U.S. patent application number 09/260198 was filed with the patent office on 2001-08-02 for multilayer combined rigid/flex printed circuit board containing flexible soldermask.
Invention is credited to CARON, A. ROLAND, JEAN, SANDRA L., KEATING, JAMES E., LARMOUTH, ROBERT S., MILLETTE, LEE J..
Application Number | 20010010303 09/260198 |
Document ID | / |
Family ID | 25179521 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010010303 |
Kind Code |
A1 |
CARON, A. ROLAND ; et
al. |
August 2, 2001 |
MULTILAYER COMBINED RIGID/FLEX PRINTED CIRCUIT BOARD CONTAINING
FLEXIBLE SOLDERMASK
Abstract
A multilayer rigid flex printed circuit board wherein the board
laminate comprises a basestock composite containing a flexible
core, formed by laminating a first conductive layer to a flexible
insulator layer, a second insulator layer affixed to the basestock,
said second insulator layer having a cutout region proximate to the
flexible core of the basestock composite to expose a portion of
said first conducting layer on said flexible core, a second
conductive layer attached to said second insulator layer said
second conductive layer having a cutout region proximate to the
flexible core of the basestock composite, and a photo-imageable
soldermask applied to the exposed portion said first conducting
layer, and to the second conductive layer, wherein said
photoimageable soldermask allows for photo definition of openings
on the conductive layers to which it is applied.
Inventors: |
CARON, A. ROLAND; (HUDSON,
NH) ; JEAN, SANDRA L.; (HUDSON, NH) ; KEATING,
JAMES E.; (LITCHFIELD, NH) ; LARMOUTH, ROBERT S.;
(LONDONDERRY, NH) ; MILLETTE, LEE J.; (LITCHFIELD,
NH) |
Correspondence
Address: |
NORMAN P SOLOWAY
HAYES SOLOWAY HENNESSEY
GROSSMAN & HAGE
175 CANAL STREET
MANCHESTER
NH
03101
|
Family ID: |
25179521 |
Appl. No.: |
09/260198 |
Filed: |
March 1, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09260198 |
Mar 1, 1999 |
|
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|
08800844 |
Feb 14, 1997 |
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Current U.S.
Class: |
216/18 ; 174/254;
216/20; 216/36; 427/97.2; 430/312; 430/314 |
Current CPC
Class: |
H05K 3/4691 20130101;
H05K 3/28 20130101; H05K 3/0023 20130101; H05K 2203/0759
20130101 |
Class at
Publication: |
216/18 ; 216/20;
216/36; 427/96; 174/254 |
International
Class: |
H05K 001/00; H05K
003/46; B05D 005/12; H01B 013/00 |
Claims
What is claimed is:
1. A multilayer rigid flex printed circuit board wherein the board
laminate comprises a basestock composite containing a flexible
core, formed by laminating a first conductive layer to a flexible
insulator layer; a second insulator layer affixed to the basestock,
said second insulator layer having a cutout region proximate to the
flexible core of the basestock composite to expose a portion of
said first conducting layer on said flexible core; a second
conductive layer attached to said second insulator layer said
second conductive layer having a cutout region proximate to the
flexible core of the basestock composite; and a photo-imageable
soldermask applied to the exposed portion of said first conducting
layer, and to the second conductive layer, wherein said
photoimageable soldermask allows for photo definition of openings
therein.
2. The multilayer rigid flex printed circuit board of claim 1
wherein said first and second conductive layers comprise about 9
micron to two ounce copper sheet and said flexible insulator layer
comprises fiberglass sheet impregnated with an epoxy adhesive,
polyimide film, or combination thereof.
3. The multilayer rigid flex printed circuit board of claim 1
wherein said photo-imageable solder mask comprises a flexible
thermal curing liquid photoimageable soldermask which thickness is
adjusted relative to the thickness of said conductive layer, and is
about 1-3 mils thick.
4. The multilayer rigid flex printed circuit board of claim 1
wherein said first conductive layer is imaged and etched and
contains conductor patterns.
5. The multilayer rigid flex printed circuit board of claim 1
wherein said second insulator layer affixed to the basestock
contains a overlaps a section of said flexible core.
6. A multilayer rigid flex printed circuit board wherein the board
laminate comprises a basestock composite containing a flexible
core, formed by laminating two conductive layers to a flexible
insulator layer; a second insulator layer affixed to the basestock,
said second insulator layer having a cutout region proximate to the
flexible core of the basestock composite to expose said first
conducting layer on said flexible core; a second conductive layer
attached to said second insulator layer comprising an upper and
lower conductive sheet separated by an insulating layer and having
a cutout region proximate to the flexible core of the basestock
composite; and a photo-imageable soldermask applied to said first
conductive layer and said upper conductive sheet of said second
conductive layer wherein said photoimageable soldermask allows for
photo definition of openings on either said first or second
conductive layers.
7. The multilayer rigid flex printed circuit board of claim 6
wherein said first and said second conductive layers comprise
copper sheet of about 9 micron to two ounce copper, and said
flexible insulator layer comprises fiberglass sheet impregnated
with an epoxy adhesive, polyimide film, or combination thereof.
8. The multilayer rigid flex printed circuit board of claim 6
wherein said photo-imageable solder mask comprises a flexible
thermal hardening two component liquid photoimageable solder
mask.
9. The multilayer rigid flex printed circuit board of claim 6
wherein said first conductive layer and said second conductive
layer is imaged and etched and contains conductor patterns.
10. The multilayer rigid flex printed circuit board of claim 6
wherein said second insulator layer affixed to the basestock
overlaps a section of said flexible core.
11. The multilayer rigid flex printed circuit board of claim 6
wherein said flexible insulating layer is either epoxy prepreg of a
thickness of about 1.5 mils to 5.0 mils, or polyimide film of a
thickness of about 0.5 mils to 3.0 mils, or combination
thereof.
12. The multilayer rigid flex printed circuit board of claim 6
wherein said second insulator layer is epoxy prepreg and has a
thickness of about 1.5 to 5.0 mils.
13. A process for the preparation of a multilayer rigid flex
circuit board comprising the steps of: laminating a first conductor
layer to a first flexible insulating layer to form a basestock
composite wherein said first conductor layer contains a flexible
core section; imaging and etching said first conductor layer to
form conductor patterns; laminating a second conductive layer to a
second insulating layer wherein said second conductive layer
contains a cover section thereof which covers said flexible core
section and which does not bond to said flexible core section;
laminating said first conductor layer and said second conductive
layers together to form a rigid section; removing said cover
section covering said flexible core section to expose said flexible
core section; and coating said second conductive layer and said
exposed flexible core section with a photo-imageable solder mask
and photo defining openings therein.
14. The process of claim 13 wherein coating with a photo-imageable
solder mask comprises the steps of depositing a liquid precursor of
said photo-imageable solder mask and curing the liquid precursor to
form said photo-imageable solder mask as a coating in both the
rigid and flexible sections of the rigid-flex printed circuit
board.
15. The process of claim 13 wherein said liquid precursor is
applied by spray coating.
16. The process of claim 13 wherein said cover section which covers
said flexible core section and which does not bond to said flexible
core section comprises copper or insulating material.
17. The process of claim 13 wherein holes are drilled in said rigid
section, followed by cleaning and plating through said holes to
form conductive barrels connecting conductors in the conductor
patterns of said rigid section.
18. The process of claim 13, including the step of surface coating
the photo defined openings said surface coating acting to prevent
oxidation.
19. The process of claim 18 wherein said surface coating comprises
a tin/lead coating, a gold coating, or an organic coating.
20. The process of claim 13 wherein said cover section which covers
said flexible core section and which does not bond to said flexible
core section forms an opening as between said flexible core section
and said cover section.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the fabrication of
multilayer combined rigid and flex printed circuit boards wherein a
flexible soldermask replaces traditional polyimide film in the flex
area. In addition, the flexible soldermask, being photo-imageable,
can be placed on both the rigid and flexible sections at the same
time, thus allowing photo definition of the openings in one or both
areas. Furthermore, the method of fabrication eliminates the need
of additional process steps to fabricate the flexible inner layers
as required by prior art construction techniques.
BACKGROUND OF THE INVENTION
[0002] Techniques for making multilayer rigid flex printed circuit
boards are well known in the field. One early example of the prior
art is disclosed in U.S. Pat. No. 3,409,732 assigned to the
assignee of the present application. Typically, a rigid flex
stacked printed circuit board includes flexible printed circuit
cables extending from the periphery of the rigid section or
sections. The rigid portions of the flex cables are typically used
as sites for electronic components or mechanical hardware. It is
important to note that the copper conductor in each plane or layer
is fabricated from one continuous sheet of copper foil.
[0003] Typically, and as disclosed in detail in U.S. Pat. No.
4,800,461, also assigned to the assignee of the present invention,
in the construction of a multilayer rigid flex circuit board, the
initial processing step includes formation of a basestock by
laminating two copper sheets to an insulator layer comprising one
or two fiberglass sheets impregnated with an adhesive such as an
epoxy, commonly referred to as a prepreg. Following lamination, the
copper layers can be imaged and etched to provide copper pads and
conductors. The exposed copper conductor patterns are then treated
to enhance bondability of an epoxy prepreg to the copper. Then, two
additional insulator prepreg sheets having cutouts are positioned
on both sides of the base stock. A flexible insulator of Kapton
(polyimide) covered with a suitable adhesive which provides bonding
to copper is positioned on both sides of the cutout section. In
addition, the Kapton sheets are slightly longer than the cutout
sections in the glass layers to overlap by, for example, 0.050
inch. The sandwich formed by the foregoing sheets is then laminated
together to provide a rigid flex board, where in the Kapton
provides excellent flexibility and tear resistance characteristics
to the flex section.
[0004] Of additional general background interest, attention is
directed to U.S. Pat. No. 5,499,444, entitled "Method of
Manufacturing a Rigid Flex Printed Circuit Board". In said patent,
it is reported the fabrication of multilayer boards with the above
referenced materials has led to some persistent problems. First,
alignment of circuitry at different layers in the board is
critical, and provisions must be made to prevent sliding of any
layers in different planes with respect to other planes by more
than a tolerance of a few thousandths of an inch. Maintaining
registration of the flexible portion has also been a serious
problem, since the hard board must be cured or laminated by a
heat-press process that is likely to cause interlayer slippage as
well as thermal dimensional changes. Other problems are encountered
due to the thermal expansion of the typically used insulator
materials such as acrylic adhesive and the polyimide film utilized
in the construction of the rigid flex boards. Thus, failures occur
when the board is subjected to elevated temperatures in thermal
stress testing, hot oil-solder reflow, and the like.
[0005] Another difficulty with the use of dielectric films such as
Kapton film in the rigid board area is their absorption of
excessive moisture, on the order of up to 3 percent by weight of
water. Absorbed moisture in the circuitry, with no means of escape,
may cause unacceptable delamination in the rigid board area when it
volatilizes during fabrication or during subsequent high
temperature operation. This effect may be more destructive than
simple mismatch of thermal coefficients.
[0006] The foregoing problems have in one form or another been
addressed in the prior art, as shown for example in U.S. Pat. No.
4,800,641; U.S. Pat. No. 5,144,742 and U.S. Pat. No. 5,004,639.
Approaches in addressing one or more of the problems involve such
construction techniques as adding pads in non functional layers of
the plated through holes; utilizing a curable liquid dielectric for
certain layers or portions of layers; using temporary sheet patches
of filler material surrounding flex regions during a heat pressing
assembly stage to maintain alignment; adding glass fiber
reinforcement of the flex layer for strengthening; and finally, a
number of other changes or addition to the manufacturing process.
However, each of these solutions entails additional steps to
address any one problem, and may have adverse consequences on
another apparently unrelated problem. For example, the switch to
acrylic cements and a polyimide flexible film has been largely
responsible for moisture absorption and failure of plated through
holes in multilayer boards at high temperatures. Reducing the size
of the polyimide/acrylic components to limit thermal stress
introduces boundary problems where flexible and rigid elements are
patched together. Furthermore, many of these improvements require
more detailed manufacturing steps that can be costly as well as
time consuming.
[0007] In U.S. Pat. No. 5,499,444, a rigid flex printed circuit
board is prepared via a process wherein each circuit layer is
punched in the peripheral regions with alignment slots and all
layers are assembled without any acrylic bonding in a single hot
press operation. More specifically, a central layer is formed of a
sheet of epoxy/glass material with top and bottom copper foil
layers. This central layer is cured and punched with slots at its
edge, of which a portion thereof ultimately provides formation of
the flexible portion of the finished board. The slots allow motion
along only one of two orthogonal axis, and subsequently the punched
layers (with windows for formation of the flexible region) and the
central portion are assembled in a single press curing operation in
which process-induced motions and realignments are constrained to
occur with a small magnitude that varies with the radial position
along an alignment rosette centered on the board. A top or final
cover layer extends over both the flexible and rigid regions to
further assure uniform alignment at all levels during pressure
assembly. The '444 Patent emphasizes that by using a glass/epoxy
layer for the central flexible portion instead of some form of
polyimide or other material, an all glass construction is achieved
that is free of the major problem of z-axis thermal stresses of the
conventional polyimide-glass construction. Furthermore, it is
mentioned that on top of the copper layer in the flex section is a
cover of insulating material which may be an adhesively bonded
plastic film or a coated-on film, such as a conformal cover coat
known in the art, suitable of which is a solder mask such as a UV
curable flexible solder resist or a heat curable preparation both
of which can be applied by a screening operation. This cover
material is then described as being cured before assembly with
subsequent layers.
[0008] In U.S. Pat. No. 5,144,742 there is disclosed a rigid flex
printed circuit board fabricated by the steps of first forming
circuitry components on a rigid flex subassembly including
laminated conductive layers on opposite sides of a central
insulating layer. This is followed by depositing a liquid precursor
of flexible insulating layers over the circuitry components in
portions of the printed circuit board subassembly corresponding to
the flexible section in the final rigid flex board. The liquid
precursor in then cured to form an insulating layer as a protective
coating over the circuitry components in the flexible section,
which is followed by finally laminating a plurality of components
including at least one rigid flex subassembly and rigidizing
insulating layers to form a rigid flex printed circuit board.
[0009] Finally, attention is directed to "High Resolution
Photoimageable Covercoats for " Flex Applications, a paper
presented at "Flexcon 95" by W. J. Thatcher and P. M. Banks. As
disclosed therein, the product sold under the tradename
Imageflex.TM. has become available, and is characterized as a
thermal hardening two/pack two component liquid photoimageable
flexible soldermask that dries by evaporation to give an aqueous
processable film with a gloss or matt finish. It is mentioned
therein that Imageflex.TM. has proven suitable in a variety of
flexible circuit application. The Imageflex.TM. is said to offer
significant advantages for the product of many types of flexible
printed circuits, as an alternative to polyimide coverlay, the
Imageflex.TM. offering lower cost and higher feature resolution and
alignment accuracy. In addition, a photoimageable solder mask is
now available from Taiyo, Japan.
[0010] Accordingly, as can be seen from the above review of the
prior art, in the case of manufacturing a rigid flex printed
circuit board, there has been an on-going effort to develop the
most efficient and cost effective manufacturing process to address
the various problems in the art.
[0011] It is therefore an object of the present invention to
provide what is considered to be an even further cost effective
route for the preparation of a rigid flex circuit board, where the
rigid flex circuit board itself provides an entirely new overall
construction, and wherein the manufacturing process eliminates the
use of polyimide film in the flex section as a covercoat. More
particularly, it is an object of the present invention to develop a
process which makes use of photo-imageable solder mask, on both the
rigid and flexible sections, at the same time.
[0012] Accordingly and with regard to the method disclosed herein,
the present invention has as its further objective and result the
elimination for the need of additional process steps to fabricate
flexible inner layers in a rigid flex printed board design, and
provides an economical and cost-efficient route for assembly of
boards with characteristic rigid-flex construction.
SUMMARY OF THE INVENTION
[0013] By way of summary, the present invention comprises a
multilayer rigid flex printed circuit board wherein the board
laminate comprises a basestock composite containing a flexible
core, formed by laminating a first conductive layer to a flexible
insulator layer, along with a second insulator layer affixed to the
basestock, said second insulator layer having a cutout region
proximate to the flexible core of the basestock composite to expose
a portion of said first conducting layer on said flexible core, and
a second conductive layer is attached to said second insulator
layer, said second conductive layer also having a cutout region
proximate to the flexible core of the basestock composite. A
photo-imageable soldermask is then applied to said exposed portion
of said first conductive layer, and to the second conductive layer,
wherein said photoimageable soldermask allows for photo definition
of openings upon the conductive layers upon which it is
applied.
[0014] In method form, the present invention comprises a process
for the preparation of a multilayer rigid flex circuit board
comprising the steps of laminating a first conductor layer to a
first flexible insulating layer to form a basestock composite
wherein said first conductor layer contains a flexible core
section, imaging and etching said first conductor layer to form
conductor patterns, laminating a second conductive layer to a
second insulating layer wherein said second conductive layer
contains a cover section thereof which covers said flexible core
section and which does not bond to said flexible core section,
laminating said first conductor layer and said second conductive
layers together to form a rigid section, removing said cover
section covering said flexible core section to expose said flexible
core section and coating said second conductive layer and said
exposed flexible core section with a photo-imageable solder mask
and photo defining openings therein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Various features of the invention will be understood with
reference to the figures below and the description thereof,
wherein:
[0016] FIG. 1 illustrates a side-view of the rigid flex printed
circuit board in accordance with the present invention.
[0017] FIG. 2 illustrates a side view of the rigid flex printed
circuit board in accordance with the present invention, emphasizing
the placement of the cover section which does not bond to the
flexible core and which acts as a protective barrier over the
flexible section during fabrication.
[0018] FIG. 3 illustrates yet another side view of the rigid flex
printed circuit board of the present invention, in a preferred
embodiment thereof.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0019] With reference now to FIG. 1, the present invention
comprises a multilayer rigid flex circuit board 10 wherein the
board laminate contains a basestock composite 12 containing a
flexible core section 14 formed by laminating a first conductive
layer 16 to a flexible insulator layer 18. The flexible insulating
layer may be either epoxy prepreg or polyimide film or combination
of the two. A second insulator layer 20 is affixed to the
basestock, said second insulator layer having a cutout region 24
proximate to the flexible core 14 of the basestock composite to
expose said first conducting layer on said flexible core. A second
conductive layer 26 is attached to said second insulator layer 20
said second conductive layer also having a cutout region 24
proximate to the flexible core 14 of the basestock composite. A
photo-imageable soldermask 28 is then applied to said first and
second conductive layers wherein said photoimageable soldermask
allows for photo definition of openings on either said first or
second conductive layers.
[0020] Preferably, conductive layers 16 and 26 comprise copper
sheet of one or two ounce copper, and the flexible insulating layer
18 comprises fiberglass sheet impregnated with an epoxy adhesive.
However, conductive layers have been employed ranging from as low
as 9 micron. In the preferred process of manufacturing, the
impregnated fiberglass sheet is bonded to the conductive layer
16.
[0021] The preferred photo-imageable solder mask comprises a
flexible thermal hardening two component liquid photo-imageable
solder mask sold under the trademark Imageflex.TM. and available
from Coates ASI. Particularly preferred is the two components
system known as XV600T Imageflex.TM. Resist and XV600T
Imageflex.TM. Hardener. In process form, it has been found
preferable to employ a spray coating machine which insure more even
distribution of the material across the conductive surfaces.
Preferably, the flexible photo-imageable solder mask is applied and
cures to a thickness that is relative to the thickness of the
conductive foil layer. For example, if the conductive foil is 9
microns thick, the photoimageable solder mask is set to a thickness
of about 1 mil. On the other hand, if the conductive foil were 2
ounce copper, then the mask thickness would be about 3 mils
thick.
[0022] With regards to thickness dimensions, it should be also be
pointed out at this time that preferably, insulating layer 18 as
illustrated in FIG. 1, in the case of an adhesiveless polyimide
film, will range from about 0.5 mil to 3 mils. On the other hand if
the insulating layer 18 is prepreg, thickness will preferably range
from about 1.5 to 5.0 mils. Furthermore, insulating layer 20 also
will range from about 1.5 mils to 5.0 mils.
[0023] With respect to FIG. 1, several advantageous characteristics
of the present invention over prior art rigid flex designs are now
worthy of note. First, the flexible soldermask replaces the
traditional and expensive polyimide film covercoats in the flex
area. Furthermore, the flexible soldermask, being liquid
photo-imageable, is now placed on both the rigid and flexible
sections, thereby allowing for photo definition of openings in one
or both areas, and can be conveniently placed upon such surfaces
after final lamination. This type of construction and associated
fabrication eliminates the need of additional process steps to
fabricate the flexible inner layers which was required by prior art
techniques.
[0024] In yet another preferred construction, it has been found
that if insulating layer 30 is made to overlap a section of the
flexible core 14, as is shown in greater detail in FIG. 2, the
transition from the flexible region to the rigid section is less
severe, in the sense that such construction reduces any stresses
that may occur as between the rigid and flexible sections when such
sections are bent or deflected relative to one another.
[0025] For the manufacture of the rigid flex circuit of the present
invention, and again with reference to FIG. 1, the process begins
with the steps of laminating a first conductor layer 16 to a first
flexible insulating layer 18 to form a basestock composite wherein
said first conductor layer ultimately will define a flexible core
section 14. That is, a portion of this basestock will act as the
flexible section thereof, while the peripheral ends become
sandwiched within the rigid board regions.
[0026] The above steps are preferably followed by imaging and
etching said first conductor layer to form conductor patterns on
conductor surface 16. Attention is now directed to FIG. 2, which
more clearly illustrates the above referenced optional embodiment
wherein one laminates a second insulating layer of fiberglass/epoxy
prepreg 30 to conductive layer 16 so that said second insulating
layer overlaps a section of the flexible core 14. As noted above,
pursuant to this now illustrated construction, stress is reduced as
between the rigid section and flexible section in when the
multilayer rigid flex board is employed in final application.
[0027] Conductive layers 32 and 34 (see again, FIG. 2) are then
laminated to insulating layer 36 wherein conductive layer 34 is
designed to extend across the flexible core section 14 and to form
a cover section 38 which covers said flexible core section, and
which does not bond to said flexible core section in final
lamination. That is, one laminates said first conductor layer 16 to
insulating layer 30 and to said second conductive layers 32 and 34
to form a rigid section, as shown in FIG. 2, with conductive layer
34 spanning over flexible section 14.
[0028] While in preferred embodiment, conductive layer 34 is
illustrated as covering flexible section 14, it will be appreciated
that any suitable cover material will function in like manner,
provided said covering material does not bond to the flexible
region 14 during lamination. In this regard, cover section 38 may
for example comprise an insulating material, provided such
insulating material does not bond to the conductor surface 16 of
the flexible section in the step of final lamination.
[0029] In the final step of manufacture, the cover section 38
covering said flexible core section is then removed to expose the
flexible core section 14 followed by coating of said second
conductive layer 34 which is the rigid section and said exposed
conductive surfaces of flexible core section 14 with a
photo-imageable solder mask and photo defining openings
therein.
[0030] In yet another alternative embodiment, and as illustrated in
FIG. 3, a multi-layer rigid flex printed circuit board can be
prepared pursuant to the above process of manufacture. As shown
therein, an inner flexible insulating inner layer contains
conducting surfaces 42 and 44. Laminated thereon is a layer of
prepreg 46 which extends partially into the flex region. This is
followed by a rigid cap material comprising conducting layers 48
and 52 with insulating layer 50 therebetween. Finally, the flexible
photoimageable solder mask covers 54 the exposed conducting
surfaces.
* * * * *