Materials Transactions, Vol. 45, No. 3 (2004) pp. 783 to 789
Special Issue on Lead-Free Soldering in Electronics
#2004 The Japan Institute of Metals
Microstructure and Strength of Sn-Bi Coated Sn-3.5 mass%Ag Solder Alloy
Jaesik Lee1 , Woongho Bang2 , Jaepil Jung1 and Kyuhwan Oh2
1
2
Department of Materials Science and Engineering, University of Seoul, Seoul 130-743, Korea
Department of Materials Science and Engineering, Seoul National University, Seoul 151-741, Korea
Sn-Bi coated Sn-3.5 mass%Ag solder alloy was investigated as a possible low melting temperature solder. Electroplating method was used
to form Sn-Bi coated solder alloy on the Sn-3.5 mass%Ag alloy. Sn-Bi coated solders were bumped on the FR-4 substrate and Cu plates by a
reflow machine at different temperatures. The die shear strength and microstructure were evaluated with scanning electron microscope (SEM).
The compositions of Sn-Bi coated solder, and Bi distribution of solder were studied by electron probe X-ray analyzer (EPMA). The Sn-Bi coated
Sn-Ag solder was possible to be bonded at low temperature such as 473-523 K and it is comparable to Sn-37%Pb solder. Intermetallic
compounds (IMC’s) along the bonded interface were well formed in the coated solder composed of high Bi and the compositions consisted of
59.49 at%Sn-37.20 at%Cu-3.31 at%Ag. Cu6 Sn5 and Ag3 Sn intermetallics were observed at the interface as well as inside of the solder. Bi
segregation in the solder and at the IMC was not observed. The die shear strength was 3.6384 N and increased as coated Sn-Bi thickness
increases.
(Received September 22, 2003; Accepted February 4, 2004)
Keywords: tin-bismath, coated solder, lead-free solder, microstructure, joint strength
1.
Introduction
A Sn-Pb solder, which has a low melting temperature and
excellent wettability, has been used as the most effective
solder material for decades. Recently, however, environmental concerns surrounding the use of Pb in the semiconductor industry have led to the need for a Pb-free solution
for solder materials.1) Much effort to find a replacement of
the Sn-Pb solder have been attempted. Alloys such as Sn3.5Ag, Sn-0.7Cu, Sn-3.5Ag-0.7Cu, Sn-58Bi, Sn-8.8Zn have
been developed as possible candidates to replace the Sn-Pb
conventional solder. To apply those solders on the real
process, those solders should have physical and chemical
properties correspondent to a eutectic Sn-Pb solder as follow.
1) Melting Temperature comparable to the Sn-Pb solder
2) Wetting Property comparable to the Sn-Pb solder
3) Thermo-Mechanical Reliability comparable to the Sn-Pb
solder
Sn-8.8Zn (Tm : 472 K) and Sn-58Bi (Tm : 411 K) alloys,
which have similar melting temperature to the eutectic Sn-Pb
solder have poor wettability, and they lead low shear strength
at the interface between the solder and the substrate. On the
other hand, Sn-3.5Ag (Tm : 494 K) and Sn-0.7Cu (Tm : 500 K)
alloys have comparable wettability and thermo-mechanical
reliability and are considered to be possible candidates for
applying the soldering process in electronics. However, these
solder alloys have higher melting points than the eutectic SnPb solder (456 K). Thus, their reflow process temperatures
are 30-40 K higher than that of the eutectic Sn-Pb solder.
Higher process temperature causes a severe, negative impact
on component performance such as; thermal damage on the
chip and printed circuit board (PCB), and makes them
difficult to apply to conventional processing devices.2–4) High
melting temperature of the solders acts on the bigger obstacle
to replace the eutectic Sn-Pb solder with the Pb-free solders.
Therefore, there is an increased demand for a solder with low
melting temperature in electronics.
This paper reports the application of diffusion soldering
like transient liquid phase diffusion bonding (TLP) process
for soldering.5–7) The Sn-Bi layer, where Bi is acted as a
melting depressant, was coated on Sn-3.5 mass%Ag alloy as
an effort to lower soldering temperature. The effect of
electroplated Sn-Bi layer thickness and composition on the
shear strength and microstructure was studied.
2.
Experimental Procedure
2.1 Electroplating
Electroplating was used to fabricate a melting depressant
layer on the Sn-3.5 mass%Ag solder surface. The composition of the plating solution is presented in Table 1. In order to
study characteristics of the electroplated Sn-Bi layer, Bi
composition ranged from 25 mass% to 97 mass% was
electroplated on the Sn-3.5 mass%Ag rolled plate. For the
purpose of assisting the plating, the Sn-3.5 mass%Ag plate
was polished using 500, 1000, and 2400 grit sand papers
followed by a finish using 1 mm and 0,04 mm SiO2 colloidal
suspension. The rolled solder was cleaned with 10% HCl
solution for 10 seconds, and then with ethanol to remove
oxide on the Sn-3.5 mass%Ag plate. The plating bath was
stirred at 250 rpm to maintain uniform distribution of the
solution. The cathode polarization curve was measured by
Xeno System WPG100 Potentiostat to understand the plating
behavior of Sn and Bi. The electric currents used for the
plating to form an identical thickness of Sn-Bi layers were 2,
Table 1
Methane sulfonate
acid
Composition of plating solution.
Volume fraction
Function
0.28
Solvent
SLOTLOY SNB
22
0.04
SLOTLOY SNB
14
0.01
D.I. Water
Sn2+
Bal.
40 g/liter (solute)
Bi3+
5 g/liter (solute)
Grain Refiner
Ion Homogenization
784
J. Lee, W. Bang, J. Jung and K. Oh
0.5K/sec
Zone D;
Melting and Wetting
of Solder
Tm+
20~30K
Tm
Zone C;
Temperature Rise
0.5K/sec
433K
0.3K/sec
1.3K/sec
Zone B;
Oxide removal by flux
403K
Zone A;
Temperature Rise
Fig. 1 Temperature profile during reflow soldering process.
4, 6 A/dm2 for the plating time of 2.5, 5, and 10 minitues,
respectively. The Sn-Bi electroplated Sn-3.5 mass%Ag alloy
was rinsed with DI water at 353 K. The compositions of the
electroplated Sn-Bi layers were analyzed by an electron
probe X-ray analyzer (EPMA).
(a)
2.2 Soldering
The Si-Bi electroplated Sn-3.5 mass%Ag rolled plates
(1 cm 1 cm) were used to look for the optimistic soldering
temperature of the cored alloy, mounted on a Cu coupon and
reflowed by an infrared reflow machine with RMA-typed
BGA flux to evaluate the effect of Sn-Bi composition and
thickness on the microstructure. The soldering temperatures
used were 473, 493, and 523 K and the temperature profile is
showed in Fig. 1. The microstructures of the solder joints
were investigated by SEM and EDX. Bi distribution on the
solder joint and inner solder was analyzed by EPMA area
analysis.
2.3 Mechanical strength
To evaluate the possibility of applying the Sn-Bi coated
Sn-3.5 mass%Ag solder at a real soldering process, the Sn-Bi
layer was electroplated on the Sn-3.5 mass%Ag solder balls.
In addition, the Si-Bi layer was electroplated at 4 A/dm2 for
the plating time of 2.5, 5, and 10 minutes to observe the
effects of the Sn-Bi thickness on the shear strength. The
electroplated solder balls were placed on the substrate, and
were air-reflowed at 473, 493, and 523 K. The UBM
deposited by electroplating method on the glass epoxy
substrate (FR-4) consisted of Au(0.5 nm)/Ni(5 mm)/
Cu(18 mm) from the top. The diameter of the pad and the
solder balls used were 300 mm, and 500 mm, respectively. The
SEM images of UBM pad (a), and reflowed solder balls (b)
were shown in Fig. 2. An infrared reflow machine and RMAtyped BGA flux were employed. The soldered balls were
shear-tested using a micro-shear tester. The distance between
a shear tip and the substrate was 10 mm, and shear speed was
200 mm/s. The shear test was carried out 20 times per one
soldering condition, and its average value was regarded as
shear strength. The microstructure of the soldered joints was
investigated by SEM and EDX.
3.
Results and Discussion
3.1 Characteristics of electroplated layer
Plating behavior of the solder varies with bath temperature,
current density, total metal concentration, the ratio of metals
(b)
Fig. 2 SEM images showing UBM pad (a) and solder ball reflowed at
493 K (b).
of the bath constituents (metal salts, additives, buffer),
temperature and PH.8) On this study, polarization curve was
measured for Sn4þ and Bi3þ to reveal the effect of currents,
which strongly affect the composition of the plated layer.
Figure 3 illustrates cathodic polarization curve of each
solution on the Sn-3.5 mass%Ag plate. The figure shows that
at the low current density region lower than 20 A/dm2
methane sulfonate solution including Bi3þ has the current
density equal to Sn4þ with applying lower voltage. However,
as current density increases, it is realized that the higher
voltage should be applied to make the current density in
methane sulfonate including Bi3þ equal to Sn4þ . From this
result, it is explained that Bi3þ is easy to be electroplated in
the low current density region and Sn4þ is easy in the high
current density region.
It is believed that the plating thickness is proportional to
the plating time in general electroplating.9) The linear
correlation between time and plating thickness was observed
as presented in Fig. 4. It appears that application of the date
shown in Fig. 4 made it possible to predict roughly the plating
time required to achieve aimed the thickness of the plating
layer.
The composition of the plated layer is affected by the both
Microstructure and Strength of Sn-Bi Coated Sn-3.5 mass%Ag Solder Alloy
785
0
Bi solution
Sn solution
E/mV
-300
Solder
-600
IMC
-900
-1200
Cu plate
-1500
0
10
20
30
40
Current Density, d/A.dm -2
Fig. 3 Cathodic polarization curve of each solution on Sn-3.5 mass%Ag
plate.
(a)
25
Plating Thickness, t/µ m
20
15
10
5
0
2
4
6
8
10
12
14
16
18
20
22
(b)
Plating Time, t/min
Fig. 4
Solder alloy deposition thickness as a function of time at 2 A/dm2 .
Table 2 Chemical composition of Sn-Bi electroplated layer.
(mass%)
Current Density
Sn
Bi
2
2 A/dm
4 A/dm2
4.3
19.7
95.7
80.3
6 A/dm2
72.0
28.0
composition, current density and additives. The effect of
current density on the composition of the plated layer is
demonstrated in Table 2. Plated layers are composed of
4.3Sn-95.7Bi, 19.7Sn-80.3Bi, 72.0Sn-28.0Bi at 2, 4, 6 A/
dm2 , respectively. This result implies that the composition of
the Sn-Bi plated layer can be controlled by applying current
density, and solders having different melting temperatures
can be fabricated.
In order to confirm solderability at the low temperature, the
electroplated solder balls were placed on the Cu coupon and
soldered. Figures 5, 6 and 7 show the cross sections of the
soldered joints and intermetallic compounds between electroplated solder balls and Cu-substrates, which were bonded
at 473, 493 and 523 K in air. The plated layers were formed
properly in the low current density region of 2, 4 and 6 A/
dm2 in each. The bonded interfaces show that the electro-
(c)
Fig. 5 SEM images showing the cross-sectional views of Sn-Bi electrodeposited Sn-3.5 mass%Ag plate at 2 A/dm2 after reflowed at (a) 473 K,
(b) 493 K, and (c) 523 K) (deposition layer: Sn-95.7 mass%Bi).
plated solder balls can be bonded even in a low temperature
of 473 K as well as 523 K. Figures 5(a), 6(a) and 7(a) show
intermetallic compounds (IMC) formed at 473 K. The
intermetallic compounds along the bonded interface consisted of Sn, Cu, and Ag (refer to Fig. 9). The intermetallic
compounds were formed, and were observed as large size in
the high current density region (at 6 A/dm2 ). In the case of
786
J. Lee, W. Bang, J. Jung and K. Oh
Solder
Solder
IMC
Cu plate
Cu plate
(a)
(a)
(b)
(b)
IMC
Cu plate
(c)
(c)
Fig. 6 SEM images showing the cross-sectional views of Sn-Bi electrodeposited Sn-3.5 mass%Ag plate at 4 A/dm2 after reflowed at (a) 473 K,
(b) 493 K, and (c) 523 K (deposition layer: Sn-80.3 mass%Bi).
Fig. 7 SEM images showing the cross-sectional views of Sn-Bi electrodeposited Sn-3.5 mass%Ag plate at 6 A/dm2 after reflowed at (a) 473 K,
(b) 493 K, and (c) 523 K (deposition layer: Sn-28 mass%Bi).
493 K, as shown in Figs. 5(b), 6(b) and 7(b), the intermetallic
compounds were also formed but they seemed relatively thin
compared to 523 K. Figs. 5(b), (c) and 6(b), (c) show the well
developed intermetallic compounds between Cu plate and
solders, and the composition was analyzed as 59.49 at%Sn37.20 at%Cu-3.31 at%Ag.
As seen in Fig. 8, the compositions of the electroplated
layers at 2 A/dm2 and 4 A/dm2 are located on x2 and x4 at
low current density region. While heating up, temperature in
the reflow machine increases up to T1, the isothermal
reaction between the electroplated layer and the Sn-3.5 mass%Ag rolled solder takes place. The composition hence
changes x2 , x4 ! eutectic point ! x. During this reaction, it
is believed that Bi was dissolved to inner solder. On the other
hand, The solder composition varies from x6 to x at high
current density of 6 A/dm2 . Because the reaction at 6 A/dm2
Microstructure and Strength of Sn-Bi Coated Sn-3.5 mass%Ag Solder Alloy
by spalling of Cu6 Sn5 10) or Cu diffusion11) to the solder.
Ag3 Sn12,13) intermetallics formed due to existence of Ag in
the solder. Bi, however, was not observed on the layer by
EPMA mapping. Therefore, it could be understood that Bi,
the melting depressant, is well dissolved and homogenized
into the solder.
Atomic percent Bismuth
544.442K
Final stage(x)
L
Temperature K
504.9681K
6A/dm2 (x6)
T1
787
2A/dm2(x2)
4A/dm2 (x4)
(Bi)
411.0K
(flSn)
0
Sn
99.9
21.0
28.0
50
Bismuth(mass%)
80.3
95.7 100
Bi
Fig. 8 Chemical composition change path of Sn-Bi deposition layer during
air-reflow soldering.
did not undergo eutectic reaction, and the temperature was
not low enough to melt the solders, it seems that intermetallic
compounds did not form enough.
To clarity the existence of Sn, Cu, and Ag elements on the
intermetallic compounds, and to investigate the distribution
of Bi, which affects the mechanical strength of the solder,
EPMA area analysis was employed after reflowed at 493 K.
Figure 9 shows the intermetallic compounds and Bi distribution on the cross-sectioned solder joint and inner solder.
Sn, Cu, and Ag elements are composed of the intermetalic
compounds. Ag and Cu elements are observed in the inner
solder. It is not clear that Cu6 Sn5 intermetallics were built up
3.2 Mechanical strength of Sn-Bi coated solder ball
The Sn-Bi layer was electroplated on the Sn-3.5 mass%Ag
solder ball to evaluate the possibility of applying the Sn-Bi
coated Sn-3.5 mass%Ag solder at a real soldering process. As
seen at the cross sectional view shown in Fig. 10, Sn-Bi was
electroplated properly on the Sn-3.5 mass%Ag solder ball,
and needle-shaped surface morphology was observed. In
addition, the Sn-Bi layer shows uniform appearance in shape.
The thickness of the Sn-Bi layer reached to 14 mm after
electroplated for 10 min. Furthermore, the Sn-Bi coated
solder balls were bonded well on the glass epoxy substrate
(FR-4) at the reflow temperature (493 K) as seen in Fig. 2(b).
To evaluate the mechanical strength of the reflowed Sn-Bi
coated solder ball and observe the effect of thickness of the
Sn-Bi layer on the mechanical strength, the shear test was
employed. The results were shown in Fig. 11. The Sn37 mass%Pb solder was used as a reference to compare with
the shear strength of the Sn-Bi coated Sn-3.5 mass%Ag
solder. The shear strength of the Sn-Bi coated Sn-3.5 mass%Ag solder reflowed at 493 K showed 3.6384 N at the
plating time of 10 min, and it has nearly similar strength as
Sn
Ag
Cu
Fig. 9 EPMA mapping image showing the intermetallic compounds and Bi distribution on cross-sectioned solder joint and inner solder of
Sn-Bi electroplated Sn-3.5 mass%Ag plate at 4 A/dm2 after reflowed at 493 K.
788
J. Lee, W. Bang, J. Jung and K. Oh
Sn-3.5mass%Ag
Sn-Bi
(a)
(a)
SnAg eutectic
Sn
(Cu,Ni)6 Sn5, (Ni,Cu)3 Sn4, Ag3 Sn
(b)
(b)
Fig. 10 SEM observation showing (a) cross-section, and (b) surface
morphology of the Sn-Bi electroplated Sn-3.5 wt%Ag ball, (current
density: 4 A/dm2 , plating time: 10 min).
5.0
4.5
Shear strength, /N
4.0
3.5
3.0
2.5
2.0
2.5min
5min
10min
Sn37Pb
1.5
1.0
470
480
490
500
510
520
530
Soldering temperature, K
Fig. 11 Comparison of shear strengths due to the various plating times.
high as that of Sn-37 mass%Pb. Interestingly, as the reflow
temperature increased from 493 K to 523 K, the shear
strength tended to slightly decrease from 3.6484 N to
3.3717 N at the plating time of 10 min. In addition, it was
observed that the shear strength after reflow-soldered at
493 K was getting higher such as 2.3985, 3.466, and 3.6384 N
in thickness of the Sn-Bi layer.
Fig. 12 SEM image showing cross-sectional structure of Sn-Bi coated
solder ball after reflowed at 493 K ((a)150, (b)1000).
Figure 12 shows the microstructure of cored solder balls
after air-reflowed at 493 K. It is shown in Fig. 12(a) that the
Si-Bi coated solder was well-bonded on the glass epoxy
substrate (FR-4). In the solder and Au/Ni/Cu substrate
reaction, (Cu, Ni)6 Sn5 , (Ni, Cu)3 Sn, and Ag3 Sn intermetallic
compounds formed at interface as shown in Fig. 12(b). As
seen in Fig. 9, it is observed that Cu, Sn, Ag composition
coexist in the interface between the solder and substrate.
Chen et al.14) reported that (Cu, Ni)6 Sn5 and (Ni, Cu)3 Sn4
formed in the Cu content between Sn-0.4Cu and Sn-0.7Cu. In
addition, it is generally known that Ag3 Sn intermetallics
formed in the interface between Ag and Sn.12,13) Eutectic
SnAg and Sn were observed in the solder and it is thought
that the eutectic SnAg particle in the solder could strengthen
mechanical strength.
4.
Conclusion
Sn-Bi electroplated on Sn-3.5 mass%Ag alloy was applied
to lower soldering temperature. The following conclusions
were obtained.
(1) Bi is easy to be plated at low current density, while Sn is
easy at high current density.
Microstructure and Strength of Sn-Bi Coated Sn-3.5 mass%Ag Solder Alloy
(2) Various compositions of Sn-Bi are effectively electroplated and plated layers are composed of 4.3Sn-95.7Bi,
19.7Sn-80.3Bi, 72.0Sn-28.0Bi at electroplating density
of 2, 4, 6 A/dm2 , respectively.
(3) Any defects on the interface were not observed even
after soldered at 473 K and the intermetallic compounds
were formed along the interface between the solder and
the Cu plate. The compounds consisted of 59.49 at%Sn37.20 at%Cu-3.31 at%Ag.
(4) The Bi segregation was not observed by EPMA
mapping. Therefore, it could be understood that Bi is
well dissolved and homogenized into the solder.
(5) The shear strength had the highest value when the Sn-Bi
coated ball was reflowed at 493 K, and was nearly
similar with that of Sn-37 mass%Pb (3.7793 N). The
IMC’s formed between the Sn-Bi coated ball and the
substrate pad was (Cu, Ni)6 Sn5 , (Ni, Cu)3 Sn, and
Ag3 Sn.
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
13)
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