U.S. patent application number 13/227566 was filed with the patent office on 2011-12-29 for silver-coated ball and method for manufacturing same.
This patent application is currently assigned to NEOMAX MATERIALS CO., LTD.. Invention is credited to Ken ASADA, Fumiaki KIKUI.
Application Number | 20110318484 13/227566 |
Document ID | / |
Family ID | 37451952 |
Filed Date | 2011-12-29 |
United States Patent
Application |
20110318484 |
Kind Code |
A1 |
ASADA; Ken ; et al. |
December 29, 2011 |
SILVER-COATED BALL AND METHOD FOR MANUFACTURING SAME
Abstract
A silver-coated ball 10 according to the present invention
includes: a spherical core 1; and a coating layer 2 including
silver superfine particles, which is arranged so as to surround the
core 1. The silver superfine particles included in the coating
layer 2 have a mean particle size of 1 nm to 50 nm.
Inventors: |
ASADA; Ken; (Osaka, JP)
; KIKUI; Fumiaki; (Osaka, JP) |
Assignee: |
NEOMAX MATERIALS CO., LTD.
Osaka
JP
|
Family ID: |
37451952 |
Appl. No.: |
13/227566 |
Filed: |
September 8, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11915659 |
Nov 27, 2007 |
8039107 |
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PCT/JP2006/310227 |
May 23, 2006 |
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13227566 |
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Current U.S.
Class: |
427/212 ;
977/773 |
Current CPC
Class: |
H01L 2924/01079
20130101; H01L 2224/1357 20130101; H01L 2224/13639 20130101; B23K
35/3006 20130101; H01L 2224/48091 20130101; H01L 2924/01006
20130101; H01L 2924/181 20130101; H01L 2224/48227 20130101; B22F
1/02 20130101; H01L 2224/13083 20130101; H01L 2924/01004 20130101;
H01L 2924/01075 20130101; H05K 2201/0257 20130101; H01L 2224/1308
20130101; Y10T 428/2991 20150115; H01L 23/49816 20130101; H01L
2224/13144 20130101; H01L 2924/15182 20130101; H01L 2224/13155
20130101; H01L 2924/01016 20130101; H01L 2924/01029 20130101; H01L
21/4853 20130101; H01L 2924/014 20130101; H01L 2924/00014 20130101;
H01L 2924/01013 20130101; H01L 2924/01005 20130101; H01L 2924/01027
20130101; H01L 24/10 20130101; Y02P 70/50 20151101; H01L 2924/15184
20130101; H01L 2924/01047 20130101; H01L 2924/15311 20130101; Y10T
428/12181 20150115; Y10T 428/2998 20150115; H01L 2924/01033
20130101; H01L 2924/01082 20130101; B23K 35/40 20130101; H05K
2201/0215 20130101; H01L 24/48 20130101; H05K 2201/10234 20130101;
Y02P 70/613 20151101; H01L 2924/00013 20130101; H01L 24/13
20130101; H01L 2224/13147 20130101; B23K 35/02 20130101; H01L
2924/01012 20130101; H01L 2924/01078 20130101; H05K 3/3463
20130101; B23K 35/0244 20130101; H01L 2224/13084 20130101; H01L
2224/131 20130101; H05K 3/3436 20130101; H01L 2224/13 20130101;
H01L 2224/48091 20130101; H01L 2924/00014 20130101; H01L 2224/13144
20130101; H01L 2924/00014 20130101; H01L 2224/13147 20130101; H01L
2924/00014 20130101; H01L 2224/13155 20130101; H01L 2924/00014
20130101; H01L 2224/13639 20130101; H01L 2924/00014 20130101; H01L
2224/131 20130101; H01L 2924/014 20130101; H01L 2224/1308 20130101;
H01L 2224/131 20130101; H01L 2924/014 20130101; H01L 2924/00013
20130101; H01L 2224/13099 20130101; H01L 2224/13 20130101; H01L
2924/00 20130101; H01L 2924/181 20130101; H01L 2924/00012 20130101;
H01L 2924/00014 20130101; H01L 2224/45099 20130101; H01L 2924/00014
20130101; H01L 2224/45015 20130101; H01L 2924/207 20130101 |
Class at
Publication: |
427/212 ;
977/773 |
International
Class: |
B05D 7/00 20060101
B05D007/00; B05D 3/00 20060101 B05D003/00; B05D 1/18 20060101
B05D001/18 |
Foreign Application Data
Date |
Code |
Application Number |
May 27, 2005 |
JP |
2005-156226 |
Claims
1. A method of making a silver-coated ball, the method comprising
the steps of: providing a dispersion including a spherical core,
silver superfine particles, and a solvent; forming a film of the
dispersion on the surface of the core; and removing the solvent of
the dispersion from the film of the dispersion, thereby forming a
coating layer, including the silver superfine particles, on the
surface of the core, wherein the silver superfine particles have a
mean particle size of 1 nm to 50 nm, and wherein the solvent
includes a nonpolar hydrocarbon solvent, and wherein the ratio in
mass percentage of the silver superfine particles to the solvent is
40-85 mass % to 15-60 mass %.
2. The method of claim 1, wherein the step of forming the film of
the dispersion on the surface of the core includes immersing the
core in the dispersion.
3. The method of claim 1, wherein the step of forming the coating
layer including the silver superfine particles includes putting the
ball, on which the film of the dispersion has been formed, on a
slope and rolling the ball on the slope.
4. The method of claim 1, wherein the solvent includes a solvent
with a boiling point higher than about 100.degree. C. and a solvent
with a boiling point that is equal to or lower than about
100.degree. C.
5. The method of claim 1, wherein the nonpolar hydrocarbon solvent
includes xylene.
Description
TECHNICAL FIELD
[0001] The present invention relates to a silver-coated ball, and
more particularly relates to a silver-coated ball in which the
surface of the core is covered with a coating layer including
silver superfine particles with a mean particle size of 1 nm to 50
nm.
BACKGROUND ART
[0002] Solder balls are mainly used to connect a number of
electric/electronic components together. Specifically, solder balls
are used as input/output terminals for a quad flatpack package
(QFP) with lead terminals around its components and semiconductor
packages such as a ball grid array (BGA) and a chip size package
(CSP), which are relatively small in size and which can cope with
multiple-pin applications.
[0003] FIGS. 10(a) and 10(b) are respectively a perspective view
and a cross-sectional view of a BGA that uses solder balls. As
shown in FIGS. 10(a) and 10(b), a BGA is an LSI package in which
silver-coated balls 50 are bonded onto the lower surface of an LSI
chip with an interposer 62 interposed between them. The
silver-coated balls 50 are arranged in matrix on one side of the
interposer 62, and are used as input/output terminals for the
package. Each of these silver-coated balls 50 is a tiny metallic
sphere with a diameter of about 0.1 mm to about 1.0 mm, and may be
obtained by forming a solder layer, including lead (Pb), on the
surface of the metallic ball, for example.
[0004] In recent years, solder including lead is being replaced
with solder including no lead (which is also called "Pb-free
solder") in order to handle environmental issues. In view of these
circumstances, the applicant of the present application disclosed a
solder ball, of which the surface is coated with a tin-silver
(Sn--AG) based solder layer with no lead and in which the creation
of voids is minimized even when being heated and melted (see Patent
Documents Nos. 1 and 2).
[0005] In general, solders are roughly classifiable, according to
their soldering temperature, into medium-low temperature solders
with melting temperatures of approximately 150.degree. C. to
approximately 250.degree. C. and high-temperature solders with
melting temperatures of approximately 250.degree. C. to
approximately 300.degree. C. The medium-low temperature solder is
mostly used to connect an electronic component onto a printed
circuit board, while the high-temperature solder is often used to
connect the interconnects of an electronic component together.
[0006] The Sn--Ag based solder layer mentioned above has a melting
point of approximately 216.degree. C., and therefore, solder balls
with this solder layer can be used effectively in a soldering
process at a medium to low temperature. In a high temperature range
of about 250.degree. C. to about 300.degree. C., however, the
Sn--Ag based solder layer would melt again to deform the balls, for
example. For that reason, the Sn--Ag based solder layer cannot be
used for a high-temperature soldering process. That is why a
lead-free solder ball applicable to such a high-temperature
soldering process is now in high demand.
[0007] Meanwhile, it is known that a metal in the form of
nanoparticles (i.e., superfine particles with particle sizes of
several nm to several hundreds of nm) has physical properties that
are quite different from those of the same metal in bulk. For
example, it is known that silver nanoparticles get sintered at a
far lower temperature than silver in bulk. As for silver
nanoparticles, Patent Document No. 3 discloses a method of making a
silver colloid organosol including silver nanoparticles with a mean
particle size of approximately 32 nm (see Examples of Patent
Document No. 3). [0008] Patent Document No. 1: Japanese Patent
Application Laid-Open Publication No. 2004-114123 [0009] Patent
Document No. 2: Japanese Patent Application Laid-Open Publication
No. 2004-128262 [0010] Patent Document No. 3: Japanese Patent
Application Laid-Open Publication No. 2003-159525
DISCLOSURE OF INVENTION
Problems to be Solved by the Invention
[0011] The present inventors carried out experiments to explore the
possibility of using silver nanoparticles as a high-temperature
solder material for solder balls.
[0012] A primary object of the present invention is to provide a
silver-coated ball including a coating layer of silver
nanoparticles and a method of making such a ball.
Means for Solving the Problems
[0013] A silver-coated ball according to the present invention
includes: a spherical core; and a coating layer including silver
superfine particles, which is arranged so as to surround the core.
The silver superfine particles included in the coating layer have a
mean particle size of 1 nm to 50 nm.
[0014] In one preferred embodiment, the silver-coated ball has 0.01
mass % to 1 mass % of carbon.
[0015] In another preferred embodiment, the coating layer has a
thickness of 0.1 .mu.m to 50 .mu.m.
[0016] In another preferred embodiment, the core is made of copper
or resin.
[0017] In another preferred embodiment, the core has a mean
particle size of 0.05 mm to 1.5 mm.
[0018] A method of making a silver-coated ball according to the
present invention includes the steps of: providing a dispersion
including a spherical core, silver superfine particles, and a
solvent; forming a film of the dispersion on the surface of the
core; and removing the solvent of the dispersion from the film of
the dispersion, thereby forming a coating layer, including the
silver superfine particles, on the surface of the core. The silver
superfine particles have a mean particle size of 1 nm to 50 nm. The
solvent includes a nonpolar hydrocarbon solvent. And the ratio in
mass percentage of the silver superfine particles to the solvent is
40-85 mass % to 15-60 mass %.
[0019] In one preferred embodiment, the step of forming the film of
the dispersion on the surface of the core includes immersing the
core in the dispersion.
[0020] In another preferred embodiment, the step of forming the
coating layer including the silver superfine particles includes
putting the ball, on which the film of the dispersion has been
formed, on a slope and rolling the ball on the slope.
[0021] In another preferred embodiment, the solvent includes a
solvent with a boiling point higher than about 100.degree. C. and a
solvent with a boiling point that is equal to or lower than about
100.degree. C.
[0022] In another preferred embodiment, the nonpolar hydrocarbon
solvent includes xylene.
Effects of the Invention
[0023] In the silver-coated ball of the present invention, its
spherical core is covered with a coating layer including silver
superfine particles with a mean particle size of about 1 nm to
about 50 nm. These silver superfine particles have a melting point
of about 250.degree. C. to about 300.degree. C. That is why the
silver-coated ball of the present invention can be used as a
lead-free solder material in a high-temperature soldering process.
Once melted as a result of the soldering process, the silver never
melts again until the temperature reaches the melting point of
silver at about 960.degree. C. Consequently, the present invention
provides a semiconductor package, of which the bonding strength
with the silver-coated balls can be increased at high
temperatures.
BRIEF DESCRIPTION OF DRAWINGS
[0024] FIG. 1 is a cross-sectional view schematically illustrating
a silver-coated ball 10 according to a preferred embodiment of the
present invention.
[0025] FIG. 2 schematically illustrates a device for making a
silver-coated ball from a dispersion film coated ball.
[0026] FIGS. 3(a) and 3(b) illustrate an exemplary method of
forming a semiconductor interconnect structure according to the
present invention.
[0027] FIG. 4 is a photograph that was shot by observing
silver-coated copper balls representing a first specific example of
the present invention with a stereoscopic microscope.
[0028] FIG. 5 is a photograph that was shot by observing
silver-coated copper balls representing a first comparative example
with a stereoscopic microscope.
[0029] FIG. 6 is a photograph that was shot by observing copper
balls with a stereoscopic microscope.
[0030] FIG. 7 is a stereoscopic micrograph showing how the
silver-coated copper balls of the first specific example looked
after having been heated and melted at 300.degree. C. for two hours
within a nitrogen atmosphere.
[0031] FIG. 8 shows a DTA curve of the silver-coated copper balls
of the first specific example.
[0032] FIG. 9 shows a DTA curve associated with Dispersion A.
[0033] FIGS. 10(a) and 10(b) are respectively a perspective view
and a cross-sectional view illustrating a BGA that uses solder
balls.
DESCRIPTION OF THE REFERENCE NUMERALS
[0034] 1 core [0035] 2 coating layer [0036] 4A molten solder layer
[0037] 10 silver-coated ball [0038] 12 Cu layer [0039] 14 Ni
plating layer [0040] 16 Au plating layer [0041] 18 pad [0042] 20
substrate [0043] 31 slope [0044] 32 base [0045] 50 silver-coated
ball [0046] 62 interposer
BEST MODE FOR CARRYING OUT THE INVENTION
[0047] To provide a silver-coated ball of which the surface of the
core is uniformly covered with a coating layer including silver
superfine particles (which will be sometimes referred to herein as
a "silver coating layer"), the present inventors carries out
experiments with special attention paid to a dispersion of silver
superfine particles.
[0048] Generally speaking, silver superfine particles have such
high surface activity as to coagulate together easily at room
temperature. That is why the composition of the dispersion is
appropriately controlled according to the intended application, for
example, such that silver superfine particles with a desired
particle size distribution do not coagulate together, but can keep
chemically stabilized, in the dispersion. The dispersion usually
includes a solvent to dissolve the silver superfine particles and a
surfactant and may further include a deoxidizer and a protective
colloid agent as needed.
[0049] For example, Patent Document No. 3 cited above discloses a
composite gel in which a noble metal compound such as silver
superfine particles and a surfactant are mixed together at a
predetermined ratio. This composite gel can be used effectively as
a material to make a noble metal colloid organosol including
single-dispersion noble metal colloid particles at a high
concentration, and is preferably used as conductive paste for
electronic components and as a pigment for fibers, for example.
Other than that, ink, paste and so on, including silver superfine
particles at a high concentration and having excellent dispersion
stability and good sinterability at low temperatures, are also
available. For example, Nano Metal Ink, which is conductive ink for
fine interconnects produced by Vacuum Metallurgical Inc. (which is
now called ULVAC Materials, Inc.), and Nano Paste, which is
metallic paste for fine interconnects also produced by the same
company, are available.
[0050] However, none of these dispersions that have been proposed
so far considers application onto the surface of a sphere, which is
quite different from preferred embodiments of the present
invention. For that reason, the present inventors discovered and
confirmed via experiments that even if the conventional dispersion
was used, the desired silver coating layer could not be formed
uniformly on the surface of the core but the silver superfine
particles coagulated together or the coating layer partially peeled
off (see specific examples of the present invention to be described
later).
[0051] Based on such results of experiments, the present inventors
carried out further researches with the compositions of the
dispersion and other parameters changed. As a result, the present
inventors discovered that if a film of a dispersion, including a
solvent and silver superfine particles at a predetermined ratio,
was formed on the surface of the core and then subjected to a
prescribed solvent removal process to obtain a desired silver fine
particle containing coating layer, the silver superfine particles
did not coagulate together but the solvent vaporized uniformly,
thereby achieving our objects perfectly. The present inventors
acquired the basic idea of the present invention in this
manner.
[0052] The dispersion for use in this preferred embodiment includes
silver superfine particles and a solvent at an appropriately
controlled ratio, and therefore, can be adsorbed onto (i.e., can
make close contact with) the surface of the sphere just as
intended. Besides, the dispersion preferably includes a
high-boiling-point solvent with a boiling point higher than about
100.degree. C., and vaporizes slowly. That is why the silver
superfine particles can be dispersed with good stability in this
dispersion almost without coagulating together.
[0053] Furthermore, since the solvent removal process of this
preferred embodiment is controlled such that the solvent vaporizes
at a constant rate, the dispersion would never be distributed
unevenly around the core.
[0054] Consequently, according to this preferred embodiment, the
surface of the core can be covered with a coating layer including
silver superfine particles with good adhesiveness and with a
uniform thickness.
Preferred Embodiments
[0055] FIG. 1 is a cross-sectional view illustrating a
silver-coated ball 10 according to a preferred embodiment of the
present invention. As shown in FIG. 1, the silver-coated ball 10 of
this preferred embodiment includes a spherical core 1 and a coating
layer 2 including silver superfine particles with a mean particle
size of 1 nm to 50 nm and surrounding the core 1.
[0056] In the silver-coated ball 10 of this preferred embodiment,
the surface of the core 1 is covered with silver superfine
particles with such a mean particle size. These silver superfine
particles have a melting point of about 250.degree. C. to about
300.degree. C., and therefore, can be used to perform a soldering
process in a high temperature range. Besides, once melted through
heating, the silver never melts again until the temperature reaches
its own melting point of about 960.degree. C. Consequently, a
semiconductor package, of which the bonding strength with the
silver-coated balls is excellent even at high temperatures, is
provided.
[0057] The silver superfine particles included in the coating layer
2 have a mean particle size of 1 nm to 50 nm. The silver superfine
particles may have any mean particle size as long as those
particles achieve the performance described above. However, this
range is set tentatively in view of the dispersion stability. The
silver superfine particles preferably have a mean particle size of
8 nm to 20 nm. Considering possible variations in particle size,
the silver superfine particles may have a mean particle size of 8
nm.+-.2 nm to 20 nm.+-.2 nm. The mean particle size was figured out
herein by calculating the equivalent area circle diameters of
silver particles, falling within the viewing range of 100 nm
square, using an image processor and working out the average
thereof.
[0058] The silver superfine particles do not have to have a
monodispersity with a narrow particle size distribution. To form a
dense coating layer on the surface of the sphere, the silver
superfine particles should rather have a polydispersity in which
the particle size distribution has two peaks.
[0059] The coating layer 2 may include 0.01 mass % to 1 mass % of C
(carbon). Most of C would derive from a solvent for use to make the
silver-coated ball of this preferred embodiment. As will be
described later, according to this preferred embodiment, the
solvent is included at a higher percentage than a normal silver
superfine particle containing dispersion in order to have the
silver superfine particles make good contact with the surface of
the sphere. And the dispersion preferably includes a high boiling
point solvent with a boiling point that is higher than about
100.degree. C. That is why a lot of C would be introduced into the
coating layer. The content of C is measured by high frequency
combustion infrared absorption method that uses a carbon/sulfur
analyzer.
[0060] The coating layer 2 preferably has a thickness of 0.1 .mu.m
to 50 .mu.m. This thickness range is preferred for the following
reasons. Specifically, if the thickness of the coating layer 2 were
less than 0.1 .mu.m, the coating layer 2 would not function as a
solder layer effectively. The coating layer 2 preferably has a
thickness of 1.5 .mu.m or more. Nevertheless, if the thickness of
the coating layer 2 exceeded 50 .mu.m, then the coating layer might
melt and be misaligned after the silver-coated ball has been bonded
onto the substrate. The thickness of the coating layer 2 may be
figured out by measuring the diameter (i.e., the equivalent area
circle diameter) of the ball, of which the surface of the core 1
has already been coated with the coating layer 2, and that of the
ball, which has not been coated with the coating layer 2 yet, using
a microscope and then calculating the difference between these two
diameters.
[0061] The differential thermal analysis (DTA) curve of the
silver-coated ball 10 preferably has a peak of heat absorption with
a maximum value within the range of approximately 100.degree. C. to
approximately 200.degree. C. As will be described in detail by way
of specific examples later, the DTA curve of the silver-coated ball
of this preferred embodiment has not only a peak of heat absorption
corresponding to the melting point of silver superfine particles
(at about 240.degree. C. to about 250.degree. C.) but also another
peak of heat absorption that has a maximum value at about
150.degree. C. (see FIG. 8). The latter peak of heat absorption
would derive from a high boiling point solvent with a boiling point
that is higher than approximately 100.degree. C. for use to prepare
the silver-coated ball (e.g., xylene with a boiling point of
approximately 140.degree. C. in the specific examples to be
described later). It is not yet quite clear exactly how the desired
silver coating layer is formed uniformly according to this
preferred embodiment. This is probably mainly because by using a
dispersion including the high boiling point solvent described
above, the solvent would vaporize at an appropriate rate, thus
preventing the silver superfine particles from being distributed
non-uniformly (or coagulating) on the surface of the core.
[0062] As shown in FIG. 1, the coating layer 2 preferably has a
single-layer structure including silver superfine particles.
[0063] Alternatively, the coating layer 2 may also have a
multilayer structure consisting of a number of metal layers as long
as the silver superfine particles can achieve the performance
described above. For example, the coating layer 2 may include a
first metal layer including silver superfine particles and a second
metal layer (plating layer) surrounding the first layer. In such a
multilayer structure, the surface of the silver superfine particles
is coated with the second layer for plating. That is why even when
heated and melted at high temperatures, the silver superfine
particles will never be oxidized and lose their own properties. The
second metal layer preferably includes a metal such as Sn or In
that will melt at a lower temperature than the silver superfine
particles.
[0064] The core 1 may be anything as long as it is normally used to
make a solder ball.
[0065] For example, the core 1 may be made of Cu, Al or any other
suitable metal and is preferably made of Cu, among other things.
This is because Cu has a high melting point, high thermal
conductivity and low electrical resistance, and can be used
effectively as a connector material for a semiconductor
package.
[0066] The core 1 may also be made of resin. In that case, to
increase the thermal conductivity and form the coating layer 2 more
easily, a metal layer of Ni, for example, is preferably deposited
on the surface of the core 1 before the coating layer 2 is formed
thereon.
[0067] The core 1 preferably has a mean particle size of 0.05 mm to
1.5 mm. The mean particle size is appropriately adjusted according
to the number of pins of a BGA, for example.
[0068] Next, it will be described how to make a silver-coated ball
10 according to this preferred embodiment.
[0069] The manufacturing process of this preferred embodiment
includes the steps of providing a dispersion including a spherical
core, silver superfine particles and a solvent, forming a film of
the dispersion on the surface of the core, and removing the solvent
of the dispersion from the film of the dispersion, thereby forming
a coating layer, including the silver superfine particles, on the
surface of the core,
[0070] Hereinafter, the respective process steps will be described
in detail one by one.
[0071] First, a spherical core and dispersion are provided.
[0072] The dispersion includes silver superfine particles and a
solvent. As will be described later, the dispersion for use in this
preferred embodiment has a composition that is qualified to make a
desired silver-coated ball.
[0073] The dispersion includes 40 mass % to 85 mass % of silver
superfine particles and 15 mass % to 60 mass % of solvent, and
generally has a higher percentage of solvent than most of
dispersions that have ever been proposed. For that reason, a
coating layer with a uniform thickness can be formed on the surface
of the sphere so as to make close contact with the surface without
coagulating the silver superfine particles. If the ratio of the
silver superfine particles to the solvent fell outside of the range
defined above, then the silver superfine particles could not make
good contact with the surface of the core and possibly come off. A
preferred content ratio of the silver superfine particles to the
solvent would be 50-70 mass % to 30-50 mass %.
[0074] Any solvent may be used as long as the solvent can dissolve
the silver superfine particles. Both a nonpolar solvent and a polar
solvent will do. However, to form a coating layer including silver
superfine particles on the surface of the core such that the
particles can make good contact with the surface, a nonpolar
solvent is preferred. Among other things, a nonpolar hydrocarbon
solvent is particularly preferable.
[0075] The nonpolar hydrocarbon solvent is typically a paraffin
hydrocarbon or an aromatic hydrocarbon. Examples of paraffin
hydrocarbons include hexane (with a boiling point of approximately
69.degree. C.), octane (with a boiling point of approximately
126.degree. C.), cyclohexane (with a boiling point of approximately
81.degree. C.) and cyclopentane (with a boiling point of
approximately 51.degree. C.). Examples of aromatic hydrocarbons
include xylene (with a boiling point of approximately 140.degree.
C.), toluene (with a boiling point of approximately 110.degree. C.)
and benzene (with a boiling point of approximately 81.degree. C.).
Halogenated aromatic hydrocarbons such as chlorobenzene are also
included. Any of these hydrocarbons may be used either by itself or
in combination. The solvent for use in this preferred embodiment
preferably includes xylene at least.
[0076] In this preferred embodiment, the solvent preferably
includes a solvent with a boiling point that is higher than
100.degree. C. (which will be referred to herein as a "high boiling
point solvent") and a solvent with a boiling point that is equal to
or lower than 100.degree. C. (which will be referred to herein as a
"low boiling point solvent"). Among other things, a high boiling
point solvent has an appropriate vaporizing rate that is high
enough to form the desired silver superfine particle coating layer,
and should be useful. Optionally, the solvent consists essentially
of the high boiling point solvent alone.
[0077] The dispersion may include not just the silver superfine
particles and solvent mentioned above but also other additives
(such as a surfactant, an antifoaming agent, and an anticorrosion
agent), which are usually included in a silver superfine particle
containing dispersion, unless the functions of this preferred
embodiment are ruined.
[0078] Next, a film of the dispersion is formed on the surface of
the core. For the sake of convenience, the ball obtained in this
process step will be referred to herein as a "dispersion film
coated ball" to be distinguished from the "silver-coated ball", in
which the surface of the core is coated with a silver coating layer
and which should be eventually obtained by the method of this
preferred embodiment.
[0079] The film of the dispersion is preferably formed by an
immersion process. Specifically, the core is immersed for a
predetermined amount of time in a dispersion that has been heated
to a temperature of approximately 30.degree. C. The immersion
process time can be appropriately controlled according to the
composition of the dispersion or any other parameter but is
preferably three minutes or less. It should be noted that before
immersed in the dispersion, the core is preferably degreased in
advance. Then, the dispersion can make better contact with the
surface of the core.
[0080] In the dispersion film coated ball thus obtained, adjacent
cores are bridged together with the dispersion, and therefore, the
dispersion is distributed mostly around the cores. If the solvent
were vaporized as it is, a lot of silver superfine particles might
be left where the dispersion is distributed a lot.
[0081] In view of this consideration, according to this preferred
embodiment, the solvent is removed from the film of the dispersion
in the dispersion film coated ball, thereby forming a coating layer
including silver superfine particles on the surface of the core. As
a result, a desired silver-coated ball can be obtained.
[0082] Specifically, the silver-coated ball is preferably made
using a device such as that shown in FIG. 2, for example. This
device includes a slope 31 to roll the dispersion film coated ball
and a base 32 to support the slope.
[0083] First, the dispersion film coated ball is put on the slope
31 and the core is rolled on the slope 31. By rolling the
dispersion film coated ball along the slope 31 continuously, a
dispersion film with a uniform thickness is formed on the surface
of the core. As a result, a coating layer including silver
superfine particles is deposited to a uniform thickness on the
surface of the core. Such a solvent removal function will be
further promoted by using a slope of glass, for example.
Optionally, the solvent vaporizing rate can be adjusted by changing
the angles of the slope 31.
[0084] In this preferred embodiment, to obtain a silver coating
layer with an even smaller variation in thickness, the solvent is
preferably controlled so as to vaporize uniformly. For example, to
accelerate the vaporization of the solvent, the excessive solvent
on the surface may be absorbed into and removed with a piece of
paper (such as a Kimwipe) or cloth before the dispersion film
coated ball is put on the slope. Alternatively, the surface of the
ball may be dried with a dryer. Optionally, in the process step of
rolling the dispersion film coated ball on the slope, the surface
of the ball may also be dried with a dryer, for example.
[0085] Hereinafter, a method for forming a semiconductor
interconnect structure, including the silver-coated balls of the
present invention, will be described with reference to FIG. 3. In
the following description, various interconnect structures, in
which silver-coated balls may be used for an element or device
including a semiconductor chip at least, will be collectively
referred to herein as "semiconductor interconnect structures".
[0086] First, as shown in FIG. 3(a), silver-coated balls 50 and a
desired substrate 20, on which the silver-coated balls 50 will be
bonded, are provided. The substrate 20 may be used as an interposer
for a BGA (see FIG. 10) or a CSP. On the principal surface of the
substrate 20, arranged are pads 18 of conductive materials. Each of
these pads 18 may be a stack of a Cu layer 12, a Ni plating layer
14 and an Au plating layer 16, for example. Next, the silver-coated
balls 50 on the pads 18 are heated, thereby melting the coating
layer 2 as shown in FIG. 3(b), where the molten solder layer is
identified by the reference numeral 4A. Then, the molten coating
layer 4A is cooled, solidified, and thereby bonded onto the pads
18. By performing these process steps, a semiconductor interconnect
structure is formed.
[0087] In this semiconductor interconnect structure, the
silver-coated balls 50 are bonded to the substrate 20 so strongly
that misalignment and other inconveniences are rarely caused. As a
result, a highly reliable semiconductor interconnect structure is
provided.
EXAMPLES
[0088] In the specific examples to be described below, spherical
copper cores were used and it was examined how the degree of
contact of the silver superfine particles changed according to the
composition of the dispersion. Specifically, using two types of
copper cores (with diameters of 0.35 mm and 0.75 mm, respectively)
and dispersions A and B with the following compositions,
silver-coated copper balls representing specific examples #1 and #2
of the present invention and silver-coated copper balls
representing comparative examples #1 and #2 were made by the method
to be described later.
[0089] (Dispersion A)
[0090] Dispersion A includes approximately 90 mass % of silver
superfine particles (with a mean particle size of about 3 nm to
about 15 nm) and approximately 10 mass % of solvent. Dispersion A
does not meet the content ratio of silver superfine particles to
solvent as defined for the preferred embodiment described above.
The solvent consists essentially of xylene and toluene and includes
more xylene than toluene.
[0091] (Dispersion B)
[0092] Dispersion B is prepared by further adding xylene to
Dispersion A and includes approximately 60 mass % of silver
superfine particles (with a mean particle size of about 3 nm to
about 15 nm) and approximately 40 mass % of solvent. Dispersion B
satisfies the content ratio of silver superfine particles to
solvent as defined for the preferred embodiment described
above.
Example 1
[0093] First, a copper core with a diameter of 0.75 mm was
degreased with a neutral degreaser 506 (produced by Ishihara
Chemical Co., Ltd.) as preprocessing. Specifically, the copper core
was immersed in the neutral degreaser at 35.degree. C. for
approximately five minutes, cleaned with pure water for
approximately three minutes at room temperature, and then further
cleaned with running water for approximately one minute.
Thereafter, the core was immersed in ethanol for approximately two
minutes and then dried.
[0094] Next, Dispersion B was heated to about 30.degree. C. and the
copper core that had been pre-processed as described above was
immersed in the dispersion for approximately two minutes. As a
result, a dispersion film coated copper ball, in which a film of
the dispersion was formed on the surface of the copper core, could
be obtained.
[0095] After the immersion, the excessive dispersion remaining on
the surface of the dispersion film coated copper ball was wiped
away with a Kimwipe.
[0096] This copper ball was loaded into the device shown in FIG. 2
and put and rolled on a petri dish arranged in the device, thereby
making the thickness of the coating layer uniform.
[0097] In this manner, a silver-coated copper ball representing a
first specific example of the present invention (in which the
coating layer of the silver superfine particles had a thickness of
approximately 0.4 .mu.m) was obtained.
Example 2
[0098] A silver-coated copper ball representing a second specific
example of the present invention was made in the same way as in the
first specific example described above except that a copper ball
with a diameter of 0.35 mm was used instead of the copper ball with
a diameter of 0.75 mm. In the silver-coated copper ball of the
second specific example, the coating layer of the silver superfine
particles had a thickness of approximately 0.7 .mu.m.
Comparative Example 1
[0099] A silver-coated copper ball representing a first comparative
example was made in the same way as the first specific example
described above except that Dispersion A was used instead of
Dispersion B.
[0100] (Observation of Silver Coating Layer)
[0101] FIGS. 4 and 5 are photographs that were shot by observing
silver-coated copper balls representing the first specific example
of the present invention and the first comparative example,
respectively, with a stereoscopic microscope. FIG. 6 is a
micrograph of copper balls, on which the silver coating layer had
not been formed yet, just for reference.
[0102] As can be seen from FIG. 4, in the silver-coated copper
balls of the first specific example that used Dispersion B of the
preferred embodiment described above, the silver superfine
particles did not coagulate but a coating layer with a uniform
thickness was formed on the surface of the copper balls so as to
make good contact with the balls.
[0103] On the other hand, in the silver-coated copper balls of the
first comparative example that were made without using Dispersion B
of this preferred embodiment, the silver superfine particles
coagulated and no uniform coating layer could be formed as shown in
FIG. 5.
[0104] FIG. 7 is a stereoscopic micrograph showing how the
silver-coated copper balls of the first specific example looked
after having been heated and melted at 300.degree. C. for two hours
within a nitrogen atmosphere just for reference. As can be seen
from FIG. 7, even after the silver-coated copper balls of the first
specific example had been heated and melted at high temperatures,
the surface of the copper balls were still covered with those
silver superfine particles that still maintained a good degree of
contact with the surface. Thus, it can be seen that the
silver-coated copper balls of the first specific example can be
used effectively as a lead-free solder material for a
high-temperature soldering process.
[0105] (Analysis on C Content)
[0106] The contents of C (carbon) in the silver-coated balls
representing the first and second specific examples of the present
invention were measured by the high frequency combustion infrared
absorption method. The samples under test had a mass of
approximately 0.2 g.
[0107] For the purpose of comparison, the contents of C in the
copper balls (with diameters of 0.75 mm and 0.35 mm, respectively)
that were used in the first and second specific examples were also
measured in the same way.
[0108] The results are shown in the following Table 1, in which the
unit mass (g/kpcs) means a unit mass (g) per 1,000 silver-coated
balls:
TABLE-US-00001 TABLE 1 Sample under test Unit mass C content Sample
# Diameter Coating layer? (g/kpsc) (mass %) 1* 0.75 mm YES 0.193
0.125 2 0.75 mm NO 0.191 <0.001 3** 0.35 mm YES 0.020 0.172 4
0.35 mm NO 0.020 0.001 *Sample #1 corresponds to silver-coated
copper ball of Example #1 **Sample #3 corresponds to silver-coated
copper ball of Example #2
[0109] Comparing the C contents before and after the silver
superfine particle coating layer was formed on the surface of the
copper balls (i.e., those of Samples #2 and #1 and those of Samples
#4 and #3), it can be seen that the C contents of the silver-coated
balls of the first and second specific examples increased due to
the formation of the silver coating layer. The C content would have
been increased mainly by the solvent that was used to form the
silver superfine particle coating layer.
[0110] It should be noted that the silver-coated copper balls
representing the first comparative example that was made without
using Dispersion B of this preferred embodiment had no uniform
coating layer and its C content could not be measured.
[0111] (DTA Curve)
[0112] FIG. 8 shows the DTA curve of the silver-coated copper balls
representing the first specific example. Specifically, the DTA
curve was plotted when the silver-coated copper balls (25 mg) were
heated in the air at a temperature increase rate of 5.degree.
C./min. The DTA curve associated with Dispersion A is shown in FIG.
9 for reference.
[0113] As shown in FIG. 9, the DTA curve associated with Dispersion
A has a single peak of heat absorption (at 240.degree. C. to
250.degree. C.) corresponding to the melting point (of
approximately 260.degree. C.) of silver superfine particles. On the
other hand, the DTA curve of the silver-coated balls that were made
using Dispersion B had not only that peak of heat absorption but
also another peak of heat absorption that has a maximum value at
around 150.degree. C. The peak of heat absorption at around
150.degree. C. would mostly derive from xylene (with a boiling
point of approximately 140.degree. C.).
INDUSTRIAL APPLICABILITY
[0114] The present invention provides a silver-coated ball that
contributes to realizing a soldering process in a high temperature
range of approximately 250.degree. C. to approximately 300.degree.
C. The silver-coated balls of the present invention can be used
effectively as input/output terminals for a semiconductor package
such as a BGA or a CSP.
* * * * *