U.S. patent application number 09/920192 was filed with the patent office on 2002-01-03 for method for making an electronic circuit assembly.
Invention is credited to Baker, Jay DeAvis, Belke, Robert Edward, Goenka, Lakhi Nandlal, Hu, Jun Ming, Jairazbhoy, Vivek Amir, Krautheim, Thomas B., Li, Delin, McLeskey, Edward, Paruchuri, Mohan R., Pham, Cuong Van.
Application Number | 20020000331 09/920192 |
Document ID | / |
Family ID | 22524081 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020000331 |
Kind Code |
A1 |
Jairazbhoy, Vivek Amir ; et
al. |
January 3, 2002 |
Method for making an electronic circuit assembly
Abstract
A method for producing an electronic circuit assembly (e.g., a
circuit board) from an etched tri-metal-layer structure which
provides air bridge crossovers and specially designed bumps etched
from a middle layer of the tri-metal-layer structure. The bumps are
formed at particular circuit locations in order to provide
interconnects for (1) heavy wirebonding, (2) fine wirebonding, or
(3) direct chip attachment; or, to provide (4) lifters for assuring
a minimum solder joint standoff height or (5) barriers for
retarding solder joint crack propagation.
Inventors: |
Jairazbhoy, Vivek Amir;
(Farmington Hills, MI) ; Krautheim, Thomas B.;
(Belleville, MI) ; Paruchuri, Mohan R.; (Canton,
MI) ; Goenka, Lakhi Nandlal; (Ann Arbor, MI) ;
Hu, Jun Ming; (Canton, MI) ; Baker, Jay DeAvis;
(W. Bloomfield, MI) ; McLeskey, Edward;
(Waterford, MI) ; Li, Delin; (Canton, MI) ;
Pham, Cuong Van; (Belleville, MI) ; Belke, Robert
Edward; (West Bloomfield, MI) |
Correspondence
Address: |
Brinks Hofer Gilson & Lione
P.O. Box 10395
Chicago
IL
60610
US
|
Family ID: |
22524081 |
Appl. No.: |
09/920192 |
Filed: |
August 1, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09920192 |
Aug 1, 2001 |
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09148061 |
Sep 4, 1998 |
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Current U.S.
Class: |
174/263 ;
174/260; 29/830; 29/832; 29/840; 29/843; 29/847 |
Current CPC
Class: |
H05K 3/4007 20130101;
H05K 2201/0367 20130101; Y10T 29/49126 20150115; H05K 3/303
20130101; Y02P 70/50 20151101; Y10T 29/49149 20150115; H05K 1/111
20130101; H05K 3/4685 20130101; Y10T 29/49133 20150115; H05K
2201/10636 20130101; H05K 2203/049 20130101; Y10T 29/49155
20150115; H05K 2201/0361 20130101; H05K 3/062 20130101; Y10T
29/49128 20150115; Y10T 29/4913 20150115; Y02P 70/613 20151101;
Y10T 29/49144 20150115; H05K 3/328 20130101; Y02P 70/611 20151101;
Y10T 29/49156 20150115; H05K 3/3442 20130101; H05K 2201/2036
20130101; H05K 2201/0373 20130101; H05K 2201/09781 20130101 |
Class at
Publication: |
174/263 ;
174/260; 29/830; 29/832; 29/847; 29/840; 29/843 |
International
Class: |
H05K 001/11; H01R
012/04; H05K 001/16; H05K 003/36; H05K 003/34 |
Claims
What is claimed is:
1. A method for making an electronic circuit assembly, comprising
the steps of: (a) providing a precircuit comprising: (i) a metallic
sheet made of a first metal and having a top surface and a bottom
surface; (ii) a first conductor pattern attached to the bottom
surface of the metallic sheet and made of a second metal, wherein
the first conductor pattern comprises: (A) a base pad having a
first predetermined size and shape and a base pad perimeter
thereabout, (B) a circuit trace, and (C) first and second pedestal
pads disposed proximate the circuit trace on opposite sides
thereof; (iii) a second conductor pattern attached to the top
surface of the metallic sheet and made of a third metal, wherein
the second conductor pattern comprises: (A) a plurality of masking
pads arranged generally within the base pad perimeter, wherein each
masking pad has a second predetermined size and shape smaller than
the base pad, and (B) a bridging element having first and second
enlarged ends and a constricted portion therebetween, the bridging
element being oriented generally transverse to the circuit trace
with each enlarged end disposed opposite a respective one of the
pedestal pads; and (iv) a substrate having an electrically
insulative surface to which the first conductor pattern is
attached; (b) etching the precircuit in an etchant which etches
substantially only the first metal, so as to form undercuts in the
metallic sheet directly beneath the masking pads; and (c)
continuing to etch the precircuit and undercut the masking pads to
create a circuit, (i) such that the metallic sheet beneath each
masking pad is substantially completely undercut causing the
masking pads to detach from the metallic sheet, thereby providing a
plurality of bumps made of the first metal disposed atop the base
pad generally within the base pad perimeter, (ii) wherein a
pedestal made of the first metal is formed between each pedestal
pad and its respective enlarged end of the bridging element,
thereby providing an air bridge crossover above the circuit
trace.
2. A method according to claim 1, wherein the second and third
metals are the same metal.
3. A method according to claim 1, wherein the first metal is
aluminum and the second and third metals are copper.
4. A method according to claim 1, wherein the plurality of bumps is
distributed generally evenly across substantially all of the base
pad.
5. A method according to claim 4, further comprising the step of:
(d1) wirebonding an end of a wirebond wire to the plurality of
bumps atop the base pad.
6. A method according to claim 1, wherein the first conductor
pattern includes at least two base pads to which a surface mount
electronic component may be soldered and wherein the second
conductor pattern includes a plurality of masking pads for each of
the at least two base pads, wherein each plurality of bumps for the
at least two base pads is arranged proximate an edge of its
respective base pad within a projected footprint of the surface
mount electronic component.
7. A method according to claim 6, further comprising the steps of:
(d2) placing the surface mount electronic component atop the at
least two base pads such that a termination of the component rests
generally atop each plurality of bumps atop its respective base
pad; and (e) soldering each component termination to its respective
base pad, such that each plurality of bumps is generally enclosed
within a solder joint connecting each component termination with
its respective base pad.
8. A method for making an electronic circuit assembly, comprising
the steps of: (a) providing a precircuit comprising: (i) a metallic
sheet made of a first metal and having a top surface and a bottom
surface; (ii) a first conductor pattern attached to the bottom
surface of the metallic sheet and made of a second metal, wherein
the first conductor pattern comprises: (A) a plurality of base pads
each having a first predetermined size and shape and a base pad
perimeter thereabout, (B) a circuit trace, and (C) first and second
pedestal pads disposed proximate the circuit trace on opposite
sides thereof; (iii) a second conductor pattern attached to the top
surface of the metallic sheet and made of a third metal, wherein
the second conductor pattern comprises: (A) a masking pad generally
centered opposite each base pad, each masking pad having a second
predetermined size and shape generally congruent in size with the
first predetermined size and shape of its respective base pad, and
(B) a bridging element having first and second enlarged ends and a
constricted portion therebetween, the bridging element being
oriented generally transverse to the circuit trace with each
enlarged end disposed opposite a respective one of the pedestal
pads; and (iv) a substrate having an electrically insulative
surface to which the first conductor pattern is attached; (b)
etching the precircuit in an etchant which etches substantially
only the first metal, so as to form undercuts in the metallic sheet
directly beneath the masking pads; and (c) continuing to etch the
precircuit and undercut the masking pads to create a circuit, (i)
such that the metallic sheet beneath each masking pad is
substantially completely undercut causing the masking pads to
detach from the metallic sheet, thereby providing a plurality of
bumps made of the first metal, each bump being disposed atop a
respective one of the base pads, (ii) wherein a pedestal made of
the first metal is formed between each pedestal pad and its
respective enlarged end of the bridging element, thereby providing
an air bridge crossover above the circuit trace.
9. A method according to claim 8, wherein the bumps are disposed
generally about a projected footprint of a surface mount electronic
component.
10. A method according to claim 9, further comprising the steps of:
attaching the surface mount electronic component to the circuit
amid the bumps, wherein the component has I/O bond pads arranged on
a top surface thereof; and attaching a wirebond wire between each
I/O bond pad and a respective bump.
11. A method according to claim 8, wherein the bumps are disposed
generally within a projected footprint of a surface mount
electronic component in matched relation with a plurality of
respective I/O bond pads on a face of the component.
12. A method according to claim 11, further comprising the steps
of: orienting the electronic component such that each I/O bond pad
rests atop a respective one of the bumps; and simultaneously
bonding each I/O bond pad to its respective bump.
13. A method according to claim 8, wherein the first conductor
pattern further includes two mounting pads to which a surface mount
electronic component may be soldered, such that the bumps are
disposed generally between the two mounting pads and generally
within a projected footprint of the surface mount electronic
component.
14. A method according to claim 13, further comprising the steps
of: orienting the electronic component such that a body portion
thereof rests atop the bumps with a termination of the component
registered atop each of the two mounting pads; and soldering each
component termination to its respective mounting pad.
15. A method according to claim 8, wherein the second and third
metals are the same metal.
16. A method according to claim 8, wherein the first metal is
aluminum and the second and third metals are copper.
17. A method for making an electronic circuit assembly, comprising
the steps of: (a) providing a precircuit comprising: (i) a metallic
sheet made of a first metal and having a top surface and a bottom
surface; (ii) a first conductor pattern attached to the bottom
surface of the metallic sheet and made of a second metal, wherein
the first conductor pattern comprises: (A) a base pad having a
first predetermined size and shape and a base pad perimeter
thereabout, (B) a circuit trace, and (C) first and second pedestal
pads disposed proximate the circuit trace on opposite sides
thereof; (iii) a second conductor pattern attached to the top
surface of the metallic sheet and made of a third metal, wherein
the second conductor pattern comprises: a bridging element having
first and second enlarged ends and a constricted portion
therebetween, the bridging element being oriented generally
transverse to the circuit trace with each enlarged end disposed
opposite a respective one of the pedestal pads; and (iv) a
substrate having an electrically insulative surface to which the
first conductor pattern is attached; (b) attaching at least one
masking pad to the top surface of the metallic sheet generally
within the base pad perimeter, wherein each masking pad has a
second predetermined size and shape and is made of a masking
material which may be stripped without significantly attacking the
first, second, and third metals; (c) etching the precircuit in an
etchant which etches substantially only the first metal, so as to
form a circuit having, (i) a plurality of bumps made of the first
metal, each bump being disposed beneath a respective one of the
masking pads, and (ii) a pedestal made of the first metal formed
between each pedestal pad and its respective enlarged end of the
bridging element, thus providing an air bridge crossover above the
circuit trace; and (d) stripping the masking pads from the circuit,
thereby exposing the bumps.
18. A method according to claim 17, wherein the first metal is
aluminum and the second and third metals are copper.
19. A method for making an electronic circuit assembly, comprising
the steps of: (a) providing a precircuit comprising: (i) a metallic
sheet made of a first metal and having a top surface and a bottom
surface; (ii) a first conductor pattern attached to the bottom
surface of the metallic sheet and made of a second metal, wherein
the first conductor pattern comprises: (A) a base pad having a
first predetermined size and shape and a base pad perimeter
thereabout, (B) a circuit trace, and (C) first and second pedestal
pads disposed proximate the circuit trace on opposite sides
thereof; (iii) a second conductor pattern attached to the top
surface of the metallic sheet and made of a third metal, wherein
the second conductor pattern comprises: (A) at least one masking
pad arranged generally within the base pad perimeter, wherein each
masking pad has a second predetermined size and shape, and (B) a
bridging element having first and second enlarged ends and a
constricted portion therebetween, the bridging element being
oriented generally transverse to the circuit trace with each
enlarged end disposed opposite a respective one of the pedestal
pads; and (iv) a substrate having an electrically insulative
surface to which the first conductor pattern is attached; (b)
etching the precircuit in an etchant which etches substantially
only the first metal, so as to form a circuit having, (i) a
plurality of bumps made of the first metal, each bump being
disposed beneath a respective one of the masking pads, and (ii) a
pedestal made of the first metal formed between each pedestal pad
and its respective enlarged end of the bridging element, thus
providing an air bridge crossover above the circuit trace; and (c)
selectively etching away only the masking pads, thereby exposing
the bumps.
20. A method according to claim 19, wherein the first metal is
aluminum and the second and third metals are copper.
21. An electronic circuit assembly for connecting an electronic
component thereto, comprising: an electrically insulative
substrate; at least two mounting pads disposed on said substrate in
matched relation with respective terminations of the electronic
component; and at least one metallic bump attached to each mounting
pad within a projected footprint of the electronic component.
22. An electronic circuit assembly according to claim 21, wherein
said at least one bump on each mounting pad is/are arranged
generally symmetrically thereon with respect to a central
longitudinal axis of said projected footprint.
23. An electronic circuit assembly according to claim 21, wherein
said mounting pads are made of a first metal and said metallic
bumps are made of a second metal.
24. An electronic circuit assembly according to claim 23, wherein
said first metal is copper and said second metal is aluminum.
25. An electronic circuit and component assembly, comprising: an
electrically insulative substrate; at least two mounting pads
arranged on said substrate; at least one metallic bump attached to
a top surface of each mounting pad; an electronic component having
at least two terminations thereon, said component being disposed
such that each termination rests atop a respective one of said
mounting pads in contact with said at least one metallic bump atop
each pad; and a joint of electrically conductive bonding material
connecting each termination with a respective one of said mounting
pads.
26. An electronic circuit assembly according to claim 25, wherein
said mounting pads are made of copper and said bumps are made of
aluminum.
27. An electronic circuit assembly according to claim 25, wherein
said bonding material is solder or electrically conductive
adhesive.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to electronic
circuit assemblies, and more particularly to a method for making an
electronic circuit assembly out of etched tri-metal-layered circuit
structures.
[0003] 2. Disclosure Information
[0004] U.S. Pat. No. 3,801,388 to Akiyama et al. (hereinafter
"Akiyama"), U.S. Pat. No. 4,404,059 to Livshits et al. (hereinafter
"Livshits"), and U.S. Pat. No. 5,738,797 to Belke, Jr. et al.
(hereinafter "Belke"), all of which are incorporated herein by
reference, disclose various methods for making electronic circuits
which feature circuit crossovers or "air bridges" using a
combination of plating and etching steps.
[0005] The air bridge structures disclosed in these patents are
useful in offering design flexibility and printed circuit board
real estate savings as far as routing and layout of circuit traces;
however, they do not disclose or suggest any approach for
accommodating certain circuit board interconnect processes, such as
heavy wirebonding (e.g., using 5- to 20-mil aluminum wire, such as
in wirebonding power transistor dice to leadframes or mounting
pads), fine wirebonding (e.g., using less-than-5-mil gold wire,
such as in connecting the I/O pads of bare integrated circuit dice
to their respective circuit board mounting pads), or direct
component attachment (e.g., bonding of flip-chips, BGAs (ball grid
arrays), and the like directly to traces/pads on a circuit board
substrate).
[0006] It would be desirable, therefore, to provide a method for
using the aforementioned air bridge circuit structure with such
interconnect processes as heavy and fine wirebonding and direct
component attachment.
SUMMARY OF THE INVENTION
[0007] The present invention overcomes the deficiencies of prior
art approaches by providing a method for making an electronic
circuit assembly comprising the steps of: (a) providing a specially
designed tri-metal-layer precircuit, (b) selectively etching the
precircuit so as to urge the formation of a particular type of
undercut in the tri-metal precircuit structure, and (c) continuing
to etch the precircuit until a finished circuit is formed, wherein
the circuit includes circuit traces, air bridge crossovers, and
base pads having one or more etched bumps thereon. These bumps may
then be used to facilitate heavy and fine wirebonding and direct
chip attachment.
[0008] It is an object and advantage that the present invention
provides the aforementioned air bridge crossover circuit structure
while also providing bumps specially etched from the precircuit
structure which may be advantageously utilized to accommodate heavy
and fine wirebonding and direct chip attachment.
[0009] Another advantage is that the bumps provided by the present
invention may be used to assure a minimum solder joint standoff
height before, during, and after reflow soldering of a surface
mount electronic component.
[0010] Yet another advantage is that the bumps of the present
invention may be arranged so as to retard solder joint crack
propagation in reflowed electronic components.
[0011] These and other advantages, features and objects of the
invention will become apparent from the drawings, detailed
description and claims which follow.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIGS. 1a-c are top, side, and bottom views, respectively, of
a precircuit according to a first embodiment of the present
invention.
[0013] FIGS. 2a-e are side views of a precircuit undergoing
additive process construction according to the prior art.
[0014] FIGS. 3a-e are side views of a precircuit undergoing
subtractive process construction according to the prior art.
[0015] FIG. 4 is a magnified side view of a portion of the
precircuit of FIG. 1 after initial etching, showing undercut
masking pads.
[0016] FIGS. 5a-c are top, side, and side section views,
respectively, of a circuit according to a first configuration of
the first embodiment, particularly designed for heavy
wirebonding.
[0017] FIG. 6 is a side view of the configuration shown in FIG. 5b,
after heavy wirebonding.
[0018] FIGS. 7a-b are top and side views, respectively, of a
circuit according to a second configuration of the first
embodiment, particularly designed for arresting solder joint crack
propagation.
[0019] FIG. 8 is a side view of the configuration shown in FIG. 7b,
after component mounting and reflow soldering.
[0020] FIGS. 9a-b are top views of the first and second
configurations of the first embodiment, respectively, having bumps
shaped as elongated strips.
[0021] FIGS. 10a-b are top views of a first configuration of a
second embodiment of the present invention before and after fine
wirebonding, respectively.
[0022] FIGS. 11a-b are top views of a second configuration of a
second embodiment of the present invention before and after direct
chip attachment, respectively.
[0023] FIG. 11c is a side view of a portion of the configuration
shown in FIG. 11b.
[0024] FIGS. 12a-b are top and side views of a third configuration
of a second embodiment of the present invention before and after
component reflow soldering, respectively.
[0025] FIG. 13 is a top view of various configurations of the first
and second embodiments of the present invention before component or
bondwire attachment.
[0026] FIGS. 14a-d are side views of a precircuit undergoing a
first alternative method for forming bumps according to the present
invention.
[0027] FIGS. 15a-d are side views of a precircuit undergoing a
second alternative method for forming bumps according to the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] Referring now to the drawings, FIGS. 1-9 illustrate an
electronic circuit assembly according to a first embodiment 100 of
the present invention and the process steps for making the same.
The assembly begins as a pre-circuit 91 as shown in FIGS. 1a-c, and
is selectively etched so as to form the final structure 100 shown
in FIGS. 5a-c and 7a-c.
[0029] To assist the reader in understanding the present invention,
all reference numbers used herein are summarized in the table
below, along with the elements they represent:
1 10 = Metallic sheet/first metal layer 12 = Bottom surface of
metallic sheet 14 = Top surface of metallic sheet 16 = Bumps formed
from metallic sheet 13 = Bridges of first metal between bumps 20 =
Base pad on bottom surface of metallic sheet 22 = Perimeter of base
pad 24 = Perimeter of base pad projected onto top surface 26 =
Second (bottom) metal layer 28 = Edge of base pad within component
footprint 29 = Mounting pad (for Config. 2C) 30 = Undercuts in
metallic sheet under masking pads 40 = Masking pad on top surface
of metallic sheet 46 = Third (top) metal layer 52 = Bottom etch
resist pattern 53 = Apertures or etch resist-free regions in 52/54
54 = Top etch resist pattern 56 = Masking material resistant to
first metal etchant 57 = Bottom pattern plating mask 58 = Apertures
or mask-free regions in 57/59 59 = Top pattern plating mask 60 =
Circuit trace 62 = Pedestal pad 64 = Bridging element 65 = Enlarged
end of bridging element 66 = Constricted portion of bridging
element 68 = Pedestal 69 = Air bridge crossover 71 = Heavy
wirebonding wire 72 = Fine wirebonding wire 80 = Substrate 82 =
Electrically insulative surface of substrate 91 = Precircuit for
first embodiment 92 = Precircuit for second embodiment 93 = Surface
mount electronic component 95 = Termination of electronic component
97 = I/O bond pad of chip component 98 = Body portion of surface
mount component 99 = Solder joint 100 = Final structure of first
embodiment 200 = Final structure of second embodiment F = Footprint
of electronic component H = Solder joint standoff height S =
Spacing between adjacent masking pads X = Direction tangential to
metallic sheet Z = Direction orthogonal to metallic sheet 1A =
Heavy wirebonding configuration 1B = Solder joint crack-arresting
configuration 2A = Fine wirebonding configuration 2B = Direct chip
attach configuration 2C = Lifter configuration
[0030] The precircuit 91 for the first embodiment 100 generally
comprises four basic layers of structure: (1) a metallic sheet 10
made of a first metal and having a bottom surface 12 and a top
surface 14, (2) a first conductor pattern attached to the bottom
surface 12 of the metallic sheet and made of a second metal, (3) a
second conductor pattern attached to the top surface 14 of the
metallic sheet and made of a third metal, and (4) a substrate 80
having an electrically insulative surface 82 to which the first
conductor pattern is attached. The first conductor pattern in turn
comprises: a base pad 20 having a first predetermined size and
shape and a base pad perimeter 22 thereabout (which defines a
respective projected base pad perimeter 24 on the top surface 14 of
the metallic sheet), at least one circuit trace 60, and first and
second pedestal pads 62 disposed proximate the circuit trace 60 on
opposite sides thereof. The second conductor pattern comprises: a
plurality of masking pads 40 arranged generally within the base pad
perimeter 22 (or, more precisely, within the projected perimeter
24), wherein each masking pad 40 has a second predetermined size
and shape smaller than the base pad 20; and a bridging element 64
having first and second enlarged ends 65 and a constricted portion
66 between the ends 65, wherein the bridging element 64 is oriented
generally transverse to the circuit trace 60 with each enlarged end
65 disposed opposite a respective one of the pedestal pads 62, as
illustrated in FIGS. 1a-c.
[0031] The precircuit 91 may be created using one of many different
processes, such as the "additive" process of Belke or Livshits, or
the "subtractive" process of Akiyama. An additive approach is
illustrated in FIGS. 2a-e, involving the steps of: (1) providing a
metallic sheet 10 made of the first metal having top and bottom
surfaces 14/12 thereon (FIG. 2a); (2) applying a pattern plating
mask 57/59 to each of the top and bottom surfaces 12/14, wherein
each mask 57/59 has apertures or mask-free regions 58 therein which
correspond to the respective first and second conductor patterns
(FIG. 2b); (3) plating or depositing the second and third metals
through the apertures/mask-free regions 58 in the respective masks
57/59 so as to form the first and second conductor patterns on the
metallic sheet 10 (FIG. 2c); (4) stripping the masks 57/59 (FIG.
2d); and (5) attaching the first conductor pattern to the
dielectric surface 82 of a suitable substrate 80 (FIG. 2e). These
additive process steps are further described in Belke and
Livshits.
[0032] A subtractive approach for creating the precircuit 91 is
illustrated in FIGS. 3a-e, involving the steps of: (1) providing a
tri-metal laminate comprising a metallic sheet 10 made of a first
metal, a bottom metal layer 26 attached to the bottom surface 12 of
the metallic sheet and made of a second metal, and a top metal
layer 46 attached to the top surface 14 of the metallic sheet and
made of a third metal (FIG. 3a); (2) applying a bottom etch resist
pattern 52 (e.g., exposed photoresist) to the bottom metal layer
with apertures or etch resist-free regions 53 therein conforming to
the first conductor pattern to be formed thereon (FIG. 3b); (3)
applying a top etch resist pattern 54 to the top metal layer also
having apertures/etch resist-free regions 53 therein conforming to
the second conductor pattern to be formed thereon (FIG. 3b); (4)
etching the exposed portions (i.e., not covered by etch resist) of
the bottom metal layer 26 in an etchant which etches substantially
only the second metal so as to form the first conductor pattern
(FIG. 3c); (5) etching the exposed portion of the top metal layer
46 in an etchant which etches substantially only the third metal so
as to form the second conductive pattern (FIG. 3c); (6) stripping
the top and bottom etch resist patterns 52/54 so as to expose the
conductor patterns (FIG. 3d); and (7) attaching the first conductor
pattern to the dielectric surface 82 of a suitable substrate 80
(FIG. 3e). Regardless of whether an additive or a subtractive
approach is used to create the precircuit, the same precircuit
structure 91 will result.
[0033] Once the precircuit 91 is created, it is then exposed for a
predetermined time to an etchant which etches substantially only
the first metal, so as to form undercuts 30 in the first metal 10
directly underneath the masking pads 40, as shown in FIG. 4. The
next step is to continue to etch the precircuit and undercut the
masking pads to create a circuit 100, such that the metallic sheet
10 region underneath each masking pad 40 is substantially
completely undercut, causing the pads 40 to become detached from
the metallic sheet 10 and thereby providing a plurality of bumps 16
made of the first metal disposed atop the base pad 20 generally
within the perimeter 22 thereof, as shown in FIGS. 5a-b. At the
same time, the continued etching etches away those portions of the
metallic sheet 10 that are exposed (i.e., not covered by the first
and second conductor patterns); however, wherever the metallic
sheet 10 is shielded from the etchant by portions of the first and
second conductor patterns, the metallic sheet/first metal in such
regions remains unetched or only minimally etched (depending on the
thickness, geometry, and relative positioning of the first and
second pattern features thereat, the strength of and exposure time
to the etchants, the relative thicknesses of the tri-metal layers
10/26/46, etc.). While the bumps 16 are being formed, a pedestal 68
made of the first metal sheet 10 is also being formed between each
pedestal pad 62 and its respective enlarged end 65 of the bridging
element 64, thereby providing an air bridge crossover 69 above the
circuit trace 60, as shown in FIGS. 5a-c (particularly in FIG.
5c).
[0034] In order for the masking pads 40 to become undercut so as to
become detached from the gradually forming first metal bumps 16,
while at the same time causing the enlarged ends 65 of the bridging
element 64 to become undercut but remaining attached to the
gradually forming pedestals 68, the shape and size of each masking
pad 40, bridging element feature (i.e., ends 65 and constricted
portion 66), and base pad 20 must be carefully selected so that the
circuit 100 ends up as described herein. Guidelines for selecting
the relative sizes and shapes for these features may be found in
Livshits, Akiyama, Belke, and below.
[0035] Two different configurations of bump layouts are shown in
FIGS. 5a-b and 7a-b. In the first configuration (labeled "Config.
1A" in FIGS. 5a-b), the plurality of bumps 16 is distributed
generally evenly across substantially all of the base pad 20. With
the bumps 16 distributed thusly, an aluminum or other metal
wirebonding wire 71 may be wirebonded to the plurality of bumps, as
illustrated in FIG. 6, thus providing a way to effect heavy
wirebonding interconnects.
[0036] In the second configuration (labeled "Config. 1B" in FIGS.
7a-b), the first conductor pattern includes at least two closely
spaced base pads 20 (rather than only one), wherein these pads
serve as mounting pads 20 to which a surface mount electronic
component 93 (e.g., a resistor chip) may be soldered. Also, the
second conductor pattern includes a plurality of bumps 16 for each
of the at least two mounting pads 20. Here, each plurality of bumps
16 is arranged proximate an edge 28 of its respective mounting pad
20, within a projected footprint F the component 93. (As used
herein, the "footprint" F of a component 93 refers to the region of
the circuit which is covered by the component when the component is
placed thereon.) With the bumps arranged in this way, the component
93 may be placed atop the two or more pads 20 such that a component
termination 95 rests generally atop each plurality or cluster of
bumps 16, as shown in FIG. 7b. Then, each termination 95 may be
soldered to its respective mounting pad 20, such that each cluster
of bumps 16 is generally enclosed within a solder joint 99
connecting each component termination 95 with its pad 20, as shown
in FIG. 8. In this configuration, the bumps 16 provide two
advantages: (1) they provide a minimum standoff height H, which
typically improves solder joint resistance to thermal and physical
stress, and (2) they provide a barrier in the otherwise normal
solder joint crack propagation path, thereby helping to arrest or
at least retard further crack propagation so as to prolong solder
joint service life.
[0037] A wide variety of metals may be used in the present
invention, such as copper, aluminum, nickel, steel, and so on.
Typically, the second and third metals are the same metal, although
they may optionally be different. The metals to be used for a given
circuit structure are usually chosen according to the ability of
the metals to be clad together or plated onto each other, and by
their relative reaction rates with known etchants.
[0038] An exemplary configuration for the present embodiment 100
would provide a first metal sheet 10 (and hence bumps 16) made of
6-mil-thick aluminum, and the second and third metals (and hence
first and second conductor patterns) made of 2-mil-thick copper.
For this copper/aluminum/copper combination, the aluminum etchant
may be, inter alia, NaOH or KOH, plus an oxidizing agent (e.g.,
NaNO3, NaNO2, or NaBrO3); the copper etchant may be, inter alia,
chromic acid, nitric acid, or peroxysulfuric acid. The
predetermined shape and size of the base pad 20 may conform with
conventional shape and size guidelines used to design pads for
heavy wirebonding or component reflow soldering, or they may vary
therefrom, depending on the given design constraints; for example,
the base pad 20 for either application may me a 60.times.80-mil
rectangle. The predetermined size and shape of the masking pads 40
is more non-intuitive, since there is no analog to these pads under
conventional wirebonding or reflowing practice. The shape of these
pads 40 is preferably circular or square, but may be any other
desired shape (e.g., elongated strips, as in FIGS. 9a-b). The size
of each pad 40 is preferably 18 mils or less across in diameter or
width, and most preferably 10 mils or less. This size is kept small
to permit the etchant to form undercuts 30 in the first metal
beneath the masking pads 40 in the "X" direction enough so that the
connection between the pads 40 and the bumps 16 is weakened and the
pad 40 is lifted away without etching too deeply in the "Z"
direction, as shown in FIG. 4. The spacing S between adjacent pads
40 should be selected so as to allow enough space therebetween so
the etchant may etch the first metal effectively. This spacing S is
dependent on the type of etchant, the strength of the etchant, the
process (e.g., dipping versus spraying), the etchant flow/spray
rates, and so forth. A general rule of thumb is that the spacing S
between adjacent pads 40 should be about one-half the pad diameter
or width. For example, in FIGS. 5a-b, the masking pads 40 are
generally square in shape, are 10 mils in width, and are spaced 5
mils apart on a 40.times.70-mil rectangular base pad 20, and
eventually form first metal bumps 16 that are 3-6 mils high and 10
mils in diameter.
[0039] A second embodiment of the present invention is illustrated
in FIGS. 10-12, comprising three different configurations (i.e.,
Configs. 2A, 2B and 2C). While the general size of the masking pads
40 is the same for both embodiments (100/200) and all of the
configurations (1A-B and 2A-C), an important distinction between
the first and second embodiments is that in the former case
(Configs. 1A-B) there is preferably a plurality of masking pads 40
and bumps 16 atop each base pad 20, while in the latter case
(Configs. 2A-C) there is only one masking pad 40 and bump 16 per
base pad 20. Also, as illustrated in FIGS. 10-12, the base pads 20
of the second embodiment 2A-C are much smaller than those 20 of the
first embodiment 1A-B, are generally about the same size as their
corresponding masking pads 40, and are generally concentric with
the pads 40. It may also be noted that while most of the bumps 16
of Configs. 1A-B are shown in FIGS. 5 and 7 as being discrete, it
is permissible in the first embodiment that there be bridges 18 of
the first metal connecting adjacent bumps 16; however, in Configs.
2A and 2B of the second embodiment, no bridges 18 of the first
metal 10 are allowed to connect adjacent bumps 16, as will be
explained in further detail below.
[0040] Because the basic structure for the second embodiment 200 is
similar to that for the first embodiment 100, the same process
steps for creating the first precircuit 91 may be used for creating
the second precircuit 92, with the only difference being in where
the apertures/mask-free regions 58 or etch resist patterns 52/54
are placed.
[0041] As illustrated in FIGS. 10a-b, the arrangement of bumps 16
in Config. 2A is best suited for fine wirebonding. Here, the bumps
16 are arranged generally about (i.e., outside) and proximate a
projected footprint F of a surface mount electronic component 93,
such as a bare die having I/O bond pads 97 thereon. With the bumps
16 arranged in this way, the component 93 may be attached to the
circuit amid the bumps 16 (e.g., using an adhesive) with the
component's I/O bond pads 97 arranged on the component's top
surface. Then, each bond pad 97 may be fine-wirebonded to its
respective bump 16 (e.g., using 3- to 5-mil diameter gold wire
72),/as shown in FIG. 10b. An optional step of plating the bumps 16
may also be performed, such as by electrolytic, electroless, or
immersion processing. For example, if the metallic sheet/bumps
10/16 are made of aluminum, a coating of immersion silver may be
placed atop the bumps 16, and then a fine gold wire may be
wirebonded to the silver bump coating.
[0042] Configuration 2B, shown in FIGS. 11a-c, is similar to 2A,
but here the bumps 16 are arranged generally within the component
footprint F, rather than about/outside the footprint. The bumps 16
are also arranged in matched relation with the plurality of
respective I/O bond pads 97 on the face of the component 93. Here,
the component 93 is preferably a bare die flip-chip, which may be
oriented "face down" in conventional flip-chip fashion such that
each I/O pad 97 rests atop a respective one of the bumps 16,
whereupon the pads 97 are simultaneously bonded to the bumps 16,
such as by thermocompression, ultrasonic bonding, or thermosonic
bonding. If the metal layer 10 of the precircuit 92 is made of
aluminum, as suggested above, then the I/O bond pads--which are
typically also made of aluminum--may be easily bonded to the
aluminum bumps 16, such as by conventional thermocompression
bonding.
[0043] Config. 2C, illustrated in FIGS. 12a-b, is somewhat
analogous to Config. 1B of the first embodiment, in that the bumps
16 in Config. 2C serve as "lifters" which hold the component 93 at
a certain standoff height H above the mounting pads 29. However,
while in Config. 1B the bumps 16 are formed atop each base pad 20
(with the base pad also serving as a mounting pad for the
component), in Config. 2C two or more separate mounting pads 29 are
provided with the bumps/base pads 16/20 being formed between/amid
the mounting pads 29, but not atop these pads 29. Here, separate
mounting pads 29 serve as mounting pads for the respective
terminations 95 of the surface mount component 93. With the bumps
16 arranged as described, the component 93 may be oriented with its
body portion 98 resting atop the bumps 16 and its terminations 95
registered atop their respective mounting pads 29, as shown in FIG.
12b, whereupon the terminations may be soldered to their respective
pads (e.g., by reflow soldering, molten solder dispensing, etc.).
In this arrangement, the bumps 16 serve to maintain the component
93 at a given standoff height H before, during, and after formation
of the solder joints 99. For best results, the bumps 16 should be
arranged in a pattern that supports the component 93 in a generally
level orientation, as illustrated in FIG. 12b.
[0044] The process for creating the structures of Configs. 2A-C is
similar to that for Configs. 1A-B, and begins by providing a
precircuit 92 comprising: (1) a metallic sheet 10 made of a first
metal and having a bottom surface 12 and a top surface 14; (2) a
first conductor pattern attached to the bottom surface 12 of the
first metal layer 10 and made of a second metal; (3) a second
conductor pattern attached to the top surface 14 of the first metal
layer 10 and made of a third metal; and (4) a substrate 80 having
an electrically insulative surface 82 to which the first conductor
pattern is attached. The first conductor pattern comprises: a
plurality of base pads 20 each having a first predetermined size
and shape and a base pad perimeter 22 thereabout; a circuit trace
60; and first and second pedestal pads 62 disposed proximate the
circuit trace 60 on opposite sides thereof. The second
conductor-pattern comprises: a masking pad 40 generally centered
opposite each base pad 20, each masking pad 40 having a second
predetermined size and shape generally congruent in size with the
first predetermined size and shape of its respective base pad 20;
and a bridging element 64 having first and second enlarged ends 65
and a constricted portion 66 therebetween, wherein the element 64
is oriented generally transverse to the circuit trace with each
enlarged end disposed opposite a respective one of the pedestal
pads 62. The specific placement of the masking pads 40 and base
pads 20 (and hence the placement of the first metal bumps 16 formed
therebetween) is determined according to which of the three
configurations is desired; these placement schemes are described
above and illustrated in the drawings. After providing the
precircuit 92, it is then etched in a manner similar to the method
for forming the first embodiment 100, so as to undercut and remove
the masking pads 40 and form a bump 16 made of the first metal
disposed atop each base/mounting pad 20, as well as forming the air
bridge crossover(s) 69.
[0045] It should be noted that the bumps 16 for Configs. 2A-B
should be discrete; that is, no bridges 18 of the first metal may
connect any such bump 16 with another. This is because in these
configurations, each bump 16 is eventually connected with a single
chip I/O bond pad 97, so electrical isolation is required among
each bump 16 and any adjacent bumps 16. However, in Config. 2C it
is permissible for bridges 18 of the first metal to connect any
bumps together, since these bumps do not connect with any
electrical termination or bond pad of the component 93, but are
merely physical lifters.
[0046] In order to provide a comparison among all the
configurations of the two embodiments and to present an example of
their respective scales, FIG. 13 shows the various configurations
for both embodiments 100/200 in one drawing.
[0047] Two alternative methods for forming the bumps 16 in either
of the two embodiments 100/200 are illustrated in FIGS. 14-15. The
first alternative method is shown in FIGS. 14a-d, which begins by
providing either of the two precircuits 91/92 as described above,
except that the masking pad(s) 40 is/are made of a masking material
56 that is different from the first, second and third metals, is
resistant to the first metal etchant, and can be etched in an
etchant (or removed by some means) that does not significantly
attack/etch the first, second, and third metals. This material 56
may be a metal, a polymer, etc., and should be selected in light of
the first, second, and third metals chosen and their respective
etchants. For example, the masking material 56 may be an organic
photoimageable etch resist that is not significantly
attacked/etched by the first metal etchant, and which can be
stripped using a solvent that does not appreciably attack the
first, second, and third metals. As another example, the masking
material 56 may be an etch-resistant adhesive film that may be
subsequently peeled away from metal sheet 10. The masking material
pads 40/56 are attached to the top surface 14 of the first metal
sheet 10 as a step separate from the attachment/formation of the
bridging elements, preferably thereafter, as illustrated in FIG.
14b. After the modified precircuit is provided, the precircuit is
exposed to an etchant which etches substantially only the first
metal layer 10, thereby etching away substantially all of the first
metal layer 10 except: (1) a bump 16 made of the first metal 10
underneath each masking material pad 40/56, and (2) a pedestal 68
made of the first metal 10 between each pedestal pad and its
respective enlarged end of the bridging element, thus providing an
air bridge crossover above the circuit trace 60, as illustrated in
FIG. 14c. The masking material 56 may then be stripped away, thus
exposing the first metal bumps 16, as shown in FIG. 14d. Note that
in this alternative approach, it is not necessary that the masking
pads 40 be sized such that they become undercut and detached from
the first metal bumps 16, because they are instead stripped away as
a separate process step after formation of the circuit's bumps and
air bridge(s).
[0048] The second alternative method is shown in FIGS. 15a-d. Here,
the same precircuit 91/92 as in Configs. 1A-B and 2A-C is
provided--i.e., the masking pads 40 are made of the third metal, as
are the other second conductor pattern elements--however, the
masking pads 40 do not have to be sized so as to become drastically
undercut and thus detached from the first metal bumps 16. Like the
first alternative approach, after the modified precircuit is
provided, the precircuit is exposed to an etchant which etches
substantially only the first metal layer 10, thereby etching away
substantially all of the first metal layer 10 except: (1) a bump 16
made of the first metal 10 underneath each masking pad 40, and (2)
a pedestal 68 made of the first metal 10 between each pedestal pad
and its respective enlarged end of the bridging element, thus
providing an air bridge crossover above the circuit trace 60, as
illustrated in FIG. 15b. Then, only the masking pads 40 are
selectively etched, in an etchant which etches substantially only
the third metal, thereby exposing the bumps 16 underneath, as shown
in FIGS. 15c-d. This latter step may be accomplished by masking the
rest of the second conductor pattern with an etch resist that is
resistant to the third metal etchant, or (as illustrated in FIG.
15c) by sealing off the masking pads 40 from the rest of the second
conductor pattern and exposing only the sealed off pads 40 to a
third metal etchant.
[0049] Various other modifications to the present invention may
occur to those skilled in the art to which the present invention
pertains. For example, although circuit traces 60 are not shown in
some drawings (for clarity), one skilled in the art will appreciate
that a circuit trace or conductive via would typically be connected
to each base pad 20, mounting pad 20/29, pedestal pad 62, and bump
16 in Configs. 2A-B for connection with other
pads/traces/components in the electronic circuit assembly or
printed circuit board. Also, although reference is made herein to
"soldering" an electronic component to the circuit, equivalent
processes such as conductive adhesive bonding (e.g., applying and
curing a heat-activated silver-filled epoxy) may instead be used.
Additionally, "soldering" may include reflow soldering, molten
solder dispensing, or any other process used to connect component
terminations to their associated mounting pads. Moreover, the "I/O
bond pads" include not only the typical aluminum bond pads found on
bare dice in flip-chip applications, but may also include balls or
bumps made of gold, solder, and the like which serve as device I/O
interconnects; the bond pads/balls/bumps may be arranged about the
outer periphery of the device, or may be distributed generally
evenly across a face of the device (e.g., BGAs). Additionally, it
will be appreciated that the substrate 80 to which the first
conductor pattern is attached may be made of metal, polymer,
ceramic, or other materials, so long as the surface 82 thereof to
which the conductor pattern is connected is electrically
insulative. For example, the substrate 80 may comprise an aluminum
plate with a coating of electrically insulative epoxy, adhesive,
curable film, or the like. The surface 82 may be an adhesive in
itself, which effects the bonding of the first conductor pattern to
the substrate 80, or a separate adhesive may be interposed between
the substrate surface 82 and the conductor pattern. Also, two or
more of the configurations disclosed herein may be combined as
desired, such as in combining Configs. 1B and 2C, which would
provide bumps 16 both on the mounting pads 20 and between the
mounting pads 20. Other modifications not explicitly mentioned
herein are also possible and within the scope of the present
invention. It is the following claims, including all equivalents,
which define the scope of the present invention.
* * * * *