Transactions of The Japan Institute of Electronics Packaging Vol. 8, No. 1, 2015
[Technical Paper]
Evaluation on Mechanical Properties of Sn-Bi-Ag Solder and
Reliability of the Solder Joint
Hanae Shimokawa*,***, Tasao Soga*, Koji Serizawa*, Kaoru Katayama**, and Ikuo Shohji***
*Processing Innovation Research Dept., Yokohama Research Laboratory, Hitachi. Ltd. 292 Yoshida-cho, Totsuka-ku, Yokohama 244-0817, Japan
**Global MONOZUKURI Division, Hitachi, Ltd., Information & Telecommunication Systems Company, Hitachi Omori 2nd Bldg., 27-18, 6 chome
Minamiohi, Shinagawa-ku, Tokyo 144-8572, Japan
***Division of Mechanical Science and Technology, Faculty of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515,
Japan
(Received March 31, 2015; accepted October 13, 2015)
Abstract
This paper presents low-temperature Pb-free soldering technology using Sn-57Bi-1Ag (mass%). Here, the effects of hightemperature annealing on the mechanical properties of the solder such as tensile strength and elongation are investigated. The experimental results show that during annealing, the sizes of both of Sn and Bi phases coarsen, however the
mechanical properties do not deteriorate. The deformation behavior of Sn-57Bi-1Ag is found to be dependent on sliding
at grain boundaries between Sn and Bi phases, and this behavior remains consistent even after coarsening. The creep
strength of solder joint at high temperature is also studied, and it is found that Sn-57Bi-1Ag exhibits superior creep
strength at temperature below approximately 100°C compared to the Sn-37Pb (mass%) solder.
The thermal cycling test of Sn-57Bi-1Ag solder joint is also conducted under the condition between 0°C and 90°C. The
result shows that the length of crack is shorter than Sn-37Pb in the same conditions, which means Sn-57Bi-1Ag is an
effective material for low temperature soldering.
Keywords:Sn-Bi-Ag Pb Free Solder, Mechanical Properties, Elongation, Grain Boundary Sliding, Creep Strength
1. Introduction
health concern by the addition of Sb,[8] Sn-Bi-Ag solder
A tin-lead solder such as Sn-37Pb (mass%) was the sol-
system was studied here. Regarding the amount of Ag,
der most commonly used in industry to fabricate elec-
Ueda et al. reported that 1 mass% addition of Ag increases
tronic equipment. In the light of environmental and health
the elongation of eutectic Sn-Bi solder alloy.[3]
concerns, however, a Sn-3Ag-0.5Cu (mass%) solder was
McCormack et al. reported that 0.25 to 0.5 mass% addition
developed and has been commonly used already. Since the
in near the eutectic Sn-Bi solder improves the ductility[4]
solidus temperature of Sn-3Ag-0.5Cu is 217°C, it is difficult
and Suganuma et al. reported that, for the Sn-Bi eutectic
to replace the Sn-37Pb solder used for step soldering pro-
alloy, the addition of Ag should be less than 0.8 mass% to
cess. It is also difficult to apply Sn-3Ag-0.5Cu for soldering
inhibit the formation of large primary Ag3Sn.[7] However,
the temperature-sensitive electronic components and sub-
Suganuma et al. also reported that there is little influence
strates. As the eutectic Sn-58Bi (mass%) solder melts at
of Ag content on joining properties of QFP lead frames on
138°C and this solder is one of the possible candidates for
a circuit board because of high strength of Sn-57Bi alloy
low temperature soldering, the growth of intermetallic
itself.[7] In order to apply low temperature solder to flow
compound layer and its mechanical properties have been
soldering process in future, where the solder composition
examined.[1, 2] Several reports showed that the addition
tends to change during operation, the Sn-Bi eutectic solder
of a small amount of third element such as Ag, P and Sb
added relatively high amount of Ag, namely Sn-57Bi-1Ag
refines the microstructure of eutectic Sn-58Bi solder,
(mass%) with melting point of 138°C, was selected to
improving its ductility.[3–7] Due to the difficulty to control
study.
very small amount of P, such as 0.02 mass%,[5] and the
46
On Sn-57Bi-1Ag, due to the low melting point, it is conCopyright © The Japan Institute of Electronics Packaging
Shimokawa et al.: Mechanical Properties of Sn-Bi-Ag Solder and Reliability (2/9)
sidered that changes in the microstructure of the solder in
1,000, 2,500, and 7,500 h in an oven, and photos of the
high-temperature service will affect the mechanical prop-
microstructure at three different points in each specimen
erties and creep strength of solder joint, but these effects
were taken by SEM. Six lines were drawn in each SEM
have not been studied enough, especially about the defor-
photo and the width of the Sn and Bi phases crossing the
mation mechanism after microstructure coarsening. In
line were measured, as shown in Fig. 2. Such measure-
this study, the effects of high-temperature annealing, up to
ments were carried out at about 100 points. The average
125°C, on the microstructure and mechanical properties
size of Sn phase was plotted against annealing time.
of this Sn-57Bi-1Ag solder were investigated, and the
2.3 Creep test
deformation mechanism was discussed in a metallographic
A schematic of the creep test specimen is shown in Fig.
point of view. Moreover, although Suganuma et al.
3. The sample was prepared by soldering a Cu lead to a Cu
reported that heat-exposure below 100°C has no serious
pad on a glass epoxy substrate using solder paste. The Cu
degradation on the joint structure with Sn-Pb plated elec-
lead was coated with Ni (thickness: approximately 2 μm)
trodes,[7] the similar phenomenon has not been con-
and then Au (thickness: approximately 0.1 μm). The maxi-
firmed yet for Ni/Au plated electrodes. It is important to
mum temperature of reflow soldering was 200°C for
investigate the joint reliability with Ni/Au plated elec-
Sn-57Bi-1Ag and 220°C for Sn-37Pb. The joint area was 9
trodes because such electrodes are commonly used for
mm2. A load was applied to the Cu lead in an oven at 90
lead-free soldering. Under such background, the permissi-
and 125°C, as shown in Fig. 3, and the time until rupture
ble maximum temperature of the solder joint using the Ni/
was measured. The load was varied from 0.5 to 3.3 kg.
Au plated electrode was investigated by the creep test of
2.4 Thermal cycling test
solder joint as a function of temperature. The reliability of
In order to evaluate the reliability of Sn-57Bi-1Ag solder
solder joint was evaluated by the thermal cycling test
joint, a ceramic pin grid array (PGA) package with 1,019
under the permissible maximum temperature obtained.
pins was soldered to the hole of a glass epoxy substrate, as
shown in Fig. 4. The PGA was 42 mm × 42 mm × 2.34 mm
2. Experimental Procedures
in size, and the temperature of the solder bath was 215°C.
2.1 Tensile test
The pin was made of Kovar plated with Ni (thickness:
Sn-57Bi-1Ag and Sn-37Pb solders were melted in cruci-
approximately 2 μm) followed by Au (thickness: approxi-
bles on a hot plate and poured into carbon molds heated to
mately 0.1 μm). The pin pitch was 1.27 mm and the thick-
200°C for Sn-57Bi-1Ag, and 250°C for Sn-37Pb. The speci-
ness of the substrate was 4 mm. After soldering the PGA
mens were then cooled to room temperature. The shape
package to the substrate, the thermal cycling test were
and size of the specimens are shown in Fig. 1. The speci-
carried out on this specimen in air at between 0°C and
mens were aged at room temperature for 10 days after
casting. Tensile tests were carried out using an Instron
㼘㼕㼚㼑
mechanical testing machine (4204) at a cross-head speed
㻿㼚
㻮㼕
㼍㻝
of 0.1 mm/min at room temperature to obtain tensile
strength and elongation until rupture. The elongation was
㼍㻞
converted from the amount of displacement of crosshead.
㼍㻟
To investigate the deformation behavior of the solders,
㼍㻠
the surfaces of the specimens were polished with 0.25 μm
diamond paste and the microstructure was then observed
㻿㼕㼦㼑㻌㼛㼒㻌㼜㼔㼍㼟㼑㻩㻭㼢㼑㻔㼍㻝䡡䡡䡡㼍㼚㻕
using a scanning electron microscope (SEM), before and
Fig. 2 Schematic representation of measurement of size of
after the tensile test.
Sn phase.
2.2 Measurement of size of Sn phase
The specimens were annealed at 65, 90, and 125°C for
㻡
㻝
㻞
㻝㻜
㻳㼘㼍㼟㼟㻌㼑㼜㼛㼤㼥㻌
㼟㼡㼎㼟㼠㼞㼍㼠㼑
㻿㼛㼘㼐㼑㼞
㻔㼖㼛㼕㼚㼠㻌㼍㼞㼑㼍㻦㻌㻟㻌㼙㼙㻌㼤㻌㻟㻌㼙㼙㻕
㻟㻞
㻯㼡㻌㼘㼑㼍㼐
㻔㻺㼕㻌㻗㻌㻭㼡㻌㼜㼘㼍㼠㼕㼚㼓㻕
㻯㼡㻌㻼㼍㼐
㼇㼙㼙㼉
Fig. 1 Shape and size of specimen for tensile test.
㼃
Fig. 3 Schematic of creep test set up.
47
㼏㼑㼞㼍㼙㼕㼏㻌㻼㻳㻭
㼅
㼒㼛㼞㻌㼛㼎㼟㼑㼞㼢㼍㼠㼕㼛㼚㻌
㼍㼚㼐㻌㻲㻱㻹㻌㼍㼚㼍㼘㼥㼟㼕㼟
䞉䞉䞉䞉
㻿㼡㼎㼟㼠㼞㼍㼠㼑
㻜
㼤
Transactions of The Japan Institute of Electronics Packaging Vol. 8, No. 1, 2015
㻳㼘㼍㼟㼟㻌㼑㼜㼛㼤㼥㻌
㻜
㻔㼎㻕㻌㻿㼠㼞㼡㼏㼠㼡㼞㼑㻌㼛㼒㻌㼐㼕㼍㼓㼛㼚㼍㼘㻌㼜㼘㼍㼚㼑
㼟㼡㼎㼟㼠㼞㼍㼠㼑
㻔㼍㻕㻌㻿㼜㼑㼏㼕㼙㼑㼚㻌㼡㼟㼕㼚㼓㻌㻼㻳㻭
㼏㼑㼚㼠㼑㼞
㻝㻜㻝㻥㻌㼜㼕㼚㻌
㼏㼑㼞㼍㼙㼕㼏㻌㻼㻳㻭
㻼㻳㻭
䡡䡡䡡
㻰㼕㼍㼓㼛㼚㼍㼘㻌㼜㼘㼍㼚㼑㻌
㼒㼛㼞㻌㼛㼎㼟㼑㼞㼢㼍㼠㼕㼛㼚㻌
㼍㼚㼐㻌㻲㻱㻹㻌㼍㼚㼍㼘㼥㼟㼕㼟
㻼㻳㻭
㼅
㻼㻳㻭
䡡䡡䡡
䞉䞉䞉䞉
㻿㼡㼎㼟㼠㼞㼍㼠㼑
㻜
㻳㼘㼍㼟㼟㻌㼑㼜㼛㼤㼥㻌
㼟㼡㼎㼟㼠㼞㼍㼠㼑
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㼟㼛㼘㼐㼑㼞
㼤
㻯㼡
㻜
㻔㼎㻕㻌㻿㼠㼞㼡㼏㼠㼡㼞㼑㻌㼛㼒㻌㼐㼕㼍㼓㼛㼚㼍㼘㻌㼜㼘㼍㼚㼑
㻔㼍㻕㻌㻿㼜㼑㼏㼕㼙㼑㼚㻌㼡㼟㼕㼚㼓㻌㻼㻳㻭
㻷㼛㼢㼍㼞 㼜㼕㼚㻌
㼣㼕㼠㼔㻌㻺㼕㻛㻭㼡㻌㼜㼘㼍㼠㼕㼚㼓
㻔㼏㻕㻌㻿㼠㼞㼡㼏㼠㼡㼞㼑㻌㼛㼒㻌㼟㼛㼘㼐㼑㼞㻌㼖㼛㼕㼚㼠
Fig. 4 Shape of the specimen for reliability test.
㻼㻳㻭
㻼㻳㻭
䡡䡡䡡
㻷㼛㼢㼍㼞 㼜㼕㼚㻌
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
Table 1 Material properties
used.
㼣㼕㼠㼔㻌㻺㼕㻛㻭㼡㻌㼜㼘㼍㼠㼕㼚㼓
㻿㼡㼎㼟㼠㼞㼍㼠㼑
㻜
㼤
㼟㼛㼘㼐㼑㼞
Temperature
㻜
Material
㻔㼎㻕㻌㻿㼠㼞㼡㼏㼠㼡㼞㼑㻌㼛㼒㻌㼐㼕㼍㼓㼛㼚㼍㼘㻌㼜㼘㼍㼚㼑
(°C)
㻼㻳㻭
Sn-57Bi-1Ag
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㼟㼛㼘㼐㼑㼞
Sn-37Pb
(kgf/mm2)
Yield stress
Work-hardening
coefficient
(1/°C)
(kgf/mm2)
(kgf/mm2)
㻔㼏㻕㻌㻿㼠㼞㼡㼏㼠㼡㼞㼑㻌㼛㼒㻌㼟㼛㼘㼐㼑㼞㻌㼖㼛㼕㼚㼠
8.6
170
3,430
↑
↑
4.2
90
1,350
↑
↑
25
㻷㼛㼢㼍㼞 㼜㼕㼚㻌
㼣㼕㼠㼔㻌㻺㼕㻛㻭㼡㻌㼜㼘㼍㼠㼕㼚㼓
25
CTE
1.38 × 10-5
3,970
125
Poisson
ratio
㻯㼡
0.35
-50
-60
㻯㼡
Young’s
modulus
3,090
0.3
↑
2,940
0.7
35
-5
4.11
114.2
-5
2.16
62.6
-5
0.92
28.7
2.10 × 10
2.24 × 10
㻔㼏㻕㻌㻿㼠㼞㼡㼏㼠㼡㼞㼑㻌㼛㼒㻌㼟㼛㼘㼐㼑㼞㻌㼖㼛㼕㼚㼠
120
1,810
↑
-55
18,400
0.23
2.45 × 10-5
20
18,340
↑
2.70 × 10-5
100
18,300
↑
2.83 × 10-5
-55
2,300
0.23
1.56 × 10-5
20
2,300
↑
↑
100
2,300
↑
↑
PGA
Substrate
2.44 × 10
㼀㼑㼚㼟㼕㼘㼑㻌㼟㼠㼞㼑㼚㼓㼠㼔㻌㻔㻹㻼㼍㻕
90°C, with a hold time at each temperature of 30 min and
heating-up and cooling-down period of about 10 min. Along
the diagonal line of PGA package, the cross-sectional
microstructure of the soldered joints and the length of
cracks were then observed by SEM. In parallel, a 2D plane
model of the solder joint was established for finite element
analysis and the amplitude of equivalent strain occurred at
the solder joint along the diagonal line was calculated.
Table 1 shows the material properties used in the model-
㻤㻜
㻤㻜
㼀㼑㼚㼟㼕㼘㼑㻌㼟㼠㼞㼑㼚㼓㼠㼔
㻱㼘㼛㼚㼓㼍㼠㼕㼛㼚
㻢㻜
㻢㻜
㻠㻜
㻠㻜
㻞㻜
㻞㻜
㻜
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㻿㼚㻙㻟㻣㻼㼎
㻱㼘㼛㼚㼓㼍㼠㼕㼛㼚㻌㻔㻑㻕
㼅
㻜
Fig. 5 Tensile strength and elongation of Sn-57Bi-1Ag and
Sn-37Pb.
ing. The principal stress in x direction was also obtained.
The results were compared with that of Sn-37Pb solder
sidered to be generally brittle, the origin of the high elon-
joint.
gation of the Sn-57Bi-1Ag solder was investigated. Figure 6
shows the microstructures of the surfaces of Sn-57Bi-1Ag
3. Results and Discussion
and Sn-37Pb specimens before and after the tensile test.
3.1 Mechanical properties and deformation behav-
The initial microstructure of Sn-57Bi-1Ag is fine lamellar
ior
structure. The light phases in Fig. 6(a) are Bi phases, and
The tensile strength and elongation of Sn-57Bi-1Ag and
the gray regions are the Sn-rich phases. Ag is dispersed as
Sn-37Pb are shown in Fig. 5. The tensile strength of
Ag3Sn at the boundary of the Sn and Bi phases. As there is
Sn-57Bi-1Ag is higher than that of Sn-37Pb, however the
a little Ag in the Sn-57Bi-1Ag solder, Ag3Sn is not clearly
elongation of both solders is almost the same. As Bi is con-
observed in Fig. 6. Primary crystals of Sn in the initial
48
Shimokawa et al.: Mechanical Properties of Sn-Bi-Ag Solder and Reliability (4/9)
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㻿㼛㼘㼐㼑㼞
㻮㼑㼒㼛㼞㼑
㼠㼑㼚㼟㼕㼘㼑
㼠㼑㼟㼠
㻝㻡㻌μm
㻔㼍㻕
㻮㼕
㻿㼚㻙㻟㻣㻼㼎
㻢㻌μm
㻝㻡㻌μm
㻔㼒㻕
㻔㼎㻕
㻿㼚
㻿㼚
㻿㼚
㻮㼕
㻿㼚
㼜㼞㼕㼙㼍㼞㼥㻌
㼏㼞㼥㼟㼠㼍㼘
㻿㼚 㼜㼞㼕㼙㼍㼞㼥㻌
㼏㼞㼥㼟㼠㼍㼘
㻼㼛㼕㼚㼠㻌㼛㼒㻌
㼛㼎㼟㼑㼞㼢㼍㼠㼕㼛㼚㻌
㻔㼏㻕
㻼㼎
㻔㼐㻕
㻔㼑㻕
㻔㼓㻕
㻭㼒㼠㼑㼞
㼠㼑㼚㼟㼕㼘㼑
㼠㼑㼟㼠
㻯㼞㼍㼏㼗
㻮㼕
㻿㼚
㻿㼘㼕㼐㼕㼚㼓㻌㼍㼠㻌㼓㼞㼍㼕㼚㻌㼎㼛㼡㼚㼐㼍㼞㼥㻌㼎㼑㼠㼣㼑㼑㼚㻌㻿㼚㻌㼍㼚㼐㻌㻮㼕㻌㼜㼔㼍㼟㼑㼟
Fig. 6 Microstructures of Sn-57Bi-1Ag and Sn-37Pb before and after tensile test.
microstructure of Sn-57Bi-1Ag are also observed, consid-
㻔㼎㻕
㻔㼍㻕
ered to have crystallized in localized areas of higher Sn
concentration. In these primary crystals of Sn, Bi is finely
precipitated. Figures 6(c) and (d) show the microstructure
changes from Figs. 6(a) and (b), respectively, following
the tensile test. The direction of tensile distortion is shown
in Fig. 6. These figures show that sliding occurs at grain
boundaries between Sn and Bi phases. Figure 6(e) shows
the microstructure of another point on the same specimen
㻝㻡㻌μm
after tensile test. This figure also shows that sliding occurs
Fig. 7 Microstructures of Sn-57Bi-1Ag
at grain boundaries between Sn and Bi phases. Primary
(a) Initial.
crystals of Sn, however, exhibit little deformation under
(b) After annealing at 90°C for 2,500 h.
tensile strain, due to the precipitation hardening of Sn by
smaller Bi particles. These results indicate that elongation
Sn phases. Since sliding is not clearly observed at the
will decrease when the amount of Sn in Sn-57Bi-1Ag is
boundary between Sn and Pb phases, the deformation in
increased because the proportion of precipitation-hard-
Sn-37Pb seems to occur as a result of sliding within Sn and
ened primary crystal increases. Therefore, the relatively
Pb phases. Kitamura et al. reported that boundary sliding
high elongation of Sn-57Bi-1Ag is attributable to sliding at
also occurs at the interface between colonies in the initial
grain boundaries in the area of fine lamellar microstruc-
deformation stage.[10]
ture and the contribution of the sliding at the grain bound-
The relatively high elongation of the Sn-57Bi-1Ag solder
ary of the fine Bi inside primary crystal of Sn to the defor-
suggests that this solder will be useful in service. How-
mation is negligible. From the report of mechanical
ever, its low melting point (138°C) means that the Sn-57Bi-
properties of the Sn-57Bi eutectic solder,[9] it was shown
1Ag solder will be particularly sensitive to high-tempera-
that the dominant factor of creep deformation at 75°C is
tures. High-temperature may induce microstructure
sliding at grain boundary. Although the evaluation method
coarsening, with the likely result that the mechanical prop-
is different from our test, the deformation behavior would
erties will change because Sn-57Bi-1Ag is deformed by
not be changed so much by the addition of 1 mass% Ag.
sliding at fine grain boundaries in the area of lamellar
Figures 6(f) and (g) show the microstructures of Sn-
microstructure. Therefore, the microstructure and
37Pb before and after the tensile test. The microstructure
mechanical properties of the Sn-57Bi-1Ag solder were
of Sn-37Pb is consisted of two phases; the lighter areas are
examined as a function of annealing temperature.
associated with Pb phases, whereas the darker areas are
Figure 7 shows the microstructural change of the same
49
㻢
㻢㻡䉝
㻠
㻥㻜䉝
㻞
㻝㻞㻡䉝
㻜
㻜
㻡㻜㻜㻜
㻝㻜㻜㻜㻜
㻤㻜
㻤㻜
㼀㼑㼚㼟㼕㼘㼑㻌㼟㼠㼞㼑㼚㼓㼠㼔
㻢㻜
㻢㻜
㻠㻜
㻠㻜
㻱㼘㼛㼚㼓㼍㼠㼕㼛㼚
㻞㻜
㻞㻜
㻱㼘㼛㼚㼓㼍㼠㼕㼛㼚㻌㻔㻑㻕
㻤
㼀㼑㼚㼟㼕㼘㼑㻌㼟㼠㼞㼑㼚㼓㼠㼔㻌㻔㻹㻼㼍㻕
㻳㼞㼍㼕㼚㻌㼟㼕㼦㼑㻌㼛㼒㻌㻿㼚㻌㼜㼔㼍㼟㼑㻌㻔μ㼙㻕
Transactions of The Japan Institute of Electronics Packaging Vol. 8, No. 1, 2015
㻜
㻜
㻜
㼀㼕㼙㼑㻌㼍㼚㼚㼑㼍㼘㼑㼐㻌䠄䡄䠅
㻞
㻠
㻢
㻿㼕㼦㼑㻌㼛㼒㻌㻿㼚㻌㼜㼔㼍㼟㼑㻌㻔μm㻕
Fig. 8 Change of size of Sn phase with annealing for Sn-57Bi-
Fig. 9 Change in tensile strength and elongation with size of
1Ag solder.
Sn phase.
㻔㼎㻕
㻔㼍㻕
㻼㼛㼕㼚㼠㻌㼛㼒
㼛㼎㼟㼑㼞㼢㼍㼠㼕㼛㼚
㻤
㻔㼏㻕
㻮㼕
㻿㼚 㼜㼞㼕㼙㼍㼞㼥㻌
㼏㼞㼥㼟㼠㼍㼘
㻝㻡㻌μm
Fig. 10 Microstructures of Sn-57Bi-1Ag annealed at 90°C for 7,500 h and change
after tensile test.
(a) Before tensile test
(b) After tensile test (same point with (a))
(c) Another point after tensile test
area after high-temperature annealing. Figure 7(a) is the
mary crystal still does not deform, due to precipitation-
initial microstructure, and (b) is after annealing at 90°C for
hardening by Bi. However, sliding lines are observed very
2,500 h. Coarsening of both Sn and Bi phases is observed.
locally in the Bi phase shown in Fig. 10(c), because the
The sizes of the phases were measured as described in
solid solution of Sn to Bi is very limited. The cause of elon-
Fig. 2 for each annealing condition, with the results shown
gation becoming lower at a Sn grain size of approximately
in Fig. 8. Sn phase can be seen to coarsen with increasing
2 μm compared to the initially fine structure is not clear.
temperature and time. Tensile tests were carried out for
However it is considered that rapid cooling such as that in
each annealing condition in order to examine the mechani-
soldering processes does not allow the solder to solidify
cal properties of coarsened Sn-57Bi-1Ag. The change in
along the line of Sn-Bi equilibrium, and excess Sn initially
tensile strength and elongation according to the average
soluted in the Bi phase may precipitate during Sn coarsen-
size of the Sn phase is shown in Fig. 9. As the size of the
ing, according to a report on the effects of annealing on
Sn phase becomes approximately 2 μm, the tensile
Sn-Bi eutectic solder by Hu et al.[11] It has been also
strength increased while elongation decreased. When the
reported that, in the case of the Sn-Bi eutectic solder, the
size of Sn phases becomes larger than approximately 4
solid solution hardening of Bi into Sn-rich solid solution
μm, elongation can be seen to increase. In order to deter-
causes the increase of the hardness at high temperature,
mine the reason that Sn-57Bi-1Ag coarsened by high-tem-
such as 100°C, by Miyazawa et al.[12] These microstruc-
perature annealing has the same elongation as unannealed
tural changes are expected to occur also in the case of
Sn-57Bi-1Ag, which has a fine microstructure, the defor-
Sn-57Bi-1Ag, resulting in lowering the elongation at a Sn
mation behavior of annealed Sn-57Bi-1Ag was examined.
grain size of approximately 2 μm. These results were
Figure 10 shows the microstructures of Sn-57Bi-1Ag
obtained from tensile tests at cross-head speed of 0.1 mm/
annealed at 90°C for 7,500 h, by tensile test. Deformation
min, in which the strain rate was estimated to be about
still occurs at grain boundaries between Sn and Bi phases,
1.7 × 10-4 s-1. However, at higher test speed of 500 mm/
the same as in unannealed Sn-57Bi-1Ag, and the Sn pri-
min, where the strain rate is about 8.3 × 10-1 s-1, Sn-57Bi-
50
Shimokawa et al.: Mechanical Properties of Sn-Bi-Ag Solder and Reliability (6/9)
㻠
㻠㻜
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㻿㼚㻙㻟㻣㻼㼎
㻿㼔㼑㼍㼞㻌㼟㼠㼞㼑㼟㼟㻌䠄㻹㻼㼍䠅
㻱㼘㼛㼚㼓㼍㼠㼕㼛㼚㻌㻔㻑㻕
㻢㻜
㻿㼚㻙㻟㻣㻼㼎
㻞㻜
㻜
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㻜㻚㻝
㻡㻜㻜
㻯㼞㼛㼟㼟㻌㼔㼑㼍㼐㻌㼟㼜㼑㼑㼐㻌㻔㼙㼙㻛㼙㼕㼚㻕
䖃 Sn-57Bi-1Ag at 125Υ
䕿 Sn-57Bi-1Ag at 90Υ
䕦 Sn-37Pb at 125Υ
䕧 Sn-37Pb at 90Υ
㻟
㻞
㻖
㻝
㻖
㻜
Fig. 11 The relationship of elongation and test speed.
㻖㼟㼠㼕㼘㼘㻌㼘㼕㼢㼕㼚㼓
㻝㻜㻞㻌
㻝㻜㻟㻌
㻝㻜㻢㻌
㻝㻜㻠
㻝㻜㻡㻌
㻾㼡㼜㼠㼡㼞㼑㻌㼠㼕㼙㼑㻌㻔㼙㼕㼚㻕
㻔㼍㻕㻌㻾㼡㼜㼠㼡㼞㼑㻌㼠㼕㼙㼑㻌㼛㼒㻌㼟㼛㼘㼐㼑㼞㻌㼖㼛㼕㼚㼠㻌㼍㼓㼍㼕㼚㼟㼠㻌㼟㼔㼑㼍㼞㻌㼟㼠㼞㼑㼟㼟
1Ag becomes brittle. Figure 11 shows the elongation of
Sn-57Bi-1Ag and Sn-37Pb at a cross-head speed of 0.1 and
㻔㻝㻕㻌㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㻔㻞㻕㻌㻿㼚㻙㻟㻣㻼㼎
500 mm/min. In the case of the Sn-57Bi-1Ag specimen
tested at 500 mm/min, elongation becomes significantly
lower than that at 0.1 mm/min, whereas in the case of the
Sn-37Pb specimen tested at 500 mm/min, elongation
becomes lower than that at 0.1 mm/min, but the differ-
㻝㻜㻌μ㼙
ence of the elongation between two cross head speed is
㻔㼎㻕㻌㻲㼞㼍㼏㼠㼡㼞㼑㻌㼟㼡㼞㼒㼍㼏㼑㼟㻌㼛㼒㻌㼟㼛㼘㼐㼑㼞㻌㼖㼛㼕㼚㼠㼟㻌
small. From these results, Sn-57Bi-1Ag is considered to be
Fig. 12 Rupture time by creep test.
essentially brittle, however, it has a useful property that
grain boundary sliding is likely to occur at low strain rates
at room temperature. This property is probably attributhigh homologous temperature at room temperature. This
indicates that Sn-57Bi-1Ag would be useful in applications
where low strain rates prevail, such as by thermomechanical fatigue, yet would not be suitable for joints subjected to
㻾㼡㼜㼠㼡㼞㼑㻌㼠㼕㼙㼑㻌㻔㼙㼕㼚䠅
able to its low melting point, 138°C, giving the solder a
㻝㻜㻢㻌
1.E+05
㻝㻜㻡㻌
1.E+04
㻝㻜㻠
㻿㼚㻙㻟㻣㻼㼎
㻝㻜㻟㻌
1.E+03
high strain rates, such as by rapid heating and cooling or
impact.
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㻝㻜㻞㻌
1.E+02
㻣㻜
3.2 Creep properties of solder joint
Figure 12 shows the rupture time for solder joint under
shear stress. The rupture time for Sn-57Bi-1Ag is shorter
㻥㻜
㻝㻝㻜
㻝㻟㻜
㻝㻡㻜
㼀㼑㼙㼜㼑㼞㼍㼠㼡㼞㼑㻌㻔䉝㻕
Fig. 13 Effect of temperature on rupture time by shear
stress.
than that for Sn-37Pb at 125°C, yet longer than Sn-37Pb at
90°C. Rupture occurred inside both the Sn-57Bi-1Ag and
Above 100°C, the Sn-57Bi-1Ag solder becomes very soft;
Sn-37Pb solder layer, as shown in Fig. 12(b). Figure 13
the hardness at 21°C is 20 HV, yet the hardness at 100°C
shows the dependence of temperature on rupture time at
lowers about 75% to 5 HV by the measurement of Sn-57Bi-
shear stresses of around 1.6 MPa. In this evaluation, only
1Ag solder ingot.
two temperature conditions were conducted. However,
Until now, the creep strength of the Sn-Bi eutectic solder
from the assumption that the properties change almost
has been often studied and compared with Sn-37Pb. From
proportionally in the range between 90°C and 125°C as
the report of Mei et al.,[13, 14] at 65°C, the steady state
both temperatures are above half absolute temperature of
shear strain rate of Sn-58Bi solder is smaller than that of
those melting points, the dotted lines were described in
Sn-37Pb, and from the report of Jin et al.,[15] at 100°C, the
Fig. 13. By taking the intersection of these lines, creep
creep resistance of Sn-37Pb solder is inferior to that of the
strength of the solder joint of Sn-57Bi-1Ag and Sn-37Pb are
Sn-57Bi solder. Taking into consideration of these results
almost the same at approximately 100°C, and at below
on creep tests on the Sn-Bi eutectic solder together,
approximately 100°C, the creep strength of Sn-57Bi-1Ag is
Sn-57Bi-1Ag solder seems to be able to use up to the tem-
superior to that of Sn-37Pb, indicating that the permissible
perature, approximately 100°C.
maximum temperature of the Sn-57Bi-1Ag solder joint
3.3 Reliability of solder joint
using Ni/Au plated electrodes is approximately 100°C.
Because the Sn-57Bi-1Ag solder joint using Ni/Au plated
51
Transactions of The Japan Institute of Electronics Packaging Vol. 8, No. 1, 2015
㻞㻜㻜
㻺㼕
㻸㼑㼚㼓㼠㼔㻌㼛㼒㻌㼏㼞㼍㼏㼗㻌㻔μ㼙㻕
㼏㼞㼍㼏㼗
㻝㻢㻜
㻿㼚㻙㻟㻣㻼㼎
㻝㻞㻜
㻤㻜
㻠㻜
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㻜
㻜
㻡
㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓㻌㼟㼛㼘㼐㼑㼞
㻝㻜
㻝㻡
㻞㻜
㻺㼡㼙㼎㼑㼞㻌㼛㼒㻌㼜㼕㼚㻌㼒㼞㼛㼙㻌㼏㼑㼚㼠㼑㼞
Fig. 15 Comparison of length of crack between Sn-57Bi-1Ag
25 μm
and Sn-37Pb (0~90°C, 1,000 cycles).
Fig. 14 Crack occurred at Sn-57Bi-1Ag solder joint (0~90°C,
Amplitude of equivalent strain
1,000 cycles).
electrodes has excellent creep properties below approximately 100°C comparing with Sn-37Pb as mentioned
above, the thermal cycling test was conducted between 0
and 90°C, for 1,000 cycles to evaluate the reliability of the
solder joint. Figure 14 shows the crack occurred at the
㻜㻚㻜㻝㻠
㻜㻚㻜㻝㻞
㻜㻚㻜㻜㻤
㻜㻚㻜㻜㻢
㻜㻚㻜㻜㻠
solder and the Ni layer. The length of the crack at the each
pin along the diagonal line of PGA package is shown in
Sn-57Bi-1Ag
㻜㻚㻜㻜㻞
㻜
Sn-57Bi-1Ag solder joint after 1,000 cycles. The crack propagates in the solder layer, not at the interface between the
Sn-37Pb
㻜㻚㻜㻝
㻜
㻡
㻝㻜
㻝㻡
㻞㻜
Number of pin from center
Fig. 16 Comparison of the amplitude of equivalent strain
between Sn-57Bi-1Ag and Sn-37Pb (0~90°C).
Fig. 15, revealing that the length of crack of Sn-57Bi-1Ag
㼄㻙㼐㼕㼞㼑㼏㼠㼕㼛㼚 㼜㼞㼕㼚㼏㼕㼜㼍㼘㻌㼟㼠㼞㼑㼟㼟㻌
㻔㼗㼓㼒㻛㼙㼙㻞㻕㻌
solder is shorter than that of Sn-37Pb solder.
In order to clarify the relationship between the life of
solder joint and the strain occurred at the solder joint, the
elasto-plastic finite element analysis was conducted and
the amplitude of equivalent strain at the solder joint along
the diagonal line of PGA package was obtained. Figure 16
shows the amplitude of equivalent strain of the Sn-57Bi-
㻟㻚㻡
㻟㻚㻜
Sn-57Bi-1Ag
㻞㻚㻡
㻞㻚㻜
Sn-37Pb
㻝㻚㻡
㻝㻚㻜
㻜㻚㻡
㻜㻚㻜
㻜
1Ag and Sn-37Pb solder joints. It shows the amplitude of
㻡
㻝㻜
㻝㻡
㻞㻜
㻺㼡㼙㼎㼑㼞㻌㼛㼒㻌㼜㼕㼚㻌㼒㼞㼛㼙㻌㼏㼑㼚㼠㼑㼞㻌
equivalent strain of Sn-57Bi-1Ag solder joint is smaller than
Fig. 17 Comparison of x-direction principal stress between
that of Sn-37Pb solder joint, due to the smaller Young’s
Sn-57Bi-1Ag and Sn-37Pb (0~90°C).
range between 0 and 90°C. From the above FEM analysis,
the principal stress in x direction at the interface between
solder and Ni was also obtained, as shown in Fig. 17. The x
direction principal stress of Sn-57Bi-1Ag solder joint at the
No. 15 pin, at most external side of PGA package, is about
1.7 times of the Sn-37Pb solder joint. This result means
that it is important for the Sn-57Bi-1Ag solder joint to
choose appropriate materials of pin to avoid peeling at the
interface, because of relatively high stress at the interface
than the conventional solder joint.
㻭㼙㼜㼘㼕㼠㼡㼐㼑㻌㼛㼒㻌㼑㼝㼡㼕㼢㼍㼘㼑㼚㼠㻌㼟㼠㼞㼍㼕㼚
modulus of Sn-37Pb than Sn-57Bi-1Ag in the considerable
㻜㻚㻝㻜㻜
䖃 㻿㼚㻙㻟㻣㻼㼎
㻜㻚㻜㻝㻜
䕧 㻿㼚㻙㻡㻣㻮㼕㻙㻝㻭㼓
㻜㻚㻜㻜㻝
㻝㻜㻜
㻝㻜㻜㻜
㻝㻜㻜㻜㻜
㻝㻜㻜㻜㻜㻜
㻝㻜㻜㻜㻜㻜㻜
㻲㼍㼠㼕㼓㼡㼑㻌㼘㼕㼒㼑㻌㻔㼏㼥㼏㼘㼑㼟㻕
Fig. 18 Fatigue life of Sn-57Bi-1Ag and Sn-37Pb (0~90°C).
As respects the life of solder joint, we decided that the
life of the solder joint is the cycle when the crack became
amplitude of the equivalent strain. This result indicates
300 μm long, the half length of the depth, 600 μm, of the
that if the strain occurred in the solder joint is same, the
hole. Figure 18 shows the relationship of the life and the
life of Sn-37Pb solder joint may be longer. However, due to
52
Shimokawa et al.: Mechanical Properties of Sn-Bi-Ag Solder and Reliability (8/9)
the mechanical properties of Sn-57Bi-1Ag, the strain
of Bi-Sn Solder Alloys by Ag-Doping,” Journal of
occurred in the solder joint is smaller than Sn-37Pb and
Electronic Materials, Vol. 26, No. 8, pp. 954–958,
finally the life of Sn-57Bi-1Ag solder joint could be longer
1997.
than that of Sn-37Pb. Therefore, the Sn-57Bi-1Ag solder is
[5] Y. Nakahara, J. Matsunaga, and R. Ninomiya, “Effect
effective for soldering the electronic component and sub-
of Trace Elements on the Mechanical Properties of
strate with low temperature resistance, in moderate envi-
Sn-Bi Solder,” Proc. 6th Symposium on Microjoining
ronment conditions like under approximately 100°C.
and Assembly Technology in Electronics, pp. 251–
254, 2000.
[6] S. Sakuyama, T. Akamatsu, K. Uenishi, and T. Sato,
4. Conclusions
In this study, the effects of high-temperature annealing,
“Effects of a Third Element on Microstructure and
up to 125°C, on the microstructure and mechanical prop-
Mechanical Properties of Eutectic Sn-Bi Solder,”
erties of the Sn-57Bi-1Ag solder were investigated, includ-
Transactions of The Japan Institute of Electronics
ing an investigation of the creep strength of solder joint as
Packaging, Vol. 2, No. 1, pp. 98–103, 2009.
a function of temperature and time. The reliability of solder
[7] K. Suganuma, T. Sakai, K. Kim, Y. Takagi, J.
joint was evaluated by the thermal cycling test under the
Sugimoto, and M. Ueshima, “Thermal and Mechani-
permissible maximum temperature obtained. The results
cal Stability of Soldering QFP With Sn-Bi-Ag Lead-
can be summarized as follows:
Free Alloy,” IEEE Transactions of Electronics Pack-
1. The deformation behavior of Sn-57Bi-1Ag depends
on sliding at grain boundaries between Sn and Bi
phases in the area of lamellar structure, and this
behavior remains consistent even after coarsening.
aging Manufacturing, Vol. 25, No. 4, pp. 257–261,
2002.
[8] M. Okamoto, T. Nakatsuka, O. Ikeda, K. Serizawa,
and H. Shimokawa, “Lead-Free Soldering Technolo-
2. The creep strength of Sn-57Bi-1Ag is superior to Sn-
gies Meet the Restriction of the Use of Certain Haz-
37Pb solder at temperatures below approximately
ardous Substances in Regions Including EU,”
100°C when soldered to a lead plated with Ni and
Hitachihyoron, Vol. 88, No. 12, pp. 66–69, 2006.
[9] Y. Kariya, “Mechanical Properties,” Journal of Japan
Au.
3. As the results of thermal cycling test under the con-
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[10] T. Kitamura, S. Kikuchi, and M. Koiwa, “Nonuniform
Sn-57Bi-1Ag is shorter than that in Sn-37Pb, which
Deformation and Dynamic Recrystallization of As-
means Sn-57Bi-1Ag is effective material for low tem-
cast Pb-Sn Eutectic Alloys,” Journal of the Society of
perature soldering.
Materials Science Japan, Vol. 40, No. 448, pp. 15–20,
1991.
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Hanae Shimokawa
Tasao Soga
Koji Serizawa*
Kaoru Katayama
Ikuo Shohji
*Koji Serizawa
Current address: Senju Metal Industry Co., Ltd
54