion of
the PCB 300 of FIG. 3A. A conductive chip pad 305 is attached to
the electrical component 308. For simplicity, the PCB 300 of FIG.
3B shows only one of the conductive pads 320 attached to the PCB
300 of FIG. 3A. As shown in FIG. 3B, the conductive chip pad 305 of
the electrical component 308 can be electrically connected to the
conductive pad 320 of the PCB 300 using the soldering material 307.
The assembly can be heated, for example using a reflow oven or an
infrared heater, to melt the solder ball 307 and to thereby
mechanically couple the electrical component 308 with the PCB 300.
Once coupled, the electrical component 308 and the PCB 300 are
electrically connected, and electric signals from the conductive
pad 320 can flow to the electrical component 308 through the
soldering material 307 and the electrically conductive chip pad
305.
[0045] During the manufacturing of PCBs, an electrical component
assembly process, or normal operation of PCBs, cracks can occur on
one or more layers of the PCBs. One cause of such cracks is the
considerable thermal stress (e.g., including mechanical stress
arising from changes in temperature) to which PCBs are subjected,
for example, during the manufacturing process including heating of
the soldering material. Various materials used in the assembly
processes, such as insulating layers, conductive layers, soldering
metals, and electrical components can have different coefficients
of thermal expansion (CTE), potentially causing these materials to
expand and contract at different rates in response to changes in
temperature. As such, thermal stress during the PCB assembly
process can arise from mismatches in CTE, both between the
electrical components, including soldering material, and the PCB
boards onto which the electrical components are mounted, and
between the different materials which make up the PCB. In the case
of a type of crack called pad cratering, thermal mismatch or
Coefficient of Thermal Expansion (CTE) mismatch, for example
between cap layers and surface conductive layers, can cause a
defect such as a crack in the cap layers as the cap layers and the
surface conductive layers respond (e.g., expand or contract) to
temperature changes at unequal rates. For example, when heat is
applied to soldering material 307, CTE mismatch between the
conductive pad 320 and portions of the cap layer underneath the
conductive pad 320 can cause portions of the cap layers to move
relative to the conductive pad 320 (e.g., opposite direction),
separating some portions of the cap layer from the conductive pad
320. Pad cratering can cause portions of the PCB to be separated or
fall off, resulting in mechanical failure in the PCB, or can create
a defect in the flow of electricity in the PCB, causing an
electrical failure. In certain embodiments, thermal stress can
cause the conductive pads 320 to partially, substantially, or fully
separate from the underlying cap layer. The at least partially
separated conductive pads 320 can remove portions of the cap layer
still attached to portions of the at least partially separated
conductive pads 320, thereby creating holes or craters the cap
layer from which the at least partially separated conductive pads
320 separate. Strain such as thermal strain can also cause a defect
by applying stress to the junction connecting the cap layer and the
conductive pad 320 without forming a crack in the cap layer to
potentially cause intermittent or thermally sensitive electrical or
mechanical failures.
[0046] Still with reference to 3B, the recent trend of using
lead-free PCB manufacturing processes including lead-free soldering
has exacerbated pad cratering. The leading lead-free alloys used in
the PCB assembly process such as tin, bismuth, copper, various
proprietary mixtures of some of these alloys, and/or other
materials have higher melting points than lead-based soldering
material, necessitating the use of higher temperatures to melt the
soldering material 307 to couple, for example, the semiconductor
electrical component 308 and the PCB 300. As CTEs are a function of
temperature, the application of higher temperatures to the various
layers of the PCB 300 and the electrical component 308 can put even
more strain on the PCB 300, the various layers of the PCB 300
(e.g., between insulating and conductive layers), the electrical
component 308, and the various layers of the electrical component
308 by increasing differences due to thermal expansion, thereby
increasing thermal stress.
[0047] Still with reference to 3B, the PCB 300 can also be
subjected to more mechanical strain, including during the
manufacturing process, as a result of rising manufacturing
temperatures. The use of increasing reflow-soldering temperatures
can correspondingly increase the hardness of insulating layers,
including cap layers, making these insulating layers more brittle
and more susceptible to mechanical stress. Further, the leading
non-lead based soldering materials typically have harder and
stiffer properties than lead-based soldering materials and,
therefore, can generate higher mechanical forces on the conductive
pads 320 or insulating layers of the PCB 300 including the cap
layers engaging the conductive pads 320. Alone or in combination,
these factors can increase the frequency of cracks, including pad
cratering, that can occur in insulating cap layers engaging the
conductive pads 320. In certain embodiments, strain as described
herein applies stress to junctions connecting insulating layers and
conductive layers, causing defects in the connection (e.g., sever
partially or completely, undermine connectivity, etc.) creating
electrical or mechanical failures.
[0048] FIGS. 3C-3E illustrate an embodiment of pad cratering that
can occur, for example, in an insulating layer 325 positioned below
surface conductive layers of PCBs, including, for example, the
conductive pad 320 of FIG. 3B. Although the pad cratering embodied
in FIGS. 3C-3E is shown as occurring in the insulating layer 325
underneath the conductive pad 320 of FIG. 3B, FIGS. 3C-3E
illustrate only one embodiment and cracks can occur in different
layers, including, without limitation, the insulating inner layers
135, 135a of FIG. 2, and in insulating layers engaging different
conductive layers, such as, without limitation, the conductive
inner layers 130, 130a, 130b. FIG. 3C shows the conductive pad 320
and the insulating layer 325 (for example, the first insulating
layer 125 of FIG. 2) engaging the conductive pad 320 under normal
circumstances. FIG. 3D shows the connection between the conductive
pad 320 and the insulating layer 325, wherein the insulating layer
325 is beginning to form a crack 370 (pad cratering) as a result of
the strain exerted onto the conductive pad 320. As can be seen in
FIG. 3D, the crater 370 separates at least one end of the
insulating layer 325 into top portion 175 and bottom portion 185.
FIG. 3E illustrates a more substantial pad cratering 380 wherein
the top portion 175 of the insulating layer 325 is separated from
the bottom portion 185, likely causing failure in the PCB 300 or
portions of the PCB 300.
B. Detailed Descriptions of Non-Limiting Embodiments
[0049] Methods and systems for use with manufacturing, assembling,
and using PCBs, including rigid and partially rigid PCBs,
comprising more stable (e.g., thermally, mechanically, physically,
etc.) strain resistant layers to resist damage that can arise from
strain and/or stress (e.g., mechanical, thermal, physical, and the
like) will now be described with reference to the accompanying
drawings.
[0050] FIG. 4 illustrates a multilayer PCB 400 comprising the PCB
100 of FIG. 1 and further comprising first and second strain
resistant layers 450, 450a. The first surface conductive layer 120
is over the first strain resistant layer 450, the latter of which
is over the first insulating layer 125. The top surface of the
first strain resistant layer 450 engages the first surface
conductive layer 120 and the bottom surface of the first strain
resistant layer 450 engages the first insulating layer 125. The
second strain resistant layer 450a is over the second surface
conductive layer 120a and the second insulating inner layer 125a is
over the second strain resistant layer 450a. The second strain
resistant layer 450a is between the second surface conductive layer
120a and the second insulating layer 125a and the first strain
resistant layer 450 is between the first surface conductive layer
120. The bottom surface of the second strain resistant layer 450a
engages the second surface conductive layer 120a and the top
surface of the second strain resistant layer 450a engages the
second insulating layer 125a. In some embodiments, a pre-formed
laminate component comprising the first strain resistant layer 450
engaging the first surface conductive layer 120 or a pre-formed
laminate component comprising the second strain resistant layer
450a engaging the second surface conductive layer 450b can be used
to manufacture the PCB 400 by attaching the pre-formed laminate
component to the remaining layers of the PCB 400 (e.g., the first
or second insulating layers 125, 125a).
[0051] In some embodiments, the strain resistant layers 450, 450a
comprise suitable commercially available materials, such as, for
example, Kapton.RTM. polyimide film, available from E. I. du Pont
de Nemours and Company (DuPont). In certain embodiments, the PCB
400 comprises pre-formed layers of the first strain resistant layer
450 engaging the first surface conductive layer 120 and/or the
second strain resistant layer 450a engaging the second surface
conductive layer 450b. In some embodiments, the PCB 400 comprises
commercially available pre-formed components of conductive and
strain resistant layers such as, for example, R/Flex 1000.RTM.
available from Rogers Corporation and Pyralux.RTM. LF, Pyralux.RTM.
AC, Pyralux.RTM. FR, available from DuPont, and the like.
[0052] In FIG. 4, the PCB 400 comprises the strain resistant layers
450, 450a, the surface conductive layers 120, 120a, the insulating
layers 125, 125a, the conductive inner layers 130, 130a, 130b, and
the insulating inner layers 135, 135a. However, the PCB 400 may
comprise more or fewer layers of materials or structures, for
example materials or structures not illustrated in FIG. 4. In some
embodiments, the strain resistant layers 450, 450a are configured
to engage the first and second surface conductive layers 120, 120a,
respectively, with other layers of the PCB 400, such as, for
example, the insulating layers 125, 125a, respectively. The PCB 400
may comprise additional layers not shown in FIG. 4 such as cover
coats to protect the PCB 400 against corrosion and contamination,
other materials that for example bond various layers of the PCB
400, and the like. In certain embodiments, a solder resist layer
can be over the top surface of one or both of the outermost surface
conductive layers 120, 120a. In some embodiments, the PCB 400 can
comprise a through-hole penetrating one or more of the layers of
the PCB 400. In certain embodiments, the PCB 400 comprises one or
more via holes plated with conductive material. Other various
configurations are also possible. In some embodiments, the PCB 400
is a single sided and single layer PCB. In some embodiments, the
PCB 400 is a double sided and single layer PCB In an embodiment,
the PCB 400 comprises at least one rigid insulating layer extending
throughout the entire length of the PCB 400 such that the PCB 400
comprises entirely rigid portions.
[0053] Still with reference to FIG. 4, the surface conductive
layers 120, 120a can be manufactured using one or more suitable
processes. In certain embodiments, the surface conductive layers
120, 120a comprise electrodeposited copper made by, for example,
plating copper from a copper anode to a cathode. In some
embodiments, the PCB 400 comprises surface conductive layers 120,
120a comprising rolled-annealed copper foil. Rolled-annealed copper
foil may be made, for example, by heating copper ingots and rolling
and annealing the ingots by passing the ingots through a serious of
rollers. Certain non-rigid PCBs can comprise surface conductive
layers made of rolled-annealed copper because rolled-annealed
copper comprises qualities such as ductility (ability to stretch
without breaking given as a ratio of length of stretched portion
and original length) make rolled-annealed copper suitable for
non-rigid purposes such as, for example, flexibility. In some
embodiments, ductility of rolled-annealed copper is about 20-45%,
including 25%, 30%, 35%, and 40%. Rolled-annealed copper has not
been used with rigid PCBs for several reasons, including higher
production costs (e.g., compared with electrodeposited copper),
lack of availability of various thicknesses and widths, and the
perceived lack of benefit of the flexibility of rolled-annealed
copper when used in connection with rigid PCBs comprising
non-bending portions. The Applicant has recognized that certain
qualities exhibited by rolled-annealed copper, such as ductility,
hardness, resistance, etc. can make rolled-annealed copper suitable
for rigid PCBs (e.g., may minimize defects from forming in the
rigid PCB during lead-free manufacturing). For example, the PCB 400
comprising the surface conductive layer 120 comprising
rolled-annealed copper can absorb some thermal stress as elastic
deformation, thereby likely reducing defects such as pad cratering
from occurring, for example, in portions of the cap layer
underneath the surface conductive layer 120. In other embodiments,
the ability of the PCB 400 to absorb thermal stress may be
increased by using the surface conductive layers 120, 120a
comprising rolled-annealed copper in combination with the strain
resistant layers 450, 450a comprising thermal characteristics
(e.g., ductility) as further described herein. The surface
conductive layers 120, 120a can be manufactured using one or more
the above processes, other processes, and combinations thereof.
[0054] With continued reference to FIG. 4, the surface conductive
layers 120, 120a and the strain resistant layers 450, 450a can be
manufactured using one or more suitable processes or methods. In
some embodiments, a method of manufacturing the PCB 400 comprises
attaching the surface conductive layer 120 to the strain resistant
layer 450 using an intermediate layer such as, for example, a
bonding intermediate layer (e.g., adhesive).
[0055] In some embodiments, a method of manufacturing the PCB 400
comprises adhesivelessly attaching the surface conductive layers
120, 120a to the strain resistant layers 450, 450a, respectively.
The surface conductive layers 120, 120a can be adhesivelessly
attached to the strain resistant layers 450, 450a, respectively,
using one or more suitable methods. The surface conductive layers
120, 120a and the strain resistant layers 450, 450a can be
adhesivelessly attached using a "cast to foil" process wherein a
solution of strain resistant material, such as, for example,
polyimide, is applied to the conductive layers 120, 120a and
heated, resulting the strain resistant layer 450, 450a over the
surface conductive layers 120, 120a, respectively. In one
embodiment, the PCB 400 comprises polyimide (e.g., the strain
resistant layer 450) cast-on-copper (e.g., the surface conductive
layer 120). The surface conductive layers 120, 120a and the strain
resistant layers 450, 450a can also be adhesivelessly attached
using a "sputtering" process wherein conductive cathode (e.g.,
copper) is bombarded with ions to cause conductive particles
impinge on each of the strain resistant layers 450, 450a such that
the surface conductive layers 120, 120a are over the strain
resistant layers 450, 450a, respectively. In certain embodiments, a
method of manufacturing the PCB 400 comprises plating (e.g.,
electroless plating) conductive material on each of the strain
resistant layers 450, 450a to form the surface conductive layers
120, 120a adhesivelessly engaging the strain resistant layers 450,
450a, respectively. In some embodiments, a "vapor deposition"
method of making the PCB 400 comprises vaporizing conductive
material such as copper in a vacuum chamber and depositing the
metal vapor on strain resistant material, thereby forming the
strain resistant layers 450, 450a adhesivelessly engaging the
surface conductive layers 120, 120a, respectively. Further still, a
method of making the PCB 400 can comprise further treating the
surface conductive layers 120, 120a engaging the strain resistant
layers 450, 450a, either adhesivelessly or using an intermediate
bonding layer such as an adhesive, with other processes such as,
for example, bonding, stabilizing, etc.
[0056] Although FIG. 4 illustrates the strain resistant layers 450,
450a positioned as cap layers and engaging the surface conductive
layers 120, 120a, respectively, the strain resistant layers 450,
450a can be suitably used with the PCB 400 in other configurations.
For example, although FIG. 4 shows the strain resistant layer 450
between the surface conductive layer 120 and the insulating layer
125, the strain resistant layer 450 can be the only dielectric
between the surface conductive layer 120 and the first electrically
conductive inner layer 130. In some embodiments, additional strain
resist materials comprising, for example, polyester, liquid crystal
polymer, polyimide, etc. can also be suitably used as binding
material in the central, normally rigid, glass layer/prepreg
portions of PCBs. In still some embodiments, one or more of the
strain resistant layers may be formed (e.g., deposited) elsewhere
in the PCB 400. In one embodiment, strain resistant layers can be
between and/or engage inner conductive layers, for example, the
conductive inner layer 130a, and inner insulating layers, such as,
for example, the insulating inner layer 135a. In some embodiments,
the strain resistant layers 450, 450a are configured to
respectively engage the first and second surface conductive layers
120, 120a with other layers of the PCB 400, such as, for example,
the conductive inner layers 130, 130a.
[0057] With continued reference to FIG. 4, the strain resistant
layers 450, 450a in certain embodiments comprise at least
substantially fiberglass-free material. The strain resistant layers
450, 450a comprising fiberglass-free material can help to reduce
(e.g., minimize, eliminate) the occurrence of electrical failure
arising from cathodic/anodic filament (CAF) growth. CAF growth can
result in an electrical shorting failure when dendritic metal
filaments grow along insulating interfaces (typically layers
comprising glass fiber/epoxy resin interface), such as, for
example, the insulating inner layers 135, 135a and/or the
insulating layers 125, 125a, creating an electrical path between
two or more layers of the PCB 400 that should remain electrically
isolated (for example, portions of the surface conductive layer 120
and portions of the conductive inner layer 130 or portions of the
conductive inner layer 130 and portions of the conductive inner
layer 130a). Strain resistant layers in accordance with embodiments
disclosed herein can reduce (e.g., minimize, eliminate) CAF growth
because, as previously mentioned, the strain resistant layers (for
example, the strain resistant layers 450, 450a) do not comprise or
are substantially free of fiberglass material that might otherwise
provide the surface along which the dendritic metal filaments may
grow. In certain embodiments, one or more strain resistant layers
can be used instead of one or more rigid insulating layers of PCBs,
including rigid PCBs (e.g., instead of one or more of the
insulating inner layers 135, 135a and/or the insulating layers 125,
125a).
[0058] In accordance to certain embodiments, each of the strain
resistant layers 450, 450a can have a thickness in the range of
about 10-30 microns, including about 10-15 microns, 12-15 microns,
15-17 microns, 15-20 microns, 15-25 microns, and 20-30 microns. In
certain embodiments, each of the strain resistant layers 450, 450a
can have a thickness of about 12 microns, including about 15
microns, 17 microns, 18 microns, 20 microns, 22microns, 25 microns,
and 28 microns. The strain resistant layers 450, 450a can comprise
thicknesses in the range of about 5-100 microns, including about
10-20 microns, 20-30 microns, 30-40 microns, 40-50 microns, 50-60
microns, 60-70 microns, 70-80 microns, 80-90 microns, 90-100
microns, 100-120 microns, 110-130 microns, and the like. In certain
embodiments, each of the strain resistant layers 450, 450a can have
a thickness of about 35 microns, including about 45 microns, 55
microns, 65 microns, 75 microns, 85 microns, 95 microns, 105
microns, and the like. In one embodiment, each of the strain
resistant layers 450, 450a have thicknesses of less than 10 microns
(e.g., 8 microns), thicknesses of greater than 30 microns (e.g.,
about 32 microns), or both. In one embodiment, each of the strain
resistant layers 450, 450a have thicknesses of less than 5 microns
(e.g., 1 micron), thicknesses of greater than 130 microns (e.g.,
about 150 microns), or both. In accordance to certain embodiments,
each of the surface conductive layers 120, 120a can have a
thickness in the range of about 15-25 microns, including about
15-17 microns, 16-18 microns, 17-19 microns, 16-19 microns, and
18-20 microns. The surface conductive layers 120, 120a can comprise
thicknesses in the range of about 5-100 microns, including about
10-20 microns, 20-30 microns, 30-40 microns, 40-50 microns, 50-60
microns, 60-70 microns, 70-80 microns, 80-90 microns, 90-100
microns, 100-120 microns, 110-130 microns, and the like. In certain
embodiments, each of the surface conductive layers 120, 120a can
have a thickness of about 35 microns, including about 45 microns,
55 microns, 65 microns, 75 microns, 85 microns, 95 microns, 105
microns, and the like. In accordance to various embodiments, each
of the surface conductive layers 120, 120a can have a thickness of
about 16 microns, including 17 microns, 18 microns, 19 microns, 20
microns, 25 microns, 26 microns, etc. In an embodiment, each of the
surface conductive layers 120, 120a can have thicknesses of less
than about 15 microns (e.g., about 14 microns), a thicknesses of
greater than about 25 microns (e.g., 27 microns), or both. In one
embodiment, each of the surface conductive layers 120, 120a can
have thicknesses of less than about 5 microns (e.g., about 1
micron), a thicknesses of greater than about 130 microns (e.g.,
about 150 microns), or both. In an embodiment, the strain resistant
layers 450, 450a can be about 0.0005 inches thick.
[0059] FIG. 5 depicts another embodiment of a multilayer PCB 500
comprising the PCB 200 of FIG. 2 and further comprising first and
second strain resistant layers 550, 550a. In some embodiments, the
first and second strain resistant layers 550, 550a each comprise a
strain resistant material (e.g., polyimide). The first strain
resistant layer 550 is between the first surface conductive layer
120 and the first insulating layer 125. In some embodiments, the
top surface of the first strain resistant layer 550 engages the
first surface conductive layer 120 and the bottom surface of the
first strain resistant layer 550 engages other insulating layers of
the PCB 500, such as the first insulating layer 125. The first
level via holes 250 are formed on the PCB 500 penetrating the first
surface conductive layer 120, the first strain resistant layer 550,
and the first insulating layer 125. In some embodiments, the first
and second strain resistant layers 550, 550a are configured to
engage the first and second surface conductive layers 120, 120a
with other layers of the PCB 500, such as, for example, the
conductive inner layers 130, 130a.
[0060] In some embodiments, the strain resistant material comprises
polyimide. In certain embodiments, the surface conductive layer 120
and the first strain resistant layer 550 are pre-formed by
electrodepositing conductive material on the first strain resistant
layer 550 and the second surface conductive layer 120a and the
second strain resistant layer 550a are pre-formed by
electrodepositing conductive material on the second strain
resistant layer 550a. In some embodiments, the pre-formed layers
each comprise commercially available copper and polyimide copper
clad laminates such as, for example, Pyralux.RTM. AF, available
from DuPont.
[0061] In some embodiments, the first and second strain resistant
layers 550, 550a engage the first and second insulating layers
125,125, respectively, using other mechanisms. In certain
embodiments, additional strain resistant layers may be positioned
elsewhere in the PCB 500. For example, a strain resistant layer
(not shown) may be used as a non-cap layer to engage with one or
more of the conductive inner layers 130, 130a, 130b. In some
embodiments, strain resistant layers may be employed instead of or
in combination with one or more of the insulating inner layers 135,
135a and/or the insulating layers 125, 125a.
[0062] In accordance with various embodiments disclosed herein, a
number of processes and methods can be used to manufacture rigid
PCBs (or rigid portions of partially rigid PCBs) and assemblies
comprising semiconductor and/or circuit components. In some
embodiments, a methods of manufacturing a rigid printed circuit
board assembly comprises providing a first component comprising a
conductive layer and a strain resistant layer, providing a stack of
laminates comprising at least one insulating layer, and attaching
the first component to the stack of laminates, thereby at least
partially (e.g., fully) forming a printed circuit board. In one
embodiment, the stack of laminates comprises at last one rigid
insulating layer. The conductive layer of the first component can
be a surface conductive layer and/or the strain resistant layer of
the first component can be a cap layer. In certain embodiments, the
method of manufacturing PCBs further comprises mounting or
attaching a circuit component on the printed circuit board to
thereby form a rigid printed circuit board assembly. In some
embodiments, providing the first component comprises providing
pre-formed layers of the surface conductive layer 120 and the first
strain resistant layer 550 and/or the surface conductive layer 120a
and the second strain resistant layer 550a. In some embodiments,
providing the first component comprises adhesivelessly attaching
the conductive layer to the strain resistant layer. In certain
embodiments, the conductive layer of the first component comprises
rolled-annealed copper and/or the strain resistant layer of the
first component comprises polyimide.
[0063] In some configurations, providing a stack of laminates
comprises providing at least one insulating layer comprising a
rigid material. In certain embodiments, providing a stack of
laminates comprises forming one or more internal conductive layers
engaging the one or more insulating layers. In some embodiments,
providing the stack of laminates further comprises etching the one
or more internal conductive layers, thereby forming circuit
patterns (e.g., the circuit patterns 123, 123a, 123b). With respect
to FIG. 5, the first and second insulating layers 125, 125a, the
first, second, and third conductive inner layers 130, 130a, 130b,
and first and second insulating inner layers 135, 135a can form the
stack of laminates. In some arrangements, attaching the first
component to the stack of laminates comprises connecting the strain
resistant layer of the first component with the stack of laminates.
In an embodiment, a strain resistant layer (e.g., the strain
resistant layer 550) comprises a first surface and a second
surface, wherein the first surface is substantially opposite from
the second surface (e.g., the top surface of the strain resistant
layer 550 engaging the bottom surface of the surface copper layer
120 is substantially opposite from the bottom surface of the strain
resistant layer 550 engaging the top surface of the inner
insulating layer 125). In some arrangements, attaching the first
component to the stack of laminates comprises attaching the bottom
surface of the strain resistant layer to the top surface of the
stack (e.g., top surface of a first insulating layer of the stack
such as the inner insulating layer 125 of FIG. 5). In some
configurations, mounting the circuit component onto the rigid
printed circuit board to form a rigid printed circuit board
assembly comprises connecting the circuit component to the
conductive layer of the first component using soldering
material.
[0064] FIG. 6 illustrates an embodiment of methods and components
for manufacturing PCBs, including rigid and partially rigid PCBs,
some portions of which may be disclosed in U.S. Pat. No. 5,674,596,
the entire content of which is expressly incorporated herein by
reference. A stack 600 of laminates for use with rigid PCBs
comprises three components 610, 610a, 610b, each comprising a
discardable separator layer 630, and two PCB laminated layers 611,
612. For illustrative purposes only, the PCB laminated layers 611,
612 are illustrated similarly to the PCB 100 of FIG. 1 (e.g., each
comprising insulating outer layers 125, 125a, inner conductive
layers 130, 130a, and inner insulating layers 135, 135a) but
without the surface conductive layers 120, 120a of FIG. 1. The
components 610, 610a, 610b each can comprise a discardable layer
630 comprising discardable materials such as metals (e.g.,
aluminum) between two conductive layers 620, 640. The conductive
layers 620, 640 can comprise material different from the
discardable layer 630 (e.g., comprising copper when the discardable
layer 630 comprises aluminum). The surfaces of the conductive
layers 620, 640 facing the discardable layer 630 can be processed
to be substantially free of particles or defects, and are protected
from exposure to various contaminants, such as, for example,
airborne particles and resin dust, by the discardable layer 630.
Although the stack 600 shows the three components 610, 610a, 610b
and the two PCB laminated layers 611, 612, the configuration is for
illustrative purposes only and the stack 600 can comprise more or
fewer numbers of components and/or PCB laminate layers.
[0065] In an embodiment of manufacturing one or more PCBs,
including rigid PCBs, a method comprises releasing the conductive
layers 620, 640 from the discardable layer 630 and to form the
outer conductive layers of PCBs as described herein. For example,
the method can comprise attaching the conductive layers 620, 640 of
the component 610a as surface conductive layers to the insulating
outer layer 125a of the PCB laminate layer 611 and the insulating
outer layer 125 of the PCB laminate layer 612, respectively. An
embodiment of the method can comprise at least partially separating
one or both of the conductive layers 620, 640 of the component 610a
from the discardable layer 630 of the component 610a. The
conductive layer 640 of the component 610 can attach as a surface
conductive layer to the insulating outer layer 125 of the PCB
laminate layer 611. The conductive layer 640 of the component 610
can then at least partially separate from the discardable layer 630
of the component 610. In certain such embodiments, the method can
also comprise attaching the conductive layer 620 of the component
610 as a surface conductive layer to the insulating outer layer of
another PCB laminate layer (not shown above the component 610), and
the conductive layer 620 of the component 610 can then at least
partially separate from the discardable layer 630 of the component
610. The conductive layer 620 of the component 610b can attach as a
surface conductive layer to the insulating layer 125a of the PCB
laminate layer 612. The conductive layer 620 can then at least
partially separate from the discardable layer 630 of the component
610b. In certain such embodiments, the method can comprise
attaching the conductive layer 640 as a surface conductive layer to
the insulating outer layer of another PCB laminate layer (not shown
below the component 610b), and then at least partially separating
the conductive layer 640 of the component 610b from the discardable
layer 630 of the component 610b. After separation from the
conductive layers 620, 640 of the components 610, 610a, 610b, the
method can comprise discarding the discardable layers 630 of the
components 610, 610a, 610b (e.g., by selective etching of the
material of the discardable layers 630). In certain embodiments,
the conductive layers of the components comprising discardable
layer (e.g., the conductive layer 620 of the component 610a
comprising the discardable layer 630) are first released from the
components by, for example, at least partially separating the
conductive layers from the discardable layer, and then attached to
the conductive layers as surface conductive layers of PCB laminates
(e.g., to the insulating layer 125a of PCB laminate 611).
[0066] FIG. 7A shows a component 700 for manufacturing PCBs
comprising two conductive layers 720, 720a and a discardable layer
730 comprising discardable materials, including metals, such as,
for example, aluminum. The two conductive layers 720, 720a (e.g.,
each comprising copper) are over opposite sides of the discardable
layer 730. The component 700 further comprises stress resistant
layers 750, 750a over the outer surfaces of the conductive layers
720, 720a. Returning back to FIG. 6, the component 700 of FIG. 7
can be used instead of one or more of the components 610, 610a,
610b, thereby attaching the strain resistant layers 750, 750a, as
well as the conductive layers 720, 720a, onto one or more of the
PCB laminate layers 611, 612 and/or other PCB laminate layers (not
shown). For example, when using the component 700 in place of the
component 610a, the method of manufacturing PCBs can comprise
attaching the strain resistant layers 750, 750a to the insulating
outer layer 125a of the PCB laminate layer 611 and the insulating
outer layer 125 of the PCB laminate layer 612, respectively. The
conductive layers 720, 720a of the component 700, which still are
attached to the strain resistant layers 750, 750a, respectively,
also attach to the PCB laminate layers 611, 612, respectively, as
surface conductive layers. In certain embodiments, the method can
comprise at least partially separating one or both of the
conductive layers 720, 720a of the component 700 from the
discardable layer 730 of the component 700. In an embodiment, the
discardable layer 730 of the component 700 can be discarded (e.g.,
by selective etching of the material of the discardable layer 730).
In a further embodiment, the discardable layer 730 is discarded
after one or both of the conductive layers 720, 720a at least
partially separate from the discardable layer 730. In this manner,
the conductive layers 720, 720a and the strain resistant layers
750, 750a can be effectively attached to PCB laminates 611, 622,
respectively. In some embodiments, the component 700 comprises
pre-formed layers of the first conductive layer 720 engaging the
strain resistant outer layers 750, the second conductive layer 720a
engaging the second stress resistant outer layers 750a, or both. In
other embodiments, the pre-formed layers comprise laminated
products, such as, for example, DuPont's Pyralux.RTM. FR,
Pyralux.RTM. LF, etc.
[0067] FIG. 7B shows an embodiment of another component 710 for use
with manufacturing PCBs, including rigid PCBs. The component 710
comprises a discardable separator 730b, a conductive layer 720b,
and a strain resistant layer 750b. The conductive layer 720b is
over the discardable layer 730b and engages the discardable layer
730b. The strain resistant layer 750b is over the conductive layer
720b and engages the conductive layer 720b. Returning back to FIG.
6, the component 710 can be used to provide the strain resistant
layer 750b and the conductive layer 720b over the outermost PCB
laminates 611, 612 of the stack 600. For example, if the PCB
laminate layer 612 is the outermost PCB laminate layer on the
bottom of the stack 600, using double-sided components such as the
component 700 could cause at least one of the conductive layers
720, 720a of the component 700 and at least one of the strain
resistant layers 750, 750a of the component 700 to be discarded
without attaching to anything. The component 710 of FIG. 7B can
advantageously be used on outer PCB laminates of the stack 600
instead of the component 700, thereby eliminating the unnecessary
discarding of conductive layers and/or strain resistant layers. The
strain resistant layer 750b, as well as the conductive layer 720b,
of the component 610b can attach to the insulating layer 125a of
the outer most PCB laminate layer, (e.g., the PCB layer 612). The
conductive layer 720b can then at least partially separate from the
discardable layer 730b of the component 710. The discardable layer
730b of the component 710, for example after at least partial
separation from the conductive layer 720b of the component 710, can
be discarded (e.g., by selective etching of the material of the
discardable layer 730).
[0068] In certain preferred embodiments of the components disclosed
in FIGS. 6, 7A, and 7B, the conductive layers 620, 640, 750a, 750b
comprise copper and/or the discardable layers 630, 730 comprise
aluminum. However, the conductive layers 620, 640, 750a, 750b
and/or the discardable layers 630, 730 can comprise any suitable
metals, including, without limitation, gold, nickel, copper,
aluminum, nickel, kovar, steel, and alloys and combinations
thereof, without departing from the embodiments disclosed in the
present application.
C. Strain Resistant Materials and Material Characteristics
[0069] Material characteristics and other properties of the strain
resistant layers disclosed in various embodiments herein will now
be discussed. The strain resistant layers comprise, among others,
mechanical and thermal characteristics that may resist, for
example, damage caused by stress, including, without limitation,
thermal and mechanical strain. In certain embodiments, the strain
resistant layers may comprise more stable material (e.g.,
thermally, physically, mechanically, etc.) than rigid insulating
layers as described herein. In some embodiments, the strain
resistant layers can be more dimensionally stable, for example,
under high temperatures, than rigid insulating layers. In some
embodiments, the strain resistant layers comprise a material
suitable for manufacturing PCBs comprising non-rigid bendable
portions, such as, for example, polyester, polyimide, aromatic
polyimide, combinations thereof, and the like. In some embodiments,
the strain resistant layers can be manufactured from a mixture of
the above-mentioned or other materials. In some embodiments, the
strain resistant layers comprise resin such as polyimide having one
or more of the following characteristics: fully cured,
substantially halogen free, non-glass reinforced, substantially
lead free, and substantially fiberglass free. In some embodiments,
the strain resistant layers comprise resin such as polyimide
comprising at least two of the following characteristics: fully
cured, substantially halogen free, non-glass reinforced,
substantially lead free, and substantially fiberglass free. In some
embodiments, the strain resistant layers comprise plastics such as
polyimide including a halogen and/or fiberglass. In certain
embodiments, the strain resistant layers are substantially free of
at least one of the following materials: halogen, fiberglass, and
lead. The resistant layers can be substantially free of lead. In
one embodiment, the strain resistant layers are substantially free
of halogen. In a certain embodiment, the strain resistant layers
are substantially free of fiberglass. In some embodiments, the
strain resistant layers comprise partially, substantially, or fully
cured or uncured polyimide. The strain resistant layers can also
comprise material that is at least partially reinforced with some
fiberglass.
[0070] The strain resistant layers in accordance with embodiments
disclosed herein may provide one or more advantages, including when
used in connection with rigid PCBs. Strain resistant layers and
materials as described herein have not been used with rigid PCBs
for several reasons, including higher production costs, lack of
availability of various thicknesses and widths, the perceived lack
of benefit of characteristics of these materials when used in
connection with rigid PCBs comprising non-bending portions, and the
like. The Applicant has recognized that certain qualities exhibited
by strain resistant materials (e.g., one or more of ductility,
hardness, resistance, and the like) can make the strain resistant
layers suitable for rigid PCBs as further described herein. For
example, strain resistant layers comprising low loss material such
as polyimide can dissipate less power along longer lengths than
non-low loss materials and can allow for higher density circuits.
In another example, strain resistant layers in accordance with
various embodiments herein can have higher electrical resistance.
Strain resistant layers comprising material having higher
electrical resistance can perform better under high temperatures by
retaining insulating properties under high temperatures that may
degrade insulating qualities of other types of insulating layers
(for example, epoxies). The strain resistant layers comprising
material having higher electrical resistance, therefore, can help
reduce (e.g., minimize, eliminate) electrical failures caused by,
among others, insulating layers rendered defective by high
temperatures.
[0071] FIG. 8 illustrates various characteristics of example
insulating layers. In accordance with various embodiments disclosed
herein, strain resistant layers comprising resins such as, without
limitation, polyimide or polyimide-based materials can be
configured to reduce (e.g., minimize, eliminate), among other
defects, pad cratering. FIG. 8 illustrates typical properties for
an example strain resistant layer comprising polyimide as well as
typical values for FR-4 and High-Temp FR-4 rigid insulating layers.
The values in FIG. 8 were obtained using methods in accordance with
IPC TM-650 (Association Connecting Electronics Industries Test
Method Manual by HIS) and/or ASTM International Standards Worldwide
(e.g., ASTM D-190, ASTM D-696, etc.).
[0072] As illustrated in FIG. 8, the strain resistant layers, for
example comprising polyimide, can be advantageously more ductile
than non-strain resistant layers. The ductility (sometimes also
referred to as elongation) of an embodiment of a strain resistant
layer, for example comprising polyimide, can be in the range of
about 15-80%, and also can be about 20%, 25%, 30%, 40%, 45%, 50%,
60%, 65%, 70%, and 75%. In certain embodiments, the strain
resistant layer comprises ductility in the range of about 15-20%,
20-30%, 30-40%, 20-60%, 40-80%, 50-80%, 15-35%, and the like.
Further still, the strain resistant layers can comprise ductility
different from the ranges provided herein, and can comprise
ductility less than about 15% (e.g., about 10%), greater than about
80% (e.g., about 90%). In some embodiments, the strain resistant
layers comprise ductility of at least about 15%.
[0073] The strain resistant layers can comprise higher ductility
properties than other insulating materials (for example, ductility
of less than 5% for both FR-4 and High-Temp FR-4 epoxies). As
previously mentioned, thermal stress resulting from changes in
temperature can cause unequal responses (e.g., rates of expansion,
contraction, etc.) in insulating and non-insulating layers,
including cap layers, of PCBs and other materials (e.g., surface
conductive layers, soldering materials, electrical component
conductive pads, etc.). In some situations, thermal stress can
cause the surface conductive layers of PCBs and cap layers
underneath the surface conductive layers to move relative to each
other (e.g., in opposite directions, in other directions placing
strain on the connection between the cap layers and the surface
copper layers, etc.) such that defects such as pad cratering form
in or around, among others, the cap layers. The strain resistant
layers in accordance with embodiments disclosed herein are more
ductile than other insulating layers (e.g., FR-4), and can reduce
(e.g., minimize, eliminate) pad cratering by, among others, at
least absorbing some of the thermal stress as elastic deformation.
Embodiments of strain resistant layers comprising ductile material
as disclosed herein can also absorb mechanical stress exerted onto
components of PCBs (e.g., cap layers), for example, by other more
rigid PCB components such as lead-free soldering material, further
reducing the occurring of defects such as pad cratering in the PCB
components including in the cap layers.
[0074] Although FIG. 8 illustrates characteristics of certain
strain resistant layers, including strain resistant layers
comprising polyimide (a polyimide cap layer), these values are
representative of only the one example of a strain resistant layer
and are not a comprehensive representation of all possible strain
resistant layers. The various disclosed PCB embodiments can
suitably comprise strain resistant layers having different mixtures
of polyimide, strain resistant layers comprising other strain
resistant materials such as, for example, liquid crystal polymer,
train resistant layers comprising mixtures of polyimide and other
strain resistant materials, or a combination thereof. In some
embodiments, strain resistant layers comprising a different mix of
polyimide and/or other materials may have elongation and other
properties that are different from FIG. 8 (for example, a strain
resistant layer may have a Tg outside the range of about
220-420.degree. C., ductility outside the range of about 15-80%,
etc.).
[0075] In accordance with certain embodiments disclosed herein,
strain resistant layers (e.g., comprising polyimide) can have
favorable properties that may reduce (e.g., minimize, eliminate)
failures caused by, among others, pad cratering. In particular and
as can be seen in FIG. 8, strain resistant layers comprising
polyimide can have a Glass Transition Temperature (Tg) range of
about 220-420.degree. C., and can also have Tg values of about
240.degree. C., 290.degree. C., 340.degree. C., 390.degree. C.,
440.degree. C., and 490.degree. C. In some embodiments, the strain
resistant layer comprises Tg values of about 210.degree. C.,
including Tg values of about 215.degree. C., 230.degree. C.,
235.degree. C., 245.degree. C., 250.degree. C., 255.degree. C.,
265.degree. C., 270.degree. C., 285.degree. C., 300.degree. C.,
305.degree. C., 315.degree. C., 330.degree. C., 350.degree. C.,
360.degree. C., 370.degree. C., 375.degree. C., 385.degree. C.,
400.degree. C., 405.degree. C., 415.degree. C., 430.degree. C.,
445.degree. C., 455.degree. C., 460.degree. C., 470.degree. C., and
the like. In certain embodiments, the strain resistant layer
comprising material such as polyimide comprise Tg values in the
range of about 220-450.degree. C., including about 250-300.degree.
C., 300-350.degree. C., 350-400.degree. C., 400-450.degree. C.,
450-500.degree. C. and the like. In certain embodiments, the strain
resistant layer can comprise Tg values in the range of about
220-230.degree. C., including about 230-240.degree. C.,
235-245.degree. C., 245-260.degree. C., 260-280.degree. C.,
280-290.degree. C., 290-310.degree. C., 310-320.degree. C.,
320-340.degree. C., 340-360.degree. C., 360-370.degree. C.,
375-395.degree. C., 400-420.degree. C., 405-415.degree. C.,
410-430.degree. C., 430-460.degree. C., 250-350.degree. C.,
350-450.degree. C., and the like. Further still, the strain
resistant layers can comprise Tg values different from the ranges
provided herein, and can comprise Tg less than 220.degree. C.
(e.g., 200.degree. C.), greater than 420.degree. C. (e.g.,
500.degree. C.), or both. In some embodiments, the strain resistant
layers comprise material such as polyimide having Tg of at least
220.degree. C. The strain resistant materials can comprise higher
Tg values than other rigid insulating layers (e.g., roughly
170.degree. C. and 180.degree. C. for FR-4 and High-Temp FR-4,
respectively.
[0076] As illustrated in FIG. 8, the total expansion due to
temperature up to solder reflow temperature is lower for a material
having higher Tg range (e.g., polyimide) than for a material having
a lower Tg (e.g., FR-4). Materials with higher Tg ranges can be
advantageous for use as insulating layers, including for use as cap
layers in rigid PCBs, because such materials can maintain their
dimensional stability (e.g., expand and/or contract less in
response to, among others, changes in temperature) over a wider
temperature range, potentially reducing the likelihood that cracks
will occur in various PCB layers, including cap layers.
[0077] As shown in FIG. 8, certain strain resisting layers
comprising polyimide comprise Coefficients of Thermal Expansion
(CTE), including CTE values in the lateral (X and Y) directions
(CTE X,Y) configured to, among others, resist defects such as pad
cratering that occur in insulating layers, including cap layers.
CTE X,Y values for certain such strain resistant layers can be
lower than the CTE X,Y values for other insulating layers,
including rigid insulating layers. Materials having lower CTE X,Y
characteristics (e.g., polyimide) are more stable than materials
having higher CTE X,Y characteristics (e.g., FR-4, High-Temp FR-4)
because materials having lower CTE X,Y values generally are less
responsive (e.g., expand, contract, etc.) to temperature changes.
For example, FIG. 8 illustrates that CTE X,Y for polyimide above
the Tg is in the range of about 20 parts per million per .degree.
C. in temperature (ppm/.degree. C.) and about 42 ppm/.degree. C.,
including CTE X,Y of about 20 ppm/.degree. C., 25 ppm/.degree. C.,
30 ppm/.degree. C., 35 ppm/.degree. C., 38 ppm/.degree. C., 39
ppm/.degree. C., 40 ppm/.degree. C., 41 ppm/.degree. C., and 42
ppm/.degree. C. By comparison, FR-4 has a higher CTE X,Y of 140
ppm/.degree. C. and High-Temp FR-4 had a higher CTE X,Y of 45
ppm/.degree. C. Thus, the strain resistant layers comprising
polyimide expands less than other insulating layers such as rigid
cap layers even under similar temperature conditions, and
therefore, the strain resistant layers can reduce thermal forces
that apply stress on various components of PCBs, including cap
layers.
[0078] In certain embodiments, the strain resistant layer
comprising material such as polyimide comprise CTE X,Y values in
the range of about 15-45 ppm/.degree. C., including about 20-25
ppm/.degree. C., 20-30 ppm/.degree. C., 25-30 ppm/.degree. C.,
20-25 ppm/.degree. C., 25-35 ppm/.degree. C., and the like. Further
still, the strain resistant layers can comprise CTE X,Y values
different from the ranges provided herein, and can comprise CTE X,Y
less than about 20 ppm/.degree. C. (e.g., about 15 ppm/.degree.
C.), greater than about 42 ppm/.degree. C. (e.g., about 50
ppm/.degree. C.), or both. In some embodiments, the strain
resistant layers comprise material having CTE X,Y of about 45
ppm/.degree. C. or less. The strain resistant materials can also
comprise CTE of about 25 ppm/.degree. C. or less, including about
23 ppm/.degree. C., 21 ppm/.degree. C., 19 ppm/.degree. C., 18
ppm/.degree. C., 10 ppm/.degree. C., and the like.
[0079] In certain embodiments, the example strain resistant layers
comprise CTE, XY characteristics that better match CTE, XY
characteristics of other components such as surface copper layers.
For example, typical CTE X,Y values for copper and tin-lead solder
are around 16 ppm/.degree. C. and 27 ppm/.degree. C., respectively.
The CTE X,Y range of about 20 to 42 ppm/.degree. C. (e.g., 25
ppm/.degree. C.) of the example strain resistant layers are closer
to the CTE X,Y values of copper and tin-lead than the CTE X,Y
values of rigid insulating layers such as FR-4, and therefore, the
strain resistant layers and the other components (e.g., surface
copper layer) exhibit similar thermal responses under similar
thermal conditions (e.g., expand similarly under high temperature).
Two layers of different PCB materials (e.g., a cap layer and a
surface copper layer) comprising similar thermal coefficients
generally exhibit similar thermal behaviors and thus can reduce
thermal forces that move one layer relative to the other layer so
as to create a defect such as a crack in one layer (e.g., the cap
layer). Strain resistant layers comprising CTE values closer to CTE
values of surface copper layers than other rigid insulating layers
may be more dimensionally stable than the other rigid insulating
layers. Thermally matching the strain resistant layers and other
materials such as surface copper layers and solder materials can
reduce thermal stress on the strain resistant layers (cap layers as
shown in FIG. 4) or other insulating layers of PCBs, and the
reduced thermal stress may also reduce (e.g., minimize, eliminate)
the occurrence of pad cratering in the cap layers or other layers.
Further, the strain resistant layers can comprise other material
(e.g., inorganic filler) to reduce the difference of CTE between
the strain resistant layers and other layers (e.g., surface
conductive layers).
[0080] The strain resistant layers in accordance with embodiments
discloses herein advantageously comprise tensile strength
characteristics suited for reducing (e.g., minimizing, eliminating,
etc.) damage occurring in, among others, insulating layers such as
cap layers. Tensile strength indicates the level at which stress
causes sufficient change in the material (e.g., at least partially
break, deform, decompose, etc.) such that the change at least
interferes with the normal operation of the material and/or the PCB
in which the material is located, for example, by causing
electrical or mechanical failure. Therefore, insulating layers
comprising strain resistant materials having higher tensile
strengths perform better because the strain resistant layers can
operate under stress levels that otherwise cause defects in
materials having lower tensile strengths.
[0081] As illustrated in FIG. 8, the strain resistant materials can
comprise polyimide having tensile strength in the range of about
10,000-50,000 psi and can include tensile strengths of about 15,000
psi, 20,000 psi, 25,000 psi, 30,000 psi, 35,000 psi, 40,000 psi,
45,000 psi, 50,000 psi, 53,000 psi, etc. In certain embodiments,
the strain resistant layer comprising material such as polyimide
comprise tensile strength in the range of about 10,000-20,000 psi,
including about 25,000-35,000 psi, 35,000-45,000 psi, 15,000-30,000
psi, 25,000-45,000 psi, and the like. Further still, the strain
resistant layers can comprise tensile strength values different
from the ranges provided herein, and can comprise tensile strength
less than about 10,000 psi (e.g., about 5,000 psi), greater than
about 50,000 psi (e.g., about 75,000 psi), or both. In some
embodiments, the strain resistant layers comprise material such as
polyimide having tensile strength of at least about 10,000 psi.
[0082] In accordance with certain embodiments, the strain resistant
layers comprise strain resistant material having a unique
combination of two or more of the aforementioned characteristics
(e.g., tensile strength, ductility, CTE X,Y, etc.). Although strain
resistant layers comprising, for example, one of the aforementioned
characteristics (e.g., ductility) can help reduce or eliminate
various defects, strain resistant layers comprising two or more of
theses qualities are even more suited to reduce (e.g., minimize,
eliminate, etc.) various types of damage, including pad cratering,
that occur in insulating layers such as cap layers. For example,
the strain resistant layers 450,450a of FIG. 4 comprising two or
more of these characteristic (e.g., tensile strength, ductility,
CTE X,Y, etc.) are more apt to resist damage such as pad cratering
from occurring than other insulating layers comprising, for
example, fewer than two of these characteristics.
[0083] In accordance with such certain embodiments, the strain
resistant layers comprise strain resistant materials having two or
more of the following characteristics: ductility in the range of
about 15-80%, including about 15-20%, 20-30%, 30-40%, 20-60%,
40-80%, 50-80%, and 15-35%; Tg in range of about 220-420.degree.
C., including about 220-250.degree. C., including about
250-300.degree. C., 300-350.degree. C., and 350-400.degree. C.; and
tensile strength in the range of about 10,000-50,000 psi, including
about 10,000-20,000 psi, 25,000-35,000 psi, 35,000-45,000 psi,
15,000-30,000 psi, 25,000-45,000 psi, and the like.
[0084] In accordance with further certain embodiments, the strain
resistant layers comprise two or more of the following
characteristics: ductility of at least about 15%, including about
20%, 25%, 30%, 40%, 45%, 50%, 60%, 65%, 70%, 75%, and 85%; Tg of at
least about 220.degree. C., including about 240.degree. C.,
290.degree. C., 340.degree. C., 390.degree. C., 440.degree. C., and
490.degree. C.; and tensile strength of at least about 10,000 psi,
including about 15,000 psi, 20,000 psi, 25,000 psi, 30,000 psi,
35,000 psi, 40,000 psi, 45,000 psi, 50,000 psi, and 53,000 psi. In
other embodiments, the strain resistant layers comprise two or more
of the following characteristics: ductility of at least about 15%;
Tg of at least about 220.degree. C.; tensile strength of at least
about 10,000 psi; and CTE X,Y of 45 ppm/.degree. C. or less.
[0085] FIG. 9 illustrates Z-axis expansion for example strain
resistant layers comprising polyimide relative to similarly sized
FR-4 and High-Temp FR-4, all of which having 1 inch (25.4
millimeter) by 1 inch X, Y dimensions. PCBs comprising the various
types of insulating layers were subjected to high temperatures
during soldering electronic components to the PCBs, for example,
about 215.degree. C. during the leaded solder reflow process and
245.degree. C. during the lead-free solder reflow process. At
temperatures ranging from 20.degree. C. to Tg, the example strain
resistant layer, FR-4, and High-Temp FR-4 expanded by about
0.094107 millimeters (mm), about 0.053340 mm, about 0.065024 mm,
respectively. At temperatures ranging from Tg to the leaded solder
reflow temperature (215.degree. C.), FR-4 expanded by about
0.160020 mm and High-Temp FR-4 expanded by about 0.040005 mm
whereas the example resistant layer expanded by an insignificant
amount. For temperatures ranging from Tg to lead-free reflow
temperature (245.degree. C.), the example strain resistant layer
expanded by about 0.025400 mm, whereas both the FR-4 and the
High-Temp FR-4 expanded by higher values of about 0.266700 mm and
about 0.074232, respectively. Total expansion for the example
strain resistant material for leaded solder reflow was about
0.094107 mm, which was less than each of the total leaded solder
reflow expansions of FR-4 and High-Temp FR-4 (about 0.119507 mm and
about 0.105029 mm, respectively). Total expansion for the example
strain resistant material for lead-free solder reflow was about
0.119507 mm, which was less than each of the total lead free solder
reflow expansions of FR-4 and High-Temp FR-4 (about 0.320040 mm and
about 0.139319 mm, respectively). In one embodiment, the strain
resistant layers comprise aromatic polyimide. As such, cap layers
and other insulating layers comprising the strain resistant layers
(e.g., polyimide) as disclosed herein can advantageously maintain
dimensional stability over a wide temperature range.
D. Detailed Descriptions of Further Non-Limiting Embodiments
[0086] FIGS. 10A and 10B illustrates a schematic diagram of an
embodiment of a rigid PCB 1000. In FIG. 10A, the PCB 1000 comprises
a rigid dielectric 1010, a conductive surface layer 1020, and a
solder material 1040. The PCB 1000 optionally comprises a
conductive plate 1030 between the conductive surface layer 1020 and
the solder material 1040. The rigid dielectric 1010 comprises a
rigid material such as, without limitation, standard FR-4 Epoxy or
High-Temp FR-4 Epoxy. The conductive surface layer 1020 comprises a
conductive material such as copper and can be formed by etching the
conductive material. The rigid dielectric 1010 engages the surface
conductive layer 1020. The conductive plate 1030 is over the
conductive surface layer 1020. The solder material 1040 is
deposited over the conductive plate 1030. The solder material 1040
connects an electrical component (not shown) with the PCB 1000,
thereby forming a rigid printed circuit board assembly. As already
mentioned, high temperatures, for example, in excess of 400.degree.
C. used in lead-free soldering processes, are applied to the
soldering material 1040 to connect the electrical component with
the PCB 1000. Subjecting the PCB 1000 and the rigid dielectric
layer 1010 to high temperatures can cause defects to occur in the
rigid dielectric 1010, for example, underneath the conductive
surface layer 1020.
[0087] FIG. 10B shows the PCB 1000 of FIG. 10A comprising an
insulating layer 1050 between the conductive surface layer 1020 and
the rigid dielectric 1010. The top surface of the insulating layer
1050 is configured to engage the conductive surface layer 1020 and
the bottom surface of the insulating layer 1050 is configured to
engage the rigid dielectric 1010. As previously mentioned, the
insulating layer 1050 comprises materials having better
characteristics (e.g., thermal, mechanical, physical, etc.) than
the rigid dielectric layer 1010, such as, for example, higher Tg
(greater than or equal to about 220.degree. C.). In some
embodiments, the insulating layer 1050 has ductility of greater
than or equal to about 15%. In various embodiments, the insulating
layer 1050 has tensile strength greater than or equal to about
10,000 psi. In some embodiments, the insulating layer 1050
comprises polyimide. In certain embodiments, the insulating layer
1050 comprises another strain resistant material (e.g., liquid
crystal polymer, polyester, etc.). As mentioned above, the
insulating layer 1050 comprises materials that can perform better
under high temperatures (e.g., expand less, absorb stress, etc.)
and therefore a PCB comprising the insulating layer 1050 engaging
the surface conductive layer 1010 can reduce (e.g., minimize,
eliminate) the occurrence of defects underneath the conductive
surface pad 1020 including, for example, pad cratering. In some
embodiments, a circuit component (not shown), for example an
Integrated Circuit (IC) chip component, can be mounted on the PCB
1000 using the solder material 1040, thereby forming a printed
circuit board assembly.
[0088] Further non-limiting specific embodiments of strain
resistant layers as disclosed herein can be found, for example, at
FIGS. 1 and 2 and pages 3-6, of the above-specified U.S.
Provisional App. No. 61/016,292, the content of which is hereby
entirely incorporated by reference, and FIGS. 1-7 of the
above-specified U.S. Provisional App. No. 61/078,315, the entire
content of which is hereby entirely incorporated by reference.
[0089] The various embodiments described herein can also be
combined to provide further embodiments. Related methods,
apparatuses, and systems utilizing strain resistant layers,
including, without limitation, polyimide layers, in rigid printed
circuit boards, are described in the above-referenced provisional
applications to which this application claims priority, the
entireties of each of which are hereby expressly incorporated by
reference: U.S. Provisional Patent Application Ser. No. 61/016,292,
which was filed on Dec. 21, 2007, and U.S. Provisional Application
No. 61/078,315, which was filed on Jul. 3, 2008. While the
above-listed applications may have been incorporated by reference
for particular subject matter as described earlier in this
application, the Applicant intends the entire disclosures of the
above-identified applications to be incorporated by reference into
the present application, in that any and all of the disclosures in
these incorporated applications may be combined and incorporated
with the embodiments described in the present application.
[0090] Although the foregoing description has shown, described, and
pointed out the fundamental novel features of the embodiments
disclosed herein, it will be understood that various omissions,
substitutions, and changes in the form of the detail of the
apparatus as illustrated, as well as the uses thereof, may be made
by those skilled in the art, without departing from the spirit or
scope of the disclosed embodiments. Consequently, the scope of the
present application should not be limited to the foregoing
discussion, but should be defined by the appended claims.
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