U.S. patent application number 09/609626 was filed with the patent office on 2002-05-30 for method and apparatus for applying a protective over-coating to a ball-grid-array (bga) structure.
Invention is credited to Ong, E. C..
Application Number | 20020064931 09/609626 |
Document ID | / |
Family ID | 24441596 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020064931 |
Kind Code |
A1 |
Ong, E. C. |
May 30, 2002 |
Method and apparatus for applying a protective over-coating to a
ball-grid-array (BGA) structure
Abstract
A vacuum-application and coating apparatus for applying a
protective coating to at least one ball-grid-array assembly is
provided. The apparatus comprises an upper plate having at least
one injection port forming the upper chamber wall, and a lower
plate having at least one vacuum port forming the lower chamber
wall of the vacuum-application and coating apparatus when
assembled. A compliant layer of material is provided on the
chamber-side surface of the upper plate and a sealing mechanism for
enabling a vacuum seal is also provided. At least one
ball-grid-array assembly is placed on the chamber surface of the
lower plate during assembly of the vacuum-application and coating
apparatus, which forms a vacuum chamber. The ball-grid-array
assemblies held in the chamber are protected from receiving any
coating on the upper portions of connected solder balls during
processing by virtue of intimate contact between the solder balls
and the compliant layer of material. In other aspects methods are
provided for adding a protective coating to ball-grid-array
assemblies and subsequently providing opening for access to the die
pads. In another aspect a process is provided for completely
encapsulating balls, then exposing a portion and applying a new
grid array.
Inventors: |
Ong, E. C.; (Sunnyvale,
CA) |
Correspondence
Address: |
CENTRAL COAST PATENT AGENCY
PO BOX 187
AROMAS
CA
95004
US
|
Family ID: |
24441596 |
Appl. No.: |
09/609626 |
Filed: |
July 3, 2000 |
Current U.S.
Class: |
438/613 ;
257/E21.508; 257/E23.021; 257/E23.069 |
Current CPC
Class: |
H01L 2224/05022
20130101; H01L 2924/14 20130101; H05K 2203/025 20130101; H01L
2924/01078 20130101; H01L 2924/01033 20130101; H01L 2224/11334
20130101; H01L 2924/01027 20130101; H01L 2224/131 20130101; H01L
2924/01006 20130101; H01L 24/11 20130101; H01L 2924/01082 20130101;
H01L 2924/00013 20130101; H01L 24/13 20130101; H01L 21/4853
20130101; H01L 2924/01075 20130101; H05K 3/3436 20130101; H01L
2224/05001 20130101; H01L 2924/01005 20130101; H01L 2224/1191
20130101; H01L 2924/01079 20130101; H01L 2924/00014 20130101; H01L
24/03 20130101; H01L 2224/16 20130101; H01L 24/05 20130101; H01L
2924/01029 20130101; H01L 2224/13144 20130101; Y02P 70/613
20151101; Y10T 29/49149 20150115; H05K 2201/10977 20130101; Y10T
29/49144 20150115; H01L 2924/01039 20130101; H01L 24/94 20130101;
H01L 2224/45144 20130101; H01L 2224/274 20130101; H01L 2224/05573
20130101; H01L 2224/05568 20130101; H01L 2924/014 20130101; H05K
2201/0379 20130101; H01L 23/49816 20130101; H01L 2224/13025
20130101; Y02P 70/50 20151101; H01L 2924/12042 20130101; H01L
2224/13144 20130101; H01L 2924/00014 20130101; H01L 2224/131
20130101; H01L 2924/014 20130101; H01L 2924/00013 20130101; H01L
2224/13099 20130101; H01L 2924/12042 20130101; H01L 2924/00
20130101; H01L 2224/45144 20130101; H01L 2924/00 20130101; H01L
2924/00014 20130101; H01L 2224/05599 20130101; H01L 2924/00014
20130101; H01L 2224/05099 20130101 |
Class at
Publication: |
438/613 |
International
Class: |
H01L 021/44; H01L
021/48; H01L 021/50 |
Claims
What is claimed is:
1. A coating mold for forming a protective coating on a
ball-grid-array assembly having solder balls extending above a base
surface of the ball-grid-array assembly, comprising: a first
portion having a substantially flat area for supporting the ball
grid array assembly on a back surface; a second portion having a
substantially flat compliant layer; an injection port passing
through one or the other of the first and second portions; a vacuum
pumping port passing through one or the other of the first and
second portions; and a sealing mechanism for sealing the first
portion to the second portion, enclosing the ball-grid-array
assembly; characterized in that with the first portion closed on
the second portion the compliant layer contacts the solder balls of
the ball grid array, and a space is formed between the second
portion and the base surface of the ball-grid-array.
2. The coating mold of claim 1 wherein the sealing mechanism is an
o-ring.
3. The coating mold of claim 1 further comprising a clamping or
bolting mechanism for keeping the mold closed in process.
4. The coating mold of claim 1 wherein the compliant layer is a
flexible polymer material.
5. A method for applying a protective coating to a ball-grid-array
assembly having solder balls extending above a base surface,
comprising steps of: (a) placing the ball-grid-array assembly,
including solder balls, into a coating mold having a first surface
for supporting the ball-grid-array assembly on a back surface and a
second portion having a substantially flat compliant layer; (b)
closing and sealing the mold such that the compliant layer contacts
the balls of the ball grid array, leaving a space between the
compliant layer and the base surface of the ball-grid -array
assembly; (c) creating a vacuum in the space formed in step (b);
(d) injecting a polymer-based coating into the space formed; and
(e) curing the polymer material, such that, when opened the
ball-grid-array assembly is coated while leaving an upper portion
of each of the solder balls exposed.
6. The method of claim 5 wherein the mold closes on an o-ring
seal.
7. A method for providing a protective polymer coating to a
ball-grid-array assembly, before placing solder balls on the die
pads of the assembly, comprising the steps of: (a) overcoating the
assembly with a polymer material; and (b) opening each die pad area
to the die pad through the polymer material by a removal process to
expose the die pads for placement of the solder balls.
8. The method of claim 7 wherein, in step (b) the removal process
comprises laser machining.
9. The method of claim 7 wherein the removal process comprises
chemical etching.
10. The method of claim 7 wherein the removal process comprises
physical etching.
11. A method for providing a protective polymer coating to a
ball-grid-array assembly, before placing solder balls on the die
pads of the assembly, comprising the steps of: (a) screen printing
photoresist onto the ball-grid-array assembly; (b) masking the
ball-grid-array assembly to protect areas of photoresist over the
die pads; (c) developing and removing the photoresist exposed
through the mask, leaving the photoresist over the die pads as
protective photoresist islands; (d) applying a protective coat to
the ball-grid-array assembly to the thickness of the photoresist
coating; and (e) developing and removing the remaining photoresist
islands to expose the underlying die pads.
12. A method for protecting and strengthening a ball-grid-array
assembly having solder balls extending above a base surface,
comprising steps of: (a) applying a protective material layer over
the solder balls to a level at or above the level of the top of the
solder balls, providing thereby a new upper surface for the
assembly; (b) removing a portion of the new upper surface to an
extent that a portion of each of the original solder balls is
exposed as a flat region in a planar upper surface; and (c)
applying new solder material over each of the flat exposed solder
ball regions.
13. The method of claim 12 wherein, in step (b), removal is by
machining.
14. The method of claim 12 wherein, in step (a) the protective
material is applied by one of screening, spraying, or dispense and
spinning.
15. The method of claim 12 comprising an additional step (d) for
reflowing the new solder material.
Description
FIELD OF THE INVENTION
[0001] The present invention is in the field of semiconductor and
printed-circuit-board (PCB) manufacturing including surface mount
technologies (SMT), and pertains more particularly to methods and
apparatus for applying protective coatings to structures meant for
connection by BGA techniques.
BACKGROUND OF THE INVENTION
[0002] The field of integrated circuit interconnection and
packaging is one of the most rapidly-evolving technologies
associated with semiconductor manufacturing. As demand for devices
that are smaller and more powerful continues to increase, pressures
are put on manufacturers to develop better and more efficient ways
to assemble and package IC products. One of the more recently
developed methods for assembling and packaging IC products is known
as Ball-Grid-Array (BGA) technology. Motorola.TM. inc. is one of
the noted pioneers of BGA technology. Currently there are many
companies that license BGA technology developed by Motorola.TM.,
and Motorola and other companies continue to develop BGA
technology.
[0003] BGA technology provides several advantages over more
mainstream technologies such as Fine-Pitch-Technology (FTP), and
Pin-Grid-Array (PGA). One obvious advantage is that there are no
leads that can be damaged during handling. Another obvious
advantage is that the solder balls are typically self-centering on
die pads. Still other advantages are smaller size, better thermal
and electrical performances, better package yields, and so on.
[0004] In BGA technology, wafers or substrates are typically
protected with a non-conductive material such as a nitride layer.
The die pads are exposed through the nitride layer by means of
chemical etching, or by other known methods. The protective nitride
layer is intended to protect the substrates from contaminants and
damage. One problem with prior-art protective coatings such as a
nitride layer is that it is ultra-thin and does not offer any
protection to the die pads themselves nor to the connection points
between solder balls in the die pads.
[0005] It has occurred in the inventor that an additional
protective coating, such as a protective polymer-based coating,
would offer a measure of protection not provided with prior-art
coatings. For example, it is desired that in addition to protecting
the substrates itself, die pads and soldered connections may also
benefit logically from protection. However, in order to obtain the
added, protective benefits from an additional coating, a unique
application process must be conceived. It is to such a process that
the method and apparatus of the present invention is directed.
[0006] What is clearly needed is a method and apparatus for
applying a protective overcoat to a Ball-Grid-Array (BGA) device
such that exposed die-pad areas, soldered connections, and exposed
areas of solder balls in the assembly are protected from
exposure.
SUMMARY OF THE INVENTION
[0007] In a preferred embodiment of the present invention a coating
mold for forming a protective coating on a ball-grid-array assembly
having solder balls extending above a base surface of the
ball-grid-array assembly is provided, comprising a first portion
having a substantially flat area for supporting the ball grid array
assembly on a back surface; a second portion having a substantially
flat compliant layer; an injection port passing through one or the
other of the first and second portions; a vacuum pumping port
passing through one or the other of the first and second portions;
and a sealing mechanism for sealing the first portion to the second
portion, enclosing the ball-grid-array assembly. The mold is
characterized in that with the first portion closed on the second
portion the compliant layer contacts the solder balls of the ball
grid array, and a space is formed between the second portion and
the base surface of the ball-grid-array.
[0008] In some embodiments the sealing mechanism is an o-ring. Also
in some embodiments there is a clamping or bolting mechanism for
keeping the mold closed in process. Preferably the compliant layer
is a flexible polymer material.
[0009] In another aspect of the invention a method for applying a
protective coating to a ball-grid-array assembly having solder
balls extending above a base surface is provided, comprising steps
of (a) placing the ball-grid-array assembly, including solder
balls, into a coating mold having a first surface for supporting
the ball-grid-array assembly on a back surface and a second portion
having a substantially flat compliant layer; (b) closing and
sealing the mold such that the compliant layer contacts the balls
of the ball grid array, leaving a space between the compliant layer
and the base surface of the ball-grid -array assembly; (c) creating
a vacuum in the space formed in step (b); (d) injecting a
polymer-based coating into the space formed; and (e) curing the
polymer material, such that, when opened the ball-grid-array
assembly is coated while leaving an upper portion of each of the
solder balls exposed. In preferred embodiments of the method the
mold closes on an o-ring seal.
[0010] In yet another aspect of the invention a method for
providing a protective polymer coating to a ball-grid-array
assembly, before placing solder balls on the die pads of the
assembly is provided, comprising the steps of (a) overcoating the
assembly with a polymer material; and (b) opening each die pad area
to the die pad through the polymer material by a removal process to
expose the die pads for placement of the solder balls.
[0011] In a preferred embodiment of this method, in step (b) the
removal process comprises laser machining. In another embodiment
the removal process comprises chemical etching, and in yet another
embodiment the removal process comprises physical etching.
[0012] In still another aspect of the invention a method for
providing a protective polymer coating to a ball-grid-array
assembly, before placing solder balls on the die pads of the
assembly is provided, comprising the steps of (a) screen printing
photoresist onto the ball-grid-array assembly; (b) masking the
ball-grid-array assembly to protect areas of photoresist over the
die pads; (c) developing and removing the photoresist exposed
through the mask, leaving the photoresist over the die pads as
protective photoresist islands; (d) applying a protective coat to
the ball-grid-array assembly to the thickness of the photoresist
coating; and (e) developing and removing the remaining photoresist
islands to expose the underlying die pads.
[0013] In yet another aspect of the invention a method for
protecting and strengthening a ball-grid-array assembly having
solder balls extending above a base surface is provided, comprising
steps of (a) applying a protective material layer over the solder
balls to a level at or above the level of the top of the solder
balls, providing thereby a new upper surface for the assembly; (b)
removing a portion of the new upper surface to an extent that a
portion of each of the original solder balls is exposed as a flat
region in a planar upper surface; and (c) applying new solder
material over each of the flat exposed solder ball regions.
[0014] In some embodiments, in step (b), removal is by machining.
The protective material may be applied in a number of different
ways, such as by screening, spraying, or dispense and spinning.
There may also be an additional step for reflowing the new solder
material.
[0015] Now, for the first time a method and apparatus for applying
a protective overcoat to a Ball-Grid-Array (BGA) is provided that
protects exposed die-pad areas, soldered connections, and exposed
areas of solder balls from exposure and damage.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0016] FIG. 1A is a perspective view of a wafer with die pads
according to prior art.
[0017] FIG. 1B is an expanded and broken view of the wafer of FIG.
1A illustrating a die-pad exposed through a nitride coating.
[0018] FIG. 2 is a broken view of a BGA assembly with a protective
overcoat according to an embodiment of the present invention.
[0019] FIG. 3A is a plan view of the wafer of FIG. 2 with a
protective overcoat applied as a first step according to an
embodiment of the present invention.
[0020] FIG. 3B is a plan view of the coated wafer of FIG. 3A with
coated areas removed in areas to expose the die pads.
[0021] FIG. 3C is a plan view of the coated wafer of FIGS. 3A and
3B with solder balls in place according to a third step.
[0022] FIG. 4 is a process diagram illustrating processing steps a
through e for coating and creating die pad openings according to
another embodiment of the present invention.
[0023] FIG. 5A is a section view of a vacuum enhanced coating
apparatus for applying a protective overcoat to a BGA assembly
according to a preferred embodiment of the present invention.
[0024] FIG. 5B is a detailed view of a portion of FIG. 5A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] FIG. 1A is a idealized perspective view of a coated wafer 9
with die pads 11 according to the prior art art. The skilled
artisan will recognize that the pads have been very much
exaggerated in this view to be able to provide some detail. In this
example of prior art wafer 9 is coated with a thin, protective
layer that is nonconductive, such as a nitride layer 13. Die pads
11 are illustrated in an array on wafer 9. Typically, die pads 11
are nitride coated along with wafer 9, which may be a rectangular
substrate instead of an actual wafer. After nitride coating, die
pads 11 are exposed by such as an etching process.
[0026] FIG. 1B is an expanded and broken view of one pad 11 of FIG.
1A, shown in perspective, illustrating the pad exposed through the
nitride layer. In this detail, a die pad 11 can be seen recessed
beneath the thickness of nitride coating 13. It is noted herein,
that die pad 11 is completely exposed, meaning that there is no
protective layer above any of the land occupied by die pad 11. When
a solder ball (not shown) is placed on die pad 11, certain real
estate of die pad 11 along with the soldered area between the ball
and die pad 11 will be exposed, and therefore vulnerable to damage
and contamination. A goal of the present invention is to provide a
process that according to various embodiments, which are described
in enabling detail below, may be used to successfully apply a
protective coating layer in addition to the standard hard
protective layer such as the nitride layer described above.
[0027] FIG. 2 is a broken view of a portion of a BGA assembly 14
with a protective overcoat 17 according to an embodiment of the
present invention. In this example of the present invention, BGA
assembly 14 exhibits 2 die pads 11 having solder balls 15 adhered
thereto. A protective coating 17 is, in a preferred embodiment, a
polymer-based coating such as a polyamide coating. In other
embodiments, other polymer-based coatings may be used such as are
known in the art and available to the inventor. This example
illustrates a preferred embodiment, wherein protective coating 17
coats the substrate and the normally exposed area of each die pad
11 around solder balls 15 and also around the perimeter of each
solder ball 15.
[0028] A nitride coating 13, which is illustrated in FIGS. 1A and
B, is illustrated here as coating the substrate portion of assembly
14 with the coating extending up over the attached die pads. It may
be assumed herein, that a portion of coating 13 has been removed by
any one of several known methods in order to clear to an
appropriate area on the upper surf aces of each die pad 11 for
placement and reflow of solder balls 15. Protective coating 17 is
illustrated as over coating nitride layer 13 and encompassing the
lower peripheral areas of solder balls 15. A height dimension D
illustrates the thickness of coating 17, which may be anywhere from
1 to 3 mils thick in a preferred embodiment. Overcoat 17 functions
to protect any exposed pad areas as well as a portion of solder
balls 15.
[0029] In practice of the present invention, the inventor has
isolated three basic processes that are useful to successfully
apply protective coating 17 to BGA assembly 14. FIG. 3A is a
plan-broken view of wafer 14 of FIG. 2 with a protective overcoat
applied as a first step according to an embodiment of the present
invention. FIG. 3B is a plan-broken view of coated wafer 14 of FIG.
3A undergoing a process to expose covered die pads in a second
step. FIG. 3C is a plan-broken view of coated wafer 14 of FIGS. 3A
and 3C with solder balls in place according to a third step. The
examples of FIGS. 3A, 3B, and 3C illustrate a general 3-part
process for the over coating wafer 14, removing material to expose
die pads, and then screening the solder balls into place for a
re-flow operation.
[0030] Referring now to FIG. 3A, wafer 14 is illustrated with
protective coating 17 already applied. It may be assumed herein,
although not specifically illustrated, that die pads 11 of FIG. 2
and nitride coating 13 of FIG. 2 are present on wafer 14 before
application of protective coating 17. Coating 17 in a first step
completely covers die pads 11 and nitride coating 13. Coating 17
may be a Polyamide coating or a similar polymer-based coating as
described above. Coating 17 may be applied by any one of several
processes, such as by vacuum deposition process, a spin-on process,
or by virtue of other known methods.
[0031] Referring now to FIG. 3B, protective coating 17 is partially
removed over the land areas above each die pad attached to wafer
14. This process may be a laser process, a plasma-etch process, or
a chemical-etch process. In both the plasma-etch and chemical-etch
processes, a mask is used to protect portions of coating 17 not
covering die pads. These portions are represented herein by element
number 19. Areas where material has been removed are represented
herein by element number 21. Once die pads are exposed, they are
ready to accept solder balls.
[0032] Referring now to FIG. 3C, wafer 14 is illustrated with
solder balls 15 screened in place and ready to be re-flowed onto
the associated die pads. A re-flow process uses heat to effect the
solder connections between balls 15 and associated die pads. The
process described above with respect to FIGS. 3A-3C may be used to
according to one embodiment, to protect any BGA assembly.
[0033] FIG. 4 is a process diagram illustrating processing steps a
through e for coating and creating die pad openings according to
another embodiment of the present invention. In step a, wafer 14 is
coated with a photoresist coating represented herein by element
number 23. As described in FIG. 3A above, it may be assumed that
die pads (11) and a standard nitride layer (13) are present in this
step. This photoresist process may be accomplished using a standard
screen-printing technique. It is noted herein that photoresist 23
is applied before applying a protective coating (17).
[0034] In step b, a masking technique is used to cover areas of
photoresist that are directly over die pads (11). Through
development of photoresist (23) with a protective mask applied,
resist islands are formed as represented by element number 25 in
this step. Resist islands 25 are present areas of photoresist left
directly over die pads (11) after developing.
[0035] In step c, protective coating 17 is applied at substantially
the same thickness as photoresist 25. This process of coating fills
in the areas inbetween resist islands 25, such areas representing
real estate of wafer 14 not occupied by a die pad (11).
[0036] In step d, a second masking technique is used to protect the
areas coated with protective coating 17 in step c. At this point in
the process, resist islands 25 are chemically developed, and then
etched away exposing associated die pads (11) leaving all other
real estate untouched. In step e, solder balls 15 are screened in
place over die pads (11) as described with reference to FIG. 3C. At
this point of the process, a re-flow operation to permanently
attach solder balls 15 to die pads (11) may begin. The process
represented herein by FIG. 4, illustrates a process for applying
protective coating 17 according to yet another embodiment of the
present invention.
[0037] FIG. 5A is a section view of a vacuum-application and
coating apparatus 27 for applying protective overcoat 17 to a BGA
assembly according to a preferred embodiment of the present
invention. Vacuum-application and coating apparatus 27, hereinafter
referred to as simply apparatus 27, is provided and adapted to
enable an automated coating process to be performed on a BGA
assembly after re-flow. Apparatus 27 comprises an upper plate 29, a
lower plate 31, and a vacuum seal 33. In a preferred embodiment
both plate 29 and 31 are manufactured of stainless-steel or other
durable metals. Plates 29 and 31 may be circular, or rectangular in
shape. Other shapes may be employed as well.
[0038] In operation a BGA assembly 32, with solder balls in place,
is enclosed by plate 29 and 31 fitted together using a seal 33. It
may be assumed herein that either plate 29 or plate 31 has an
o-ring-style groove provided on its mating surface, generally
around the perimeter, such that seal 33 may be properly retained
and facilitated. In one embodiment, both mating surfaces of plates
29 and 31 may be grooved to facilitate seal 33. In still another
embodiment, a metallic sealing apparatus may be used instead of an
o-ring.
[0039] Plate 29 and 31 are fitted together over seal 33 to form
apparatus 27, and the plates may be held together by any of several
methods, such as by bolts or by clamp mechanisms. A chamber formed
within apparatus 27 after assembling contains at least one BGA
assembly. In one embodiment, many BGA assemblies may be introduced
into the formed chamber for processing. The height of an internal
processing area formed within apparatus 27 after assembly is
sufficient to accommodate the height of a BGA assembly without
damaging the assembly.
[0040] Plate 29 has a compliant layer of material, illustrated
herein as compliant layer 37 affixed thereto and covering the area
over the ball array of an enclosed part. This compliant layer 37
may be a rubberized material, a polymer-based material, or any
other suitable material having compliant characteristics. The
purpose of compliant layer 37 on plate 29 is to protect the upper
portions of solder balls (15) of a BGA assembly or assemblies
inserted into apparatus 27 for processing. The dimensions of the
plates are such that, when the plates are closed, the compliant
layer forms over the upper portion of each solder ball as may be
seen in FIG. 5B.
[0041] Upper plate 29 has an injection port 37 provided
therethrough, which opens into the vacuum chamber formed within
apparatus 27. Port 37 is adapted to enable injection of an uncured
protective coating material 17, in liquid form, into the vacuum
chamber during processing. In one embodiment, there may be more
than 1 injection port 37 provided within plate 29. Lower plate 31
has a vacuum port 35 providing therethrough, which opens into the
vacuum chamber formed within apparatus 27. Port 35 is adapted to
connect a vacuum pumping apparatus (not shown) to enable a vacuum
to be drawn within apparatus 27. In one embodiment, there may be
more than one vacuum port provided within plate 31.
[0042] In practice of the present invention, at least one BGA
assembly complete with re-flowed solder balls is placed onto the
surface of plate 31. Plate 29 is urged into to plate 31 over seal
33 and bolted or clamped together with the BGA assembly or
assemblies inside. A vacuum is then drawn by virtue of port 35. The
protective coating 17 is injected through port(s) 37 to the
internal chamber coating the inserted BGA assembly or
assemblies.
[0043] FIG. 5B is an expanded view of one edge of the assembly
shown in FIG. 5A. In this expanded view, wafer 14 is shown with one
solder ball 15. Compliant layer 37 forms over the top of solder
ball 15 and protects the covered area of ball 15 from being coated
with injected coating 17, in a manner that, when released, the
solder balls will be exposed on the coated parts. The top surface
of solder ball 15 is required to be free of coating as this area is
used for lead connection. However, the remaining real estate of
wafer 14 and solder ball 15 is covered with protective coating 17
during this back-filling operation. after back-filling with the
protective coating material in liquid form, the material is cured
before the molds are opened.
[0044] It will be apparent to one with skill in the art that
apparatus 27 may be manufactured of a size such as to facilitate
the processing of a number of BGA assemblies simultaneously. In one
embodiment apparatus 27 may process only a few assemblies, or
perhaps one assembly at a time. Once processing is completed within
apparatus 27, BGA assemblies are removed from apparatus 27 by
unbolting or unclamping apparatus and pulling apart plates 29 and
31 revealing completed BGA assemblies. A tracking operation may be
used to remove excess coating.
[0045] In yet another embodiment of the invention for a method is
provided for protecting a BGA assembly in a manner that increased
strength is also provided. This method is illustrated herein with
the aid of FIGS. 6a through 6f. FIG. 6a illustrates a wafer 41 with
balls 45 placed and soldered to solder pads, with a nitride layer
43 in place, as is known in the art. FIG. 6b shows the assembly of
FIG. 6a with a protective layer 47 applied according to embodiments
to the present invention as described above. Layer 47 may be
applied by screening, spraying, dispense and spinning, by
backfilling, or in any of several other ways. Preferably, layer 47
completely covers all balls in the ball grid array.
[0046] In FIG. 6c a machining operation is illustrated using a
grinding or cutting wheel 49 to remove a portion of layer 47 and
enough of each ball in the ball grid array that each ball is now
exposed as a flat pad even with the upper machined surface of layer
47. FIG. 6d shows the assembly of FIG. 6c completely
planarized.
[0047] After planarization, solder material is applied over each
exposed solder ball machined surface. FIG. 6e illustrates a solder
pad 51 in place over each solder ball in the assembly. Solder
islands 51 may be applied by screen printing paste, by plating, or
by direct solder ball attachment. Preferably the new solder
material may have a melting point equal to that of the original
solder balls, or a lower melting point.
[0048] After the new solder material is applied, that material is
re-flowed, such that the new ball grid array surface is created
over the original. The original solder balls are now completely
encapsulated in the material of layer 47, and the original wafer
surface and all of the elements of that surface are very well
protected. Additionally, the new array is much more robust and
strong than the original, because all stress points have now been
redistributed away from the wafer surface.
[0049] It will be apparent to one with skill in the art that the
method and apparatus of the present invention may be provided for a
wide variety of shapes and sizes of BGA assemblies without
departing from the spirit and scope of the present invention.
Similarly, the method and apparatus of the present invention may be
applied to BGA assemblies of varying materials. The method and
apparatus of the present invention provides an automated and
efficient way to apply an additional protective coating to BGA
assemblies. The method and apparatus of the present invention
should be afforded the broadest scope possible under examination.
The spirit and scope of the present invention should be limited
only by the claims that follow.
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