U.S. patent application number 16/102130 was filed with the patent office on 2019-03-14 for lead-free solder alloy.
The applicant listed for this patent is Senju Metal Industry Co., Ltd.. Invention is credited to Kyu-oh Lee, Hikaru Nomura, Tsukasa Ohnishi, Ken Tachibana, Yoshie Yamanaka, Shunsaku Yoshikawa.
Application Number | 20190076966 16/102130 |
Document ID | / |
Family ID | 52427801 |
Filed Date | 2019-03-14 |
United States Patent
Application |
20190076966 |
Kind Code |
A1 |
Ohnishi; Tsukasa ; et
al. |
March 14, 2019 |
Lead-Free Solder Alloy
Abstract
A lead-free solder alloy capable of forming solder joints in
which electromigration and an increase in resistance during
electric conduction at a high current density are suppressed has an
alloy composition consisting essentially of 1.0-13.0 mass % of In,
0.1-4.0 mass % of Ag, 0.3-1.0 mass % of Cu, a remainder of Sn. The
solder alloy has excellent tensile properties even at a high
temperature exceeding 100.degree. C. and can be used not only for
CPUs but also for power semiconductors.
Inventors: |
Ohnishi; Tsukasa; (Tokyo,
JP) ; Yoshikawa; Shunsaku; (Tokyo, JP) ;
Tachibana; Ken; (Tokyo, JP) ; Yamanaka; Yoshie;
(Tokyo, JP) ; Nomura; Hikaru; (Tokyo, JP) ;
Lee; Kyu-oh; (Chandler, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Senju Metal Industry Co., Ltd. |
Tokyo |
|
JP |
|
|
Family ID: |
52427801 |
Appl. No.: |
16/102130 |
Filed: |
August 13, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13959321 |
Aug 5, 2013 |
10076808 |
|
|
16102130 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K 35/0227 20130101;
Y10T 403/479 20150115; B23K 35/0222 20130101; B23K 35/262 20130101;
H01L 24/81 20130101; B23K 35/26 20130101; H01L 24/13 20130101; B23K
35/0244 20130101; B23K 2101/40 20180801; B23K 1/0016 20130101; B23K
35/025 20130101; C22C 13/00 20130101 |
International
Class: |
B23K 35/26 20060101
B23K035/26; B23K 35/02 20060101 B23K035/02; H01L 23/00 20060101
H01L023/00; B23K 1/00 20060101 B23K001/00; C22C 13/00 20060101
C22C013/00 |
Claims
1. A lead-free solder alloy having an alloy composition consisting
essentially of, in mass percent, In: 1.0-2.0%, Ag: 0.1-0.3%, Cu:
0.3-1.0%, and a remainder of Sn.
2. A lead-free solder alloy as set forth in claim 1 wherein the Cu
is 0.5-1.0%, in mass percent.
3. A lead-free solder alloy as set forth in claim 1 wherein the Cu
is 0.5-0.7%, in mass percent.
4. A lead-free solder alloy as set forth in claim 1 wherein the Cu
is 0.3-0.5%, in mass percent.
5. A solder joint made from a lead-free solder alloy as set forth
in claim 1.
6. A solder joint as set forth in claim 5, wherein when the
lead-free solder alloy is used as solder joint under an energizing
condition at a current density of 0.12 mA/.mu.m2 in air at 165
degrees C., the percent increase in a resistance value of the
solder joint from a resistance value before the energization to a
resistance value when 2500 hours have elapsed after the start of
the energization is not more than 30% and a difference between the
percent increase in a resistance value and a percent increase in a
resistance value when 500 hours have elapsed from the start of the
energization is not more than 5%.
7. A method for suppressing electromigration of a solder joint
during electrical conduction comprising forming a solder joint
using a lead-free solder alloy as set forth in claim 1.
8. A solder joint made from a lead-free solder alloy as set forth
in claim 1, wherein the solder joint is included in power
conversion equipment for solar power generation.
9. A solder joint made from a lead-free solder alloy as set forth
in claim 1, wherein the solder joint is included in a high-current
inverter.
10. A solder joint made from a lead-free solder alloy as set forth
in claim 1, wherein the solder joint is included in a power
semiconductor on a motor control inverter for an electric vehicle
or plug-in hybrid vehicle.
11. A solder joint made from a lead-free solder alloy as set forth
in claim 1, wherein the solder joint is included in a CPU, in which
the current density per terminal is around 104-105 A/cm2.
12. A solder joint made from a lead-free solder alloy as set forth
in claim 1, wherein a current density of 0.12 mA/.mu.m2 passes
through the solder joint.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. patent
application Ser. No. 13/959,321 filed Aug. 5, 2013, the disclosure
of which is hereby incorporated by reference in its entirety
herein.
TECHNICAL FIELD
[0002] This invention relates to a lead-free Sn--Ag--Cu based
solder alloy which can be used in a high temperature, high current
density environment.
BACKGROUND ART
[0003] In recent years, due to reductions in the size and increases
in the performance of central processing units (CPUs) which are
mainly used for computers, the current density per terminal of
semiconductor elements mounted in CPUs has been increasing. It is
said that the current density will reach the order of around
10.sup.4-10.sup.5 A/cm.sup.2 in the future. As the current density
increases, the heat generated by passage of current increases,
thereby elevating the temperature of the terminals and increasing
the thermal vibrations of atoms in the terminals. As a result, the
occurrence of electromigration developed in solder joints becomes
marked, eventually leading to failure of the solder joints.
[0004] Electromigration (which may hereinafter be abbreviated as
EM) is a phenomenon which occurs when a current is flowing through
a conductor such as a solder joint. Atoms in the solder joint which
are undergoing thermal vibrations collide with electrons forming
the electric current, and momentum is transferred from the
electrons to the atoms, thereby increasing the momentum of the
atoms. The atoms having an increased momentum migrate toward the
anode side of the solder joint by going along the flow of
electrons. When atoms migrate toward the anode side of the solder
joint, lattice vacancies develop on the cathode side of the solder
joint. These lattice vacancies accumulate to form voids. Growth of
the voids eventually causes failure of the solder joint. In this
manner, electromigration develops in locations where electrical
conduction takes place, and it has become a problem even inside
solder joints.
[0005] The environment of use of a solder joint which is envisaged
in this description is an environment at the time of operating CPUs
with a high current density and is referred to below as a high
current density environment. Evaluation of the reliability of a
solder joint in such an environment can be carried out by an
electromigration test (also referred to as an EM test) in which a
current with a high current density of 0.12 mA/.mu.m.sup.2 is
continuously passed through a solder joint for 2500 hours in air at
165.degree. C.
[0006] Sn--Cu solder alloys and Sn--Ag--Cu solder alloys have been
widely used as lead-free solder alloys. Sn--Cu solder alloys and
Sn--Ag--Cu solder alloys easily develop electromigration because
Sn, which is the main component of these alloys, has a large
effective charge number. As a result, solder joints made of these
alloys readily fail in a high current environment.
[0007] Patent Document 1 discloses a Sn--Ag--Cu--In solder alloy
which has improved resistance to thermal fatigue and thereby
suppresses the occurrence of cracks. The Sn--Ag--Cu--In solder
alloy disclosed in Patent Document 1 has improved wettability due
to the addition of a small amount of In. As a result, the
occurrence of cracks and fracture of solder joints are
suppressed.
PRIOR ART DOCUMENTS
Patent Documents
[0008] Patent Document 1: JP 2002-307187 A
SUMMARY OF THE INVENTION
[0009] However, Patent Document 1 does not contain any suggestion
or teaching concerning suppressing the occurrence of
electromigration and suppressing failure of solder joints by adding
In to a Sn--Ag--Cu solder alloy. Although Patent Document 1 refers
to resistance to thermal fatigue, it does not contain sufficient
discussion of the effects when electrical conduction takes place
for a long period in a high current density environment at a high
temperature. Thus, the environment of operation of CPUs is not
faithfully reproduced in Patent Document 1.
[0010] The specific solder alloys which are investigated in Patent
Document 1 are ones having alloy compositions containing 0.5% In.
These alloy compositions have an extremely low In content, which
makes it impossible to confirm whether resistance to thermal
fatigue is sufficiently improved by the addition of In. It is not
verified at all whether these alloy compositions can avoid
electromigration when electrical conduction takes place for a long
period in a high current density environment at a high temperature.
Accordingly, the solder alloy disclosed in Patent Document 1 cannot
be said to solve the problem of increased electromigration due to
the increase in current density in recent years.
[0011] The object of the present invention is to provide a
lead-free solder alloy which can suppress an increase in the
interconnection resistance of solder joints by suppressing of the
growth of voids due to electromigration in a high temperature, high
current density environment.
[0012] The present inventors performed detail investigation of the
composition of a Sn--Ag--Cu--In solder alloy from the standpoint of
suppressing the occurrence of electromigration. Namely, they
considered that when Sn migrated toward the anode side along with
the flow of electrons to leave lattice vacancies in the cathode
side, In might fill the lattice vacancies and could suppress the
growth of the lattice vacancies. As a result, they found that an In
content of 1.0-13.0% in a Sn--Ag--Cu--In solder alloy can
effectively suppress the formation of lattice vacancies and the
growth of voids due to electromigration, and they thereby completed
the present invention.
[0013] The present invention provides a lead-free solder alloy
having an alloy composition consisting essentially of, in mass
percent, In: 1.0-13.0%, Ag: 0.1-4.0%, Cu: 0.3-1.0%, and a remainder
of Sn.
[0014] In one embodiment of the present invention, the Ag content
of the lead-free solder alloy is 0.3-3.0% by mass.
[0015] In another embodiment of the present invention, the
lead-free solder alloy contains, in mass percent, In: 2.0-13.0%,
Ag: 0.3-3.0%, and Cu: 0.5-0.7%.
[0016] In yet another embodiment of the present invention, the
lead-free solder alloy contains, in mass percent, In: 5.0-10.0%,
Ag: 0.1-1.5%, and Cu: 0.3-1.0%.
[0017] The present invention also provides a solder joint made from
any of the lead-free solder alloys described above.
[0018] A solder joint according to the present invention has a
value of percent increase in the resistance after 2500 hours of
conduction in air at 165.degree. C. with a current density of 0.12
mA/.mu.m.sup.2 which is at most 30% compared to the resistance
before the start of conduction and is at most 5% compared to the
resistance at 500 hours after the start of conduction.
[0019] The present invention also provides a method of suppressing
electromigration of a solder joint during electrical conduction
comprising forming a solder joint using the above-described solder
alloy.
BRIEF EXPLANATION OF THE DRAWINGS
[0020] FIG. 1 schematically shows the relationship between the In
concentration of a solder joint and the distance from the cathode
side of the solder joint at the end of an EM test.
[0021] FIGS. 2A and 2B are a schematic vertical cross-sectional
view and a schematic plan view, respectively, of a FCLGA package
used in an EM test.
[0022] FIG. 3 schematically shows the flowing directions of
electrons (e.sup.-) in soldered joints which are subjected to an EM
test.
[0023] FIG. 4 is a cross-sectional SEM photograph showing
intermetallic compounds formed at the boundary between a solder
joint made of a Sn-7In-1Ag-0.5Cu solder alloy and an electrode with
a Ni/Au coating.
[0024] FIG. 5 is a graph showing the variation in the resistance of
a solder joint made of a Sn-0.7Cu solder alloy as a comparative
example as a function of the length of time of an EM test.
[0025] FIG. 6 is a graph showing the variation in the resistance of
a solder joint made of a Sn-1Ag-0.5Cu-7In solder alloy according to
the present invention as a function of the length of time of an EM
test.
[0026] FIGS. 7A and 7B are graphs showing the variation in the
resistance of a solder joint made of a Sn-1Ag-0.5Cu-4In solder
alloy and a Sn-1Ag-0.5Cu-13In solder alloy, respectively, according
to the present invention as a function of the length of time of an
EM test.
[0027] FIG. 8 is a SEM photograph of a cross section of a solder
joint formed from a Sn-0.7Cu solder alloy as a comparative example
on a Cu electrode having a Ni/Au coating at the end of an EM test
for 2500 hours.
[0028] FIG. 9 is a SEM photograph of a cross section of a solder
joint formed from a Sn-7In-1Ag-0.5Cu solder alloy according to the
present invention on a Cu electrode having a Ni/Au coating at the
end of an EM test for 2500 hours.
[0029] FIG. 10 is a plan view and an end view of a test piece used
in a tensile test.
[0030] FIG. 11 is a graph showing the tensile strength of a
Sn-(0-15)In-1Ag-0.5Cu solder alloy as a function of the In content
thereof.
[0031] FIG. 12 is a graph showing the elongation at failure of a
Sn-(0-15)In-1Ag-0.5Cu solder alloy as a function of the In content
thereof.
MODES FOR CARRYING OUT THE INVENTION
[0032] The present invention will next be described in detail. In
this description, percent with respect to the composition of a
solder alloy means mass percent unless otherwise indicated.
[0033] A Sn--In--Ag--Cu solder alloy according to the present
invention contains 1.0-13.0% of In, which makes it possible to
suppress the growth of voids due to electromigration. Below, the
relationship between Sn, In, and electromigration will be explained
in detail.
[0034] When a solder joint made of a solder alloy according to the
present invention conducts electricity at a high temperature, Sn
preferentially migrates toward the anode side of the solder joint
along with the flow of electrons as discussed later, leaving
lattice vacancies on the cathode side. On the other hand, In fills
the lattice vacancies formed on the cathode side, leading to the
formation of a Sn-rich layer on the anode side and an In-rich layer
on the cathode side of the solder joint. As the In content of the
solder alloy increases, the above-described phenomenon becomes more
marked, and the In-rich layer and the Sn-rich layer increase in
thickness. As a result, when an EM test is carried out on a solder
joint by passing an electric current at a high current density at a
high temperature, during the period until approximately 500 hours
have elapsed, a resistance shift in which the resistance of the
solder joint increases is observed. This increase in resistance in
the initial stage of an EM test is thought to be caused by the
above-mentioned phase separation (the formation of an Sn-rich layer
and an In-rich layer).
[0035] As shown in Table 1, the resistivity of In is about 8 times
that of Sn. Therefore, the increase in resistance of a solder joint
made from a Sn--In--Ag--Cu solder alloy in the initial stage of an
EM test is thought to be attributable to the growth of an In-rich
layer. This increase in the initial stage is expected to be greater
as the thickness of the In-rich layer is thicker.
[0036] When In atoms fill the lattice vacancies formed on the
cathode side as a result of migration of Sn atoms, In atoms can
substitute for the sites of lattice vacancies which were formerly
occupied by .beta.-Sn, thereby forming a solid solution. Such
substitution of In atoms in the form of a solid solution is thought
to suppress void nucleation on the cathode side, thereby increasing
the resistance of the solder joint to electromigration (suppression
of the growth of voids due to electromigration).
TABLE-US-00001 TABLE 1 Atomic Melting radius Resistivity point
Metal (pm) (ohm cm) (.degree. C.) In 167 83.7 157 Sn 140 10.1
232
[0037] In a binary alloy, when the two elements constituting the
alloy which have different diffusivity want to migrate in the same
direction, only the one species having the larger diffusivity
preferentially migrates in the intended direction, leaving lattice
vacancies. The other species having the smaller diffusivity fills
the lattice vacancies. In general, the larger the effective charge
number, the greater the diffusivity. The effective charge numbers
for Sn and In are -18 and -2, respectively. The term "effective
charge number" used herein indicates the degree of easiness of the
occurrence of electromigration. Accordingly, Sn, which is greater
in the absolute value of effective charge number, preferentially
migrates in the intended direction along with the flow of
electrons, while In fills the lattice vacancies formed by migration
of Sn.
[0038] Therefore, the formation of an In-rich layer involves two
steps, namely, the occurrence of a flow of Sn atoms toward the
anode side which is induced by EM and the occurrence of a flow of
In atoms in the opposite direction. The flow of Sn atoms is
produced as a result of collision of Sn atoms with electrons
accompanied by momentum transfer. Due to the migration of Sn, the
other atom, In cannot remain in its initial position in the lattice
which it assumed before conduction, and it must migrate in the
opposite direction from Sn. When Sn atoms start to migrate towards
the anode side of a solder joint together with electrons, a
compressive stress develops on the anode side of the solder joint.
On the other hand, a tensile stress develops on the cathode side of
the solder joint, resulting in the formation of a stress gradient
between the anode and cathode. If the stress gradient is
sufficiently large, In, which is relatively difficult to migrate,
starts to migrate from the anode side toward the cathode side of
the solder joint. Accordingly, it takes a certain time to form an
In-rich layer which appears as an increase in resistance in the
initial stages of an EM test. As shown in FIGS. 6 and 7 which are
discussed later, the time required to form an In-rich layer is in
the range of 100 to 500 hours, for example, and it depends on the
In content of a solder alloy (the lower the In content, the
shorter). When the In content is minute as in the solder alloy
disclosed in Patent Document 1, the time required to form an
In-rich layer is reduced, but filling of lattice vacancies by In
becomes inadequate and voids end up growing.
[0039] When Sn forms a thin Sn-rich layer at the interface on the
anode side of a solder joint, almost all In migrates from the
Sn-rich layer toward the cathode side.
[0040] FIG. 1 schematically shows the relationship between the
concentration of In in a solder joint after an EM test and the
distance from the anode side of the solder joint. As shown in FIG.
1, after an electric current with a high current density is passed
through a solder joint for a long period of time, the In
concentration (C.sub.1) in the Sn-rich layer having a small
distance from the anode side of a solder joint is nearly zero, and
in the central portion of the solder joint (solder matrix) between
the cathode and anode sides (indicated as a Sn--In layer in FIG.
1), the In concentration of the solder alloy before bonding is
C.sub.2. The In concentration in the In-rich layer which is located
away from the anode side is much higher than C.sub.2. In this case,
the rate of growth of the Sn-rich layer can be expressed by the
following Equation (1):
( C 2 - C 1 ) dy dt = J Sn = C Sn .times. D Sn .times. z * .kappa.
T .times. e .times. .rho. .times. j ( 1 ) ##EQU00001##
where, y is the thickness of the Sn-rich layer, t is the conduction
time (duration of passage of current), C.sub.1 and C.sub.2 are
respectively the In concentration in the Sn-rich layer and the
solder alloy, J.sub.Sn is the EM-induced atomic flux of Sn,
C.sub.Sn, D.sub.Sn, and z* are respectively the concentration,
diffusivity, and effective charge number of Sn which is the
diffusing species in the base solder, .rho. is the resistivity of
the solder alloy, .kappa. is Boltzmann's constant, T is the
temperature, e is the electron charge, and j is the current
density.
[0041] If the EM-induced atomic flux of Sn is limiting, the growth
rate of the Sn-rich layer dy/dt should be constant, which reveals
that the growth rate of the Sn-rich layer has a linear dependency
on the conduction time. However, if the diffusion of In in the
Sn-rich layer is limiting, the growth rate of the Sn-rich layer is
controlled by the slower In diffusion and can be expressed by the
following Equation (2):
( C 2 - C 1 ) dy dt = J In = - D In dC In dx ( 2 ) ##EQU00002##
where, J.sub.In is the diffusion flux of In out of the anode side,
namely, from the Sn-rich layer, D.sub.In is the diffusivity of In,
C.sub.In is the In concentration of the solder alloy, and x is the
distance from the Sn-rich layer in the direction perpendicular to
the Sn-rich layer. Seeing that the growth rate of the Sn-rich layer
or the In-rich layer is depending upon the In concentration in the
solder alloy and obey a parabolic dependence, this is in agreement
with Equation (2).
[0042] As discussed above, with a Sn--In--Ag--Cu solder alloy
according to the present invention, due to the presence of a
sufficient amount of In, after Sn forms a Sn-rich layer on the
anode side of a solder joint as a result of conduction which causes
Sn to preferentially migrate toward the anode side, the
above-described stress gradient causes In to migrate toward the
cathode side of the solder joint and form an In-rich layer on the
cathode side. At this time, In can fill lattice vacancies formed by
migration of Sn and thereby suppress the formation of lattice
vacancies. As a result, the formation of voids due to
electromigration can be suppressed, resulting in preventing failure
of a solder joint.
[0043] The alloy composition of a solder alloy according to the
present invention is as follows.
[0044] The In content of a solder alloy according to the present
invention is at least 1.0% to at most 13.0%. Indium is effective at
suppressing the occurrence of electromigration during conduction
and improving the mechanical properties of a solder joint at high
temperatures, and it lowers the melting point of the solder alloy.
If the In content is larger than 13.0%, the mechanical properties
and particularly the ductility of a solder alloy deteriorate. If
the In content is less than 1.0%, the effect of the addition of In
cannot be adequately exhibited. The lower limit on the In content
is preferably 2.0% and more preferably 5.0%. The upper limit of the
In content is preferably 13.0% and more preferably 10.0%.
[0045] From the standpoint of obtaining excellent tensile strength
without deteriorating elongation at failure, it is particularly
preferable for the In content to be in the range of 5.0-10.0%.
After soldering is carried out with a Sn--In--Ag--Cu solder alloy
according to the present invention and an electric current is
passed through the resulting solder joint, as shown in FIG. 1, the
initial composition of the solder alloy is maintained in the
central portion between the cathode and anode sides (indicated as a
Sn--In layer in FIG. 1) of the solder joint. In a solder alloy
according to the present invention, due to the form of In which is
in solid solution in a Sn phase, the solder alloy has a high
tensile strength at a high temperature, and a decrease in
elongation at failure can also be suppressed. Accordingly, if a
solder alloy according to the present invention has an In content
of 5.0-10.0%, not only is it possible to suppress failure of a
solder joint due to electromigration, it is also possible to obtain
excellent mechanical properties at high temperatures.
[0046] The Ag content of a solder alloy according to the present
invention is 0.1-4.0%. Ag is effective at improving the wettability
and mechanical properties such as the tensile strength of a solder
alloy. If the Ag content is greater than 4.0%, the liquidus
temperature (also referred to liquidus line temperature or LL) of
the solder alloy ends up increasing. If the Ag content is less than
0.1%, wettability deteriorates. The Ag content is preferably at
least 0.3% to at most 3.0%.
[0047] The Cu content of a solder alloy according to the present
invention is 0.3-1.0%. Cu is effective at improving the wettability
and mechanical properties such as the tensile strength of a solder
alloy and suppressing Cu erosion of electrodes or terminals which
are typically made of Cu. If the Cu content is greater than 1.0%,
the wettability of the solder alloy deteriorates and the liquidus
temperature thereof increases. If the Cu content is less than 0.3%,
the bonding strength of a solder joint deteriorates. The Cu content
is preferably at least 0.5% to at most 0.7%.
[0048] A solder alloy according to the present invention preferably
has an alloy composition consisting essentially of In: 1.0-13.0%,
Ag: 0.3-3.0%, Cu: 0.3-1.0%, and a remainder of Sn, more preferably
In: 2.0-13.0%, Ag: 0.3-3.0%, Cu: 0.5-0.7%, and a remainder of Sn.
Another preferably solder alloy according to the present invention
has an alloy composition consisting essentially of In: 5.0-10.0%,
Ag: 0.1-1.5%, Cu: 0.3-1.0%, and a remainder of Sn.
[0049] Bonding using a solder alloy according to the present
invention does not require special conditions and may be carried
out by the reflow method in a conventional manner. Specifically,
reflow soldering is generally carried out at a temperature of from
a few degrees to around 20.degree. C. higher than the liquidus
temperature of the solder alloy.
[0050] A solder joint according to the present invention is
suitable for use to connect an IC chip to its substrate (an
interposer) in a semiconductor package or connect a semiconductor
package to a printed circuit board. The term "solder joint" which
connects between two terminals means the portion from one terminal
to the other terminal.
[0051] When a solder joint according to the present invention
undergoes conduction with a current density of 0.12 mA/.mu.m.sup.2
in air at 165.degree. C., it is preferable that the percent
increase in the resistance after 2500 hours of conduction be at
most 30% when compared to the resistance before the start of
conduction and at most 5% when compared to the resistance after 500
hours of conduction. As a result, even when a solder joint
according to the present invention is used for conduction for long
periods in such a high temperature, high current density
environment, breakdown due to electromigration does not take
place.
[0052] A solder joint according to the present invention is thought
to have excellent heat resistance at high temperatures when not
conducting. Therefore, a solder joint according to the present
invention can be formed by the above-described bonding method using
typical soldering conditions.
[0053] The suppression of electromigration according to the present
invention is realized by forming a solder joint using a solder
alloy according to the present invention for bonding between a
semiconductor element and a substrate, for example. In the present
invention, it is possible to suppress the formation of voids due to
electromigration in a solder joint during electrical conduction,
which can takes place in solder joints which carry electric current
inside a CPU during its operation.
[0054] A solder alloy according to the present invention can be
used in the form of a preform, a wire, a solder paste, a solder
ball (also called solder sphere), or the like. For example, solder
balls may have a diameter in the range of 1-100 .mu.m.
[0055] A solder alloy according to the present invention can also
be used to manufacture low .alpha.-ray solder balls by use of low
.alpha.-ray materials in the preparation of the solder alloy. Such
low .alpha.-ray solder balls can suppress software errors when used
to form solder bumps in the periphery of memories.
EXAMPLE
[0056] Various lead-free Sn--Ag--Cu--In solder alloys were prepared
in order to investigate electromigration of solder joints formed
from the solder alloys and evaluate the mechanical properties of
the solder alloys in the following manner.
1. Measurement of Electromigration (EM Test)
[0057] Prior to an EM test, first, the solderability performance of
a Sn-4In-1Ag-0.5Cu solder alloy, a Sn-7In-1Ag-0.5Cu solder alloy,
and a Sn-13In-1Ag-0.5Cu solder alloy was investigated. Solder balls
of a solder alloy to be tested were placed on Cu pads having a
Ni/Au coating and soldered by reflow using a water-soluble
non-halogenated flux to form solder bumps.
[0058] FIG. 4 is a SEM photograph of a cross section of a solder
bump made of a Sn-7In-1Ag-0.5Cu solder alloy at a magnification of
5000.times.. An intermetallic compound 43 was formed at the
interface between a solder bump 41 and a Ni coating 42. The
intermetallic compound 43 is a typical acicular Ni--Sn
intermetallic compound, indicating that the soldering reaction
between the solder alloy and the Ni/Au coating is very good. Thus,
it was confirmed that the solderability of this lead-free solder
alloy was excellent. The formation of a Ni--Sn intermetallic
compound as shown in FIG. 4 was also observed in solder joints made
of the other solder alloys according to the present invention
having an In content of 4% or 13%. Accordingly, the solderability
performance of all these solder alloys according to the present
invention is excellent.
[0059] FIG. 2A shows a schematic vertical cross sectional view, and
FIG. 2B shows a schematic plan view (b) of a FCLGA (flip chip land
grid array) package 10 used for an EM test. As shown in FIGS. 2A
and 2B by way of example, the FCLGA package 10 comprised a die 11
measuring 10 mm.times.10 mm.times.750 .mu.m in thickness, a die
substrate 12 measuring 22 mm.times.22 mm.times.1.0 mm in thickness,
and an organic laminate substrate 13 measuring 35 mm.times.35
mm.times.1.2 mm in thickness. The die 11 and the die substrate 12
are bonded by flip chip (FC) bonding 14, while the die substrate 12
and the organic laminate substrate 13 are bonded by BGA
bonding.
[0060] In this example, an EM test of solder joints was carried out
using an FCLGA package having FC-bonded solder joints formed from
solder balls made of a solder alloy according to the present
invention or a comparative solder alloy by the above-described
method. The solder alloy according to the present invention had a
composition of Sn-(4, 7, or 13)In-1Ag-0.5Cu, and the comparative
solder alloy had a composition of Sn-0.7Cu.
[0061] As schematically shown in FIG. 3, in the EM test, the part
device was designated so that a whole row of solder bumps could be
tested in the same polarity and with the same current. Thus, in
this part, the overall resistance of the solcer joints in a row
having the same direction of current flow could be measured.
[0062] The EM test was performed in a chamber. The chamber in which
a FCLGA package to be tested was already placed was heated to
165.degree. C. and maintained at that temperature. Once a steady
state of the temperature had been reached, a constant current of
950 mA was applied to the test part (the FCLGA package), and the
resistance was continuously monitored in situ for 2500 hours or
longer. The current density in each solder bump during conduction
was 0.12 mA/.mu.m.sup.2 (calculated by the equation: 950
mA/(.pi..times.(100 .mu.m/2).sup.2).
[0063] The test results for a Sn-7In-1Ag-0.5Cu solder alloy are
shown in Table 2 along with the results of the Sn-0.7Cu comparative
solder alloy. Table 2 shows the percent increase in resistance at
different lengths of time after the start of the EM test. Each
value in Table 2 is the average for 5 rows of solder joints.
TABLE-US-00002 TABLE 2 % Increase in Resistance Length of time of
test [hr] Alloy Composition 0 10 500 900 1500 2500 Sn--0.7Cu 0 [%]
0.5 [%] 8 [%] 17 [%] 62 [%] Conduction failure Sn--7In--1Ag--0.5Cu
0 [%] 1.5 [%] 23 [%] 26 [%] 27 [%] 28 [%]
[0064] FIG. 5 is a graph showing the relationship between the
length of time of the test and the resistance of solder joints made
of a Sn-0.7Cu solder alloy as a comparative example. FIG. 6 is a
graph showing the relationship between the length of time of the
test and the resistance of solder joints made from a
Sn-7In-1Ag-0.5Cu solder alloy according to the present invention.
FIGS. 5 and 6 each show the results of the electromigration for 5
rows of solder joints of each solder composition. The data shown in
Table 2 were determined from the data in these graphs as an average
for the 5 rows of solder joints.
[0065] As shown in FIG. 5, when a certain length of time passed for
the comparative Sn-0.7Cu solder alloy, a typical electromigration
resistance shift in the form of an abrupt increase in resistance
was observed. Specifically, also as shown in Table 2, the increase
in resistance was very gentle until approximately 900 hours after
the start of the test. This length of time of 900 hours is thought
to be the time until nucleation of voids starts and the voids grow
to such an extent that causes an abrupt quality degradation. When
900 hours are exceeded, the resistance abruptly increased with the
percent increase at 1500 hours being around 62%, and conduction
failure occurred in two rows of the solder joints before 2500
hours. In addition, all the resistance of the 5 rows of solder
joints showed a significant fluctuation after 900 hours and
particularly after 1200 hours, indicating that the resistance
became significantly unstable. The dispersion of the results in the
5 rows of solder joints was also large.
[0066] In contrast, as shown in FIG. 6 and Table 2, the resistance
of solder joints made of a Sn-7In-1Ag-0.5Cu solder alloy according
to the present invention increased by approximately 2% in the
extremely early stage of an EM test (in around 10 hours). The
resistance at 500 hours then increased by 23% compared to the start
of the test. The resistance after 500 hours was nearly constant,
and it remained stable as evidenced by the fact that the percent
increase in resistance compared to the start of the test was only
28% even at 2500 hours. In addition, the dispersion in resistance
in the 5 rows of solder joints and the fluctuation of the
resistance in each joint were much smaller compared to FIG. 5. As
shown in FIGS. 7A and 7B, solder joints made of the other solder
alloys according to the present invention which contains 4% or 13%
In exhibited the same tendency as shown in FIG. 6. Namely, after
the initial stage increase in resistance, the resistance remained
stable even at 2500 hours. As the In content was higher, the
initial stage increase in resistance became greater.
[0067] The mechanism of electromigration was investigated in a
cross section of a solder joint after the EM test using a scanning
electron microscope (SEM) and an electron probe microanalyzer
(EPMA).
[0068] FIGS. 8 and 9 are SEM photographs of a cross section of a
solder joint made of a comparative Sn-0.7Cu solder alloy and a
Sn-7In-1Ag-0.5Cu solder alloy according to the present invention,
respectively, after the EM test was carried out for 2500 hours. As
shown in these figures, a solder joint 53 or 63 was formed from a
solder ball so as to connect a Cu pad 56 or 66 having a Ni/Au
plating formed on a die substrate and a Cu pad 51 or 61 formed on a
die. In the EM test, a current was passed through the solder joints
53 or 63 so as to cause electrons to flow in the direction from the
Cu pad 56 or 66 on the die substrate toward the Cu pad 51 or 61 on
the die. Therefore, the Cu pad 56 and 66 on the die substrate
functioned as a cathode while the Cu pad 51 or 61 on the die
functioned as an anode.
[0069] As shown in FIGS. 8 and 9, a Cu--Sn intermetallic compound
52 or 62 was formed on each of the Cu pads 51 and 61 which was an
anode. A Ni plating layer 55 or 65 was observed on each of the Cu
pads 56 and 66 which was a cathode. The Cu--Sn intermetallic
compound 52 or 62 and the Ni plating layer 55 or 65 were connected
by the solder joints 53 or 63. Voids 54 or 64 were formed in each
of the solder joints 53 and 63 between the cathode 56 or 66 and the
Ni plating layer 55 or 65.
[0070] As shown in FIG. 8, when using a comparative Sn-0.7Cu solder
alloy, voids 54 were formed in the solder joint 53 so as to make a
line or a layer in the vicinity of the Ni plating layer 55 on the
cathode 56. The formation of voids in such a state in a solder
joint may cause the solder joint to be readily broken when
undergoing a stress. In contrast, as shown in FIG. 9, in the case
of a Sn-7In-1Ag-0.5Cu solder alloy according to the present
invention, voids 64 formed in the solder joint 63 in the vicinity
of the Ni plating layer 65 on the cathode 66 did not form a layer
but formed separated masses of voids. Therefore, there was little
concern of failure of conduction through the solder joint even
after 2500 hours of an EM test.
[0071] In order to identify the atomic flow during the EM test,
EPMA data was collected after completion of the EM test for 2500
hours. From this result, it was found that a solder joint made of a
Sn-0.7Cu solder alloy behaved in a typical electromigration
mechanism. Namely, when high density electron flows bombarded Sn
atoms, Sn atoms were caused to migrate toward the anode side while
voids were moved against the flow of electrons and accumulated in
the cathode side.
[0072] In an In-containing Sn--Ag--Cu alloy according to the
present invention, Sn atoms migrate along with electrons while In
atoms migrate against the flow of electrons, thereby forming an
Sn-rich layer in the anode side and an In-rich layer in the cathode
side. The formation of these layers is thought to take place by the
previously discussed mechanism.
2. Mechanical Properties
[0073] The solder alloys having compositions shown in Table 3
[Sn-xIn-1Ag-0.5Cu (x=0-15)] and Table 4 [Sn-xIn-(0 or 1)Ag-0.7Cu
(x=0-15)] were cast into molds to prepare test pieces having the
shape shown in FIG. 10. In this figure, the numerals show
dimensions in mm. For example, the parallel portion of each test
piece had a diameter of 8 mm and a length of 30 mm. The test piece
was prepared by casting each solder alloy at a temperature
100.degree. C. above the liquidus temperature of the composition
into a split mold which was processed to the shape shown in FIG. 10
followed by air cooling to room temperature and removal of the
resulting cast piece from the split mold.
[0074] Using a tensile tester, tension was applied to the test
piece in air at either room temperature (RT) or 125.degree. C. at a
speed of 6 mm per minute, and the tensile strength and the
elongation at failure were calculated from the load and
displacement read from a load cell. The results are shown in Tables
3 and 4.
TABLE-US-00003 TABLE 3 Tensile Strength Elongation In SL LL (MPa)
(%) (mass %) (.degree. C.) (.degree. C.) RT 125.degree. C. RT
125.degree. C. 0 216.9 225.7 35.7 17.6 74.9 59.6 1 213.1 224.0 33.5
17.2 70.2 63.8 2 209.1 221.6 36.1 18.2 69.6 65.9 3 208.4 219.2 39.8
19.0 63.7 63.7 4 205.8 217.5 51.3 25.5 37.4 44.6 5 203.0 215.8 55.2
26.4 45.3 46.5 6 199.0 213.8 55.8 26.7 50.2 48.4 7 196.5 212.5 57.2
26.5 45.7 53.5 9 191.2 209.8 56.8 25.1 40.8 46.3 10 187.5 207.8
57.0 24.4 41.1 47.3 13 180.0 202.7 52.9 20.5 36.4 41.4 15 177.2
200.9 45.7 16.4 38.9 7.3 Alloy composition (mass %):
Sn--xIn--1Ag--0.5Cu (x = 0-15) SL = solidus line temperature; LL =
liquidus line temperature
TABLE-US-00004 TABLE 4 Tensile Strength Elongation Ag In SL LL
(MPa) (%) (mass %) (mass %) (.degree. C.) (.degree. C.) RT
125.degree. C. RT 125.degree. C. 0 0 227.3 229.2 32.0 75.6 1 0
216.7 224.0 34.4 16.2 73.3 66.8 1 1 213.1 222.7 35.3 17.1 82.9 64.7
1 2 209.7 221.0 42.2 18.0 79.4 69.6 1 3 208.3 218.8 42.7 20.0 71.8
59.5 1 4 205.6 215.5 47.7 23.0 56.3 56.5 1 5 203.1 214.3 55.3 27.2
53.4 47.1 1 6 199.1 212.9 56.8 26.6 48.2 50.3 1 7 196.6 211.9 57.7
26.5 42.4 53.5 1 9 191.2 208.5 58.2 26.6 37.4 56.1 1 10 186.0 207.0
58.0 23.9 38.0 52.4 1 13 176.5 202.1 53.1 20.7 38.2 48.0 1 15 177.9
199.0 46.7 16.5 40.7 16.8 Alloy composition (mass %): Sn--xIn-(0 or
1)Ag--0.7Cu (x = 0-15) SL = solidus line temperature; LL = liquidus
line temperature
[0075] As shown in Tables 3 and 4, alloy compositions having a Cu
content of either 0.5% or 0.7% exhibited the same tendency. In
particular, a solder alloy having an In content of at least 4% for
the case of a Cu content of 0.5% or at least 5% for the case of a
Cu content of 0.7% exhibited a high tensile strength both at room
temperature and at a high temperature. At 125.degree. C.,
elongation at failure abruptly deteriorated if the In content
exceeded 13%.
[0076] FIG. 11 is a graph showing the relationship between the
tensile strength and the In content of Sn-(0-15)In-1Ag-0.5Cu solder
alloys at room temperature and 125.degree. C. FIG. 12 is a graph
showing the relationship between elongation at failure and the In
content of Sn-(0-15)In-1Ag-0.5Cu solder alloys at room temperature
and 125.degree. C. The measurement temperature at a high
temperature was made 125.degree. C. because the operating
temperature of semiconductor elements in recent years has reached
around 100.degree. C., so it was necessary to perform measurement
in a more severe environment.
[0077] As shown in FIG. 11, the tensile strength abruptly increased
when the In content exceeded 4%, and at an In content of 5-10%, the
tensile strength exhibited a high value even at a high temperature
of 125.degree. C. On the other hand, the tensile strength
deteriorated when the In content exceeded 13%. As shown in FIG. 12,
elongation at failure gradually worsened as the In content
increased, and at 125.degree. C., it abruptly deteriorated if the
In content exceeded 13%. The same tendency as illustrated in FIGS.
11 and 12 for tensile strength and elongation at failure was
exhibited with Sn-(0-15)In-1Ag-0.7Cu solder alloys as shown in
Table 4.
[0078] As set forth above, because a solder alloy according to the
present invention contains 1.0-13.0% of In in a Sn--Ag--Cu solder
alloy, the occurrence of electromigration which tends to develop
with such a lead free solder alloy and which eventually causes
failure of a solder joint can be suppressed. In addition, when the
In content of the solder alloy is 5-10%, not only can the
occurrence of electromigration be suppressed more effectively, but
the solder alloy can exhibit excellent mechanical properties at
high temperatures.
[0079] Accordingly, a solder alloy according to the present
invention can be used not only in CPUs but also in equipment which
involves high voltages and high currents such as power conversion
equipment for solar power generation or high-current inverters for
industrial motors. For the solder alloy having an In content of
5-10%, due to its excellent mechanical properties, it can also be
used for power semiconductors mounted on motor control inverters
for electric vehicles (EV) and plug-in hybrid vehicles (PHV).
[0080] While certain embodiments of the invention are shown in the
accompanying figures and described herein above in detail, other
embodiments will be apparent to and readily made by those skilled
in the art without departing from the scope and spirit of the
invention. For example, it is to be understood that this disclosure
contemplates that to the extent possible, one or more features of
any embodiment can be combined with one or more features of the
other embodiment. Accordingly, the foregoing description is
intended to be illustrative rather than restrictive.
* * * * *