U.S. patent application number 09/746786 was filed with the patent office on 2001-05-10 for flip chip with integrated flux and underfill.
Invention is credited to Gilleo, Ken.
Application Number | 20010000929 09/746786 |
Document ID | / |
Family ID | 22075633 |
Filed Date | 2001-05-10 |
United States Patent
Application |
20010000929 |
Kind Code |
A1 |
Gilleo, Ken |
May 10, 2001 |
Flip chip with integrated flux and underfill
Abstract
A flip chip having solder bumps, an integrated underfill, and a
separate flux coating, as well as methods for making such a device,
is described. The resulting device is well suited for a simple
one-step application to a printed circuit board, thereby
simplifying flip chip manufacturing processes which heretofore have
required a separate underfilling step.
Inventors: |
Gilleo, Ken; (Chepachet,
RI) |
Correspondence
Address: |
MINTZ, LEVIN, COHN, FERRIS,
GLOVSKY and POPEO, P.C.
One Financial Center
Boston
MA
02111
US
|
Family ID: |
22075633 |
Appl. No.: |
09/746786 |
Filed: |
December 21, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09746786 |
Dec 21, 2000 |
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09067381 |
Apr 27, 1998 |
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Current U.S.
Class: |
257/795 ;
257/738; 257/779; 257/E21.503; 257/E23.021 |
Current CPC
Class: |
H01L 2224/13099
20130101; H01L 2924/01077 20130101; H01L 2924/01013 20130101; H01L
2924/01004 20130101; H01L 24/11 20130101; H01L 2924/01078 20130101;
H01L 2924/01006 20130101; H01L 2224/11 20130101; H01L 2924/0103
20130101; H01L 2924/12042 20130101; H01L 2924/01029 20130101; H01L
24/05 20130101; H01L 2924/14 20130101; H01L 2924/01027 20130101;
H01L 2924/19041 20130101; H01L 2224/73203 20130101; H01L 2924/01033
20130101; H01L 2224/16 20130101; H01L 2224/274 20130101; H01L
2924/01322 20130101; H01L 2224/13 20130101; H01L 2924/01005
20130101; H01L 2224/73104 20130101; H01L 21/563 20130101; H01L
2924/01075 20130101; H01L 2924/19042 20130101; H01L 2924/01023
20130101; H01L 24/13 20130101; H01L 2224/1148 20130101; H01L
2924/01082 20130101; H01L 2924/0105 20130101; H01L 2924/014
20130101; H01L 2924/19043 20130101; H01L 24/28 20130101; H01L
2224/1147 20130101; H01L 2224/83856 20130101; H01L 2224/13
20130101; H01L 2924/00 20130101; H01L 2224/11 20130101; H01L
2924/00 20130101; H01L 2924/12042 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
257/795 ;
257/738; 257/779 |
International
Class: |
H01L 023/48 |
Claims
What is claimed is:
1. An integrated circuit assembly which comprises: a) a substrate
having a plurality of solderable contact sites on a surface
thereof; b) a plurality of solder bumps positioned on the substrate
such that each of the solderable contact sites has one solder bump
associated therewith, the solder bumps being affixed to the
solderable contact sites; c) a flux material which covers at least
a portion of each solder bump; and d) an underfill material applied
to the surface of the substrate, the underfill material occupying a
space defined between each of the solder bumps and being of a depth
such that at least a flux covered portion of each solder bump
extends therethrough.
2. The integrated circuit assembly of claim 1 wherein the substrate
comprises a semiconductor wafer.
3. The integrated circuit assembly of claim 2 wherein the substrate
comprises a semiconductor chip.
4. The integrated circuit assembly of claim 3 wherein the substrate
comprises a flip chip.
5. The integrated circuit assembly of claim 1 wherein the flux
covers substantially all of each solder bump.
6. The integrated circuit assembly of claim 1 wherein the flux
comprises an epoxy resin and a material selected from the group
consisting of carboxylic acids, anhydrides and combinations
thereof.
7. The integrated circuit assembly of claim 1 wherein the underfill
material is reworkable.
8. The integrated circuit assembly of claim 7 wherein the underfill
material comprises a thermoplastic material.
9. The integrated circuit assembly of claim 8 wherein the
thermoplastic material is selected from the group consisting of
phenoxy resins, acrylic resins, methacrylic resins, polycarbonate
resins, polyamide resins, polybutene resins, polyester resins,
polyolefin resins and mixtures thereof.
10. A method for making an integrated circuit assembly which
comprises: a) providing a substrate having a plurality of
solderable contact sites on a surface thereof; b) positioning a
plurality of solder bumps on the substrate such that each of the
solderable contact sites has one solder bump associated therewith;
c) affixing each solder bump to its associated contact site; d)
applying a flux material to the solder bumps in a manner such that
at least a portion of each solder bump is provided with flux; and
e) applying an underfill material to the surface of the substrate
in a manner such that it occupies a space defined between each of
the solder bumps and is of a depth such that at least a flux
covered portion of each solder bump extends therethrough.
11. The method for making an integrated circuit assembly of claim
10 wherein the substrate comprises a semiconductor wafer.
12. The method for making an integrated circuit assembly of claim
11 wherein the substrate comprises a semiconductor chip.
13. The method for making an integrated circuit assembly of claim
12 wherein the substrate comprises a flip chip.
14. The method for making an integrated circuit assembly of claim
10 wherein the flux covers substantially all of each solder
bump.
15. The method for making an integrated circuit assembly of claim
10 wherein the flux comprises an epoxy resin and a material
selected from the group consisting of carboxylic acids, anhydrides
and combinations thereof.
16. The method for making an integrated circuit assembly of claim
10 wherein the underfill material is reworkable.
17. The method for making an integrated circuit assembly of claim
16 wherein the underfill material comprises a thermoplastic
material.
18. The method for making an integrated circuit assembly of claim
17 wherein the thermoplastic material is selected from the group
consisting of phenoxy resins, acrylic resins, methacrylic resins,
polycarbonate resins, polyamide resins, polybutene resins,
polyester resins, polyolefin resins and mixtures thereof.
19. A method for affixing a flip chip to a circuit board which
comprises the steps of: a) providing a printed circuit board having
a plurality of solderable contact sites on a surface thereof; b)
providing an integrated circuit chip having a plurality of
solderable contact sites on a surface thereof, each solderable
contact site on the integrated circuit chip having a corresponding
solderable contact site on the surface of the printed circuit
board, the integrated circuit chip further characterized in that it
includes: 1) a plurality of solder bumps positioned on the
integrated circuit chip such that each of the solderable contact
sites located on the surface of the integrated circuit chip has one
solder bump associated therewith, the solder bumps being affixed to
the solderable contact sites; 2) a flux material which covers at
least a portion of each solder bump; and 3) an underfill material
applied to the surface of the substrate, the underfill material
occupying a space defined between each of the solder bumps and
being of a depth such that at least a flux covered portion of each
solder bump extends therethrough; c) positioning the integrated
circuit chip relative to the printed circuit board such that each
solder bump is in contact with a solderable contact site on the
printed circuit board; d) heating the integrated circuit chip to a
temperature sufficiently high to melt the solder and the underfill
material; and e) allowing the solder and underfill material to
solidify.
20. An integrated circuit assembly which comprises: a) a substrate
having a plurality of solderable contact sites on a surface
thereof; b) a plurality of solder bumps positioned on the substrate
such that each of the solderable contact sites has one solder bump
associated therewith, the solder bumps being affixed to the
solderable contact sites; and c) an underfill material applied to
the surface of the substrate, the underfill material occupying a
space defined between each of the solder bumps and being of a depth
such that at least a portion of each solder bump extends
therethrough
Description
FIELD OF THE INVENTION
1. The present invention relates to a novel flip chip design. More
particularly, the present invention relates to a flip chip which
incorporates solder bumps, flux and an underflow material.
BACKGROUND OF THE INVENTION
2. In the electronics industry, electrical components such as
resisters, capacitors, inductors, transistors, integrated circuits,
chip carriers and the like are typically mounted on circuit boards
in one of two ways. In the first way, the components are mounted on
one side of the board and leads from the components extend through
holes in the board and are soldered on the opposite side of the
board. In the second way, the components are soldered to the same
side of the board upon which they are mounted. These latter devices
are said to be "surface-mounted."
3. Surface mounting of electronic components is a desirable
technique in that it may be used to fabricate very small circuit
structures and in that it lends itself well to process automation.
One family of surface-mounted devices, referred to as "flip chips",
comprises integrated circuit devices having numerous connecting
leads attached to pads mounted on the underside of the device. In
connection with the use of flip chips, either the circuit board or
the chip is provided with small bumps or balls of solder (hereafter
"bumps" or "solder bumps") positioned in locations which correspond
to the pads on the underside of each chip and on the surface of the
circuit board. The chip is mounted by (a) placing it in contact
with the board such that the solder bumps become sandwiched between
the pads on the board and the corresponding pads on the chip; (b)
heating the assembly to a point at which the solder is caused to
reflow (i.e., melt); and (c) cooling the assembly. Upon cooling,
the solder hardens, thereby mounting the flip chip to the board's
surface. Tolerances in devices using flip chip technology are
critical, as the spacing between individual devices as well as the
spacing between the chip and the board is typically very small. For
example, spacing of such chips from the surface of the board is
typically in the range of 0.5-3.0 mil and is expected to approach
micron spacing in the near future.
4. One problem associated with flip chip technology is that the
chips, the solder and the material forming the circuit board often
have significantly different coefficients of thermal expansion. As
a result, differing expansions as the assembly heats during use can
cause severe stresses, i.e., thermomechanical fatigue, at the chip
connections and can lead to failures which degrade device
performance or incapacitate the device entirely.
5. In order to minimize thermomechanical fatigue resulting from
different thermal expansions, thermoset epoxies have been used.
Specifically, these epoxies are used as an underflow material which
surrounds the periphery of the flip chip and occupies the space
beneath the chip between the underside of the chip and the board
which is not occupied by solder. Such epoxy systems provide a level
of protection by forming a physical barrier which resists or
reduces different expansions among the components of the
device.
6. Improved underflow materials have been developed in which the
epoxy thermoset material is provided with a silica powder filler.
By varying the amount of filler material, it is possible to cause
the coefficient of thermal expansion of the filled epoxy thermoset
to match that of the solder. In so doing, relative movement between
the underside of the flip chip and the solder connections,
resulting from their differing coefficients of thermal expansion,
is minimized. Such filled epoxy thermosets therefore reduce the
likelihood of device failure resulting from thermomechanical
fatigue during operation of the device.
7. While underfill has solved the thermal mismatch problem for flip
chips on printed circuit boards, it has created significant
difficulties in the manufacturing process. For example, the
underfill must be applied off-line using special equipment.
Typically, the underfill is applied to up to three edges of the
assembled flip chip and allowed to flow all the way under the chip.
Once the material has flowed to opposite edges and all air has been
displaced from under the chip, additional underfill is dispensed to
the outer edges so as to form a fillet making all four edges
symmetrical. This improves reliability and appearance. Next, the
assembly is baked in an oven to harden the underfill. This process,
which may take up to several hours, is necessary to harden and
fully cure the underfill. Thus, although the underfill solves the
thermal mismatch problem and provides a commercially viable
solution, a simpler manufacturing method would be desirable.
8. Recently, attempts have been made to improve and streamline the
underfill process. One method that has shown some commercial
potential involves dispensing underfill before assembling the flip
chip to the board. This method requires that the underfill allow
solder joint formation to occur. Soldering of flip chips to printed
circuit boards is generally accomplished by applying flux to the
solder bumps on the flip chip or to the circuit pads on the printed
circuit board. Thus, it has been suggested to use an underfill that
is dispensed first, prior to making solder connections. In order to
facilitate solder bonding, however, the underfill must contain flux
or have inherent properties that facilitate solder joint formation.
Flux is used since the pads on printed circuit boards often
oxidize, and since solder bumps on flip chips are always oxidized.
Thus, the flux is designed to remove the oxide layers facilitating
solder joint formation.
9. Certain underfills commonly called "dispense first underfills"
have been designed with self-contained flux chemistry.
Unfortunately, the properties required for a good flux and those
required for a good underfill are not totally compatible. As such,
a compromise of properties results. The best flux/underfill
materials typically require more than an hour to harden.
Additionally, flux-containing underfills still require the use of
special equipment including robot dispensing machines. Also, since
solder assembly and underfill application are combined into a
single step, the flip chip cannot be tested until the assembly is
complete. Thus, if the chip does not operate satisfactorily, it
cannot be removed because the underfill will have hardened, thereby
preventing reworking. In view of the above, a need still exists for
a more efficient process that reduces the need for expensive
equipment and that is compatible with existing electronic device
assembly lines. A need for a reworkable underfill exists as
well.
SUMMARY OF THE INVENTION
10. The present invention relates to an integrated circuit assembly
comprising a semiconductor wafer which includes solder bumps, flux,
and an underfill material. In a broad sense, the invention relates
to an integrated circuit assembly which includes a substrate having
a plurality of solderable contact sites on one surface and a
plurality of solder bumps positioned on that surface such that each
of the solderable contact sites has one solder bump associated with
and affixed to each solderable contact site. Each site further
includes a flux material which covers at least a portion of each
solder bump and an underfill material which occupies the space
defined between each of the solder bumps. The underfill material is
of a depth such that at least a flux covered portion of each solder
bump extends above the underfill.
11. The present invention also relates to a method for making an
integrated circuit assembly which includes the steps of providing a
substrate having a plurality of solderable contact sites on a
surface thereof, positioning a plurality of solder bumps on the
substrate such that each of the solderable contact sites has one
solder bump associated with it, and affixing each solder bump to
its associated contact site. Once the solder bumps are mounted, a
flux material is applied to the solder bumps in a manner such that
at least a portion of each solder bump is provided with flux.
Finally, an underfill material is applied to the surface of the
substrate. The underfill occupies the space defined between each of
the solder bumps and has a depth such that at least a flux covered
portion of each solder bump extends through the underfill.
12. Lastly, the invention relates to a process for affixing a flip
chip to a circuit board. The method involves providing a printed
circuit board having a plurality of solderable contact sites on a
surface, providing an integrated circuit chip of the type described
above (i.e., a chip having solder bumps, flux and an underfill
material present on its surface), and positioning the integrated
circuit chip relative to the printed circuit board such that each
solder bump is in contact with a solderable contact site on the
printed circuit board. Once positioned, the integrated circuit chip
assembly is heated to a temperature sufficiently high to melt the
solder and the underfill material. Subsequently, the assembly is
allowed to cool to a temperature which allows the solder and
underfill material to solidify.
BRIEF DESCRIPTION OF THE DRAWINGS
13. FIG. 1 is a schematic elevation of a wafer having solder bumps
thereon.
14. FIGS. 2A and 2B are schematic depictions of an apparatus for
providing a flux coating on solder bumps.
15. FIG. 3 is a schematic elevation of a flip chip having solder
bumps, each such bump having a flux coating thereon.
16. FIG. 4 is a schematic elevation of the device of FIG. 3 having
an underfill material applied between the solder bumps.
17. FIG. 5 is a schematic elevational view of an alternative
embodiment of FIG. 4 in which the flux coating entirely surrounds
each solder bump.
DETAILED DESCRIPTION OF THE INVENTION
18. The present invention provides a unique method of applying
fluxes and underfills during the flip chip mounting process.
Specifically, the present invention relates to the application of
underfill and flux at the wafer level before the wafer is divided
into individual integrated circuits. Thus, in the present invention
the underfill and flux are pre-applied and converted into a solid
state. This process differs from other types of underfill
application processes in which the underfill is present in the
liquid state and then applied to the device at the point of chip
assembly to the printed circuit board. Additionally, the underfill
and the flux are separated, rather than being mixed as a combined
flux/underfill composition. As noted previously, liquid systems
combine the flux and underfill systems into a single composition
and, thus, provide neither flux nor underfill having ideal
properties.
19. The present invention recognizes that flux is required only at
the areas of the solder bumps, and not in the spaces in between
those connecting elements. Thus, the present invention separates
the flux from the underfill in the regions between the solder
bumps. Furthermore, by maintaining the flux and underfill as
separate entities, additives tailored to each individual component
may be added to provide both the flux and the underfill with
desired properties. For example, the underfill can be a
thermoplastic that de-bonds at elevated temperatures, or the flux
layer can be designed to de-bond. As an alternative, the flux can
convert to a strongly-bonded polymer after its mission as a flux
has been accomplished, and the underfill can have the debonding
properties. As such, a system in which the flux and underfill are
maintained as separate entities is extremely versatile.
20. Likewise, an alternative embodiment employs the use of an
underfill only. In such a case, there would be a requirement that
flux be added separately to the board or to the flip chip using any
of the wide variety of processes that are currently in commercial
use. Although an additional fluxing step would be required, the use
of an underfill-only embodiment would still eliminate the necessity
for the underfill process after the chip is mounted, while allowing
the use of standard electronic component assembly equipment.
21. In one embodiment, the invention comprises the application of a
layer of hardenable underfill to a bumped wafer followed by drying
or hardening. The underfill material is preferably a thermoplastic
or a thermoset having a very low crosslink density. In either case,
the underfill material is filled with a low expansion inorganic
particulate material such as silica. The resulting underfill should
preferably have a coefficient of thermal expansion (CTE) that
approximates that of the solder joint or other joining material. In
the case of eutectic solder joints, the CTE should range from
approximately twenty to thirty parts per million per .degree.C. It
is preferred that the resin system is soluble in a safe solvent
system to allow the resin to be coated as a liquid in a viscosity
range suitable for wafer-coating methods. Although a dry polymer
film or powder could be coated onto the wafer by melting, a liquid
is preferred because of the availability of wafer dispensing and
coating equipment adapted to liquid handling processes.
Additionally, this embodiment includes a layer of flux that is
designed to be compatible with flip-chip assembly and
underfills.
22. One such flux system includes epoxy resins and an organic
carboxylic acid, an anhydride or a combination thereof, and is
commercially available from Alpha Metals under the trade name
ChipFlux 2020. This material is an anhydride system which, upon
heating, converts to carboxylic acid. Although this material is a
paste made with liquid epoxy resins, the system can be readily
modified for use in the present invention. For example, solid epoxy
resins having slightly higher molecular weight than liquid epoxies
can be substituted and used with carboxylic acid as the flux. Even
with solid epoxy resins and carboxylic acid (which is a solid at
room temperature), the system can easily be dissolved in polar
solvents and can then be coated in a liquid state and dried to a
solid film. Although the preferred flux application methods are
spin coating, spraying, or stencilling, the wafer can also be
coated using a dipping process in which the bump side of the wafer
is pressed against a thin layer of flux on a dispensing drum
consisting of a rotating platen disk and a doctor blade (described
below) to control the liquid thickness.
23. Flip chips having integrated underfill and flux can be mounted
on a printed circuit board as follows. An individual flip chip,
with integrated underfill and flux, is placed in contact with the
circuit board in a manner such that the solder bumps are aligned
with conductive pads printed on the board. The assembly is then
passed to a multi-zone re-flow soldering oven. The application of
heat causes the flux to melt and activate. For applications using
the solid ChipFlux 2020 described above and used in tests, the
material is heated to about 80.degree. C., however, the useful
range is about 40.degree. C. to about 100.degree. C. The activated
flux removes oxide on the solder bumps as well as on the circuit
board. As heating is continued to higher temperatures (typically by
moving the assembly into a higher temperature zone of the oven),
the solder bumps are caused to melt and form a metallurgical joint
between the flip chip and the printed circuit board. At that
elevated temperature, the flux becomes deactivated. For example, in
the case of a carboxylic acid/epoxy flux system, the elevated
temperature causes the acid to chemically combine with the epoxy
and become neutralized so that there will be no tendency toward
corrosion. Such fluxes are called "no clean" fluxes. These fluxes
are typically heated to about 190.degree. C. to about 220.degree.
C., however, the lower end of the temperature range is preferred
since the flux does most of its work at solder reflow temperatures.
The flux deactivation process also tends to harden the flux and
create a strong bond to the printed circuit board. Such a bond is
very desirable, and results because the typical printed circuit
board is made with epoxy, thereby enhancing the ability of the
similar epoxy-based flux to bond to the board.
24. Since the flux and underfill are contained in separate layers
in the devices of the present invention, it is not essential,
although it is preferred, that the underfill layer melt. The
underfill must soften and preferably melt so that it will wet out
and bond to the circuit substrate. Since the maximum soldering
temperature for common eutectic solder is about 220-225.degree. C.,
the underfill will have softened and/or melted upon reaching this
temperature. However, in the case where a higher melting underfill
is needed, bonding can take place at the softening point if
downward force is applied. It is preferred that only about half of
the original bump height be covered with underfill since it is
expected that the bump would typically collapse to about half of
its original height as the solder wets the conductive pad on the
printed circuit board and forms the joint. After the solder has
melted, the assembly is allowed to cool, thereby allowing the
solder to harden and to form a solid metallurgical joint between
the flip chip and the board. The resulting assembly is protected
from thermomechanical strain by the underfill and flux layers. In
one preferred embodiment, the flux may assume underfill properties
as the result of polymerization.
25. The flux can be made into an underfill-like material by adding
a sufficient low-expansion filler such as silica. It has been
recognized that the flux polymerizes during the solder reflux
process to a thermoplastic state. This means that the underfill and
the flux can be reworked by heating the chip above solder reflow
(i.e., about 200.degree. C.). This also means that any flux residue
can be removed by a polar solvent if necessary. That result is
optional, however, because most of the firm solder joint would be
encased in the underfill composition which serves to provide
optimum protective properties to the joint.
26. Alternatively, a standard flip chip bonder that can apply heat
and pressure can be employed instead of the reflow oven. In that
embodiment, the flip chip coated with the flux and underfill is
placed into contact with the conductive pads on the circuit board
and heat from the bonder head will activate the flux, form joints
by reflowing the solder bumps, and cause the underfill and flux
system to bond tightly to the board. The use of a standard flip
chip bonder would allow a flip chip to be assembled to a board that
already contained mounted components. This method could also be
used to assemble a chip at a site that is being reworked.
27. Reworking is desirable in situations in which a chip mounting
step has failed to properly position the chip on the board.
Specifically, the assembly of fine pitch, high-density components
can result in misalignments and failed connections. Furthermore,
since it is difficult to fully test an unpackaged device such as a
flip chip, it becomes desirable to be able to remove the chip if
final testing indicates that the chip is not operating optimally,
either through a fault with the chip or as a result of improper
mounting. Thermoset underfills do not allow the assembly to be
reworked since thermosets cannot be melted once they have
crosslinked.
28. The present invention eliminates the problems associated with
thermoset underfills by incorporating a thermoplastic resin as the
main component of the underfill. Thus, the chip can be removed by
raising the chip temperature to above the melting point of the
solder (approximately 183.degree. C. for tin/lead solder) and above
the de-bonding temperature of the underfill resin. Typically, the
rework temperature must be above the solder reflow temperature, but
less than about 220.degree. C. depending on the circuit substrate.
An average rework temperature would be about 200.degree. C. The
temperature can be higher if localized heat is used; for example,
in an alternate embodiment, a chip bonder could be used to remove
chips from a substrate post-bonding. In still another embodiment,
the underfill may also include a B-staged thermoset that will
de-polymerize at an elevated temperature.
29. Suitable thermoplastic resins include phenoxy, acrylic,
methacrylic, polycarbonate, polyamides, polybutene, polyesters and
some polyolefins. It is noted that the underfill does not need to
be melted, rather, it is only necessary for the underfill to soften
for de-bonding. Desirable polymers for use as thermoplastic
underfill materials include thermoplastic die attach adhesives
available from Alpha Metals under the trade name Staystik. Such
materials can be de-bonded cleanly at elevated temperatures. Thus,
when such materials are used, the thermoplastic film can be pealed
away from both the chip and the circuit at elevated temperatures,
leaving no residue.
30. Alternatively, the underfill can be made from a resin that is
known to debond when a specific solvent is applied. One such resin
system is a temporary attach adhesive available from Alpha Metals
under the trade name Staystik 393. Underfills made with Staystik
can be modified to contain a low expansion inorganic filler,
preferably, a spherically-shaped silica of about 5 to about 15
microns in diameter. In order to achieve the desired coefficient of
thermal expansion (CTE) close to that of tin/lead solder (22.5
ppm/deg. .degree.C.), the underfill should comprise about 60-70% by
weight silica and about 20-30% resin. Note that one advantage of
using Staystik 393 is that it does not dissolve, but does debond in
the presence of alcohol, thereby providing a system by which any
residue can be easily removed.
31. An underfill made with resins of the type described above would
allow the underfill to debond by adding alcohol around the chip
site. That notwithstanding, however, the solder joints would still
have to be heated to solder reflow temperatures to allow the chip
to be removed.
32. Although either one of the flux or the underfill may be applied
to the bumped chip first, it is preferred that the flux be applied
prior to application of the underfill. This is because the surface
energy properties of the interface between the bumps and the
underfill can cause the underfill to creep up the side of the bump,
thereby covering it entirely. This effect is undesirable because
the underfill becomes positioned between the solder bump and the
pad onto which the solder is intended to contact, thereby causing
the underfill to act as a contaminent. To address this situation,
flux is applied to the bumps first. In one preferred method, a
dip-transfer process using a reservoir of flux paste can be used.
This process is as described below.
33. FIG. 1 depicts a flip chip assembly 10 which comprises a wafer
12 having solder bumps 14 on its surface. Flux may be provided on
the bumps 14 by means of the apparatus and process depicted in
FIGS. 2A and 2B. Specifically, FIG. 2A is a top view of a flux
application apparatus, and FIG. 2B is a side elevational view of
that apparatus. In FIGS. 2A and 2B, the flux application apparatus
20 comprises a rotating platen 22 which communicates via a spindle
24 with a drive motor 26. A doctor blade 28 which may be adjusted
to provide a gap of a predetermined distance above the platen is
mounted on one side. A flux paste 30 is provided on the surface of
the platen 22 upstream of the doctor blade. When the platen is
rotated, the flux paste 30 is forced into the gap between the
doctor blade 28 and the platen surface 22, thereby causing the flux
paste downstream of the doctor blade to be at a predetermined and
desired thickness. In one embodiment, it is preferred that the
thickness of the flux paste 30 downstream of the doctor blade 28 is
less than that of the height of each bump 14 above the wafer 12.
The wafer is dipped into the reservoir of flux paste as can be seen
in FIGS. 2A and 2B. Since the depth of the flux paste 30 is less
than the height of the solder bumps 14, only a portion of the bumps
will become coated with the flux. Alternate flux coating methods
include screen printing, roll coating and tampo printing since only
the tops of the bumps need to be coated.
34. FIG. 3 depicts a bumped wafer that has been provided with flux
in the manner shown in FIGS. 2A and 2B. Specifically, FIG. 3 shows
a wafer 12 having solder bumps 14 thereon. On each bump is a flux
coating 16 which covers a portion of the bump. Once each bump 14
has been provided with a flux coating 16, the flux can be hardened
by drying. As noted above, one preferred flux is a solid version of
an epoxy/carboxylic acid type of flux commercially available as
Chip Flux 20/20. The flux is dissolved in a solvent and provided
with a wetting agent such as FC430, formerly available from
Minnesota Mining and Manufacturing Co., or Fluowet, a low surface
tension surfactant available from Hoechst-Celanese, to provide the
deposited flux with a low surface energy.
35. After the solder bumps 14 have been provided with a flux
coating 16, the spaces on the wafer surface between the solder
bumps 14 are provided with an underfill in the liquid phase. The
liquid underfill is applied to the wafer by spin coating, screen
printing, or any of the common methods for applying liquids to
surfaces. The resulting device is depicted in FIG. 4. Specifically,
FIG. 4 shows a wafer 12 having solder bumps 14 each having a flux
coating 16. The underfill material 18 is deposited on the wafer 12
in the spaces between the solder bumps 14. Since the flux coating
16 has a low surface energy, the underfill 18 does not become a
coating over the flux 16. This is because surface chemistry
principles require that wetting will only occur if the surface
energy of the liquid (i.e., the underfill 18) is lower than that of
the solid surface (i.e., the flux coating 16). Since the materials
are selected such that the flux liquid has a higher surface energy
than the flux coating, a receding contact angle results at the
interface between the flux coating 16, the underfill 18, and the
surrounding air. This is shown at region 15 of FIG. 4. Even though
the underfill 18 may not wet the dried flux coating 16, the flux
coating will still readily wet the bond pads on the circuit board
to which the wafer is applied. This is a result of the effect that,
when heat is applied, the flux coating 16 melts and becomes a
liquid with a low surface energy. Again, since the flux liquid will
have a lower surface energy than the bond pads, the flux liquid
readily wets them. In addition, if desired, the flux coating 16 can
be provided with various wetting additives such as the
aforementioned FC430 or Fluowet. Alternatively a silicone such as
Silwet L-77 available from Union Carbide could be employed.
36. In an alternative embodiment shown in FIG. 5, the flux coating
16 may be applied to the solder bumps 14 in a manner such that it
entirely covers them. Upon heating for the purpose of soldering the
chip to the circuit board, the flux will melt and readily wet the
bond pads. The softened underfill, being a thermoplastic material,
will flow in around the bump as the flux flows away. As such, any
residual flux will not act as a contaminant having a tendency to
reduce adhesion.
EXAMPLES
37. Flux was made by blending the following with a high speed
dispersing mixer:
38. Flux Sample 1
39. 50 wt % PMA solvent (1-methoxy-2-propyl acetate); from Dow
40. 45 wt % EPON 1001F (Bisphenol A Epoxy); Shell
41. 3 wt % Succinic Anhydride, Lonza
42. 2 wt % Thixatrol ST (thickener); from Rheox
43. Flux Sample 2
44. Same mix as above but 0.1 to 0.5 wt % wetting agent (FC430 or
Fluowet OTN) was added to reduce any tendency of the underfill to
wet over the flux-coated bumps.
45. Flux Sample 3
46. 45 grams PMA solvent 1-methoxy-2-propyl acetate); from Dow
47. 48 grams EPON 1001F (Bisphenol A Epoxy); Shell
48. 5 grams Adipic Acid, Aldrich Chemical
49. 2 grams Thixatrol ST (thickener); from Rheox
50. The resulting fluxes had viscosities of about 750 kcps.
Viscosity can easily be adjusted by varying the amount of the
thixotropic agent. A useful range for fluxes to be used in
applications of the present invention is about 250 kcps to about
1,000 kcps.
51. Flux was applied to the bumps in each case by coating out the
flux onto a glass plate so that the flux thickness was 2 to 3 mils.
The flip chips were dipped into the flux and removed with flux
clinging to the bumps. The flux was dried by placing the chip
upright in an oven at about 150.degree. C. for about 5 minutes. It
is expected that this process will also work on a wafer.
52. Flux/Underfill sample 4
53. Staytik 383 paste (no filler) was dispensed onto the bumped
side of the chip with a syringe and allowed to flow out before
drying in a vacuum oven at about 70.degree. C. for about 30
minutes. The dry film thickness was less than the bump height.
Bumps that were coated with flux containing the low surface tension
wetting agent gave the best results with little or no underfill
remaining on top of the bumps. Although soldering can still occur
even with underfill on top of the bumps, the best results occur
when only flux coats the upper bump surfaces.
54. Test Set 1
55. Flip chips, only coated with the three fluxes described above
and no underfill, were used in the first set of tests intended to
confirm good flux action. Each flux-coated chip was placed with
bumps down onto a 1" diameter copper disk that was not pre-cleaned
and therefore had a tarnished appearance. The samples were placed
on a hot plate at about 216.degree. C. for about 3 minutes. The
flux and solder bumps melted causing the chip to attach to the
copper. The copper became shiny in the areas contacted by the
flux.
56. Samples were also run through an Electrovert Atmos 2000
convection oven with a peak temperature of approximately
220.degree. C. It was found that the chips were soldered to the
copper upon removal from the oven.
57. Test 2
58. Flip chips coated with both flux and underfill were run through
the Electrovert oven at about 216.degree. C. peak temperature.
Solder joints formed at the copper interface and the underfill also
bonded.
59. Equivalents
60. From the foregoing detailed description of the specific
embodiments of the invention, it should be apparent that a unique
flip chip having an integrated flux and underfill has been
described. Although particular embodiments have been disclosed
herein in detail, this has been done by way of example for purposes
of illustration only, and is not intended to be limiting with
respect to the scope of the appended claims which follow. In
particular, it is contemplated by the inventor that various
substitutions, alterations, and modifications may be made to the
invention without departing from the spirit and scope of the
invention as defined by the claims.
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