U.S. patent application number 17/149503 was filed with the patent office on 2021-07-08 for multilayered metal nano and micron particles.
This patent application is currently assigned to ALPHA ASSEMBLY SOLUTIONS INC.. The applicant listed for this patent is ALPHA ASSEMBLY SOLUTIONS INC.. Invention is credited to Remya Chandran, Shamik Ghoshal, Sutapa Mukherjee, Ranjit Pandher, Siuli Sarkar, Bawa Singh.
Application Number | 20210205935 17/149503 |
Document ID | / |
Family ID | 1000005475709 |
Filed Date | 2021-07-08 |
United States Patent
Application |
20210205935 |
Kind Code |
A1 |
Ghoshal; Shamik ; et
al. |
July 8, 2021 |
MULTILAYERED METAL NANO AND MICRON PARTICLES
Abstract
A sintering powder, wherein a least a portion of the particles
making up the sintering powder comprise: a core comprising a first
material; and a shell at least partially coating the core, the
shell comprising a second material having a lower oxidation
potential than the first material.
Inventors: |
Ghoshal; Shamik; (South
Plainfield, NJ) ; Chandran; Remya; (South Plainfield,
NJ) ; Mukherjee; Sutapa; (South Plainfield, NJ)
; Sarkar; Siuli; (South Plainfield, NJ) ; Pandher;
Ranjit; (South Plainfield, NJ) ; Singh; Bawa;
(South Plainfield, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ALPHA ASSEMBLY SOLUTIONS INC. |
Somerset |
NJ |
US |
|
|
Assignee: |
ALPHA ASSEMBLY SOLUTIONS
INC.
Somerset
NJ
|
Family ID: |
1000005475709 |
Appl. No.: |
17/149503 |
Filed: |
January 14, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15318895 |
Dec 14, 2016 |
10894302 |
|
|
PCT/GB2015/051805 |
Jun 22, 2015 |
|
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17149503 |
|
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|
62015845 |
Jun 23, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 2998/10 20130101;
C01P 2004/84 20130101; B22F 2009/245 20130101; B23K 35/3613
20130101; B22F 2301/10 20130101; C01P 2002/72 20130101; C09C 1/62
20130101; B22F 1/025 20130101; B22F 1/0059 20130101; B22F 1/0022
20130101; C01P 2002/22 20130101; B22F 1/0014 20130101; B22F 1/0074
20130101; B22F 2301/255 20130101; B23K 35/302 20130101; B23K
35/3601 20130101; B22F 9/24 20130101; B22F 1/0018 20130101; B23K
35/025 20130101; B22F 2001/0066 20130101 |
International
Class: |
B23K 35/30 20060101
B23K035/30; B22F 1/00 20060101 B22F001/00; B22F 1/02 20060101
B22F001/02; B22F 9/24 20060101 B22F009/24; C09C 1/62 20060101
C09C001/62; B23K 35/02 20060101 B23K035/02; B23K 35/36 20060101
B23K035/36 |
Claims
1. A sintering film comprising: a sintering powder and a binder,
wherein particles of the sintering powder comprise: a core
comprising a first material comprising copper; and a shell at least
partially coating the core, the shell comprising a layer of a
second material having a lower oxidation potential than the first
material, the second material comprising silver; and a capping
agent at least partially coating the shell, and wherein the
particles have a D50 of from 1 to 30 nm.
2. The sintering film of claim 1, wherein the film is on a
polymeric substrate.
3. The sintering film of claim 1, wherein the polymeric substrate
has a release coating thereon.
4. The sintering film of claim 1, wherein the binder comprises a
thermoplastic and/or thermosetting polymer.
5. The sintering film of claim 1, wherein the binder comprises at
least one thermoplastic polymer selected from the group consisting
of poly(methyl methacrylate), polyamides, polyethylene,
polypropylene, and polystyrene.
6. The sintering film of claim 1, wherein the binder comprises at
least one thermosetting polymer selected from the group consisting
of polyurethanes, polycyanurates, epoxy resin, polyimides, melamine
resin, and bismaleimide resin.
7. The sintering film of claim 1, wherein the binder comprises an
epoxy-based resin.
8. The sintering film of claim 1, wherein the particles of the
sintering powder have a D95 of less than 100 nm.
9. The sintering film of claim 1, wherein the particles of the
sintering powder have a D95 of from 0.1 to 10 .mu.m.
10. The sintering film of claim 1, wherein the sintering powder
comprises from 0.1 to 15 wt % capping agent.
11. The sintering film of claim 1, wherein the sintering powder
comprises from 0.1 to 5 wt % capping agent.
12. The sintering film of claim 1, wherein the capping agent
comprises octylamine.
13. A method for forming a sintered joint comprising: sintering a
sintering powder, wherein particles of the sintering powder
comprise: a core comprising a first material comprising copper; and
a shell at least partially coating the core, the shell comprising a
layer of a second material having a lower oxidation potential than
the first material, the second material comprising silver; and a
capping agent at least partially coating the shell, and wherein the
particles have a D50 of from 1 to 30 nm.
14. The method of claim 13, wherein the capping agent comprises
octylamine.
15. The method of claim 13, wherein the sintering powder is a
component of a sintering film and the sintering film further
comprises a binder.
16. The method of claim 13, wherein the sintering powder is a
component of a sintering paste and the sintering paste further
comprises a binder and a solvent.
17. The method of claim 13, wherein the sintering joint is formed
on a LED, MEMS, OLED, PV cell or semiconductor.
18. The method of claim 13, wherein the sintering joint is used for
die attachment, wafer-to-wafer bonding, reflective layer printing,
hermetic and near hermetic sealing, and dispensing and the
production of interconnect lines.
19. A sintered joint formed by the method of claim 13.
20. A sintering paste comprising: a sintering powder; a solvent;
and optionally a binder and/or a rheology modifier and/or an
organosilver compound and/or an activator and/or a surfactant
and/or wetting agent and/or hydrogen peroxide or organic peroxides,
wherein particles of the sintering powder comprise: a core
comprising a first material comprising copper; and a shell at least
partially coating the core, the shell comprising a layer of a
second material having a lower oxidation potential than the first
material, the second material comprising silver; and a capping
agent at least partially coating the shell, and wherein the
particles have a D50 of from 1 to 30 nm.
21. The sintering paste of claim 20, wherein the capping agent
comprises octylamine.
22. A method of manufacturing a sintering paste of claim 20
comprising: providing a sintering powder, wherein particles of the
sintering powder comprise: a core comprising a first material
comprising copper; and a shell at least partially coating the core,
the shell comprising a layer of a second material having a lower
oxidation potential than the first material, the second material
comprising silver; and a capping agent at least partially coating
the shell, and wherein the particles have a D50 of from 1 to 30 nm;
and dispersing the sintering powder in a solvent and optionally
together with a binder and/or a rheology modifier and/or an
organosilver compound and/or an activator and/or a surfactant
and/or wetting agent and/or hydrogen peroxide or organic
peroxides.
23. A LED, MEMS, OLED, PV cell or semiconductor comprising the
sintering paste of claim 20 or sintered product thereof.
24. A LED, MEMS, OLED, PV cell or semiconductor comprising the
sintering film of claim 1 or sintered product thereof.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Pat. No.
15/318,895, filed Dec. 14, 2016, which is the U.S. national stage
application of International PCT Application No. PCT/GB2015/051805,
filed Jun. 22, 2015, which claims the benefit of U.S. Provisional
Application No. 62/015,845, filed Jun. 23, 2014, the contents of
which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates to a sintering powder and a method of
making the same.
BACKGROUND OF THE INVENTION
[0003] The demand for alternative lead-free die attach materials
with high electrical and thermal requirements is on the rise in
today's era. Highly conductive epoxies and conventional lead base
solder paste are still widely used in many of the applications. But
the main drawback of these materials is their limitation in high
temperature operation, which results in solder joint fatigue and
reliability problems like intermetallic compound growth.
[0004] Nano-structured metals are being researched extensively as
an alternative for many semiconductor interconnection challenges.
Metallic nanoparticles have been widely investigated in recent
years because they possess interesting properties that differ
significantly from those of bulk materials. Nanoparticles have high
surface energy which enables them to undergo particle coalescence
at a much lower processing temperature as compared to the bulk
materials. A sintering temperature well below the melting point for
nanoscale metals is of great advantage for semiconductor packaging.
Processing the devices at low temperature could avoid initial die
level stresses and potential circuit damages.
[0005] Nanoparticles of several elements including gold, palladium,
silver and copper have been well-studied due to their potential
applications as conducting materials in the optoelectronic or
semiconductor fields. Noble metal nanoparticles such as silver and
gold, because of their high thermal conductivity and excellent
non-oxidizing properties, have been so far the most active research
objects in the past years. However, their high cost prevents them
from widespread practical applications. So the usage of non-noble
metal nanoparticle with decent thermal stability and conductivity
is of great interest in today's electronic world. Copper
nanoparticles, due to their relatively low cost and high electrical
conductivity, exhibit high potential for replacing the noble metal
nanoparticles used in conductive materials. A major problem in
utilizing copper nanoparticles is their inherent tendency to
oxidize in ambient conditions.
[0006] In the electronics industry, semiconductor device
interconnection to the substrate is an important part of device
packaging. The main materials currently used for the die attachment
and interconnections are the low melting solders, which are not
ideal because of the low operating temperature.
[0007] Silver paste is commonly used in microelectronic packages
due to silver's high electrical and thermal performance. However,
the high cost of silver limits its usage.
[0008] The present invention seeks to tackle at least some of the
problems associated with the prior art or at least to provide a
commercially acceptable alternative solution thereto.
SUMMARY OF THE INVENTION
[0009] In a first aspect, the present invention provides a
sintering powder, wherein a least a portion of the particles making
up the sintering powder comprise: [0010] a core comprising a first
material; and [0011] a shell at least partially coating the core,
the shell comprising a second material having a lower oxidation
potential than the first material.
[0012] Each aspect or embodiment as defined herein may be combined
with any other aspect(s) or embodiment(s) unless clearly indicated
to the contrary. In particular, any features indicated as being
preferred or advantageous may be combined with any other feature
indicated as being preferred or advantageous.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The invention will now be further described with reference
to the following non-limiting Figures, in which:
[0014] FIG. 1 shows a powder X-ray diffraction (XRD) pattern of the
sintering powder of Example 1.
[0015] FIG. 2 shows a UV-vis spectrum of the sintering powder of
Example 1.
[0016] FIG. 3 shows a thermogravimetric analysis (TGA) plot of the
sintering powder of Example 1.
[0017] FIG. 4 shows a differential scanning calorimetery (DSC) plot
of the sintering powder of Example 1.
[0018] FIG. 5 shows transition electron microscope (TEM)
micrographs of the sintering powder of Example 1.
[0019] FIG. 6 shows a powder X-ray diffraction (XRD) pattern of the
sintering powder of Example 2.
[0020] FIG. 7 shows a UV-vis spectrum of the sintering powder of
Example 2.
[0021] FIG. 8 shows transmission electron microscopy (TEM) images
of the sintering powder of Example 2.
[0022] FIG. 9 shows microscopic images of the printed pattern of
Example 4.
[0023] FIG. 10 shows SEM images of the sintered layer of Example
5.
[0024] FIG. 11 shows SEM images of the sintered joints of Example
6.
[0025] FIG. 12 shows microscopic images of the dispensed pattern of
Example 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] The term "sintering powder" as used herein may encompass a
powder capable of forming a sintered joint. Sintered joints are
formed by atomic diffusion of metal particles placed between two
work pieces to be joined. The sintering powder may comprise regular
shaped particles (such as, for example, spheres) or irregular
shaped particles (such as, for example, whiskers, plates, rods or
flakes).
[0027] The term "capping agent" as used herein may encompass a
species that, when present on the surface of metal particles,
reduces agglomeration of the metal particles, enables particle size
control during powder production and reduces particles' surface
oxidation or other contamination.
[0028] The term "metal" as used herein may encompass an alloy.
[0029] The term "D95" as used herein may encompass the value of the
longest particle dimension at 95% in the cumulative distribution.
When the particles are spheres, the longest dimension will be the
diameter of the sphere. D95 values may be determined by a dynamic
light scattering method or a laser scattering method.
[0030] The term "D50" as used herein may encompass the value of the
longest particle dimension at 50% in the cumulative distribution.
When the particles are spheres, the longest dimension will be the
diameter of the sphere. D50 values may be determined by a dynamic
light scattering method or a laser scattering method.
[0031] The inventors have surprisingly found that, in comparison to
sintering powders of the prior art, the sintering powder of the
present invention may exhibit high electrical and thermal
conductivity, and low susceptibility of surface oxidation. The
sintering powders may also contain low cost materials, thereby
reducing their manufacturing costs.
[0032] The first material may exhibit high electrical and thermal
conductivity. The first material typically exhibits a thermal
conductivity of great than 5.times.10.sup.6 S/cm at 20.degree. C.,
more typically greater than 5.times.10.sup.7S/cm at 20.degree. C.
Accordingly, such high conductivities may be transferred to a
sintered joint formed using the sintering powder.
[0033] The first material may be a metal or a non-metal. The first
material, which typically makes up the majority of the sintering
powder, may be a low cost material such as, for example, copper.
Accordingly, the cost of manufacturing the sintering powder may be
reduced.
[0034] Advantageously, the second material has a lower oxidation
potential than the first material. Accordingly, the shell may serve
to reduce oxidation of the first material, thereby avoiding
degradation of the sintering powder. Preferably, the oxidation
potential of the second material is at least 0.2 V less than that
of the first material, more preferably at least 0.5 V less than
that of the first material. The first material typically has an
oxidation potential of greater than -0.4 V, preferably greater than
-0.2 V. The second material typically has an oxidation potential
less than -0.5 V, preferably less than -0.7 V.
[0035] The second material may be a metal or a non-metal.
Typically, the second material is a noble metal.
[0036] The second material typically makes up only a small part of
the sintering powder, typically less than 50 wt. % of the sintering
powder, more typically less than 25 wt. % of the sintering powder.
As a result, even though if the second material is high cost (e.g.
a noble metal such as, for example, silver) the sintering powder
may exhibit reduced degradation without significantly increasing
the manufacturing costs thereof.
[0037] At least a portion of the particles making up the sintering
powder have the above-described "core-shell" structure. Preferably,
the majority of the particles making up the sintering powder have
the core-shell structure, more preferably substantially all of the
particles making up the sintering powder have the core-shell
structure. In some preferred embodiments, the sintering powder is
substantially free of particles formed of the first material only
and/or particles formed of the second material only.
[0038] The shell at least partially coats the core. Typically, the
shell substantially coats the core, more typically completely coats
the core (i.e. the coating is continuous).
[0039] The inventors have surprisingly found that the sintering
powder as described herein may be sintered at low temperatures with
the application of only very low pressure, typically substantially
no pressure. As a result, formation of a sintered joint between
work pieces using the sintering powder may occur with reduced
damage to the work pieces. In addition, since the application of
high pressures is not required, the formation of a sintered joint
is simplified, and may be more easily automated.
[0040] The sintering powder preferably further comprises a capping
agent at least partially coating the shell. The use of a capping
agent may help to reduce agglomeration of the particles. Such
agglomeration is unfavourable, since it may increase the sintering
temperature of the sintering powder. Accordingly, the use of a
capping agent enables the formation of a sintered joint between
work pieces at lower temperatures and, therefore, may help to
reduce damage to a work piece caused by exposure to high sintering
temperatures. In addition, the use of a capping agent may help to
avoid degradation of the first and/or second material such as, for
example, damage caused by exposure of the materials to air.
[0041] The capping agent may be inorganic and/or organic. Examples
of organic capping agents include polymers and ligands.
[0042] The capping agent preferably comprises one or more of an
amine, an alcohol, a fatty acid, a thiol and a surfactant. Such
capping agents may form a weak bond with the second material.
Accordingly the temperature required to break the bonding may be
reduced, which may help to reduce the sintering temperature. In a
preferred embodiment, the capping agent comprises oleate (such as,
for example, sodium oleate) and/or octylamine.
[0043] In this embodiment, the metal particles may be substantially
coated with the capping agent, or completely coated with a capping
agent. Increasing the coverage of the capping agent on the metal
particles may help to further reduce the agglomeration of the metal
particles and, therefore, further reduce the sintering temperature.
In addition, most of the metal particles may be coated with the
capping agent, or substantially all of the metal particles may be
coated with a capping agent.
[0044] The sintering powder preferably comprises up to 15 wt. %
capping agent, more preferably from 0.1 to 5 wt. % capping agent,
even more preferably about 0.5 wt. % capping agent. The term "wt.
%" used in this regards is based on the total weight of the
sintering powder. If the sintering powder comprises more than 15
wt. % capping agent, then higher temperatures may be required to
melt the capping agent prior to sintering. Furthermore, the amount
of organics contained in the resulting sintered joint may increase.
If the sintering powder comprises less than 0.1 wt. % capping
agent, then the capping agent may not adequately cover the surface
of the metal. This may result in an increase in agglomeration of
the particles and, therefore, an increase in the sintering
temperature.
[0045] The first material preferably comprises one or more of
copper, nickel, tin, molybdenum, tungsten, aluminium, graphene,
boron nitride, boron carbide and aluminium nitride. Such materials
may exhibit a favourable combination of low cost and high
electrical conductivity.
[0046] More preferably, the first material comprises one or more of
copper, nickel, tin and molybdenum. Apart from the low cost and
utility, the use of copper, nickel, tin and molybdenum may reduce
the effect of CTE (coefficient of thermal expansion) mismatch of
adjacent materials at the interfaces of a sintered joint formed
using the sintering powder. The thermal stress caused due to CTE
mismatch is the main cause for thermal fatigue of sintered joints.
The molybdenum, nickel, copper and/or tungsten of the core may act
as a thermal bridge between the adjacent materials at the interface
in electronic packaging. In a particularly preferred embodiment,
the first material comprises copper. The first material may
comprise an alloy of one or more of copper, nickel, tin,
molybdenum, tungsten and aluminium.
[0047] The second material preferably comprises one or more of
silver, gold, palladium, platinum and graphene, more preferably
silver. Such materials are particularly effective at reducing
oxidation of the core. In addition, such materials may be
effectively coated onto nanoparticles of the first material. The
second material may comprise an alloy of one or more of silver,
gold, palladium and platinum.
[0048] In a particularly preferred embodiment, the first material
comprises copper and the second material comprises silver. Such a
combination is particularly effective at resulting in a sintering
powder exhibiting high electrical conductivity, low cost and
reduced surface oxidation. Silver and copper have excellent
electrical and thermal conductivity, and are therefore capable of
forming a sintered joint with high electrical and/or thermal
conductivity. Accordingly, the use of silver and copper makes the
sintering powder particularly suitable for use in electronics
applications, such as, for example, die attachment and
microelectronic packaging. Copper is particularly low cost. Silver
is particularly suitable for reducing oxidation of copper.
[0049] The particles preferably have a D95 of less than 100 nm,
more preferably a D50 of from 1 nm to 30 nm, even more preferably a
D50 of from 5 nm to 30 nm. Particles larger than 100 nm may result
in a low surface-to-volume ratio, thereby requiring higher
sintering temperatures and/or pressures.
[0050] In an alternative embodiment, the particles may have a D95
of from 0.1 to 10 .mu.m. Larger particle sizes may require less
capping agent. Accordingly, due to the reduction in residual
organics in the resulting joint, the resistivity is much lower. In
this alternative embodiment, the sintering powder may comprise less
than 15 wt. % capping agent, typically about 3 wt. % capping
agent.
[0051] The shell may comprise a layer of the second material and a
layer of a third material different to the second material. The
third material may comprise, for example, one or more of silver,
gold, palladium, platinum and graphene. The presence of multiple
layers in the shell may serve to reduce the thermal stress in a
sintered joint formed using the sintering powder. The additional
layer may act as a thermal bridge, thereby reducing CTE
mismatch.
[0052] In a further aspect, the present invention provides
nanoparticles comprising: [0053] a core comprising a first
material; and [0054] a shell at least partially coating the core,
the shell comprising a second material having a lower oxidation
potential than the first material.
[0055] The nanoparticles may be used as a sintering powder. The
preferred features of the first aspect of the present invention are
applicable to this aspect.
[0056] In a further aspect, the present invention provides a
sintered joint formed using the sintering powder as described
herein. Such a sintered joint may exhibit particularly high
strength and/or particularly high electrical and thermal
conductivity. Furthermore, the sintered joint may exhibit very
little change in shear strength following thermal shock, typically
substantially no change in shear strength.
[0057] In a further aspect, the present invention provides an LED
(light-emitting diode), MEMS (microelectromechanical system), OLED
(organic light-emitting diode) or PV cell (photovoltaic cell)
comprising the sintered joint described herein.
[0058] In a further aspect the present invention provides a
sintering paste comprising: [0059] the sintering powder as
described herein; [0060] a binder; [0061] a solvent; and optionally
a rheology modifier and/or an organosilver compound and/or an
activator and/or a surfactant and/or wetting agent and/or hydrogen
peroxide or organic peroxides.
[0062] The paste may be printable and/or dispensable and/or
jettable and/or pin transferable. The paste may have viscosity and
flow characteristics particularly favourable for dispensing,
meaning that the paste may be used as a one-to-one replacement for
solders.
[0063] Compared to sintering pastes known in the art, the sintering
paste of the present invention exhibits high stability at room
temperature. This means that low temperature storage of the
sintering paste is not required. This is a particularly important
advantage of the sintering paste of the present invention.
[0064] The binder and/or solvent are typically selected so that
they are able to be removed from the paste (for example by
evaporation and/or burn out) at a temperature below the targeted
sintering temperature of the sintering powder. This may help to
promote near complete sintering of the metal particles. When
organic material remains in the joint during sintering, inadequate
sintering of the metal particles may occur. This may result in a
weak sintered joint.
[0065] The binder may serve to bind the paste together so that it
is easier to handle and position accurately in the location of a
desired sintered joint. Examples of suitable binders include, but
are not restricted to, thermoplastic polymers, such as, for
example, poly(methyl methacrylate), polyamides, polyethylene,
polypropylene, polystyrene; or thermosetting polymers, such as, for
example, polyurethanes, polycyanurates, epoxy resin, polyimides,
melamine resin and bismaleimide resin. Particularly preferred
examples include hydroxypropylmethylcellulose, triacetin and
polyvinyl acetate. Preferably the binder comprises an epoxy-based
resin. Epoxy-based resin may be particularly effective at binding
the paste together so that the paste is easier to handle and may be
easier to position accurately in the location of a desired sintered
joint. Furthermore, the use of epoxy resin may result in the
formation of a stronger joint prior to sintering, meaning that
there is no requirement to hold together the work pieces to be
joined prior to sintering. The use of epoxy resin is particularly
advantageous when the capping agent comprises an amine functional
group. In this case, the amine acts as a hardener forming a
cross-linked structure. This may result in a particularly strong
joint prior to sintering.
[0066] The solvent preferably comprises a monoterpene alcohol
and/or a glycol and/or glycol ether, preferably terpineol and/or
diethylene glycol mono-n-butyl ether. Monoterpene alcohol and/or a
glycol ether may be particularly effective at dispersing the metal
particles within the paste, resulting in a homogeneous distribution
of metal particles in the matrix of organic components with reduced
cluster aggregation and/or agglomeration. The use of monoterpene
alcohol and/or a glycol ether may serve to increase the
flow-ability and printer-ability of the sintering paste.
[0067] A rheology modifier may be added to control the viscosity of
the paste. Examples of suitable rheology modifiers include, but are
not restricted to, Thixcin R, Crayvallac Super, and combinations
thereof.
[0068] During sintering, the organosilver compound may break down
to metallic silver, which may increase the thermal conductivity of
the sintered joint. The organosilver compound may comprise one or
more of short or long chain carboxylic acids (C=1 to 30), such as,
for example, silver stearate, silver palmitate, silver oleate,
silver laurate, silver neodecanoate, silver decanoate, silver
octanoate, silver hexanoate, silver lactate, silver oxalate, silver
citrate, silver acetate and silver succinate. In some embodiments,
the organosilver compound may be omitted.
[0069] An activator may be added to remove any metal oxide that may
be present from the surface being printed and/or to remove any
oxides that may be present in the sintering powder. Aryl or alkyl
carboxylic acids may be used as activators, such as, for example,
one or more of adipic acid, succinic acid and glutaric acid.
[0070] A surfactant may be added to the sintering paste to help
disperse the sintering powder in the sintering paste. Examples of
suitable surfactants include, but are not restricted to, Disperbyk
163, IGEPAL CA-630, lauryl glucoside and TritonX 100.
[0071] The sintering paste preferably further comprises a peroxide.
Examples of suitable peroxides include, but are not restricted to,
hydrogen peroxide or organic peroxides, such as, for example,
tertiary-butyl hydroperoxide and tertiary-butyl
peroxy-2-ethylhexanoate. Peroxide introduces oxygen into the paste,
which may aid sintering of the paste beneath the die area in a die
attach method. The oxygen may also enable sintering of the metal
particles under an inert atmosphere, such as, for example, a
nitrogen atmosphere. The sintering paste preferably comprises up to
3 wt. % hydrogen peroxide or organic peroxides, preferably from 0.5
to 2 wt. % hydrogen peroxide or organic peroxides, more preferably
from 0.7 to 1.8 wt. % hydrogen peroxide or organic peroxides.
Liquid peroxides are preferred to control rheology and silver
settling.
[0072] The sintering paste preferably comprises: [0073] from 1 to
15 wt. % binder; and/or [0074] from 1 to 30 wt. % solvent; and/or
[0075] up to 5 wt. % rheology modifier; and/or [0076] up to 10 wt.
% an organosilver compound; and/or [0077] up to 2 wt. % activator;
and/or [0078] up to 6 wt. % surfactant; and/or [0079] up to 2 wt. %
hydrogen peroxide or organic peroxides.
[0080] Binder and/or solvent contents within these ranges may help
to provide the sintering paste with particularly desirable
flow-ability and printer-ability. Preferably the sintering paste
comprises from 2 to 8 wt. %, binder. In one embodiment the
sintering paste comprises about 4.5 wt. % binder. Preferably the
sintering paste comprises from 5 to 30 wt. %, solvent. In one
embodiment the sintering paste comprises about 26 wt. % solvent.
The sintering paste may comprise 0 to 5 wt. % rheology modifier
and/or 0 to 2 wt. % activator and/or 0 to 6 wt. % surfactant and/or
0 to 2 hydrogen peroxide or organic peroxides. The sintering paste
may comprise from 62 to 90 wt. % sintering powder. The sintering
powder may form the balance of the sintering paste.
[0081] In a further aspect the present invention provides a
sintering paste comprising: [0082] the sintering powder as
disclosed herein; [0083] an organosilver compound; [0084] a
solvent; and [0085] optionally an activator and/or rheology
modifier and/or surfactant and/or hydrogen peroxide or organic
peroxides.
[0086] During sintering, the organosilver compound may break down
to metallic silver, which may increase the thermal conductivity of
the sintered joint. The organosilver compound may comprise one or
more of short or long chain carboxylic acids (C=1 to 30), such as,
for example, silver stearate, silver palmitate, silver oleate,
silver laurate, silver neodecanoate, silver decanoate, silver
octanoate, silver hexanoate, silver lactate, silver oxalate, silver
citrate, silver acetate and silver succinate. In some embodiments,
the organosilver compound may be omitted.
[0087] The sintering paste preferably further comprises a fatty
acid, preferably one or more of: short or long chain (C=2 to 30)
carboxylic acids or di-carboxylic acids or hydroxyl carboxylic
acids, more preferably lauric acid, stearic acid, neodecanoic acid,
stearic acid, oleic acid, oxalic acid, malonic acid, succinic acid,
adipic acid, maleic acid, citric acid or lactic acid; or short or
long chain (C=2 to 30) amines, more preferably butyl amine, hexyl
amine, octyl amine, dodecyl amine or hexadecyl amine; or
surfactants, more preferably triton X100, IGEPAL CA-630 or lauryl
glucoside. The presence of fatty acids helps to bind the paste
together. In other words, the presence of a fatty acid avoids the
need for a separate binder, such as the epoxy based resin binder
discussed above.
[0088] Accordingly, the total amount of organics in the paste is
less, resulting in a stronger final joint.
[0089] The sintering paste preferably further comprises a peroxide.
Examples of suitable peroxides include, but are not restricted to,
hydrogen peroxide or organic peroxides, such as, for example,
tertiary-butyl hydroperoxide and tertiary-butyl
peroxy-2-ethylhexanoate. Peroxide introduces oxygen into the paste,
which may aid sintering of the paste beneath the die area in a die
attach method. The oxygen may also enable sintering of the metal
particles under an inert atmosphere, such as, for example, a
nitrogen atmosphere. The sintering paste preferably comprises up to
3 wt. % hydrogen peroxide or organic peroxides, preferably from 0.5
to 2 wt. % hydrogen peroxide or organic peroxides, more preferably
from 0.7 to 1.8 wt. % hydrogen peroxide or organic peroxides.
Liquid peroxides are preferred to control rheology and silver
settling.
[0090] Preferably the sintering paste is substantially resin free,
more preferably completely resin free. The presence of resin may
reduce the thermal and electrical conductance of the silver. The
solvent preferably comprises a monoterpene alcohol and/or a glycol
and/or glycol ether, more preferably a terpineol and/or diethylene
glycol mono-n-butyl ether.
[0091] The sintering paste preferably comprises: [0092] from 1 to
30 wt. % solvent; and/or [0093] up to 50 wt. % organosilver
compound, preferably from 0.1 to 25 wt. %, more preferably from 0.1
to 10 wt. %, even more preferably from 0.1 to 9 wt. %; and/or
[0094] up to 5 wt. % rheology modifier; and/or [0095] up to 2 wt. %
activator; and/or [0096] up to 6 wt. % surfactant; and/or [0097] up
to 2 wt. % hydrogen peroxide or organic peroxides.
[0098] The sintering paste may comprise 0 to 5 wt. % rheology
modifier and/or 0 to 2 wt. % activator and/or 0 to 6 wt. %
surfactant and/or 0 to 2 wt. % hydrogen peroxide or organic
peroxides. The sintering powder may form the balance of the
sintering paste.
[0099] In a further aspect the present invention provides a
sintering film comprising the sintering powder as described herein
and a binder. The film may be applied at the wafer level, die
level, package/substrate level, and/or module level. Such a film
may be obtained, for example, by printing the sintering paste as
described herein onto a polyester sheet, heating the paste to at
least partially remove the solvent and form a film, and then
removing the film from the polyester sheet. The film as described
herein is especially advantageous since it can be transferred on
the die by simply pressing the die on to the film at slightly
elevated temperature. Transferred film is an alternate application
method, beneficially offered in certain situations. The film may be
formed on a polymeric, glass, metal or ceramic substrate or
directly on a wafer. The film may be on a polymeric substrate
comprising polyester. The film may be formed on a polymeric
substrate, wherein the polymeric substrate comprises a release
coating. The film may be produced by applying the paste
compositions by printing or casting of the material. The film may
be produced by printing in a continuous layer. Alternatively, the
film may be produced by printing to form an array of discrete
shapes.
[0100] In a further aspect the present invention provides a method
of die attachment comprising: [0101] (i) placing the sintering film
described herein between a die and a substrate to be joined; and
[0102] (ii) sintering the sintering film, wherein the sintering is
carried out without the application of pressure.
[0103] This "low pressure" or "pressure-less" sintering is
particularly advantageous, since it may make automation of the
process simpler. Furthermore, damage to the work pieces may be
reduced. Further advantages over methods employing pressured
sintering include: shorter time required for die-placement (high
UPH), low-pressure requirement for placement (highly advantageous
for processing thin wafers), compatibility with commercial
die-bonder and sintering in external heating equipment (batch
process to improve UPH).
[0104] The sintering is preferably carried out at a temperature of
from 150 to 400.degree. C. for up to 120 minutes. Such conditions
may result in particularly effective sintering of the sintering
film while avoiding damage to the work pieces.
[0105] Step (i) preferably comprises: [0106] (a) applying the
sintering film to the die to form an assembly having a die side and
a sintering film side; and [0107] (b) contacting the film side of
the assembly with the substrate.
[0108] Such a step may make automation of the process simpler, and
may be carried out, for example, by the use of a stamp.
[0109] Step (a) is preferably carried out at a temperature of from
15 to 400.degree. C. and a pressure of from 0.1 to 5 MPa for from
0.1 to 60 seconds. Such conditions may result in particularly
effective application of the sintering film while avoiding damage
to the die.
[0110] Step (b) is preferably carried out at a temperature of from
15 to 400.degree. C. and a pressure of from 0.1 to 40 MPa for from
0.1 to 60 minutes. Such conditions may result in particularly
effective contacting of the die to the substrate while avoiding
damage to the die or substrate.
[0111] In a further aspect, the present invention provides a method
of die attachment comprising: [0112] (i) placing the sintering film
described herein between a die and a substrate to be joined; and
[0113] (ii) sintering the sintering film, wherein the sintering is
carried out while applying a pressure of from 0.1 to 40 MPa.
[0114] In a further aspect, the present invention provides a method
of wafer bonding comprising: [0115] (i) placing the sintering film
described herein between two or more wafers to be joined; and
[0116] (ii) sintering the sintering film, wherein the sintering is
carried out without the application of pressure.
[0117] In a further aspect, the present invention provides a method
of transferring a sintering film to a component, comprising: [0118]
applying the sintering film described herein to a substrate to form
an assembly having a sintering film side and a substrate side;
[0119] contacting the sintering film side of the assembly with a
component; [0120] heating the assembly to a temperature of from 50
to 200.degree. C., [0121] applying a pressure of from 1 to 5 MPa to
the assembly for from 0.1 seconds to 60 minutes; and [0122]
separating the substrate from the sintering film.
[0123] The substrate may be polymeric. The sintering film may be
substantially the same size as the component. The component may be
an LED.
[0124] In a further aspect the present invention provides a method
for die-attachment, comprising: applying the sintering film
described herein to a substrate; placing a die on the film to form
an assembly; applying a pressure of less than 2 MPa to the
assembly; and sintering the assembly at a temperature of 100 to
400.degree. C. for 0.1s to 5 minutes, applying a pressure of less
than 3 MPa. The same assembly may be further sintered at a
temperature of 175 to 400.degree. C. in a pressure-less manner
using variety of processes and equipment that provide appropriate
degree of heat to initiate and complete sintering.
[0125] In a further aspect the present invention provides a method
for die-attachment, comprising: applying the sintering film
described herein to a substrate; placing a die on the film to form
an assembly; applying a pressure of less than 5 MPa to the
assembly; and sintering the assembly at a temperature of 100 to
400.degree. C. for 0.1s to 60 minutes, applying a pressure of less
than 40 MPa. The same assembly may be further sintered at a
temperature of 175 to 400.degree. C. in a pressure-less manner
using variety of processes and equipment that provide appropriate
degree of heat to initiate and complete sintering.
[0126] In a further aspect the present invention provides a method
for die-attachment, comprising: applying the sintering film
described herein on a back side of a wafer; dicing the wafer to
form a plurality of die; placing at least one die on a substrate to
form an assembly; applying a pressure of more than 1 MPa to the
assembly; and sintering the assembly at a temperature of 100 to
400.degree. C. for 0.1 s to 60 minutes. The same assembly may be
further sintered at a temperature of 175 to 400.degree. C. in a
pressure-less manner using variety of processes and equipment that
provide appropriate degree of heat to initiate and complete
sintering.
[0127] In a further aspect the present invention provides a method
for wafer bonding, comprising: applying the sintering film
described herein on a back side of a wafer; placing one more same
or different types of wafer on the sinterable Ag film containing
wafer to form an assembly; applying a pressure of more than >0.1
MPa to the assembly; and sintering the assembly at a temperature of
100 400.degree. C. for 0.25s to 120 minutes. The same assembly may
be further sintered at a temperature of 175 to 400.degree. C. in a
pressure-less manner using variety of processes and equipment that
provide appropriate degree of heat to initiate and complete
sintering.
[0128] In a further aspect the present invention provides a method
for wafer bonding, comprising: applying the sintering film on a
back side of a wafer; placing one more same or different types of
wafer on the sintering film containing wafer to form an assembly;
applying a pressure of less than 40 MPa to the assembly; and
sintering the assembly at a temperature of 100 to 400.degree. C.
for 0.25 s to 120 minutes. The same assembly may be further
sintered at a temperature of 175 to 400.degree. C. in a
pressure-less manner using variety of processes and equipment that
provide appropriate degree of heat to initiate and complete
sintering.
[0129] In a further aspect the present invention provides the use
of the sintering powder as described herein or the sintering paste
or film as described herein in a method selected from: die
attachment (e.g. chip-to-board, chip-to-substrate, chip-to-heat
sink, chip-to-fixture), wafer-to-wafer bonding (e.g. chip-to-heat
sink), reflective layer printing, hermetic and near hermetic
sealing (for example for packages and perimeter seals), the
production of interconnect lines (for example circuitry, pads), via
filling in semiconductor devices and substrates, and flip-chip and
wafer bumping.
[0130] In a further aspect, the present invention provides a method
of manufacturing the sintering powder as described herein, the
method comprising: [0131] (i) providing a solution comprising a
reducing agent and particles containing a first material; [0132]
(ii) reducing the concentration of the reducing agent in the
solution from step (i); [0133] (iii) contacting the solution from
step (ii) with a source of a second material to at least partially
coat at least some of the particles containing a first material
with the second material, the second material having a lower
oxidation potential than the first material; and [0134] (iv)
recovering at least some of the particles containing a first
material coated with the second material from the solution of step
(iii).
[0135] The solution of step (i) may be an aqueous solution. When
the second material comprises a metal, the second material source
may comprise a metal salt, for example an acetate and/or
nitrate.
[0136] The particles containing the first material are typically
nanoparticles. The term "nanoparticles" as used herein may
encompass particles having a longest dimension of from 1 to 100 nm.
The longest dimension may be determined by X-ray diffraction.
[0137] Without being bound by theory, it is considered that the
formation of the "shell-coated" particles is driven by a
transmetallation reaction. The reduction of the source of second
material (e.g. silver nitrate) occurs on the surface of the
pre-formed particles containing the first material (e.g. copper),
where the first material itself acts as a reducing agent for the
second material source. Excess of strong reducing agent (e.g.
hydrazine or borohydride) is used to prevent the first material
(e.g. copper) from being oxidised. From 0.5 to 1 mole of reducing
agent is typically used. However, if a large excess of reducing
agent remains, then simultaneous reduction of the second material
source may occur, which may disrupt the formation of the core-shell
structure.
[0138] Step (ii) may comprise removal of the reducing agent from
the solution. Alternatively, or in addition, step (ii) may comprise
chemical modification and/or deactivation of the reducing agent. In
step (ii), the concentration of reducing agent is preferably
reduced to substantially zero, more preferably zero. The reducing
agent may be destroyed in step (ii), for example using a chemical
method.
[0139] Step (i) may comprise: [0140] providing a solution
comprising a source of first material; and [0141] contacting the
solution with a reducing agent.
[0142] Contacting the solution with a reducing agent typically
comprises adding the reducing agent to the solution followed by
stirring the solution. The solution may be agitated, for example
using sonication, so as to increase the yield of particle formation
and to reduce agglomeration of the particles. When the first
material comprises a metal, the first material source may comprise
a metal salt. The solution may be heated once the reducing agent
has been added. Step (i) may be advantageously carried out without
irradiation of the solution of the source of the first material and
the reducing agent.
[0143] In step (iii), the particles containing a first material
coated with the second material are typically produced in powder
form. Accordingly, in contrast to methods in which the particles
are produced in a dispersed state, separation of the particles does
advantageously not require acidification of the dispersion.
[0144] Preferably, the first material comprises copper and the
second material comprises silver. Copper is particularly effective
at reducing the silver source without disrupting the formation of
the core-shell structure.
[0145] The reducing agent preferably comprises hydrazine (e.g.
hydrazine hydrate) and/or borohydride (e.g. sodium
borohydride).
[0146] In a preferred embodiment: [0147] the reducing agent
comprises hydrazine; and [0148] step (ii) comprises contacting the
reducing agent with acetone under basic conditions.
[0149] Excess hydrazine is destroyed by the addition of acetone in
basic medium. The excess hydrazone reacts with acetone in basic
medium to form the corresponding alkane. This works on the
principle of Wolf-Kishner reaction. In this embodiment, it is
preferred that the first material comprises copper and the second
material comprises silver. The basic conditions may be provided by,
for example, hydroxide ions.
[0150] In a preferred embodiment: [0151] the first material
comprises copper; [0152] the second material comprises silver; and
[0153] the weight ratio of silver to copper in step (iii) is from
40:60 to 85:15.
[0154] Higher levels of copper may result in the silver shell not
adequately coating the copper particles. Lower levels of copper may
result in increased manufacturing costs.
[0155] The solution of step (i) preferably comprises capping agent.
This may result in the particles being at least partially coated
with a capping agent.
[0156] Step (iv) may comprise filtration and/or centrifugation. The
recovered particles may be washed, for example with water and/or
acetone.
[0157] In a further aspect, the present invention provides a method
of manufacturing a sintering powder comprising copper-containing
nanoparticles coated with a layer comprising silver, the method
comprising: [0158] (i) providing a solution comprising a hydrazine
reducing agent and copper-containing nanoparticles; [0159] (ii)
contacting the solution from step (i) with acetone under basic
conditions to reduce the concentration of the hydrazine reducing
agent in the solution; [0160] (iii) contacting the solution from
step (ii) with a silver salt to at least partially coat at least
some of the copper-containing nanoparticles with a layer comprising
silver; and [0161] (iv) recovering at least some of the
copper-containing nanoparticles coated with the layer comprising
silver from the solution of step (iii).
[0162] In step (ii), the concentration of the reducing agent is
preferably reduced to substantially zero, more preferably zero. The
excess reducing agent, typically hydrazine, is typically destroyed
in step (ii). Step (ii) may comprise contacting the solution from
step (i) with acetone and hydroxide ions.
[0163] In a further aspect, the present invention provides a method
of manufacturing the sintering paste as described herein
comprising: [0164] providing the sintering powder as described
herein; and [0165] dispersing the sintering powder in a solvent
together with a binder and optionally a rheology modifier and/or an
organosilver compound and/or an activator and/or a surfactant
and/or wetting agent and/or hydrogen peroxide or organic
peroxides.
[0166] In a further aspect the present invention provides a method
of manufacturing a sintered joint comprising the steps: [0167]
providing the sintering powder as described herein or the sintering
paste or film as described herein in the vicinity of two or more
work pieces to be joined; and [0168] heating the sintering powder
or sintering paste or film to at least partially sinter the
metal.
[0169] Advantageously, the heating step may be carried out at
atmospheric pressure. The sintering powder or sintering paste or
film may be placed in the vicinity of the work piece under low
pressure (typically 1-5 MPa for 0.1 to 60 seconds at a temperature
of about 175 to 250.degree. C.).
[0170] The heating step is preferably carried out at a temperature
of at least 140.degree. C., more preferably from 150 to 350.degree.
C., even more preferably from 160 to 300.degree. C. Temperatures
lower than 140.degree. C. may not result in adequate sintering of
the particles in the sintering powder and/or may not result in
adequate removal of the organics by evaporation and/or burn out.
Temperatures higher than 350.degree. C. may result in damage to the
work pieces.
[0171] The invention will now be described in relation to the
following non-limiting examples.
EXAMPLE 1--Synthesis and Characterization of CoSe shell Ad--Cu
Nanoparticles
[0172] An aqueous solution of cupric acetate (154 g, 0.7713 m) was
added to an aqueous solution of sodium oleate (30 g, 0.098 m) with
continuous stirring. Hydrazine hydrate (300 g, 6 m) was added to
the above solution. The resultant reaction mixture was stirred for
45 minutes followed by the addition of sodium hydroxide and
acetone. The reaction mixture was then allowed to stir for 10
minutes. Aqueous solution of silver nitrate (70.67 g, 0.416 m) was
then added to the reaction mixture and stirred for 1 hour.
Formation of black color particle indicated the formation of
nanoparticle. The resultant nanopowder was then filtered and washed
with water and acetone.
[0173] In a similar fashion, different compositions of core shell
structure were synthesized by changing the mole ratio of the silver
and copper salt. Different composition of silver to copper
core-shell structured nanoparticles were made by changing the molar
concentration of the copper and silver precursors.
[0174] The XRD and SPR patterns confirm the structure to be "core
shell". The XRD as shown in FIG. 1 reveals the presence of both
copper and silver having a face centered cubic structure. The XRD
pattern does not show the presence of any cupric/cuprous oxide
peak. The particle size from the XRD pattern calculated by
Scherrer's formula was around 20-25 nm. This particle size was
confirmed by the use of a particle size analyzer (Microtrac
Nanotrac Ultra NPA 253) which indicated a D50 of around 20 nm. The
percentage of copper to silver was further confirmed by XRF
analysis.
[0175] The UV-vis spectrum (FIG. 2) shows the surface plasmon
resonance at 456 nm which is typical for silver which in turn
confirms the fact that silver is the shell and copper is the
core.
[0176] The TGA (FIG. 3) shows a two-step decomposition process of
the capping agent. A sharp drop is observed at 187.degree. C. with
a weight loss of around 5%. The second step involves a weight loss
of around 8% at 190.degree. C. The TGA graph clearly indicates that
no oxidation of copper takes place till 300.degree. C. A slight
weight gain in the TGA curve above 300.degree. C. reveals the
oxidation of the copper.
[0177] The DSC (FIG. 4) shows two exothermic peaks with an onset
temperature of 197.degree. C. and 210.degree. C. Both of the
exothermic peaks indicate the detachment of the capping agent from
the core shell and decomposition of the capping agent.
[0178] TEM micrographs (FIG. 5) indicate particles sizes of the
nanoparticles of from 1 nm to 25 nm. The particles appear to be
spherical in shape and do not show any agglomeration.
EXAMPLE 2--Synthesis and Characterization of Ad--Ni
Nanoparticle
[0179] An ethanol solution of nickel chloride (50 g, 0.2103 m) was
added to octylamine (10 g, 0.07 moles). The reaction mixture was
allowed to stir for 10 minute followed by the addition of NaOH and
hydrazine hydrate (250 g, 5 moles). The resultant reaction mixture
was then heated at 50-60.degree. C. for 45 minutes. The reaction
mixture was then cooled to room temperature. Glycolic solution of
silver nitrate (40 g, 0.23 moles) was then added to the reaction
mixture and stirred for 1 hour. The resultant nanopowder was then
filtered and washed with ethanol and acetone.
[0180] The XRD and SPR patterns confirm the structure to be core
shell. The XRD as shown in FIG. 6 reveals the presence of both
nickel and silver having a face centered cubic structure. The
particle size from the XRD pattern calculated by Scherrer's formula
was around 10-55 nm. This particle size was confirmed by the use of
a particle size analyzer (Microtrac Nanotrac Ultra NPA 253) which
indicated a D50 of around 12 nm. The percentage of nickel to silver
was 36:64 which was in conformity with XRF analysis.
[0181] TEM images of the silver nickel nanoparticles (FIG. 8)
reveal the formation of nanoparticles ranging from a size of 3 to
35 nm. The lattice fringes show the nature to be
polycrystalline.
EXAMPLE 3--Synthesis of nano Cu.sub.coreAq.sub.shell Paste
[0182] Sample 1: 0.1 g of lauric acid was added to 40 g of the
powder of Example 1. To the mixture, 9.463 g of solvent mixture
(4.69 g of terpineol, 4.69 g of triethylene glycol and 0.83 g of
propylene carbonate) was added and mixed in an orbital mixer. After
mixing it was milled in a three roll mill for a few minutes to
provide a homogeneous paste.
[0183] Sample 2: 0.1 g of lauric acid was added to 40 g of the
powder of Example 1. To the mixture, 0.956g of silver lactate and
9.257 g of solvent mixture (4.377 g of terpineol, 4.377 g of
triethylene glycol and 0.503g of propylene carbonate) was added and
mixed in an orbital mixer. After mixing it was milled in a three
roll mill for a few minutes to provide a homogeneous paste
[0184] Sample 3: To 40 g of the powder of Example 1, 0.264 g of
thixcin R, 5.864 g of terpineol, 5.864 g of triethylene glycol,
0.5336 g of 1,3 propanediol and 0.5336g of butyl carbitol and 0.264
g of BYK 163 was added and mixed in an orbital mixer. After mixing
it was milled in a three roll mill for a few minutes to provide a
homogeneous paste.
[0185] Sample 4: To 40 g of the powder of Example of 1, 0.264 g of
thixcin R, 5.064 g of terpineol, 5.864 g of triethylene glycol,
0.528 g of 1,3 propanediol and 0.528 g of butyl carbitol and 1.064
g of myristic acid was added and mixed in an orbital mixer. After
mixing it was milled in a three roll mill for a few minutes to
provide a homogeneous paste.
[0186] Sample 5: To 40 g of the powder of Example 1, 2.664 g of
acrylic acid, 5.328 g of octanol, 5.064 g of octadecene, 0.264 g of
thixcin R was added and mixed in an orbital mixer. After mixing it
was milled in a three roll mill for a few minutes to provide a
homogeneous paste.
[0187] Sample 6: To 40 g of the powder of Example 1, 0.2664 g of
thixcin R and 13.064 g of triethylene glycol was added and mixed in
an orbital mixer. After mixing it was milled in a three roll mill
for a few minutes to provide a homogeneous paste.
[0188] Sample 7: To 40 g of the powder of Example 1, 2.664 g of
acrylic acid, 5.328 g of octanol, 5.064 g of octadecene, 0.264 g of
thixcin R was added and mixed in an orbital mixer. After mixing it
was milled in a three roll mill for a few minutes to provide a
homogeneous paste.
[0189] Sample 8: To 40 g of the powder of Example 1, 0.2856 g of
thixcin R, 0.5713 g of polydimethyl siloxane diglycidyl ether
terminated, 14.6552 g of triethylene glycol, 1.628 g of terpineol
was added and mixed in an orbital mixer. After mixing it was milled
in a three roll mill for a few minutes to provide a homogeneous
paste.
[0190] Sample 9: To 40 g of the powder of Example 1, 0.2856 g of
thixcin R, 0.5713 g of silver oxalate and 16.28 g of triethylene
glycol was added and mixed in an orbital mixer. After mixing it was
milled in a three roll mill for a few minutes to provide a
homogeneous paste.
[0191] Sample 10: To 40 g of the powder of Example 1, 0.5333 g of
glutamic acid, 0.5333 g of 1,3 propanediol, 0.5333 g of butyl
carbitol, 5.7329 g of terpineol, 5.7329 g of triethylene glycol,
and 0.266 g of BYK163 was added and mixed in an orbital mixer.
After mixing it was milled in a three roll mill for a few minutes
to provide a homogeneous paste.
[0192] Sample 11: To 40 g of the powder of Example 1, 1.013 g of
silver lactate, 0.1066 g of Thixcin R, 0.1066 g of Lauric acid,
0.5333 g of propylene carbonate,5.786 g of terpineol and 5.786 g of
trigol was added and mixed in an orbital mixer at 1000 rpm. After
mixing it was milled in a three roll mill for a few minutes to
provide a homogeneous paste.
[0193] Sample 12: To 40 g of the powder of Example 1, 3.8604 of
nano silver powder, 0.1103 g of Lauric acid, 0.9104 g of propylene
carbonate, 5.144 g of terpineol and 5.144 g of trigol was added and
mixed in an orbital mixer at 1000 rpm. After mixing it was milled
in a three roll mill for a few minutes to provide a homogeneous
paste.
[0194] Sample 13: To 40 g of the powder of Example 1, 1.588 g of
nano silver powder, 0.5296 g of silver lactate, 0.1059 g of Lauric
acid, 0.874 g of propylene carbonate,4.94 g of terpineol and 4.94 g
of trigol was added and mixed in an orbital mixer at 1000 rpm.
After mixing it was milled in a three roll mill for a few minutes
to provide a homogeneous paste.
[0195] Sample 14: To 40 g of the powder of Example 1, 1.056 g of
nano silver powder, 1.056 of silver lactate, 0.1056 g of Lauric
acid, 0.6616 g of propylene carbonate,4.936 g of terpineol ,4.936 g
of trigol and 0.21112 g of BYK163 was added and mixed in an orbital
mixer at 1000 rpm. After mixing it was milled in a three roll mill
for a few minutes to provide a homogeneous paste.
[0196] Sample 15: To 40 g of the powder of Example 1, 3.92 g of
terpineol, 9.78 g of triethylene glycol, and 1.10 g of sodium bis
(2-ethylhexyl)sulfosuccinate was added and mixed in an orbital
mixer at 1000 rpm. After mixing it was milled in a three roll mill
for a few minutes to provide a homogeneous paste.
EXAMPLE 4--Printing
[0197] Screen/stencil printing is a widely used technology for
printed electronics and metallization of solar cells. The technique
relies on the pattern transfer from the stencil to the substrate.
AgCu, AgNi and AgMo core-shell powder-containing pastes accordingly
to the present invention were printed using a DEK printer on direct
bond copper (DBC) coated with Ni/Ag or Ni/Au using a 3 mil stencil.
The microscopic images of the printed pattern as shown in FIG. 9
and indicate good printability of the Cu.sub.core Ag.sub.shell nano
paste. The printed patterns show no undulations with a thickness of
around 75 microns.
EXAMPLE 5--Pressure Sintering and Die Attachment
[0198] To the printed pattern on direct bond copper (DBC) coated
with Ag/Ni or Au/Ni, dummy silicon die coated with Ag/Ni or Au/Ni
was placed using a Tresky die bonder with different pressure. The
SEM images as shown in FIG. 10 reveals a good densification of the
sintered layer. Both the interfaces (die side as well as the
substrate side) show diffusion of the sintered layer.
[0199] The joint strength was determined using Dage series 4000.
The shear strength was >40 MPa with a pressure ranging from 3-5
MPa. All the failures were cohesive failure. The die shear strength
with different pressure is listed below in Table 1.
TABLE-US-00001 TABLE 1 Die shear strengths for different sintering
pressures. Sintering Die shear pressure in MPa 5 >40 3 >40 2
>40 1 25-27 0.5 23-25
EXAMPLE 6--Pressure-Less Sintering and Die Attachment
[0200] To the printed pattern on direct bond copper (DBC) coated
with Ag/Ni or Au/Ni, dummy silicon die coated with Ag/Ni or Au/Ni
was placed using a die bonder. The whole was subjected to a firing
temperature of 200.degree. C.-300.degree. C. for 45-90 minutes. The
morphology of the sintered joints was cross sectioned. The
microstructure was analyzed using SEM. The SEM images as shown in
FIG. 11 reveal a good densification of the sintered layer. Both the
interfaces (die side as well as the substrate side) show diffusion
of the sintered layer. The joint strength was determined using Dage
series 4000. The die shear when done at room temperature as well as
high temperature does not show any deterioration (20-30 MPa).
EXAMPLE 7--Dispensing Application
[0201] AgCu, AgNi and AgMo particle-containing pastes according to
the present invention were dispensed using a Datacon die bonder on
direct bond copper (DBC) coated with Ni/Ag or Ni/Au and Ni/Ag or
Ni/Au lead frames. The microscopic images of the dispensed pattern
are shown in FIG. 12. The assembly was then sintered at 300.degree.
C. for 45 minutes in a box oven. The die shear obtained was around
16 MPa.
EXAMPLE 8--Film Formation
[0202] A film of around 10-50 micron was produced using a tape
caster (AgCu particle containing paste). The film was stamped at
150.degree. C. with a pressure of 2.5 MPa for 1 second. A good
transferability of film from the Mylar sheet to the die (Au/Ni
finish) was observed with no flares and cracks in the film. The
stamped die was then placed on DBC (Au/Ni finish) at 300.degree. C.
under 3 MPa and 1 MPa pressure for 90 seconds. A good die shear of
around 55 MPa and 30 MPa respectively were observed. The failure
mode was a bulk failure. One of the most promising result was the
increase in joint strength of around 25% was observed when the die
shear was done at high temperature (260.degree. C.).
EXAMPLE 9--Flip Chip Attachment
[0203] The copper core silver shell paste film (Example 3) was
explored for flip chip type of attachment. In this process, film
was casted on a Mylar sheet with the help of a Tape caster. The
casted film was then stamped on silicon wafer using a carver press.
The stamped silicon wafer was then diced using a dicing machine
followed by the UV curing. The diced wafer was then attached to the
substrate using Datacon Die bonder. The process steps were as
follows: (i) film formation, (ii) film transfer to Si wafer, (iii)
dicing of coated Si wafer, (iv) die attachment using die bonder,
(v) pressure-less sintering, and (vi) die shear and
characterisation. Further details and conditions of these steps are
set out below:
[0204] Film Casting and Stamping Conditions:
[0205] The film of copper silver paste was casted with the help of
a tape caster on Mylar sheet by rolling the paste on to the Mylar
sheet at 150.degree. C. for 15 minutes. The casted film was then
transferred to the silicon die using a Carver press where the upper
plate was set at 130.degree. C. and the low plate was set to
50.degree. C. Pressure of 10 MPa was applied for 2 minutes. The
stamped silicon wafer was then. The thickness of the film was found
to be around 25 micron.
[0206] Die Attachment and Characterization:
[0207] The diced silicon wafer was then attached to the Au finished
DBC with Datacon Die bonder. The tool temperature of 150.degree. C.
and a base plate temperature of 225.degree. C. were set and a
pressure of 0.1 MPa was applied for 1 second. The assembly was then
post cured at 225.degree. C. for 1 hour. The CSAM image of the
sintered assembly did not show any kind of delamination or
voids.
[0208] The cross section the SEM of the above mentioned assembly
showed a good diffusion of the nanoparticle on both the substrate
side and on the die. The dies were sheared and the joint strength
was found to be around 30 MPa with a bulk failure mode.
[0209] Thermal Reliability:
[0210] The assembled samples were tested for the thermal
reliability test which includes thermal shock and thermal cycling
(-40.degree. C. to 125.degree. C.) for 1000 cycles. It was found
that no deterioration was observed in joint strength as well as no
delamination at the interfaces.
[0211] The foregoing detailed description has been provided by way
of explanation and illustration, and is not intended to limit the
scope of the appended claims. Many variations in the presently
preferred embodiments illustrated herein will be apparent to one of
ordinary skill in the art and remain within the scope of the
appended claims and their equivalents.
* * * * *