U.S. patent application number 09/850808 was filed with the patent office on 2001-08-30 for flip chip with integrated mask and underfill.
Invention is credited to Gilleo, Kenneth Burton.
Application Number | 20010017414 09/850808 |
Document ID | / |
Family ID | 22075633 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017414 |
Kind Code |
A1 |
Gilleo, Kenneth Burton |
August 30, 2001 |
Flip chip with integrated mask 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 device is characterized in that the underfill
material is provided on the chip surface prior to the application
of solder bumps, and then treated to form apertures therein which
act as a mask for solder bump application. 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, Kenneth Burton;
(Cranston, RI) |
Correspondence
Address: |
MINTZ, LEVIN, COHN, FERRIS, GLOVSKY & POPEO, P.C.
One Financial Center
Boston
MA
02111
US
|
Family ID: |
22075633 |
Appl. No.: |
09/850808 |
Filed: |
May 8, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09850808 |
May 8, 2001 |
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09266166 |
Mar 10, 1999 |
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6228678 |
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09266166 |
Mar 10, 1999 |
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09067381 |
Apr 27, 1998 |
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Current U.S.
Class: |
257/737 ;
257/E21.503 |
Current CPC
Class: |
H01L 24/11 20130101;
H01L 2924/01082 20130101; H01L 2224/274 20130101; H01L 2224/83856
20130101; H01L 2224/73203 20130101; H01L 2224/11 20130101; H01L
2924/01029 20130101; H01L 2924/0103 20130101; H01L 2924/014
20130101; H01L 2924/19043 20130101; H01L 2924/01033 20130101; H01L
21/563 20130101; H01L 24/13 20130101; H01L 2924/19042 20130101;
H01L 2224/13 20130101; H01L 2224/13099 20130101; H01L 2924/01006
20130101; H01L 2924/19041 20130101; H01L 2224/1148 20130101; H01L
2924/01027 20130101; H01L 2924/12042 20130101; H01L 2224/73104
20130101; H01L 2924/01078 20130101; H01L 2924/01004 20130101; H01L
24/05 20130101; H01L 2924/01013 20130101; H01L 2224/1147 20130101;
H01L 2924/01077 20130101; H01L 2924/01322 20130101; H01L 2224/16
20130101; H01L 2924/01075 20130101; H01L 2924/14 20130101; H01L
2924/0105 20130101; H01L 24/28 20130101; H01L 2924/01005 20130101;
H01L 2924/01023 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: |
257/737 |
International
Class: |
H01L 023/48; H01L
023/52; H01L 029/40 |
Claims
What is claimed is:
1. An integrated circuit assembly which comprises: a) a substrate
having at least one solderable contact site on a surface thereof;
and b) an underfill material applied to the surface of the
substrate, the underfill material substantially entirely covering
the surface of the substrate having the solderable contact
site.
2. An integrated circuit assembly which comprises: a) a substrate
having at least one solderable contact site on a surface thereof-,
and b) an underfill material applied to the surface of the
substrate, the underfill material substantially entirely covering
the surface of the substrate having the solderable contact site
except at the solderable contact site itself, thereby allowing the
underfill to act as a mask exposing substantially only the
solderable contact site on the substrate surface.
3. A method of making an integrated circuit assembly which
comprises the steps of: a) providing a substrate having at least
one solderable contact site on a surface thereof-, b) applying an
underfill material to the substrate surface in a manner which
substantially entirely covers the substrate surface and the
solderable contact site; c) treating the underfill material to form
at least one aperture therein, the at least one aperture extending
substantially entirely through the underfill material and being
located such that it exposes only the solderable contact site; d)
applying at least one solder bump to the assembly in a manner such
that a bump occupies each aperture in the underfill, contacts the
exposed solderable contact site therein, and extends above the
underfill material; and e) applying a flux to the surface, the flux
covering at least the exposed portions of the solder bumps.
4. The method of claim 3 wherein the substrate comprises a
semiconductor wafer.
5. The method of claim 4 wherein the substrate comprises a
semiconductor chip.
6. The method of claim 5 wherein the substrate comprises a flip
chip.
7. The method of claim 3 wherein the flux covers substantially the
entire surface upon which the solder bumps are exposed.
8. The method of claim 3 wherein the flux comprises an epoxy resin
and a material selected from the group consisting of carboxylic
acids, anhydrides and combinations thereof.
9. The method of claim 3 wherein the underfill material is
reworkable.
10. The method of claim 9 wherein the underfill material comprises
a thermoplastic material.
11. The method of claim 10 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.
12. The method of claim 3 wherein the apertures are formed using
photoablation.
13. The method of claim 12 wherein the apertures are formed using a
laser.
14. The method of claim 13 wherein the laser is selected from the
group consisting of excimer lasers, UV lasers and infrared
lasers.
15. The method of claim 13 wherein the apertures are formed using a
directed laser beam.
16. The method of claim 13 wherein the apertures are formed using a
pattern mask.
17. A method for affixing an integrated circuit chip to a substrate
which comprises the steps of: a) providing a substrate 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
was made by a process including the steps of: i) providing a
semiconductor wafer having at least one solderable contact site on
a surface thereof; ii) applying an underfill material to the wafer
surface in a manner which substantially entirely covers the wafer
surface and the solderable contact site; iii) treating the
underfill material to form at least one aperture therein, the at
least one aperture extending substantially entirely through the
underfill material and being located such that it exposes only the
solderable contact site on the wafer; iv) applying at least one
solder bump to the assembly in a manner such that a bump occupies
each aperture in the underfill, contacts the exposed solderable
contact site therein, and extends above the underfill material; v)
applying a flux to the surface, the flux covering at least the
exposed portions of the solder bumps; and vi) dividing the wafer
into at least one integrated circuit chip; 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.
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of copending
application U.S. Ser. No. 09/067,381 filed Apr. 27, 1998, and
entitled Flip Chip With Integrated Flux and Underfill.
FIELD OF THE INVENTION
[0002] 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 underfill material,
wherein the underfill material acts as a mask during application of
the solder bumps.
BACKGROUND OF THE INVENTION
[0003] 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."
[0004] 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.
[0005] 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.
[0006] In order to minimize thermomechanical fatigue resulting from
different thermal expansions, thermoset epoxies have been used.
Specifically, these epoxies are used as an underfill 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.
[0007] Improved underfill 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.
[0008] 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 A 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.
[0009] 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.
[0010] 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.
[0011] Finally, certain problems have been found to arise when
applying flux/underfill materials to bumped surfaces of flip chips.
The problems result because the rough surface geometry of the
bumped surface is not readily amenable to the application of
fluids, particularly those having high viscosity. Thus, Providing
the flux/underfill directly onto a bumped surface raises at least
the possibility of discontinuities and air bubbles forming during
the flux/underfill application process. Furthermore, by eliminating
bumping prior to application of the flux/underfill layer, it may be
possible to eliminate process steps, thereby streamlining the
manufacturing process while providing chip makers with greater
design and manufacturing flexibility.
[0012] 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 also exists. A further need
exists for a flux/underfill material that can act as a mask during
the bumping steps as well.
SUMMARY OF THE INVENTION
[0013] 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. In the
present case, the underfill material is applied to the wafer prior
to applying the solder bumps. The underfill can then be processed
to convert it into a mask to assist in placement of the bumps onto
the wafer surface.
[0014] 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. An underfill material is applied to the surface of
the substrate. Subsequently, the underfill is treated to form gaps
therein at each of the solderable contact sites. The resulting
underfill mask simplifies application of the solder bumps to the
wafer in a manner such that each of the solderable contact sites
has one solder bump associated with it. Each solder bump is then
mounted 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. The resulting wafer is characterized in that 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.
[0015] 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
[0016] FIG. 1 is a schematic depiction of one embodiment of the
invention in which a wafer has an underfill material formed thereon
prior to application of solder bumps.
[0017] FIG. 2 is a schematic depiction of the device in FIG. 1 in
which the underfill material has been treated to form apertures
therein, the apertures allowing the underfill to act as a mask
during solder bump application.
[0018] FIG. 3 is a schematic depiction of the device of FIG. 2
following application of solder bumps.
[0019] FIG. 4 is a schematic depiction of the device of FIG. 3
showing application of a flux layer over the solder bumps.
[0020] FIG. 5 is a schematic depiction of an alternate embodiment
of the device of FIG. 4 showing application of a flux layer over
the solder bumps only.
DETAILED DESCRIPTION OF THE INVENTION
[0021] The present method provides a unique method of applying an
underfill to a flip chip wafer. In particular, in the present
invention, a coatable underfill material, (typically a composite of
a dissolved polymer and a solid filler in a solvent) is applied to
the wafer surface prior to application of any solder bumps. It is
important that the material selected for application has properties
suitable for use as a flip chip underfill, or at the very least,
can develop such properties during reflow solder assembly
processing. The preferred material is a thermoplastic, such as
Staystik.TM., commercially available from Alpha Metals, Inc. This
material is then modified with a predetermined amount of an
appropriate filler to provide the underfill with a coefficient of
thermal expansion (CTE) that approximates that of the solder joints
which will be formed by the bumps. A mineral filler such as silicon
dioxide is preferred. The preferred CTE of the resulting underfill
material is approximately 25 ppm/.degree. C., although values of up
to about 45 ppm/.degree. C. are also envisioned. Even after
processing the CTE of the underfill cannot become greater than
about 60 ppm/.degree. C., because this can cause detrimental
thermomechanical stresses at the solder joints.
[0022] The preferred filler material is spherical and has a
diameter less than the high of the solder bumps that will be
applied to the wafer. Thus, as typical filler ranges in size from
about 3 microns to about 15 microns. While silicon dioxide is
preferred because of its ready availability, other non-electrically
conductive materials such as aluminum nitride, aluminum oxide and
beryllium oxide can be use as well.
[0023] A solvent, or solvent blend, which is compatible with each
of the components is selected. Among the suitable solvents are
included many common oxygenated, nitrogen-containing solvents as
well as many polar aromatic solvents. The particular solvent system
chosen should have evaporation and boiling points that allow
removal of the solvent in the environment of a drying oven once the
wafer is coated with the underfill material. Suitable solvents can
be found in the Staystik pastes available from Alpha Metals,
Inc.
[0024] In use, the underfill material is applied directly to the
face, or active, side of the wafer prior to bumping. Semiconductor
wafers typically have access, or bonding, pads formed of aluminum.
Such pads are typically made solderable prior to bumping and, in
this case, prior to applying the underfill material. The access
pads are rendered solderable by depositing a solderable finish,
often referred to as under bump metallization (UBM). Several UBM
processes are commercially available, as are third-party UBM
services provided by vendors. In the UBM process, the solderable
finish is usually applied to the aluminum access pads using either
vacuum deposition or chemical plating.
[0025] The underfill solution can be formulated to have the correct
rheology for application to the wafer using any of a number of
methods. For example, since the ratio of solvent to solids in the
solution determines the viscosity of the solution, it is possible
to formulate underfill solutions that can be applied using
different methods. Since the solvent is substantially entirely
evaporated after application of the underfill solution to the
wafer, the resulting, solid underfill layer can have the
composition regardless of the initial viscosity and percent solids
of the underfill solution. This results because the solvent acts
simply as a vehicle for carrying the solids during underfill
application.
[0026] In one method, the underfill solution can be applied by spin
coating, a common semiconductor processing method in which liquid
is deposited onto a spinning wafer in order to provide a smooth and
level coating. An underfill having a viscosity in the range of
about 80-85 Kcps, measured at 2.5 RPM using an RVT #6 spindle on a
Brookfield viscometer, has been found to give good results. When
applied to a wafer, a wafer spin rate of about 1200 RPM yields a
smooth coating.
[0027] A second method is stencil printing. This method requires a
more viscous material that is produced using less solvent. The
thixotropic index, (i.e., change in viscosity as a result of
mechanical shearing), can also be adjusted, using thixotropic
additives, to improve printing characteristics. The thickness of
the stencil determines the amount of material applied to the wafer.
Likewise, the amount of solvent contained in the underfill solution
determines the amount of thickness reduction that occurs in the
underfill during drying and solvent evacuation. Thus, it is
necessary to consider both the stencil thickness and the solvent
percent of the underfill solution in order to precisely control the
thickness of the applied underfill. A dry underfill thickness range
of about 25 to about 125 microns is suitable and will depend on the
height of the bumps to be produced at a later stage. The thickness
of the underfill layer is selected to preferably be from about 50
to about 80% of the bump height to allow form the bumps to collapse
during the solder reflow step.
[0028] It should be understood that while spin coating and stencil
printing are preferred, many other methods can be used to apply the
underfill to the layer. These include, but are not limited to,
needle deposition, spraying, screen printing and others.
[0029] The coating is then dried by heating it in an oven or by
direct heating of the wafer. It has been found to be advantageous
to heat the wafer while simultaneously using a forced hot air oven
to help drive solvent out of the coating. Combined top and bottom
heating can eliminate any tendency to trap solvent in the underfill
layer by a process known as "skinning" in which the surface of the
underfill material dries prematurely and forms a film (i.e., a
skin) that acts as a barrier to further solvent evacuation. If
drying is carried out properly, the resulting underfill material is
non-tacky and amenable to handling.
[0030] Alternatively, the coating composition can be cast onto a
release paper -and then dried into a film. The film can be cut into
a proper shape, called a preform, and applied to the wafer.
Heating, with the application of pressure, will cause the underfill
layer to bond to the wafer.
[0031] In order to apply the solder bumps to the underfill-coated
wafer, openings must be formed in the underfill film at each
interconnect pad location. In one preferred embodiment, the
openings are formed using laser machining techniques. Excimer
lasers, for example, can be used to create openings in polymeric
films by a photoablation process in which ultra-violet radiation
causes the long-chain polymers to break down into small volatile
by-products. Patterning can be achieved using either a pattern mask
or a directed beam. Optical defraction grating patterning methods
are also available. Photoablation is particularly suitable for the
present application because it occurs with only a minimum amount of
heating and does not damage the wafers. The process parameters can
be set so that machining stops when the metal layer below the
underfill is exposed, thereby making the process self-limiting.
This is not always necessary, however, since many metals are
resistant to laser ablation. Although UV lasers are preferred,
other lasers, such as infrared (IR) lasers can be used as well.
[0032] Bumps of a suitable solder alloy are next applied to the
wafer using the underfill as a mask which exposes only those
section of the wafer which are to have solder bumps applied, i.e.,
the exposed pad areas of the wafer. Any bumping methods that do not
require mask removal may be employed. In one embodiment,
electroless plating can be used. Although the method does not
typically require a mask, a high aspect ratio of plated material
can be provided using the mask, and in the case of solder bumps,
such high aspect ratios are desirable. In cases in which a
zincate-electroless nickel process is employed, under bump
metallization (described above) can be eliminated since the zincate
treatment makes the aluminum platable by nickel.
[0033] Bumps made only from solder may be made by starting with
solder pastes that are widely available. Stenciling, screen
printing, pin transfer and other methods can also apply solder
paste. Once the paste has been applied, the bump is formed by
melting (i.e., reflowing) the paste. It is necessary to control the
conditions at this point, however, because the underfill can soften
or even liquefy into a viscous state if it is heated too much
during the solder reflow process.
[0034] Other solder bumping methods include metal fluid jetting, or
inverting the wafer and passing it over a solder wave or fountain
of molten solder.
[0035] Flux is deposited onto the protruding solder bumps since it
is not required on the face of the die. Since the solder bumps
protrude above the surface of the underfill material, a number of
methods can be used. For example, a thin layer of liquid flux can
be coated onto a flat plat of glass. The wafer is then placed,
bumps down, onto the thin film of flux and then withdrawn. A thin
coating of flux, which can be subsequently dried, will remain on
the bumps. Other methods such as roller coating, screen printing
and tamp printing can be used as well.
[0036] At this stage, the wafer is ready to be diced, or
singulated, to produce individual flip chips. Any of a wide variety
of the methods known in the art for dicing wafers can be employed
to that end. The sole requirement of the inventive wafers is that
the process be such that it does not interfere with the underfill
material applied to the wafer/chip surfaces.
[0037] Once diced, individual flip chips may now be bonded to
circuit boards and the like. The flip chip is placed and aligned to
the bond pads of a substrate. As used herein, the term "substrate"
is intended to mean a circuit board, a chip carrier, another
semiconductor device or a metal lead frame. It is not necessary to
add flux, although flux may be added for special reasons such as
compensating for excessive oxide on substrate pads, or the need to
hold the flip chip in place during assembly.
[0038] The positioned chip is then run through a solder reflow line
commonly used for assembly. A multi-zone oven, with individual heat
controls that permit a heating profile is preferred. The flux melts
at a temperature ranging from about 80.degree. C. to about
140.degree. C. The melting point is determined by selecting fluxes
having epoxy resins with the appropriate melting point. Flux
composition will be described in greater detail below. At higher
temperatures, the underfill softens and may also melt depending
upon the resin selected. Like the fluxes, underfill composition
will be described in greater detail below. The solder bumps finally
melt and form metallurgical joints to the substrate.
[0039] 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.
[0040] 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 filly 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] An underfill made with resins of the type described above
would allow the underfill to debond by adding alcohol around the
chip site. That not withstanding, however, the solder joints would
still have to be heated to solder reflow temperatures to allow the
chip to be removed. 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.
[0045] In one embodiment, 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.
[0046] 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 a carboxy acid system. (Anhydrides
are used in related products, although the acid derivatives provide
stronger flux activity and more consistency). 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 to
control the liquid thickness.
[0047] The invention can be further understood with reference to
the attached Figures. As can be seen schematically in FIG. 1, a
semiconductor device 10 comprises a portion of a semiconductor
wafer 12 having an underfill material 14 applied to one surface
thereof. The wafer 12 further includes a plurality of connection
pads 15 which, ultimately, will contact the solder bumps and
provide an electrical connection between the bumps and the
underlying wafer circuitry. The underfill material has been applied
to the surface upon which the solder bumps will be formed.
[0048] In FIG. 2, the underfill material 14 has been processes to
form a plurality of apertures 16 through its depth. The apertures
16 are positioned precisely at the connection pads 15 on the wafer
12. Thus, as solder material fills the apertures, it will contact
the connection pads 15 on the surface of the wafer. By providing
the apertures 16 in the underfill material 14, the underfill
material 14 is formed into a mask for applying the solder bumps. As
such, the need for a separate mask to position the solder bumps is
eliminated, because the underfill material serves the masking role
as well as the underfilling role.
[0049] In FIG. 3, the solder bumps have been applied to the device
10. In particular, each of the apertures 16 in the underfill
material 14 has been filled by a solder bump 18. The bumps 18
electrically contact the wafer 12 through the connection pads 15,
and extend a slight distance above the underfill material 14 as
well.
[0050] FIG. 4 shows the device 10 after the application of a flux
material 20. As can be seen in the Figure, the flux 20 covers the
entire exposed, bumped surface of the device 10. An alternative
embodiment is shown in FIG. 5. In that Figure, the flux 20' does
not cover the entire bumped surface of the device 10, but rather,
covers only the portion of the bumps 18 that extends above the
underfill material 14. As such, flux 20' will be present only in
the precise areas in which it is needed, rather than over the
entire exposed, bumped surface of the device 10.
[0051] The following Examples will help to illustrate the invention
further.
EXAMPLES
Example 1
Thermoplastic Reworkable Underfill Coating Solution
[0052] 100 grams of Staystik 908 (a 20% solids phenoxy solution)
was blended with 30 grams of silica filler (FB-35 from Denka Ltd.,
Tokyo, Japan) using a high shear type mixer (Cowels Dissolver run
at 2500 rpm) for 2 minutes. Entrapped air bubbles were allowed to
dissipate prior to using the solution as a coating solution.
[0053] The resulting material was spin-coated onto an unbumped
wafer mounted on a wafer coating machine (SCS Coater P6204-A). The
coating was then dried using the following profile: 50.degree. C.
for 20 minutes, 80.degree. C. for 30 minutes, 110.degree. C. for 30
minutes. The resulting fil was found to be smooth and dry.
[0054] Subsequently, openings were produced in the dry film layer
using laser machining. Eutectic solder bumps were then formed in
the openings. This was accomplished by applying solder paste to pad
areas on the wafer using stencil printing, and then reflowing the
solder to form the bumps. A flux was then applied to the bumps.
EXAMPLE 2
Flux/Underfill Preparation
[0055] 40% by weight bisphenol A epoxy resin (Shell, Epon 1007F)
and 45% by weight dipropylene glycol methyl ether acetate, were
blended together with 5% by weight hydrogenated castor oil. The
blend was cooled to 25.degree. C. Following the cooling step, 10%
by weight adipic acid was dispersed in the blend using a high speed
mixer.
[0056] The resulting flux is useful for coating onto solder bumps
and the entire exposed, bumped surface of a wafer, including the
underfill. Alternatively, the flux can be applied to the bumps
only. Once applied, the flux is converted into a solid by drying it
at about 60.degree. C. for 30 minutes.
EQUIVALENTS
[0057] From the foregoing detailed description of the specific
embodiments of the invention, it should be apparent that a unique
flip chip having an underfill which also acts as a mask for the
bump application process 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.
* * * * *