U.S. patent application number 11/626118 was filed with the patent office on 2008-05-08 for thermoplastic fluxing underfill method.
This patent application is currently assigned to Fry's Metals, Inc.. Invention is credited to David Garrett, Mark Wilson.
Application Number | 20080108178 11/626118 |
Document ID | / |
Family ID | 33510685 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080108178 |
Kind Code |
A1 |
Wilson; Mark ; et
al. |
May 8, 2008 |
THERMOPLASTIC FLUXING UNDERFILL METHOD
Abstract
A flip chip having solder bumps and an underfill that is
thermoplastic and fluxing, as well as methods for making such a
device. The resulting device is well suited for a simple one-step
application to a printed circuit board, thereby simplifying flip
chip manufacturing processes.
Inventors: |
Wilson; Mark; (Cumming,
GA) ; Garrett; David; (Marietta, GA) |
Correspondence
Address: |
SENNIGER POWERS LLP
ONE METROPOLITAN SQUARE, 16TH FLOOR
ST LOUIS
MO
63102
US
|
Assignee: |
Fry's Metals, Inc.
Jersey City
NJ
|
Family ID: |
33510685 |
Appl. No.: |
11/626118 |
Filed: |
January 23, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10458925 |
Jun 11, 2003 |
7166491 |
|
|
11626118 |
|
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Current U.S.
Class: |
438/106 ;
257/E21.503; 257/E21.505; 257/E23.119 |
Current CPC
Class: |
H01L 2924/01079
20130101; H01L 2924/01087 20130101; H01L 2924/01025 20130101; H01L
2924/09701 20130101; H01L 2924/01322 20130101; H01L 21/563
20130101; H01L 23/293 20130101; H01L 2224/73104 20130101; H01L
2924/01004 20130101; H01L 2224/73203 20130101 |
Class at
Publication: |
438/106 ;
257/E21.505 |
International
Class: |
H01L 21/58 20060101
H01L021/58 |
Claims
1. A method for forming an integrated circuit assembly for
attachment to a circuit board by soldering, the method comprising:
applying an underfill solution comprising a thermoplastic resin, a
solvent, and a fluxing agent to an integrated circuit device having
at least one solder bump on a surface thereof such that the
underfill solution is in contact with the at least one solder bump
and with the surface of the integrated circuit device; and removing
at least a portion of the solvent from the applied underfill
solution to thereby yield the integrated circuit assembly for
attachment to a circuit board, wherein the integrated circuit
assembly comprises the integrated circuit device, the at least one
solder bump, and a thermoplastic fluxing underfill in contact with
the integrated circuit device surface and in contact with the at
least one solder bump.
2. The method of claim 1 wherein removing at least a portion of the
solvent from the applied under fill solution yields a cured
thermoplastic fluxing underfill in contact with the integrated
circuit device surface.
3. The method of claim 1 wherein the thermoplastic resin has a
viscosity between about 2,500,000 cP and about 100,000 cP at a
temperature between about 80.degree. C. and about 125.degree.
C.
4. The method of claim 1 wherein the thermoplastic resin has a
viscosity between about 100,000 cP and about 500,000 cP at a
temperature between about 80.degree. C. and about 125.degree.
C.
5. The method of claim 1 wherein the thermoplastic resin has a
viscosity of less than about 30,000 cP at a temperature between
about 220.degree. C. and about 260.degree. C.
6. The method of claim 1 wherein the thermoplastic resin has a
glass transition temperature that is between about -25.degree. C.
and about 60.degree. C., a molecular weight that is between about
30,000 and about 50,000 daltons, and a viscosity that is between
about 10,000 and about 1,000 cP at a temperature that is between
about 220.degree. C. and about 260.degree. C.
7. The method of claim 6 wherein the thermoplastic resin is a
phenoxy resin having about 20 weight percent of caprolactone
grafted onto the backbone hydroxyl groups, the solvent is a polar
solvent selected from the group consisting of a ketone, an ester,
and an alcohol, and the fluxing agent is selected from the group
consisting of a monocarboxylic acid having more than 20 carbon
atoms per molecule, and a dicarboxylic acid having more than 12
carbon atoms per molecule that are liquid at room temperature and
are soluble in the solvent.
8. The method of claim 7 wherein the underfill solution comprises a
concentration of the thermoplastic resin that is between about 20
and about 60 weight percent, a concentration of the solvent that is
between about 40 and about 80 weight percent, and a concentration
of the fluxing agent that is between about 1 and about 10 weight
percent.
9. The method of claim 2 wherein the thermoplastic resin has a
viscosity between about 2,500,000 cP and about 100,000 cP at a
temperature between about 80.degree. C. and about 125.degree.
C.
10. The method of claim 2 wherein the thermoplastic resin has a
viscosity between about 100,000 cP and about 500,000 cP at a
temperature between about 80.degree. C. and about 125.degree.
C.
11. The method of claim 2 wherein the thermoplastic resin has a
viscosity of less than about 30,000 cP at a temperature between
about 220.degree. C. and about 260.degree. C.
12. The method of claim 2 wherein the thermoplastic resin has a
glass transition temperature that is between about -25.degree. C.
and about 60.degree. C., a molecular weight that is between about
30,000 and about 50,000 daltons, and a viscosity that is between
about 10,000 and about 1,000 cP at a temperature that is between
about 220.degree. C. and about 260.degree. C.
13. The method of claim 12 wherein the thermoplastic resin is a
phenoxy resin having about 20 weight percent of caprolactone
grafted onto the backbone hydroxyl groups, the solvent is a polar
solvent selected from the group consisting of a ketone, an ester,
and an alcohol, and the fluxing agent is selected from the group
consisting of a monocarboxylic acid having more than 20 carbon
atoms per molecule, and a dicarboxylic acid having more than 12
carbon atoms per molecule that are liquid at room temperature and
are soluble in the solvent.
14. The method of claim 13 wherein the underfill solution comprises
a concentration of the thermoplastic resin that is between about 20
and about 60 weight percent, a concentration of the solvent that is
between about 40 and about 80 weight percent, and a concentration
of the fluxing agent that is between about 1 and about 10 weight
percent.
15. The method of claim 1 wherein removing at least a portion of
the solvent from the applied underfill solution dries the applied
underfill solution and the process further comprises the steps of:
placing the integrated circuit assembly onto the circuit board to
yield a circuit board with the integrated circuit assembly placed
thereon; and heating the circuit board with the integrated circuit
assembly placed thereon to a reflow temperature to thereby solder
the integrated circuit device to the circuit board while the
fluxing agent fluxes the solder and to flow the cured thermoplastic
fluxing underfill thereby yielding a circuit board having the
integrated circuit device attached thereto with a metallic solder
connection and the cured thermoplastic underfill between and bonded
to the circuit board and the integrated circuit device.
16. The method of claim 2 wherein removing at least a portion of
the solvent from the applied underfill solution dries the applied
underfill solution and the process further comprises the steps of:
placing the integrated circuit assembly onto the circuit board to
yield a circuit board with the integrated circuit assembly placed
thereon; and heating the circuit board with the integrated circuit
assembly placed thereon to a reflow temperature to thereby solder
the integrated circuit device to the circuit board while the
fluxing agent fluxes the solder and to flow the cured thermoplastic
fluxing underfill thereby yielding a circuit board having the
integrated circuit device attached thereto with a metallic solder
connection and the cured thermoplastic underfill between and bonded
to the circuit board and the integrated circuit device.
17. A method for forming an integrated circuit assembly for
attachment to a circuit board by soldering, the method comprising:
applying a cured underfill film comprising a thermoplastic resin
and a fluxing agent to an integrated circuit device having at least
one solder bump on a surface thereof such that the cured underfill
film is in contact with the at least one solder bump and with the
surface of the integrated circuit device; and adhering the cured
underfill film to the integrated circuit device to yield the
integrated circuit assembly for attachment to a circuit board,
wherein the integrated circuit assembly comprises the integrated
circuit device, the at least one solder bump, and the cured
thermoplastic fluxing underfill adhered to the integrated circuit
device surface and the at least one solder bump.
18. The method of claim 17 wherein the thermoplastic resin has a
viscosity between about 2,500,000 cP and about 100,000 cP at a
temperature between about 80.degree. C. and about 125.degree.
C.
19. The method of claim 17 wherein the thermoplastic resin has a
viscosity of less than about 30,000 cP at a temperature between
about 220.degree. C. and about 260.degree. C.
20. The method of claim 17 wherein the thermoplastic resin has a
glass transition temperature that is between about -25.degree. C.
and about 60.degree. C., a molecular weight that is between about
30,000 and about 50,000 daltons, and a viscosity that is between
about 10,000 and about 1,000 cP at a temperature that is between
about 220.degree. C. and about 260.degree. C., and the fluxing
agent is selected from the group consisting of a monocarboxylic
acid having more than 20 carbon atoms per molecule, and a
dicarboxylic acid having more than 12 carbon atoms per molecule.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 10/458,925, filed on Jun. 11, 2003, issued on Jan. 23,
2007 as U.S. Pat. No. 7,166,491. Ser. No. 11/624,916, filed Jan.
19, 2007 is also a divisional of Ser. No. 10/458,925.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a flip chip design. More
particularly, the present invention relates to a flip chip design
which incorporates solder bumps and a polymeric underfill material
that is thermoplastic and provides fluxing during a solder reflow
operation
[0003] Electrical components such as resisters, capacitors,
inductors, transistors, integrated circuits, and chip carriers are
typically mounted on circuit boards according to one of two
configurations. In the first configuration, 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 configuration, 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.
A type of surface-mounted device, referred to as a flip chip, Chip
Scale Package, or Ball Grid Array comprises an integrated circuit
having numerous connecting leads attached to pads mounted on the
underside of the device. These surface-mounted devices are often
referred to as Area Array Packages. In connection with the use of
flip chips, either the circuit board or the device is provided with
small bumps or balls of solder (hereinafter "bumps" or "solder
bumps") positioned in locations which correspond to the pads on the
underside of each device and on the surface of the circuit board.
The device 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 device; (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 area array device to the board's
surface. Tolerances in area array 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 flip chips from the surface of the board to the bottom
of the die is typically between about 15 and about 75 mm and is
expected to approach about 10 mm in the near future.
[0005] One problem associated with area array 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 of the differing expansions, the heating of
the assembly during use can cause severe stresses. The stresses
imposed on the solder interconnects can lead to failures that
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 area array device 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 more closely match that of the integrated circuit and
printed circuit board substrates. 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
area array devices 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 couples the
area array device to the substrate replacing shear stresses with
bending stresses, and provides a commercially viable solution, a
simpler manufacturing method is 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 area
array device to the substrate and making solder connections. 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, the flux must be applied to the bumps before the underfill or
the underfill must contain flux or have inherent properties that
facilitate solder joint formation. Flux activity is needed to
remove the oxidation on the pads for the solder to wet the pad
metalization forming acceptable interconnects.
[0010] Certain underfills commonly called "dispense first
underfills" or no flow 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 automated
dispensing machines.
[0011] 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.
[0012] In view of the above, a need still exists for a more
efficient process which 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 harden quickly
while offering both excellent fluxing properties and excellent
underfill properties.
SUMMARY OF THE INVENTION
[0013] Briefly, therefore, the present invention is directed to a
method for forming an integrated circuit assembly for attachment to
a circuit board by soldering. The method comprises applying an
underfill solution comprising a thermoplastic resin having a glass
transition temperature that is within the range of about
-25.degree. C. to about 60.degree. C., a solvent, and a fluxing
agent to an integrated circuit device having at least one solder
bump on a surface thereof such that the underfill solution is in
contact with the at least one solder bump and with the surface of
the integrated circuit device. Then, at least a portion of the
solvent is removed from the applied underfill solution to thereby
yield the integrated circuit assembly for attachment to a circuit
board, wherein the integrated circuit assembly comprises the
integrated circuit device, the at least one solder bump, and a
thermoplastic fluxing underfill in contact with the integrated
circuit device surface and in contact with the at least one solder
bump.
[0014] The present invention is also directed to a method for
forming an integrated circuit assembly for attachment to a circuit
board by soldering. The method comprises applying an underfill
solution comprising a thermoplastic resin having a glass transition
temperature that is within the range of about -25.degree. C. to
about 60.degree. C., a solvent, and a fluxing agent to an
integrated circuit device having at least one solder bump on a
surface thereof such that the underfill solution is in contact with
the at least one solder bump and with the surface of the integrated
circuit device. Then, the solvent is removed from the applied
underfill solution to thereby yield the integrated circuit assembly
for attachment to a circuit board, wherein the integrated circuit
assembly comprises the integrated circuit device, the at least one
solder bump, and a thermoplastic fluxing underfill in contact with
the integrated circuit device surface and in contact with the at
least one solder bump.
[0015] Additionally, the present invention is directed to a method
for forming an integrated circuit assembly for attachment to a
circuit board by soldering comprising applying an a underfill
solution comprising a thermoplastic resin, a solvent, and a fluxing
agent to an integrated circuit device having at least one solder
bump on a surface thereof such that the underfill solution is in
contact with the at least one solder bump and with the surface of
the integrated circuit device. Then at least a portion of the
solvent is removed from the applied underfill solution to thereby
yield the integrated circuit assembly for attachment to a circuit
board, wherein the integrated circuit assembly comprises the
integrated circuit device, the at least one solder bump, and a
cured thermoplastic fluxing underfill in contact with the
integrated circuit device surface and in contact with the at least
one solder bump.
[0016] Further, the present invention is directed to a method for
forming an integrated circuit assembly for attachment to a circuit
board by soldering comprising applying a cured underfill film
comprising a thermoplastic resin and a fluxing agent to an
integrated circuit device having at least one solder bump on a
surface thereof such that the cured underfill film is in contact
with the at least one solder bump and with the surface of the
integrated circuit device. The method also comprises adhering the
cured underfill film to the integrated circuit device to yield the
integrated circuit assembly for attachment to a circuit board,
wherein the integrated circuit assembly comprises the integrated
circuit device, the at least one solder bump, and the cured
thermoplastic fluxing underfill adhered to the integrated circuit
device surface and the at least one solder bump.
[0017] The present invention is also directed to a method for
attaching an integrated circuit device to a circuit board by
soldering. The method comprises applying an underfill solution
comprising a thermoplastic resin having a glass transition
temperature that is within the range of about -25.degree. C. to
about 60.degree. C., a solvent, and a fluxing agent to an
integrated circuit device having at least one solder bump on a
surface thereof such that the underfill solution is in contact with
the at least one solder bump and with the surface of the integrated
circuit device. The applied underfill solution is dried to yield
the integrated circuit assembly for attachment to a circuit board,
wherein the integrated circuit assembly comprises the integrated
circuit device, the at least one solder bump, and a thermoplastic
fluxing underfill in contact with the integrated circuit device
surface and in contact with the at least one solder bump. The
integrated circuit assembly is placed onto the circuit board to
yield a circuit board with the integrated circuit assembly placed
thereon. The circuit board with the integrated circuit assembly
placed thereon is heated to a reflow temperature to thereby solder
the integrated circuit device to the circuit board while the
fluxing agent fluxes the solder and to flow the thermoplastic
fluxing underfill thereby yielding a circuit board having the
integrated circuit device attached thereto with a metallic solder
connection and the thermoplastic underfill between and bonded to
the circuit board and the integrated circuit device.
[0018] Additionally, the present invention is directed to a method
for attaching an integrated circuit device to a circuit board by
soldering. The method comprises applying an underfill solution
comprising a thermoplastic resin, a solvent, and a fluxing agent to
an integrated circuit device having at least one solder bump on a
surface thereof such that the underfill solution is in contact with
the at least one solder bump and with the surface of the integrated
circuit device. The applied underfill solution is dried to yield
the integrated circuit assembly for attachment to a circuit board,
wherein the integrated circuit assembly comprises the integrated
circuit device, the at least one solder bump, and a cured
thermoplastic fluxing underfill in contact with the integrated
circuit device surface and in contact with the at least one solder
bump. The integrated circuit assembly is placed onto the circuit
board to yield a circuit board with the integrated circuit assembly
placed thereon. The circuit board with the integrated circuit
assembly placed thereon is heated to a reflow temperature to
thereby solder the integrated circuit device to the circuit board
while the fluxing agent fluxes the solder and to flow the cured
thermoplastic fluxing underfill thereby yielding a circuit board
having the integrated circuit device attached thereto with a
metallic solder connection and the cured thermoplastic underfill
between and bonded to the circuit board and the integrated circuit
device.
[0019] Further, the present invention is directed to a method for
attaching an integrated circuit device to a circuit board by
soldering comprising placing an integrated circuit assembly
comprising an integrated circuit device having at least one solder
bump on a surface and a cured thermoplastic fluxing underfill in
contact with said surface and in contact with the at least one
solder bump onto the circuit board to yield a circuit board with
the integrated circuit assembly placed thereon. The method also
comprises heating the circuit board with the integrated circuit
assembly placed thereon to a reflow temperature to thereby solder
the integrated circuit device to the circuit board while the cured
thermoplastic fluxing underfill flows and fluxes the solder thereby
yielding a circuit board having the integrated circuit device
attached thereto with a metallic solder connection and the cured
thermoplastic underfill between and bonded to the circuit board and
the integrated circuit device.
[0020] The present invention is also directed to a thermoplastic
fluxing underfill solution for application between an integrated
circuit device and a circuit board to assist in solder assembly of
the integrated circuit device to the circuit board and to provide
shock resistance after said solder assembly of the integrated
circuit device to the circuit board. The thermoplastic fluxing
underfill solution comprises a thermoplastic resin having a glass
transition temperature within the range of about -25.degree. C. to
about 60.degree. C., and thermal stability such that the
thermoplastic resin loses less than about 10% of its weight upon
exposure to soldering conditions comprising a temperature of about
250.degree. C. for about 90 seconds. The solution also comprises a
solvent which dissolves the thermoplastic resin having said thermal
stability and a fluxing agent for fluxing a solder in solder
assembly of the integrated circuit device to the circuit board.
[0021] The foregoing and other features and advantages of the
present invention will become more apparent from the following
description and accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a schematic representation of a portion of a
semiconductor wafer having solder bumps applied to its surface.
[0023] FIG. 2 is a schematic representation of a portion of a
semiconductor wafer having solder bumps applied to its surface and
a flux/underfill material applied over the solder bumps.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The present invention is directed to a thermoplastic fluxing
underfill for use during and after solder reflow operations.
Advantageously, the thermoplastic fluxing underfill of the present
invention is a "no-flow" type underfill and may be pre-applied to a
chip and/or substrate several months (e.g., at least six months)
prior to the solder reflow operation without any decrease in the
flow, adhesion, and/or reworkability. Additionally, the
thermoplastic fluxing underfill of the present invention may be
used with lead-containing and lead-free solders. Due to these and
other characteristics, the present invention is also directed to a
unique method of forming an integrated circuit assembly comprising
an integrated circuit device (e.g., a flip chip) and the
thermoplastic fluxing underfill that is ready for end user
application (e.g., solder reflow connection onto a printed circuit
board).
[0025] In general the present invention is directed to joining any
appropriate electrical component to any appropriate printed circuit
board. In accordance with the present invention, any appropriate
type electrical component may, for example, comprise one or more of
the following: an integrated circuit device (e.g., a flip chip), a
resistor, a capacitor, an inductor, a transistor, or an area array
device). It is to be noted that hereinafter this disclosure will be
directed primarily to the joining of an integrated circuit device
to a printed circuit board. This, however, is not to be interpreted
as limiting the scope of the invention.
[0026] Appropriate substrate materials for a printed circuit board
and/or an integrated circuit device include, for example,
high-pressure laminates (i.e., layers of fibrous materials bonded
together under heat and pressure with a thermosetting resin). In
general, a laminate layer comprises an electrical-grade paper
bonded with phenolic or epoxy resin or a continuous-filament glass
cloth bonded with an epoxy-resin system. Specific examples of
laminate layers are: XXXPC which is an electrical paper impregnated
with phenolic resin; FR-2 which is similar to XXXPC with a flame
retardant property; FR-3 which is a self-extinguishing laminate of
electrical paper and epoxy resin; G-10 which is a laminate of glass
cloth sheets and epoxy resin; FR-4 which is a self-extinguishing
laminate similar to G-10; G-11 which is a glass cloth and epoxy
mixture; FR-5 which is a flame-resistant version of G-11. In one
embodiment of the present invention, the organic circuit board
material is an FR-4 laminate layer that is placed on top of, and in
intimate contact with the passive component pattern, and the two
are laminated together. In addition to laminated organic materials,
the substrate to which the integrated circuit is bonded may
comprise, for example, a semiconductor material such as silicon or
gallium arsenide, or an inorganic oxide such as alumina, titania,
or zirconia.
[0027] The selection of the solder for joining the integrated
circuit device and the printed circuit board depends upon several
factors. For example, the solder should be compatible with the
metal or metals used to form the leads of the integrated circuit
device and the printed circuit board (i.e., upon removal of oxides
from said metals by the fluxing agent the solder wets the leads
during reflow to form an electrically conductive bond).
Additionally, the selection of the solder may depend upon
environmental and/or worker safety concerns. For example, there is
an ever increasing demand for lead-free solders. Still further, the
solder alloy preferably melts at a sufficiently low temperature so
that there is no degradation of the integrated circuit device or
the printed circuit board.
[0028] Also, the solder preferably melts at a temperature at which
the thermoplastic fluxing underfill is stable. For example, in one
embodiment the solder melts at a temperature that is less than
about 300.degree. C. In another embodiment the solder melts at a
temperature between about 180.degree. C. and about 260.degree. C.
In yet another embodiment the solder melts at a temperature between
about 220.degree. C. and about 260.degree. C. Further, when
performing a reflow operation, the reflow temperature is typically
about 10.degree. C. to about 40.degree. C. higher than the solder
alloy melt temperature. For example, when reflowing a solder alloy
having a relatively high melting temperature, for example, a
melting point that is between about 210.degree. C. and about
240.degree. C., a reflow temperature that is between 220.degree. C.
about 260.degree. C. is typically preferred. When reflowing a
solder alloy having a relatively low melting temperature, for
example, a melting point that is between about 160.degree. C. and
about 190.degree. C., a relatively low reflow temperature that is
between about 170.degree. C. and about 225.degree. C. is generally
preferred.
[0029] In view of the foregoing, the thermoplastic fluxing
underfill of the present invention may be used with any
conventional leaded solders (e.g., Sn.sub.63Pb.sub.37 and
Sn.sub.62Pb.sub.36Ag.sub.2). However, it is particularly useful
with solder alloys that are substantially free of lead which are
commonly referred to as Pb-free solder alloys and typically contain
less than about 0.3 wt % of lead. Pb-free solder alloys tend to
have higher liquidus temperatures and/or reflow durations than
lead-containing solder alloys. Exemplary Pb-free solder alloys
include: Au.sub.80Sn.sub.20,
Sn.sub.96.2Ag.sub.2.5Cu.sub.0.8Sb.sub.0.5,
Sn.sub.65Ag.sub.25Sb.sub.10, Sn.sub.96.5Ag.sub.3.5,
Sn.sub.5.5Ag.sub.3.8Cu.sub.0.7, Sn.sub.96.5Ag.sub.3Cu.sub.0.5,
Sn.sub.95.5Ag.sub.4Cu.sub.0.5, Sn.sub.93.6Ag.sub.4.7Cu.sub.1.7,
Sn.sub.42Bi.sub.58, Sn.sub.90Bi.sub.9.5Cu.sub.0.5,
Sn.sub.99.3Cu.sub.0.7, Sn.sub.99Cu.sub.1, Sn.sub.97Cu.sub.3,
Sn.sub.87.1In.sub.10.5Ag.sub.2Sb.sub.0.4,
Sn.sub.77.2In.sub.20Ag.sub.2.8, Sn.sub.63.6In.sub.8.8Zn.sub.27.6,
Sn.sub.97Sb.sub.3 and Sn.sub.95Sb.sub.5. The thermoplastic fluxing
underfill of the present invention is particularly suited for
fluxing any of the foregoing Pb-free solder alloys.
[0030] The solder alloy is typically applied as a solder paste
which is a mixture of powdered solder metal alloy suspended or
dispersed in a liquid vehicle. In general, at room temperature the
solder paste is compliant enough so that it can be made to conform
to virtually any shape. At the same time, it is "tacky" enough that
it tends to adhere to any surface it is placed into contact with.
These qualities make solder paste useful for forming solder bumps
on electronic components such as ball grid array packages (BGAs) or
on the board to attach BGAs. Typically, the solder paste is
deposited by stenciling or screen printing. In one embodiment the
solder paste is deposited onto the solder-wettable pads of the
integrated circuit device. In another embodiment the solder paste
is deposited onto the solder-wettable pads of the printed circuit
board. In yet another embodiment solder paste is deposited on both
the solder-wettable pads of the integrated circuit device and the
printed circuit board.
[0031] The selection of the thermoplastic resin is based, in large
part, on its thermal-related properties. For example, the
thermoplastic resin preferably readily flows at reflow temperatures
to minimize the occurrence of voids in the underfill and thereby
maximize the bonding between the underfill, the integrated circuit
device, and the printed circuit board. More specifically, in one
embodiment of the present invention the viscosity of the
thermoplastic resin at temperatures at or above the melting point
of the solder alloy (e.g., between about 220.degree. C. and about
260.degree. C.) is less than about 30,000 cP. In another embodiment
the viscosity of the thermoplastic resin is between about 10,000
and about 1,000 cP at a temperature between about 220.degree. C.
and about 260.degree. C. In still another embodiment the viscosity
of the thermoplastic resin is between about 3,000 and about 300 cP
at a temperature between about 180.degree. C. and about 240.degree.
C.
[0032] Additionally, the thermoplastic resin is selected to have
sufficient tack to hold the integrated circuit device to the
printed circuit board when mounting the integrated circuit device
using, for example, a pick and place machine available from, e.g.,
Assemblion or Siemens. Specifically, in one embodiment the
thermoplastic resin has sufficient tack for mounting the integrated
circuit device upon being heated to between about 80.degree. C. and
about 125.degree. C. As such temperatures, the viscosity of the
thermoplastic resin is between about 2,500,000 and about 100,000
cP.
[0033] These thermal properties are, in large part, dependent upon
the glass transition temperature (Tg) and the melt temperature (Tm)
of the thermoplastic resin. The glass transition temperature is the
temperature at which the polymer transforms from being solid-like
and exhibiting an elastic deformation profile to being rubber-like
and exhibiting a viscous deformation profile. Additionally, the
transformation at Tg is typically associated with a substantial
increase in the coefficient of thermal expansion (CTE). The melt
temperature of the polymer is the point at which significant
dimensional deformation (e.g., between about 1 and about 5%) under
a load of about 25mN occurs during a static temperature ramp
utilizing a thermomechanical analyzer. It has been discovered that
thermoplastic resins having sufficient flowability at solder reflow
temperatures, sufficient tack for mounting an integrated circuit
device, and reworkability have a Tg that is between about
-25.degree. C. and about 60.degree. C. A thermoplastic resin with a
Tg lower than about -25.degree. C. would most likely have too low
of a viscosity at the maximum reflow temperature and flow out from
an integrated circuit device during the reflow operation. In
contrast, a thermoplastic resin with a Tg above about 60.degree. C.
would tend not to flow sufficiently to form the desired bond
between the integrated circuit device and printed circuit board. It
has also been discovered that appropriate thermoplastic resins
typically have a Tm that is within the range of about 50.degree. C.
to about 150.degree. C. In one embodiment of the present invention
the thermoplastic resin has a Tg that is between about -15.degree.
C. about 40.degree. C. and a Tm that is between about 60.degree. C.
and about 150.degree. C. In yet another embodiment the
thermoplastic resin has a Tg that is between about 20.degree. C.
and about 40.degree. C. and a Tm that is between about 80.degree.
C. and about 100.degree. C. In still another embodiment the
thermoplastic resin has a Tg that is between about 25.degree. C.
and about 35.degree. C. and a Tm that is between about 85.degree.
C. and about 95.degree. C. In yet another embodiment the Tg is
between about -5.degree. C. and about 10 EC and the Tm is between
about 50.degree. C. and about 65.degree. C.
[0034] In addition to temperature, the viscosity of the
thermoplastic resin is related to the molecular weight of the
polymer. In general, as the molecular weight of the polymer
increases or decreases, so does the viscosity of the thermoplastic
resin at a particular temperature. In one embodiment the molecular
weight of the thermoplastic resin is between about 30,000 and about
55,000 daltons. In other embodiment the molecular weight of the
thermoplastic resin is between about 30,000 and about 40,000
daltons. In yet another embodiment the molecular weight of the
thermoplastic resin is between about 30,000 and about 36,000
daltons. In still another embodiment the molecular weight is
between about 34,000 and about 42,000 daltons. In another
embodiment the molecular weight of the thermoplastic resin is
between about 42,000 and about 55,000 daltons.
[0035] The selection of the thermoplastic resin is also based on
its thermal stability (i.e., its resistance to degradation at
elevated temperatures). Stated another way, the selected
thermoplastic resin is considered to be thermally stable (i.e., it
does not substantially degrade during a reflow operation or a
subsequent release/rework operation). The thermal stability of a
thermoplastic resin may be quantified in terms of weight loss when
heated to a particular temperature for a particular duration. With
respect the present invention, a thermoplastic resin is considered
to be thermally stable if the weight loss is less than about 10
percent when subjected to a thermogravimetric analysis comprising
heating the resin to at least the maximum temperature at which the
desired reflow operation occurs for at least the duration at which
the maximum temperature is maintained. For example, in one
embodiment the thermoplastic resin has less than about 5 percent
weight loss when heated to about 250.degree. C. for about 60
seconds and is considered to be thermally stable. In another
embodiment the thermoplastic resin has less than a 10 percent
weight loss when heated to about 300.degree. C. for about 60
seconds and is considered to be thermally stable. Additionally, the
thermoplastic resin is also preferably moisture resistant.
[0036] It has been discovered that thermoplastic resins having
properties suitable for such a demanding application are
phenoxy-based polymers of bisphenol A (i.e., they comprise
polyhydroxyether). Other appropriate thermoplastic resins include
polysulfone. One such commercially available phenoxy-based resin is
INCHEMREZ PHENOXY PKCP-80 available from the InChem. Corporation.
This resin is a phenoxy resin having about 20 weight percent of
caprolactone grafted onto the backbone hydroxyl groups. The
INCHEMREZ PHENOXY PKCP-80 has a molecular weight of about 39,000
daltons and a glass transition temperature of about 30 EC by
differential scanning calorimetry. The caprolactone decreases the
viscosity of the thermoplastic resin. Additionally, the
caprolactone tends to decrease the Tg of the phenoxy resin which
without out the caprolactone would be about 90.degree. C.
[0037] The PKCP-80 resin adequately flows at reflow temperatures.
Specifically, the viscosity of the PKCP-80 resin is between about
7,000 and about 2,500 cP at a temperature between about 220.degree.
C. and about 260.degree. C. Also, the PKCP-80 resin has sufficient
tack for mounting an integrated circuit device. Specifically, the
viscosity of the resin is between about 100,000 and about 500,000
cP at a temperature between about 80.degree. C. and about
125.degree. C. The PKCP-80 resin is also considered to be thermally
stable. Specifically, upon being heated to a temperature of about
250.degree. C. for about 90 seconds, the resin only loses about 2%
of its weight. Further, upon being heated to a temperature of about
300.degree. C. for about 90 seconds, the resin loses less than
about 5% of its weight. With such thermal stability, the material
is considered to not decompose during a solder reflow operation.
The PKCP-80 also exhibits a low moisture uptake, specifically, less
than about 5% when heated to about 130.degree. C. while exposed to
a 85% relative humidity atmosphere.
[0038] By utilizing a thermoplastic resin system (i.e., a polymer
that softens when exposed to heat and returns to its original
condition when cooled to room temperature), the underfill of the
present invention is reworkable following the reflow operation.
Thermoplastic resins typically comprise very little cross-linking
of the polymer molecules which allows greater molecular mobility
and hence the ability to soften when heated. In contrast, many
conventional underfills comprise "thermoset" resins which are
typically highly cross-linked polymers that cannot be softened
after reflow which prevents removal or reworking of a faulty
chip.
[0039] To form a layer of thermoplastic fluxing underfill, the
thermoplastic resin is typically dissolved in an appropriate
solvent or solvent blend. The particular solvent or solvents is not
overly critical, but the solvent should readily dissolve the
thermoplastic resin and be compatible with the components in the
thermoplastic fluxing underfill. Preferably, the solvent also has
evaporation and boiling points that are high enough so that it is
considered easy and safe to handle yet low enough to allow removal
of the solvent at room temperature or a drying oven (e.g., the
evaporation point is preferably between about 70.degree. C. and
about 170.degree. C. and the boiling point is preferably between
about 90.degree. C. and about 130.degree. C.). Appropriate solvents
include many polar solvents such as ketones (e.g., acetone, methyl
ethyl ketone, methyl isobutyl ketone, cyclohexanone), esters (e.g.,
ethyl lactate, dibasic esters, ethylene glycol ethylether acetate,
diethyleneglycol ethylether acetate, propyleneglycol methylether
acetate, hexanediol diacrylate, phenoxy ethyl acrylate, ethoxyethyl
propionate), alcohols (e.g., methanol, ethanol, isopropyl alcohol,
benzyl alcohol, methylcellosolve, ethylcellosolve,
1-methoxy-2-propanol, carbitol and butylcarbitol), and combinations
thereof. In one embodiment of the present invention the solvent is
ethyl-ethoxypropionate and is commercially available from Eastman
Chemical of Kingsport, Tenn.
[0040] The amount, or concentration, of thermoplastic resin
dissolved in the solvent depends primarily upon the manner in which
the thermoplastic fluxing underfill is to be applied to the
integrated circuit device and/or printed circuit board. In general,
the concentration of resin in solution is between about 20 and
about 80 percent by weight of the thermoplastic fluxing underfill
solution prior to application. However, the concentration may be
outside the foregoing range and still be within the scope of the
present invention. Depending upon the application method, the
concentration of resin will tend to be toward the one end or the
other of the range. For example, if the thermoplastic fluxing
underfill is being deposited as a flowable liquid (e.g., it is
being dispensed by needle and syringe) the concentration of resin
is typically lower (e.g., between about 30 and about 45 weight
percent of the thermoplastic fluxing underfill solution). Whereas,
if the thermoplastic fluxing underfill is being cast into a film
prior to being applied to the integrated circuit device, the
concentration of resin in the solution tends to be higher (e.g.,
between about 40 and about 80 weight percent of the thermoplastic
underfill solution). In one embodiment, the solution comprises
about 40 weight percent of INCHEMREZ PHENOXY PKCP-80 dissolved in
ethyl-ethoxypropionate and the solution is cast into a film.
[0041] The thermoplastic fluxing underfill of the present invention
also comprises a fluxing component to remove oxides from all
surfaces involved in the soldering operation (e.g., solder pads,
solder bumps, and solder alloy powder). Further, the fluxing
component also protects against oxidation during, and for a
sufficient duration after, the reflow operation. Additionally, the
flux and/or its residues preferably do not corrode the solder metal
prior to, during, or following the soldering operation.
[0042] In addition, the fluxing component is preferably soluble or
dispersable in the solvent and thermally stable at reflow
temperatures. In general, the flux component comprises a carboxylic
acid (e.g., mono-, di- and polycarboxylic acids). Carboxylic acids
and dicarboxylic acids are preferred fluxing agents for many solder
applications, however, many lower molecular weight acids decompose
or evaporate at reflow temperatures. As such, the fluxing component
preferably comprises higher molecular weight carboxylic and
dicarboxylic acids. For example, carboxylic acids greater than C20
such as behenic acid, abietic acid, urocanic acid and dicarboxylic
acids greater than C12 such as dodecanedioic acid and
dodecanedicarboxyllic acid are preferred. Although they may be
used, many of these materials are solid at room temperature and are
not very soluble in polar solvents. Preferably, the fluxing agent
is a liquid carboxylic acid such as isostearic acid, and/or DIACID
1550 from Westvaco. In one embodiment of the present invention the
fluxing component comprises a liquid dicarboxylic acid sold under
the trade name DIACID 1550 by Westvaco Chemicals of Charleston,
S.C. The DIACID 1550 tends to be soluble in the appropriate
solvents and has an appropriate thermal stability.
[0043] To form a completely fused and strong solder joint, the
solder must adequately wet the solder pad and/or lead. Wetting
depends in large part on the metallurgical reaction between solder
and soldering surface, and on the efficacy of any fluxing
component. Thus, if the fluxing component does not adequately
remove oxides from the metals being joined during the reflow
operation, the oxides retard or prohibit the reaction.
Additionally, the joint will typically be incompletely fused, weak,
and subject to forming a void in the solder joint. Without being
held to a particular theory, it is presently believed that the
mechanism behind void formation is the entrapment of excess flux or
its vapors within the solder alloy. Thus, in addition to being
thermally stable, the concentration of fluxing component in the
thermoplastic fluxing underfill solution should be sufficient to
reduce the metal oxides in the solder alloy and on the solderable
surfaces, but not so great as to create voids. Typically, this is
accomplished with a concentration of fluxing component that is
between about 1 and about 10 weight percent of the thermoplastic
fluxing underfill solution. In another embodiment the concentration
of fluxing component is between about 4 and about 7 weight percent
of the thermoplastic fluxing underfill solution. In yet another
embodiment the concentration is about 4 weight percent of the
thermoplastic fluxing underfill solution. In still another
embodiment the concentration is about 2.5 weight percent of the
thermoplastic fluxing underfill solution.
[0044] Although not required, other additives, such as wetting
agents, defoaming agents, and coefficient of thermal expansion
(CTE) modifiers may be added to the thermoplastic fluxing
underfill. A wetting agent is typically added to improve the film
forming properties of the underfill and/or to enhance the bonding
of the underfill to the surfaces of the integrated circuit device
and printed circuit board by decreasing the surface tension of the
underfill. Appropriate wetting agents include the following classes
of materials: modified silicone resins, fluorocarbons, and acrylic
resins. The most commonly used type of wetting agent in underfills
are silanes. In one embodiment the thermoplastic fluxing underfill
comprises a commercially available silane-type wetting agent from
Byk Chemie of Wesel, Germany sold under the trade name BYK 306. The
BYK 306 wetting agent only contains 12 percent by weight wetting
agent with the remainder being solvent. If present, the
concentration of a wetting agent in the thermoplastic fluxing is
typically kept near the minimum concentration at which effective
wetting is accomplished because high concentrations can actually
decrease adhesion. In general, the concentration of wetting agent
in the underfill is between about 0.005 and about 2.0 weight
percent of the solution. In one embodiment the concentration of
wetting agent is between about 0.05 and about 0.20 weight percent
of the thermoplastic fluxing underfill solution. In one embodiment
the thermoplastic fluxing underfill comprises about 1 weight
percent of BYK 306 which in terms of what is actually added to the
thermoplastic fluxing underfill solution is about 0.12 weight
percent of the wetting agent and about 0.88 weight percent of the
associated solvent.
[0045] Defoaming agents are typically added prior to, or during,
the mixing of the thermoplastic resin and solvent to assist in the
degassing of the underfill solution. Stated another way, a
defoaming agent tends to minimize the formation of pockets of
entrapped air in the underfill solution. Such pockets of entrapped
air tend to result in the formation of voids in the cured underfill
which can degrade the adhesion and thermal stress compensation of
the underfill. Appropriate defoaming agents include the classes of
materials of polyether modified siloxanes and methylalkyl
siloxanes. The most commonly used type of defoaming agent in
underfills are modified polysiloxanes. Specific examples of
underfill defoaming agents include BYK 525, BYK 530, and BYK 535
available from Byk Chemie of Wesel, Germany. In one embodiment the
thermoplastic fluxing underfill comprises a commercially available
modified polydimethylsiloxane-type defoaming agent from Crompton of
Middlebury, Conn. sold under the trade name SAG 100. If present,
the concentration of a defoaming agent in the thermoplastic fluxing
is typically kept near the minimum concentration at which effective
degassing is accomplished because high concentrations can decrease
adhesion. In general, the concentration of defoaming agent is no
greater than about 1 weight percent of the thermoplastic fluxing
underfill solution. For example, in one embodiment the
thermoplastic fluxing underfill comprises about 1 weight percent of
SAG 100. In another embodiment the concentration of defoaming agent
is between about 0.05 and about 0.5 weight percent of the solution.
In yet another embodiment the concentration of defoaming agent is
about 0.10 weight percent of the underfill solution.
[0046] A thermoplastic resin as set forth above typically has a
coefficient of thermal expansion (CTE) that is between about 20 and
about 70 ppm/EC and acts to reduce the CTE mismatch between the
solder and the substrate materials. To further reduce any CTE
mismatch between the integrated circuit, the solder, and the
circuit board, the thermoplastic fluxing underfill of the present
invention may optionally comprise a coefficient of thermal
expansion modifier component. The CTE modifying component has a CTE
that is more compatible with the substrates (e.g., the flip chip
and circuit board) thereby decreasing the thermal stress upon
thermal cycling. The CTE modifying component is electrically
insulating and has a CTE that is preferably less than about 10
ppm/.degree. C. Exemplary CTE modifying component materials include
beryllium oxide (about 8.8 ppm/.degree. C.), aluminum oxide (about
6.5-7.0 ppm/.degree. C.), aluminum nitride (about 4.2 ppm/.degree.
C.), silicon carbide (about 4.0 ppm/.degree. C.), silicon dioxide
(about 0.5 ppm/.degree. C.), low expansion ceramic or glass powders
(between about 1.0 to about 9.0 ppm/.degree. C.), and mixtures
thereof. In one embodiment of the present invention the CTE
modifying component comprises silicon dioxide.
[0047] The maximum particle size of the CTE modifying component
(i.e., the maximum cross-sectional distance of the particle) is
preferably less than the height of the solder bumps to minimize any
negative impact on solder joint integrity. Typically, the average
particle size of the CTE modifying component is between about 3 and
about 15 .mu.m. Although the amount of the CTE modifying component
in the thermoplastic fluxing underfill depends on the particular
application, if present, the CTE modifying component typically
comprises between about 10 and about 90 wt % of the thermoplastic
fluxing underfill.
[0048] In general, the thermoplastic fluxing underfill solution is
prepared by mixing together the various constituents. Typically,
the preparation process comprises heating the solvent and
thermoplastic resin to enhance the rate of dissolution. After
dissolution of the resin in the solvent is complete, any remaining
constituents such as wetting agents, defoaming agents, and CTE
modifiers are typically dissolved or mixed into the solution.
[0049] As set forth above, the underfill solution may be formulated
to have the correct rheology for the method of application. For
example, because the ratio of solvent to solids is the primary
factor in determining the viscosity of the solution, it is possible
to formulate underfill solutions that can be applied using
different methods. Additionally, because the solvent is
substantially entirely evaporated after application of the
underfill solution to the integrated circuit device wafer, the
resulting, solid underfill layer will have the same composition
regardless of the initial viscosity and percent solids of the
underfill solution. This is because the solvent is merely a vehicle
for carrying the solids during underfill application.
[0050] In one application method, the underfill solution is applied
by spin coating. Spin coating is a common semiconductor processing
method in which liquid is deposited onto a spinning wafer in order
to provide a smooth and level coating. A typical viscosity for spin
coating an underfill is between about 80 and about 85 Kcps,
measured at 2.5 RPM using an RVT #6 spindle on a Brookfield
viscometer. When applied to a wafer, a wafer spin rate of between
about 700 and about 1500 RPM has been found to yield to uniform and
smooth coating. Good application results have been found with a
wafer spin rate of about 1200 RPM.
[0051] A second method for applying the thermoplastic fluxing
underfill is stencil printing. This method typically requires a
more viscous solution than that for spin coating. 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. Specifically, the rheology of the
solution is preferably gel-like or semi-solid if static, however,
when a shear force is applied it preferably flows like a liquid.
This allows for the underfill solution to flow through a stencil
when a force is applied using a squeegee and to maintain the
pattern of the stencil after the stencil is removed from the
surface of the substrate. Exemplary thixotropic agents include
fumed silicas. If present, the thixotropic agents typically
comprise between about 0.2 and about 9 weight percent of the
underfill solution. The thickness of the stencil determines the
amount of material applied to the wafer and the stencil should be
thicker than the bump height so that the blade applying the
underfill material does not contact the bumps. If such contact does
occur, damage to the bumps or even displacement of the bumps may
occur.
[0052] The print method employs the use of a metal stencil and an
automated stencil print machine such as those available from
Speedline. In this method, the liquid underfill is deposited on the
metal stencil which has an aperture slightly larger than the array,
and a squeegee, either metal or rubber, is used to wipe the
material over the aperture. The device to be underfilled is fitted
in a tray (i.e., a JEDEC tray) or holding device with the array
exposed in the stencil aperture. The material is deposited on the
device via the wiping of the material over the aperture. The
process parameters such as aperture height, collapse height, and
percent solids of the thermoplastic composition are typically fine
tuned to result in void free joint formation.
[0053] Other well known methods of depositing liquid underfill onto
an integrated circuit device, wafer, or other substrate include
spraying, screen printing, and needle deposition. Regardless of
which manner the liquid underfill is applied, after being applied
at least a portion of the solvent is evaporated from the underfill
solution thereby increasing the viscosity of the underfill.
Typically, the evaporation is enhanced by heating the underfill
solution 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. If a slight degree of tackiness at room temperature is
desired, however, a tackifier may be added to the underfill.
[0054] It is generally preferred that the thickness of the dried
underfill material be less than the height of the solder bumps to
allow for collapse of the bumps during the reflow operation. In one
embodiment, the thickness of the dried underfill layer is between
about 50 and about 80 percent of the solder bump height. In another
embodiment the thickness of the dried underfill layer is between
about 60 and about 70 percent of the solder bump height. 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, in addition to stencil
thickness, for example, the amount of solvent in the underfill
solution and/or the deposit thickness may be controlled in order to
control the thickness of the applied underfill. Typically, a dry
underfill thickness range of about 25 to about 125 microns is
suitable depending on the height of the solder bumps.
[0055] Alternatively, the underfill may be applied to the
integrated circuit device as a solid underfill layer. Specifically,
the underfill solution may be cast onto a release substrate (e.g.,
paper) and then dried into a film. The resulting film is then
typically cut into a proper shape called a preform and applied to
the integrated circuit device wafer (i.e., a wafer comprising a
multiplicity of integrated circuit devices). Heating, with the
application of pressure or a vacuum, is typically used to bond the
underfill layer to the wafer. The temperature of the layer
preferably is not increased above the point at which the fluxing
properties of the underfill are activated (e.g., the temperature
may be about 175.degree. C.). Pulling a vacuum is generally
preferred over applying pressure because it tends to be more
effective at preventing air from being trapped between the film and
the chip. One advantage of a solid film is that it can be easily
shipped, conveniently stored, and applied by simple mechanical
handling equipment. Like the underfill layers applied as a liquid
to the integrated circuit, the film thickness should be less than
the height of the solder bumps. In fact, the foregoing dry
thickness range is equally applicable.
[0056] Unlike systems which employ a separate flux and underfill,
the present system allows the underfill material to cover the
solder bumps since it offers fluxing properties as well as
underfill properties. In fact, it is preferred that the material
cover the bumps because, in so doing, the bumps will be protected
from oxidation, contamination, and mechanical damage. Each of the
application methods described above has the capability of covering
the bumps with the underfill material.
[0057] At this stage, the wafer is ready to be diced, or
singulated, to produce individual area array devices (e.g., 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 is
that the process does not degrade the underfill material applied to
the wafer/chip surface(s). In one embodiment dicing is achieved by
attaching the wafer to a holding tape and then sectioning the wafer
using, for example, a DISCO saw with a 5 .mu.m diamond cutting
blade operating at a speed of about 30,000 rpm. Water jet cooling
is used to keep the temperature at the cut below the softening
point of the film. The individual die or chip can then be picked
off the tape and placed into waffle packs, tape and reel packaging,
or other convenient die presentation systems used in the
industry.
[0058] Once diced, individual area array devices may now be bonded
to circuit boards and the like. Each area array device 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 (if the
underfill is not tacky at room temperature or heated until
tacky).
[0059] The area array device is then placed on the substrate using
a pick and place machine. If the underfill is not tacky at room
temperature, the substrate is preferably heated to a temperature
within the range of about 80.degree. C. to about 120.degree. C. so
that the thermoplastic has tack enough to hold the die in place.
The positioned chip and substrate assembly is then typically run
through a multi-zone oven with individual heat controls that permit
a heating profile appropriate for the specific solder. During
reflow, the flux in the underfill reduces oxides present on the
solder or the metal surface in contact with the solder and allows
solder joints to form at the substrate and circuit device pads.
Further, within the temperature range of about 60.degree. C. to
about 130.degree. C. the thermoplastic resin softens sufficiently
to flow and wet the integrated circuit device and substrate
surfaces. The assembly is cooled and the solder and underfill
harden to form a bonded assembly comprising the integrated circuit
device, the substrate, at least one solder joint, and the
underfill.
[0060] Alternatively, a flip chip bonder that can apply heat and
pressure may be employed instead of the reflow oven. In this
embodiment, the integrated circuit assembly (flip chip coated with
the thermoplastic fluxing underfill) is placed in contact with the
conductive pads on the circuit board and heat from the bonder head
softens the underfill thereby activating the flux, reflowing the
solder bumps, and softens the underfill to bond to the board and
chip. The use of a flip chip bonder allows a flip chip to be
assembled to a board that already has components mounted thereto.
This method may also be used to attach a chip to a site that is
being reworked.
[0061] Reworking is desirable, for example, if 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,
because it is difficult to fully test an unpackaged device such as
a flip chip, it is 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. The present
invention eliminates the problems associated with thermoset
underfills by incorporating a thermoplastic resin as the main
component of the underfill. Thus, a previously bonded chip may be
removed by raising the chip temperature to above the melting point
of the solder (approximately 183 EC for tin/lead solder) and above
the de-bonding temperature of the underfill resin. Typically, the
rework temperature is about 15 to about 25.degree. C. above the
solder reflow temperature. Although, the temperature may be higher
if localized heat, such as produced with a chip bonder, is
used.
[0062] The invention can be further understood with reference to
FIGS. 1 and 2. As can be seen schematically in FIG. 1, a
semiconductor device 10 comprises a portion of a semiconductor
wafer 12 having solder bumps 14 applied to its surface.
Subsequently, as represented schematically in FIG. 2, the device 10
has had a flux/underfill material 16 applied to the surface of the
wafer 12 having the solder bumps 14. The underfill material 16
occupies at least the spaces between the bumps 14 and also covers
the bumps.
[0063] With the foregoing method and compositions, a thermoplastic
fluxing underfill and an assembly comprising the underfill may be
produced. The thermoplastic fluxing underfill of the present
invention provides several advantages such as: extended shelf life
stability (e.g., greater than six months); provides mechanical
shock resistance; delays or prevents device failure do to thermal
cycling; reworkability; application to chips eliminates the need to
underfill at the end user facility; decreased manufacturing costs;
and may be used with lead-containing and lead-free solder.
[0064] The following Examples will help to illustrate the invention
further.
EXAMPLES
[0065] A thermoplastic fluxing underfill containing about 40.00 wt
% phenoxy PKCP-80 thermoplastic resin, about 55.90 wt % of ethyl
3-ethoxypropionate solvent, about 1.00 wt % of BYK 306 wetting
agent, about 0.10 wt % of SAG 100 defoaming agent, and about 4 wt %
DIACID 1550 dicarboxylic acid fluxing agent was prepared according
to the following steps. The solvent was placed in a stainless steel
beaker and heated to a temperature of about 70.degree. C. while
being stirred. Then the thermoplastic resin was added to the
solvent in approximately ten percent portions. Specifically, each
10 percent portion was added to the solvent while the stirring was
maintained until completely dissolved. At that point, the next ten
percent portion was added and mixed. This was repeated until all of
the thermoplastic resin was dissolved in the solvent at which time
the heating of the solution was discontinued. The BYK 306 and SAG
100 were then added. The solution was allowed to cool to room
temperature and the solids content was about 58 percent and the
viscosity was about 9,000 cP.
[0066] The foregoing thermoplastic underfill solution was cast onto
a paper such that the wet film had a thickness of about 0.5 mm. The
cast wet film was then dried at a temperature of about 120.degree.
C. to evaporate the solvent. The dried film had a thickness of
about 0.25 mm. A section of the dried film was cut and placed on
the solder bumps of a ball grid array. The solder balls comprised a
eutectic tin-lead alloy. The two were placed in an oven maintained
at about 165.degree. C. to attach the underfill layer to the flip
chip. The underfill coated flip chip was then placed on a printed
circuit board and subjected to a reflow operation having a maximum
temperature of about 225.degree. C. and a duration of about 90
seconds.
[0067] Integrated circuit assemblies comprising the integrated
circuit device and the thermoplastic fluxing underfill were then
evaluated for resistance to mechanical shock and thermomechanical
stress failures. Specifically, the assemblies were tested using a
drop shock test which is used to determine resistance to mechanical
shock caused by dropping a product containing the assembly. The
thermoplastic fluxing underfill improved the drop shock performance
of the area array devices by a factor of at least 10 over devices
with no underfill. Drop shock performance is a key indicator of the
robustness of the device connection to the board. The process
involves attaching a 50 g weight to a circuit board with 10 devices
and then allowing the weighted board to fall about 2 meters before
striking a horizontal surface. A failure is recorded when a device
detaches from the board or when the electrical continuity for the
device goes to open. Assemblies containing the thermoplastic
fluxing underfill were also subjected to a thermal shock
reliability test to determine the resistance to stress caused by
thermal cycling between -40.degree. C. and 125.degree. C. The
thermoplastic fluxing underfill passed this test by undergoing 1000
cycles with less than a 50 percent failure rate.
[0068] A second thermoplastic fluxing underfill solution was
prepared with a low molecular weight diluent thermoplastic resin to
reduce the overall Tg and Tm of the thermoplastic resin component
of the underfill to evaluate the applicability of the underfill for
devices which require relatively low viscosity for collapse at
typical or relatively low reflow temperatures. The formulation
comprised about 28 weight percent of PKCP-80, about 8 weight
percent of ethoxylated bisphenol A thermoplastic resin (Aldrich
Chemical), about 62 weight percent of cyclohexanone, about 2.5
weight percent of DIACID 1550, and about 1 weight percent BYK 306.
The constituents were placed in an eight ounce polypropylene jar
that was rolled at about 100 rpm for about 2 days on a ball mill
roller. Because this manufacturing process is a closed system and
uses no heat, the process reduces or eliminates the evaporation of
solvent during the incorporation process. Advantageously, because
evaporation is greatly reduced or eliminated, this method allows
for the preparation of an underfill solution in a consistent
manner. The material was then dispensed via syringe onto eutectic
bumped 10 mm.times.10 mm Amkor CABGA (daisy chained) devices. The
devices with the coating solution were dried for 1 hour at
70.degree. C. then for 1 hour at 165.degree. C. A coating of
approximately 85% of the solder ball height was achieved after the
deposition and drying process was completed twice.
[0069] The coated 10 mm.times.10 mm devices were then dipped in a
commercial flux, placed via hand on an FR4 substrate, and passed
through a reflow profile to form interconnect and to melt the pre
applied underfill. Drop shock data indicated a significant
improvement in device survivability (defined as an electrical open
in the daisy chain) to approximately 10 times that of devices
without an underfill.
[0070] It is to be understood that the above description is
intended to be illustrative and not restrictive. Many embodiments
will be apparent to those of skill in the art upon reading the
above description. The scope of the invention should therefore be
determined not with reference to the above description alone, but
should be determined with reference to the claims and the full
scope of equivalents to which such claims are entitled.
* * * * *