U.S. patent application number 09/862371 was filed with the patent office on 2002-02-28 for low temperature solder column attach by injection molded solder and structure formed.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Bolde, Lannie R., Brouillette, Guy P., Covell, James H., Danovitch, David, Gruber, Peter A., Lei, Chon C..
Application Number | 20020023945 09/862371 |
Document ID | / |
Family ID | 24604999 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020023945 |
Kind Code |
A1 |
Gruber, Peter A. ; et
al. |
February 28, 2002 |
Low temperature solder column attach by injection molded solder and
structure formed
Abstract
A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate and the structure formed by such
method are disclosed. In the method, a mold plate equipped with a
multiplicity of cavities is first filled by an injection molded
solder technique with a high temperature solder forming a
multiplicity of solder columns. The mold plate is then sandwiched
between an extraction plate and a transfer plate by utilizing a
multiplicity of displacement means equipped in the extraction plate
to displace the multiplicity of solder columns from the mold plate
into a multiplicity of apertures equipped in the transfer plate.
The multiplicity of cavities in the transfer plate each has a
straight opening and a flared opening. The flared opening is then
filled with a low temperature solder paste to encapsulate one end
of the high temperature solder column. The low temperature solder
paste is then reflown on top of a conductive pad on an electronic
substrate at a temperature lower than the melting temperature of
the high temperature solder to form a bond between the solder
column and the conductive pad.
Inventors: |
Gruber, Peter A.; (Mohegan
Lake, NY) ; Bolde, Lannie R.; (New Palte, NY)
; Brouillette, Guy P.; (Saefford, CA) ; Covell,
James H.; (Poughkeepsie, NY) ; Danovitch, David;
(Granby, CA) ; Lei, Chon C.; (Poughkeepsie,
NY) |
Correspondence
Address: |
Randy W. Tung
Tung & Associates
Suite 120
838 W. Long Lake Road
Bloomfield Hills
MI
48302
US
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
24604999 |
Appl. No.: |
09/862371 |
Filed: |
May 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
09862371 |
May 22, 2001 |
|
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09649487 |
Aug 28, 2000 |
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6276596 |
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Current U.S.
Class: |
228/225 ;
174/257; 228/180.21; 228/253; 228/56.3; 427/282; 427/97.3 |
Current CPC
Class: |
B23K 3/0638 20130101;
H05K 2203/074 20130101; H05K 2203/0338 20130101; H05K 3/3468
20130101; H01L 21/4846 20130101; B23K 35/0222 20130101; H05K
2203/128 20130101; B23K 2101/40 20180801; H05K 2203/0113 20130101;
H05K 2203/0415 20130101 |
Class at
Publication: |
228/225 ;
228/180.21; 228/56.3; 228/253; 427/96; 427/282 |
International
Class: |
B23K 031/02; B23K
035/24; B23K 035/14; B05D 001/32; B05D 005/12 |
Claims
1. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate comprising the steps of:
providing a mold plate having a multiplicity of cavities, filling
said multiplicity of cavities with a first solder having a melting
temperature of at least 240.degree. C. forming a multiplicity of
solder columns, providing an extraction plate equipped with a
multiplicity of displacement means positioned corresponding to the
positions of said multiplicity of cavities in said mold plate when
said extraction plate is positioned on top of said mold plate,
providing a transfer plate equipped with a multiplicity of
apertures for receiving solder columns, said multiplicity of
apertures each having a straight opening and a flared opening,
positioning said mold plate in between and in intimate contact with
said extraction plate and said multiplicity of apertures in said
transfer plate with said straight opening facing said mold plate,
activating said multiplicity of displacement means in said
extraction plate to displace said multiplicity of solder columns
from said mold plate into said transfer plate, filling said flared
openings of said multiplicity of apertures in said transfer plate
with a second solder having a melting temperature of less than
240.degree. C. encapsulating one end of each of said multiplicity
of solder columns to form a flared end, positioning said transfer
plate on top of an electronic substrate, heating said transfer
plate and said electronic substrate to a temperature not lower than
the melting temperature of said solder forming a bond between said
multiplicity of solder columns and said multiplicity of conductive
pads, and removing said transfer plate from said electronic
substrate.
2. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said multiplicity of displacement means comprises a multiplicity of
push pins for mechanically pressing on said multiplicity of solder
columns situated in said mold plate to displace them from said
multiplicity of cavities in said mold plate into said multiplicity
of apertures in said transfer plate.
3. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said multiplicity of displacement means comprises pneumatic means
for exerting air pressure on said multiplicity of solder columns
situated in said mold plate to displace them from said multiplicity
of cavities in said mold plate into said multiplicity of apertures
on said transfer plate.
4. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said multiplicity of displacement means comprises push pins and
pneumatic means for displacing said multiplicity of solder columns
from said multiplicity of cavities in said mold plate into said
multiplicity of apertures in said transfer plate.
5. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said multiplicity of cavities in said mold plate each having a
tapered sidewall to facilitate discharge of said multiplicity of
solder columns from said multiplicity of cavities, said tapered
sidewall produces a larger opening of said cavity in a surface of
the mold plate to be joined to said transfer plate.
6. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 5, wherein
said tapered sidewall having an angle between about 1.degree. and
about 30.degree. when measured from a vertical axis.
7. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said extraction plate, said mold plate and said transfer plate each
being equipped with an alignment means for aligning to an adjacent
plate.
8. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, further
comprising the step of providing said multiplicity of cavities
through a thickness of said mold plate.
9. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1 further
comprising the step of filling said flared openings of said
multiplicity of apertures with said multiplicity of solder columns
displaced therein with a solder paste by a printing technique.
10. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, further
comprising the step of filling said multiplicity of cavities with a
first solder by an injection molding technique.
11. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, further
comprising the step of providing a first base plate as a stop
juxtaposed to said transfer plate during the step of displacing
said multiplicity of solder columns from said mold plate into said
multiplicity of apertures in said transfer plate.
12. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1 further
comprising the step of providing a second base plate as a stop
juxtaposed to said transfer plate during said step of filling said
flared openings to prevent said multiplicity of solder columns from
falling out of said transfer plate.
13. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 12 further
comprising the step of flipping said transfer plate upside down
such that it rests on said second base plate prior to said filling
step.
14. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, further
comprising the step of flipping said transfer plate upside down
prior to the step of positioning said transfer plate on said
electronic substrate.
15. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said first solder having a melting temperature between about
240.degree. C. and about 360.degree. C.
16. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said first solder having a melting temperature between about
280.degree. C. and about 320.degree. C.
17. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said second solder having a melting temperature between about
120.degree. C. and about 239.degree. C.
18. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said second solder having a melting temperature between about
160.degree. C. and about 200.degree. C.
19. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said first solder comprises more lead than tin.
20. A method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate according to claim 1, wherein
said second solder comprises more tin than lead.
21. An electronic substrate having a multiplicity of solder columns
electrically joined thereto comprising: an electronic substrate
having a multiplicity of conductive pads on a top surface, and a
multiplicity of solder columns each formed of a first solder
electrically jointed to one of said multiplicity of conductive pads
by a second solder at a base portion of said solder column, said
second solder having a lower melting temperature than said first
solder.
22. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
conductive pads are bond pads.
23. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
first solder has a melting temperature of at least 240.degree.
C.
24. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
first solder has a melting temperature between about 240.degree. C.
and about 360.degree. C.
25. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
first solder has a melting temperature preferably between about
280.degree. C. and about 320.degree. C.
26. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
first solder contains more lead than tin.
27. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
second solder has a melting temperature of less than 240.degree.
C.
28. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
second solder has a melting temperature between about 120.degree.
C. and about 239.degree. C.
29. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
second solder has a melting temperature between about 160.degree.
C. and about 200.degree. C.
30. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
second solder has more tin than lead.
31. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 21, wherein said
multiplicity of solder columns each having a tapered shape with a
larger cross-sectional area in said base portion.
32. An electronic substrate having a multiplicity of solder columns
electrically joined thereon according to claim 31, wherein said
tapered shape forms an angle between about 1.degree. and about
30.degree. with a vertical axis.
Description
FIELD OF THE INVENTION
[0001] The present invention generally relates to an electronic
substrate that has solder column grid arrays formed thereon and
more particularly, relates to an electronic substrate that has
multi-alloy solder columns formed thereon wherein a high
temperature solder is used to form the columns and a low
temperature solder is used to join the columns to conductive pads
on the electronic substrate.
BACKGROUND OF THE INVENTION
[0002] Solder columns grid arrays (CGA) have been used as a package
in the IC industry. The high aspect ratio of the solder columns,
for instance, typically 4 to 1, allows the CGA package to tolerate
significant coefficient of thermal expansion (CTE) mismatches
between the package and the electronic substrate. Conventionally, a
high melting point solder material such as a 90Pb/10Sn solder
having a 300.degree. C. melting temperature has been used to make
connections directly to a ceramic CGA package. Since the melting
temperature of the high melting point solder material exceeds the
processing temperature of some subsequently used materials, the
process of attaching the solder columns has to be carried out in
the early stage of the manufacturing process. The solder columns
formed in the early stage are therefore prone to mechanical damages
such as bending, flaring, etc, through the remaining manufacturing
process.
[0003] More recently, a new technique of attaching the 90Pb/10Sn
solder columns with a lower temperature eutectic solder, such as
37Pb/63Sn with a melting temperature of 183.degree. C., has been
developed to attach the solder columns at the end of the
manufacturing process and thus, reducing the probability for
damage. The lower temperature solder is applied to one end of the
high temperature solder columns by means of applying a solder paste
material of 37Pb/63Sn.
[0004] In the conventional process, known as column last attached
solder process (CLASP), described in U.S. Patent No. ______,
assigned to the common assignee of the present application, solder
columns are attached at the end of the manufacturing cycle by
utilizing pre-fabricated solder columns made by cutting extruded
solder wire to a specific length. The conventional process is
therefore extremely labor intensive since it involves a process of
precision wire extrusion, wire cutting, as well as storage and
packaging. The conventional process further involves the step of
loading the solder columns into a graphite fixture positioned on a
vibration table. The loading process is again time consuming and is
prone to problems. For instance, during the wire cutting or
shipping/handling, the thin solder wires, typically of 0.02 inch
diameter are susceptible to mechanical damages, such as bending,
etc. A bent solder column cannot be loaded into the graphite
fixture.
[0005] When filling high aspect ratio, i.e. higher than 4:1, via
holes on semiconductor substrates, a screen printing technique was
found ineffective and an injection molded solder (IMS) technique
has been developed. For instance, U.S. Pat. No. 5,244,143, assigned
to the common assignee of the present invention, discloses a new
method for injecting molten solder directly into a mold, i.e. a
graphite solder column mold. The patent further discloses the mold
release and transfer methods for achieving low temperature solder
column attach by the IMS technique. An improved injection molded
solder technique, i.e. a vacuum injection molding technique was
disclosed in a co-pending application that was assigned to the
common assignee of the present invention under Ser. No. 08/518,874
which is incorporated herein by reference in its entirety. The
vacuum injection molding method utilizes a pressure differential
formed between either ambient and vacuum or positive pressure and
vacuum. The method is carried out by utilizing a shallow vacuum
link that allows a continuing evacuation of air from via holes that
have a large aspect ratio, i.e. 5:1. The vacuum link must be
sufficiently shallow such that the surface tension of molten solder
prevents cross-leaking during the operation. The shallow link
therefore effectively choke a significant part of the full pressure
differential and thus producing only partial filling of via holes
that have high aspect ratios.
[0006] It is therefore an object of the present invention to
provide a method for forming an array of multi-alloy solder columns
on an electronic substrate that does not have the drawbacks or
shortcomings of the conventional methods.
[0007] It is another object of the present invention to provide a
method for forming an array of multi-alloy solder columns by first
forming the solder columns with a high temperature solder and then
joining the solder columns to an electronic substrate with a low
temperature solder.
[0008] It is a further object of the present invention to provide a
method for forming an array of multi-alloy solder columns by an
injection molded solder technique wherein a high temperature molten
solder is injected into a solder column mold plate equipped with a
multiplicity of cavities.
[0009] It is another further object of the present invention to
provide a method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate by utilizing an extraction
plate, a mold plate and a transfer plate.
[0010] It is still another object of the present invention to
provide a method for joining a multiplicity of multi-alloy solder
columns to an electronic substrate by displacing a multiplicity of
solder columns into a multiplicity of apertures in a transfer plate
wherein the apertures are equipped with a flared opening such that
a low temperature solder paste can be screen printed therein.
[0011] It is yet another object of the present invention to provide
a method for joining a multiplicity of multi-alloy solder columns
to an electronic substrate by forming the solder columns with a
solder having a melting temperature higher than 240.degree. C. and
joining the solder columns to an electronic substrate by a solder
having a melting temperature lower than 240.degree. C.
[0012] It is still another further object of the present invention
to provide an electronic substrate which has a multiplicity of
solder columns electronically joined thereto that includes an
electronic substrate having a multiplicity of bonding pads thereon
and a multiplicity of solder columns each formed of a high
temperature solder for bonding to the bond pads by a low
temperature solder.
[0013] It is yet another further object of the present invention to
provide an electronic substrate that has a multiplicity of solder
columns electrically joined thereto wherein the solder columns are
formed with a high temperature solder and joined to the electronic
substrate with a low temperature solder.
SUMMARY OF THE INVENTION
[0014] In accordance with the present invention, a method for
joining a multiplicity of multi-alloy solder columns to an
electronic substrate and the structures formed are disclosed.
[0015] In a preferred embodiment, a method for joining a
multiplicity of multi-alloy solder columns to an electronic
substrate can be carried out by the operating steps of first
providing a mold plate that has a multiplicity of cavities, filling
the multiplicity of cavities that has a first solder with a melting
temperature of at least 240.degree. C. to form a multiplicity of
solder columns, providing an extraction plate equipped with a
multiplicity of displacement means positioned corresponding to the
positions of the multiplicity of cavities in the mold plate when
the extraction plate is positioned on top of the mold plate,
providing a transfer plate equipped with a multiplicity of
apertures for receiving solder columns, the multiplicity of
apertures each having a straight opening and a flared opening,
positioning the mold plate in between and in intimate contact with
the extraction plate and the transfer plate such that the
multiplicity of cavities in the mold plate is aligned with both the
multiplicity of displacement means in the extraction plate and the
multiplicity of apertures in the transfer plate with the straight
opening facing the mold plate, activating the multiplicity of the
displacement means in the extraction plate to displace the
multiplicity of solder columns from the mold plate into the
transfer plate, filling the flared openings of the multiplicity of
apertures in the transfer plate with a second solder that has a
melting temperature of less than 240.degree. C. encapsulating one
end of each of the multiplicity of solder columns to form a flared
end, positioning the transfer plate on top of an electronic
substrate such that the flared end of each of the multiplicity of
solder columns contacts one of a multiplicity of conductive pads on
the electronic substrate, heating the transfer plate and the
electronic substrate to a temperature not lower than the melting
temperature of the second solder forming a bond between the
multiplicity of solder columns and the multiplicity of conductive
pads, and removing the transfer plate from the electronic
substrate.
[0016] In the method for joining a multiplicity of multi-alloy
solder columns to an electronic substrate, the multiplicity of
displacement means may include a multiplicity of push pins for
mechanically pressing on the multiplicity of solder columns
situated in the mold plate to displace them from the multiplicity
of cavities in the mold plate into the multiplicity of apertures in
the transfer plate. The multiplicity of displacement means may
further include pneumatic means for exerting air pressure on the
multiplicity of solder columns situated in the mold plate to
displace them from the multiplicity of cavities in the mold plate
into the multiplicity of apertures in the transfer plate. The
multiplicity of displacement means may further include push pins
and pneumatic means for displacing the multiplicity of solder
columns from the multiplicity of cavities in the mold plate into
the multiplicity of apertures in the transfer plate. The
multiplicity of cavities in the mold plate each has a tapered
sidewall to facilitate the discharge of the multiplicity of solder
columns from the multiplicity of cavities, the tapered sidewall
produces a larger opening of the cavity in the surface of the mold
plate to be joined to the transfer plate. The tapered sidewall may
have an angle between about 1.degree. and about 30.degree. when
measured from a vertical axis.
[0017] In the method for joining a multiplicity of multi-alloy
solder columns to an electronic substrate, the extraction plate,
the mold plate and the transfer plate may each be equipped with an
alignment means for aligning to an adjacent plate. The method may
further include the step of providing the multiplicity of cavities
through a thickness of the mold plate, or the step of filling the
flared openings of the multiplicity of apertures with the
multiplicity of solder columns displaced therein with a solder
paste by a printing technique, or the step of filling the
multiplicity of cavities with a first solder by an injection
molding technique. The method may further include the step of
providing a first base plate as a stop juxtaposed to the transfer
plate during the step of displacing the multiplicity of solder
columns from the mold plate into the multiplicity of apertures in
the transfer plate, or the step of providing the second base plate
as a stop juxtaposed to the transfer plate during the step of
filling the flared openings to prevent the multiplicity of solder
columns from falling out of the transfer plate. The method may
further include the step of flipping the transfer plate upside down
such that it rests on the second base plate prior to the filling
step, or the step of flipping the transfer plate upside down prior
to the step of positioning the transfer plate on the electronic
substrate. The first solder may have a melting temperature between
about 240.degree. C. and about 360.degree. C., and preferably
between about 280.degree. C. and about 320.degree. C. The second
solder may have a melting temperature between about 120.degree. C.
and about 239.degree. C., or preferably between about 160.degree.
C. and about 200.degree. C. The first solder may include more lead
than tin, while the second solder may include more tin than
lead.
[0018] The present invention is further directed to an electronic
substrate that has a multiplicity of solder columns electrically
joined thereto which includes an electronic substrate that has a
multiplicity of conductive pads on a top surface, and a
multiplicity of solder columns each formed of a first solder
electrically joined to one of the multiplicity of conductive pads
by a second solder at a base portion of the solder column, the
second solder has a lower melting temperature than the first
solder.
[0019] In the electronic substrate that has a multiplicity of
solder columns electrically joined thereto, the conductive pads are
bond pads. The first solder has a melting temperature of at least
240.degree. C., or a melting temperature between about 240.degree.
C. and about 360.degree. C., or preferably between about
280.degree. C. and about 320.degree. C. The first solder contains
more lead than tin. The second solder has a melting temperature of
less than 240.degree. C., or between about 120.degree. C. and about
239.degree. C., or preferably between about 160.degree. C. and
about 200.degree. C. The second solder has more tin than lead. The
multiplicity of solder columns each has a tapered shape with a
larger cross-sectional area in the base portion. The tapered shape
forms an angle between about 1.degree. and about 30.degree. with a
vertical axis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] These and other objects, features and advantages of the
present invention will become apparent from the following detailed
description and the appended drawings in which:
[0021] FIG. 1 is an enlarged cross-sectional view of a present
invention solder column mold plate with a multiplicity of cavities
filled with IMS solder.
[0022] FIG. 2 are enlarged, cross-sectional view of an aligned
extraction stack formed by an extraction plate, a filled mold plate
and a transfer plate.
[0023] FIG. 3 is an enlarged, cross-sectional view of the
extraction stack of FIG. 2 in an assembled position with the push
pins pressing on the solder column, and a base plate blocking the
flared openings on the transfer plate.
[0024] FIG. 4 is an enlarged, cross-sectional view of the
extraction stack of FIG. 3 after a pneumatic force is applied and
the solder columns are displaced into the multiplicity of apertures
in the transfer plate.
[0025] FIG. 5A is an enlarged, cross-sectional view of the filled
transfer plate with a second base plate positioned on top.
[0026] FIG. 5B is an enlarged, cross-sectional view of the transfer
plate of FIG. 5A after it is flipped over on top of a second base
plate.
[0027] FIG. 6A is an enlarged, cross-sectional view of the transfer
plate of FIG. 5B with a solder paste filled into the flared
openings encapsulating the solder columns.
[0028] FIG. 6B is an enlarged, cross-sectional view of the transfer
plate of FIG. 6A after it is flipped over and positioned on top of
an electronic substrate such that the solder columns contacting the
bond pads.
[0029] FIG. 6C is an enlarged, cross-sectional view of the
electronic substrate after a solder refill process and removal of
the transfer plate.
[0030] FIG. 7 is an enlarged, cross-sectional view of a second
preferred embodiment of the present invention illustrating a
mechanical displacement means.
[0031] FIG. 8 is an enlarged, cross-sectional view of the present
invention second preferred embodiment of FIG. 7 illustrating that
the solder columns are partially pushed out of the cavities.
[0032] FIG. 9 is an enlarged, cross-sectional view of the present
invention second preferred embodiment after the mechanical
displacement means is removed from the mold plate and after the
solder columns are displaced into the transfer plate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0033] The present invention discloses a method for joining a
multiplicity of multi-alloy solder columns to an electronic
substrate and structures formed by the method.
[0034] In the method, a mold plate that has a multiplicity of
cavities is first filled by an injection molded solder technique of
a high temperature solder material, such as one that has a melting
temperature higher than 240.degree. C. The mold plate is then
positioned in between an extraction plate and a transfer plate
wherein the extraction plate is equipped with a multiplicity of
displacement means each corresponding to the position of one
cavity, while the transfer plate is equipped with a multiplicity of
apertures each corresponding to the position of the cavity in the
mold plate. The displacement means is then activated by either a
mechanical method, a pneumatic method, or a combined
mechanical/pneumatic method to discharge the solder columns molded
in the cavities into the apertures in the transfer plate. The
apertures in the transfer plate have a straight end and a flared
end. After the apertures are filled with the solder columns, the
transfer plate is flipped upside down such that the flared openings
of the apertures are facing upward for filling of a low temperature
solder paste material encapsulating one end of the solder columns.
The transfer plate is then flipped upside down again and positioned
on an electronic substrate equipped with a multiplicity of
conductive pads on a top surface such that each of the multiplicity
of solder columns contacts one of the multiplicity of conductive
pads. After a solder reflow process which bonds the low temperature
solder, and thus the solder column formed by the high temperature
solder to the conductive pad, the transfer plate is removed to
exposed an electronic substrate joined with a multiplicity of
solder columns on top.
[0035] A typical high temperature solder may be one that has
90Pb/10Sn that has a melting temperature of about 300.degree. C.
The word "about" used in this writing means a range of values of
.+-.10% of the average value given. The low temperature solder
material may be suitably a 37Pb/63Sn solder that has a melting
temperature of about 183.degree. C. The present invention
multi-alloy solder column therefor combines the best properties of
both the high temperature solder and the low temperature solder in
one device. The high temperature solder used to form the solder
columns presents superior heat endurance property for an IC
package, while the low temperature solder presents the advantage of
easy connection to an electronic substrate at a suitable low
temperature.
[0036] The present invention method eliminates all the intermediate
steps that is required in a CLASP process, and thus significantly
simplifies the overall manufacturing process. The wire extrusion,
cutting, packaging, shipping and loading steps can be encompassed
within a single IMS step. In the IMS process, an injection molded
solder is directly filled into a mold, which is a graphite solder
column mold. The present invention method therefore focuses on
disclosing the mold release and transfer methods for achieving low
temperature solder column attach by IMS. The present invention
method eliminates the problems and concern with the conventional
CLASP method, i.e. the problem that the CLASP CGA process
automation depends on a reliable means for loading solder pins into
a transfer fixture, the problem that the manual CLASP method is
slow and labor intensive in that average loading time per array is
80 seconds, and the problem that manual loading can further be
slowed down by bent, flared, or incorrect length pins.
[0037] The present invention novel method presents numerous
processing advantages. For instance, the offline casting of solder
column array into mold. The method is therefore more forgiving than
previous method of IMS due to elimination of concerns over
mold/substrate CTE mismatch, mold contamination with flux residue,
and vacuum system cross leakage. The present invention filled molds
can be inspected and stocked. Casting mold material can be
optimized for room temperature pin release and durability. The IMS
casting equipment with pressure feed is inexpensive, such that it
allows multiple casting operations ensuring continuous output. The
present invention novel method further presents the benefit of
enabling a rapid, automated pin array transfer. The speed of pin
loading into transfer fixture is only limited by array placed in
alignment time and pressed cycle speed. A further benefit made
possible by the present invention is to enable fully automated pin
array loading. The fixture alignment and pin transfer are reduced
to a pick-and-place operation with subsequent press transfer.
[0038] The present invention novel method eliminates the dependency
on pre-cut pins, length and cut tolerances, and bent pins, as well
as material cost savings. For instance, the pre-cut pin cost at 1
pound weight which includes approximately 50,000 pins is about
$70.00, while the cost for 1 pound bulk solder for IMS filled molds
is only $10. The invention further eliminates lead/tin solder
separation.
[0039] Referring initially to FIG. 1, wherein a graphite mold plate
10 is shown. The mold plate 10 is filled with solder 12 by IMS
method into the multiplicity of cavities 14. The mold plate 10
contains cavities 14 that are slightly taper along a length of the
side wall 16 to facilitate easy release. The taper of the side wall
16 from a vertical axis is between about 1.degree. and about
30.degree., and preferably between about 2.degree. and about
10.degree.. The taper allows the break-away of the contact between
the cavity 14 and the solder column 18 formed by the solder
material 12 by a minor vertical motion. The mold plate 10 further
includes alignment pins 20 and alignment holes 22 to assure
accurate alignment between the mold plate 10 and a subsequently
positioned extraction plate 30 and transfer plate 40 (Shown in FIG.
2).
[0040] An extraction stack 24 formed by the extraction plate 30,
the mold plate 10 and the transfer plate 40 is shown in FIG. 2. The
extraction plate 30 contains push pins 32 and a pressure conduit
34. The pressure conduit is used to feed a pneumatic pressure
supplied from inlet 36 into apertures 38 in each of the push pins
32. The transfer plate 40 contains a multiplicity of cavities 42
for receiving the molded solder columns 18. The multiplicity of
apertures 42 is each equipped with a straight end 44 and a flared
end, or opening 46. The flared opening 46 is provided for receiving
the application of the solder paste in a subsequent solder paste
application process. It should be noted that the extraction plate
30 and the transfer plate 40 are both equipped with similar to that
of the mold plate 10, alignment pins 20 and alignment holes 22.
[0041] FIG. 3 illustrates the addition of a first base plate 50 at
the bottom of the extraction stack 24 in an assembled position. The
function of the first base plate 40 is to stop the advancing of
solder columns 18 during the solder column displacement process.
Also shown in FIG. 3 is the joined together extraction stack 24
with the alignment pins 20 engaging the alignment holes 22. As
shown in FIG. 3, the extraction plate 30 applies only an initial
mechanical force 52 on the top surface 54 of the solder columns 18
by means of the slightly protruding push pins 32. The source of the
mechanical force 52 is applied at the top of the extraction plate
30, as shown in FIG. 3. Once the bottom surface 56 of the
extraction plate 30 contacts the top surface 58 of the mold plate
10, all solder columns 18 are loosened from the mold cavities 14
due to the tapered shape of the solder columns 18. The distance
that the solder columns 18 have traveled at this point is only a
small fraction of their overall lengths, i.e. approximately equal
to a length of the push pins of about 0.005 inch.about.0.015 inch.
Typically, a solder column 18 may have a length between about 0.05
inch and about 0.15 inch.
[0042] FIG. 4 illustrates that, once the extraction plate 30
bottoms out against the mold plate 10, a pneumatic pressure is
applied through the inlet 36 and the pressure conduit 34. The push
pins 32 contain passageways 60 that are in full communication with
the pressure conduit 34. The pneumatic pressure is thus applied to
the top surfaces 54 of all the solder columns 18. Since the solder
columns 18 have already been mechanically loosened, the solder
columns 18 are quickly pushed into the apertures 42 of the transfer
plate 40. The advancement of the solder columns 18 is stopped by
the first base plate 50.
[0043] In the next step of the process, as shown in FIG. 5A, a
second base plate 70 is added to the top of the transfer plate 40
after the mold plate 10 which originally contained the solder
columns 18 has been removed. The assembly is now flipped over and
the first base plate 50 is removed, revealing the flared opening 46
in the apertures 42 with the solder columns 18 protruding in the
middle of the flared opening 46. This is shown in FIG. 5B.
[0044] FIG. 6A illustrates the present invention transfer plate 40
with a low temperature solder paste 64 applied by screen printing,
or any other suitable method, into the flared openings 46 for
encapsulating the ends of the solder columns 18. This results in
high temperature solder columns 18 capable of being attached to an
electronic substrate 80 by low temperature solder paste 64, after
the solder paste 64 is reflown to melt the eutectic solder and to
form a metallurgical bond with the conductive pads 82 on the top
surface 84 of the substrate 80.
[0045] It should be noted that the transfer plate 40 is flipped
over from FIG. 6A to FIG. 6B such that the flared opening 46, or
the low temperature solder paste 64 are positioned juxtaposed to
the conductive pads 82 located on the electronic substrate 80. The
conductive pads 82 may be suitably bonding pads for solder. After
the transfer plate 40 is removed from the electronic substrate 80,
as shown in FIG. 6C, an electronic substrate 80 that has a
multiplicity of solder columns 18 bonded to a top surface 84 is
obtained. The reflow temperature for the low temperature solder 64
may be any temperature that is lower than a melting temperature of
the high temperature solder, i.e. a melting temperature of
183.degree. C. for the 37Pb/63Sn material.
[0046] A second preferred embodiment of the present invention is
shown in FIGS. 7, 8 and 9. As shown in FIG. 7, a one piece
extraction plate 90 which is a punch/push device, i.e. a mechanical
device is shown. The extraction plate 90 can be pushed down by
mechanical force 92 against molded solder columns 18 situated in
the mold plate 10 to displace them from the mold plate 10 into the
multiplicity of apertures 42 in the transfer plate 40. It should be
noted that, as shown in FIGS. 7 and 8, the second preferred
embodiment of the present invention does not require the use of
pneumatic force, or a high pressure air flow such as that utilized
in that preferred embodiment. After the solder columns 18 are
displaced into the transfer plate 40, the one piece extraction
plate 90 can be removed from the mold plate 10. The transfer plate
40 can then be processed similar to the method in the preferred
embodiment by filling the flared openings 46 with a low temperature
solder material.
[0047] The present invention method for joining a multiplicity of
multi-alloy solder columns to an electronic substrate and the
structures formed have therefore been amply described in the above
description and in the appended drawings of FIGS. 1-9.
[0048] While the present invention has been described in an
illustrative manner, it should be understood that the terminology
used is intended to be in a nature of words of description rather
than of limitation.
[0049] Furthermore, while the present invention has been described
in terms of two preferred embodiments, it is to be appreciated that
those skilled in the art will readily apply these teachings to
other possible variations of the inventions.
[0050] The embodiment of the invention in which an exclusive
property or privilege is claimed are defined as follows.
* * * * *