ICEPT2007 Proceedings 410
Interfacial Reactions of Ni-doped SAC105 and
SAC405 Solders on Ni-Au Finish during Multiple Reflows
Toh C.H.1, Liu Hao
1, Tu C.T
2., Chen T.D.
2, and Jessica Yeo
1
1United Test and Assembly Center Ltd, 5 Serangoon North Ave 5, SINGAPORE 554916
2Accurus Scientific Co., Ltd, 508-51, Wen Sien Road, Section 1 Jen-Der, Tainan, TAIWAN
Abstract
Solder-joint performances of SAC405 and SAC105 with 200ppm and 500ppm Ni addition were investigated for electrolytic Ni-Au
BGA pad finish. For each alloy system, ball shear tests, cross-sectional analysis and 3-D etching were performed to study the
interfacial reactions after repeated reflows. Also, the effect of solder-mask pad design on solder joint integrity was investigated. In
this study, all three systems gave rise to only (Cu,Ni)6Sn5 IMC layer at the soldering interface after multiple reflows at 245oC. The
(Cu,Ni)6Sn5 IMC had a needle-shaped structure after 1 time of reflow and the morphology remained the same after multiple reflows.
SACN0.05 appeared to be inappropriate for a BGA pad finish. It exhibited a drastic increase in the IMC layer thickness after 11
times of reflow and this corresponded to a significant increase in IMC fracture mode percentage. NSMD pad design led to ball pad
lifting because the bonding between the pad and substrate was weaker than the bulk solder strength and the IMC-pad interface
bonding.
1. Introduction
Solder composition and ball-grid array (BGA) pad finish are
some of the many factors that can affect intermetallic
compound (IMC) formation at the interface. The IMC growth
controls the strength of the solder joint. Excessive brittle
intermetallics and weak interfaces can result in solder joint
reliability concern leading to the BGA package failure.
The adoption of SAC solders doped with nickel has steadily
increased in the recent years, particularly for OSP finish. This
is developed to replace the conventional lead-free alloys such
as SAC405 for drop reliability improvement by means of
retarding the growth of Cu3Sn IMC [1, 2]. In high volume
manufacturing lines, it is always desirable to standardize
solder balls composition for BGA packages with different pad
finishes. However, there is limited report on the use of SAC
solders doped with nickel for the Ni-Au finish, a popular
industrial choice for pad finish.
In the manufacturing assembly processes, multiple reflows
are often required. During these processes, the solder joints
are subjected to the repeated melting and solidification.
Consequently, the microstructure of the BGA solder joints
can be affected by the interfacial reactions such as dissolution
of surface finish layers, compositional changes and IMC
growth. The purpose of this study is to understand how
SAC405 and two nickel doped SAC solders react with the Ni-
Au finish during multiple reflows.
2. Materials and Methods Test vehicles used in this study were BGA packages with
Au/electrolytic Ni/Cu pad. The Au thickness is around 1um,
while the Ni thickness is around 5um. Typical cross-section
SEM image for the as-plated substrate are shown in Figure. 1.
Fig. 1. Typical thickness for electrolytic Au plating
Solders balls with 0.4mm diameter from Accurus Scientific
with three types of Pb-free composition were studied as given
in Table 1. Both solder masks defined (SMD) and non-solder
mask defined (NSMD) were studied. SMD pad refers to a ball
pad defined by a solder-mask in which the copper pad is
much larger. An NSMD pad has a solder mask opening that is
larger than the copper pad area where the copper pad defined
the solder ball structure.
Electrolytic Au Electrolytic Ni
Cu Pad
ICEPT2007 Proceedings 411
Table 1: Composition of Solder Balls
Alloy Typical Composition (wt.%)
SAC405 4% Ag, 0.5%Cu, balance Sn
SACN0.02 1% Ag, 0.5%Cu, 200ppm Ni balance Sn
SACN0.05 1% Ag, 0.5%Cu, 500ppm Ni balance Sn
Solder balls were attached on the BGA substrates using a
commercial flux. The layout of the package is shown in
Figure 2. The reflow was accomplished using a hot air
furnace equipped with 6 heating zones. Reflow temperature
profile with 245oC peak temperature and a time above the
liquidus of approximately 46sec is shown in Figure 3. After 1
time of reflow, the packages were kept at a production floor
at room temperature for around one month before underwent
additional 3 times and 10 times of reflow at 245oC.
Fig. 2. Substrate layout: FBGA 4x4mm-16B, 0.8mm pitch with
0.4mm solder ball diameter.
Fig. 3. Reflow temperature profile
Fig. 3. Reflow temperature profile
A Dage 4000 tester was used for the high-speed ball zone
shear test. A constant shear speed of 50mm/sec was applied.
The gap between the substrate surfaces to the shear tool was
kept at 30um. Fracture mode distribution was studied using
optimal microscopy. Four levels of failure modes were
defined as schematically shown in Figure 4. For each alloy
composition & reflow conditions, about 80 solders balls from
various BGA substrates were sheared.
Fig. 4. Failure mode definition (a) bulk solders fracture with zero
IMC fracture; (b) IMC fracture surface > 1% and <50%; (c)IMC
fracture surface > 50%; (d) pad lifting
Specimens were cold mounted and cross-sectioned through a
row of solder balls. The specimens were then ground with
2000-grit SiC paper, and mechanical polished using 0.3 &
0.05µm Al2O3 powder. Micro-hardness tests were performed
using 50gf load on the cross-section in the middle of a solder
ball to examine the micro-hardness value after 1 time, and 11
times of reflows.
A HITACHI S-3000N scanning electron microscope (SEM)
operating at 15KeV was used to study the interfaces and IMC
microstructure. The SEM is equipped with an energy
dispersive spectrometer (EDS) to analyze the IMC and phase
composition. IMC thickness (in um) is defined as the ratio of
IMC layer area to IMC layer length as illustrates in Figure 5.
Fig. 5. Definition for IMC layer’s thickness
3. Results and Discussion
3.1 Zone Balls Shears and Failure Modes Figure 6 plots the total number of IMC fracture for the
various solders before and after multiple reflows. Only
fracture mode for the first balls are reported and compared to
avoid the neighboring balls sheared effect as schematically
shown in Figure 7. Dark shaded bar represents more than
50% of the fracture surface (individual ball) exhibiting IMC
interface fracture, whereas light shaded bar corresponds to
less than 50% of the fracture surface is IMC fracture.
80
100
120
140
160
180
200
220
240
260
0 50 100 150 200
Time (sec.)
Temperature (°C) peak temperature=245℃dw ell time above
217℃=46 sec.
IMC area
(brittle failure mode) Bulks solder area
(ductile failure mode)
(a) (b)
(c) (d)
Length of IMC layer = L (µm)
IMC layer
Ni layer
Area of IMC layer = A (µm2)
ICEPT2007 Proceedings 412
0%
20%
40%
60%
80%
100%
SAC405
(as-reflowed)
SAC405
(11x reflows)
SACN0.02
(as-reflowed)
SACN0.02
(11x reflows)
SACN0.05
(as-reflowed)
SACN0.05
(11x reflows)
0%<IMC<50% 50%<IMC<100%
Accumulation of IM
C (%) failures
Fig. 6. Accumulative IMC fracture percentage comparison after 1
time and 11times of reflow at 245oC. SAC405 with NSMD pad
designs showed 100% pad lifting after 1 time of reflow.
Solder bump Shear tool
Pad
1st 2
nd 3
rd
Fig 7. Zone balls shear failure mechanism. As the tool piece
touches the first solder ball, the solder undergoes plastic
deformation followed by fracture. The neighboring ball is
then sheared by the first ball. The process is repeated until the
whole row of solder balls is sheared.
Three failure modes were observed in this study, namely IMC
fracture, bulk solder fracture and pad lift. The IMC fracture
was with a shinny fracture surface, a typical character of
brittle fracture. Fracture in the bulk solder was with a dull
fracture surface depicting a ductile fracture. SACN0.02
solders showed the least total number of IMC fracture after 1
time and 11 times of reflow at 245oC. Although SACN0.05
solders exhibited less total number of IMC fracture than
SAC405 after 1 time of reflow, the total number of IMC
fracture for SACN0.05 increased dramatically from 15% to
65% after 11 times of reflows at 245oC.
3.2 IMC and Interfacial Microstructures The average thickness of IMC layer that formed at the
interface of a solder ball and Ni-Au overplated Cu pad with
reflow time is given in Figure 8. The IMC layer in SAC405,
SACN0.02 and SACN0.05 systems was about 0.9, 0.7 and
1.2um, respectively after 1 time of reflow. SACN0.05
exhibited a significant increase in IMC layer thickness up to
3um after 11 times of reflow while an average of 1.2um was
maintained for SAC405and SACN0.02 solders system. This
corresponded to a significant increase in IMC fracture mode
percentage as shown in Figure 6. The thick and brittle IMC
layer may provide a low energy path for crack propagation.
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
as-reflowed
4x reflows
11x reflows
as-reflowed
4x reflows
11x reflows
as-reflowed
4x reflows
11x reflows
Average(µm) Standard deviation(µm)
IMC layer thickness (µm)
SAC405 SACN0.02 SACN0.05
Fig. 8. IMC layer thickness comparison at as-reflowed, 4 times and
11times of reflow.
Figure 9 shows that the bulk solder hardness for each solder
system remained after multiple reflows. Indentations were
carried in the middle of the solder ball, at least 50um away
from the IMC layer.
0
2
4
6
8
10
12
14
16
18
20
SAC405
(as-reflowed)
SAC405
(11x reflows)
SACN0.02
(as-reflowed)
SACN0.02
(11x reflows)
SACN0.05
(as-reflowed)
SACN0.05
(11x reflows)
Max Microhardness (Hv50) Average Microhardness (Hv50)
Min Microhardness (Hv50) Stdev(Hv50)
Microhardness (Hv50)
Fig. 9. Bulk solder micro-hardness comparison after 1 time and 11
times of reflows at 245oC
Typical cross-sectional SEM images for the SAC405,
SACN0.02 and SACN0.05 system after 1, 4 and 11 times of
reflows are shown in Figure 10 and Figure 11. EDS analysis
indicated the IMC layer compositions at the soldering
interface are consistent with the stoichiometry of the
compound (Cux,Niy)6Sn5. No other IMC was detected after
multiple reflows at 245oC. The (Cux,Niy)6Sn5 is a uniform
layer conforming to the nickel surface finish.
ICEPT2007 Proceedings 413
Ni layer
Sn
Ag3Sn
(Cux,Ni1-x)6Sn5
SAC405
Ag3Sn Sn
(Cux,Ni1-x)6Sn5
Ni layer SAC405
Ni layer
Sn
Ag3Sn
(Cux,Ni1-x)6Sn5
SACN0.02
Ag3Sn
Sn
(Cux,Ni1-x)6Sn5
Ni layer
(Cux,Ni1-x)6Sn5
SACN0.02
Sn
(Cux,Ni1-x)6Sn5
Ni layer
Ag3Sn
SACN0.05
(Cux,Ni1-x)6Sn5 Ni layer
Sn
AuxCuyNizSn1-x-y-z
(Cux,Ni1-x)6Sn5
SACN0.05
Fig. 10. Solder joint microstructures comparison after 1 time of
reflow@245oC (left) and 4 times of reflows@245oC (right)
Ni layer
(Cux,Ni1-x)6Sn5
Ag3Sn Sn
SAC405
Ni layer
(Cux,Ni1-x)6Sn5
Ag3Sn
Sn
SACN0.02
Ni layer
(Cux,Ni1-x)6Sn5
Ag3Sn
Sn AuSn4
SACN0.05
Fig. 11. Solder joint microstructures comparison after 11 times of
reflow @245oC
Figure 12 shows the 3-D IMC images for each solder system.
Deep etching revealed that the morphology of (Cux,Niy)6Sn5
were needled shaped compound protruding into the solder,
making the interface very rough as seen in Figures 10-11.
Needle-shaped IMC morphology was reported by Lee et al.
[3] for SAC305 on electroplated Cu/Ni/Au after 20 times of
reflows. The morphology remained the same after repeated
reflows at 245oC up to 11 times. It is interesting to note that a
ring pattern of IMC with a relatively larger IMC was
observed for SACN0.02 and SACN0.05 after 1 time and
multiple reflows. SACN0.05 solders which exhibited a
significant growth in (Cux,Niy)6Sn5 IMC layer thickness after
11 times of reflow showed no morphology changed. The
thick IMC shown in Figure 11 was attributed to the IMC
layer thickening of (Cux,Niy)6Sn5.
acicular (Cux,Ni1-x)6Sn5
acicular (Cux,Ni1-x)6Sn5
Ag3Sn
Ag 3 Sn
acicular (Cux,Ni1-x)6Sn5
acicular (Cux,Ni1-x)6Sn5 Residual solder
Fig. 12. 3D-IMC images comparison for as-reflowed, 4 times and 11
times of reflow at 245oC.
(a) SAC405 (as reflowed)
(b) SAC405 (4x reflows)
(c) SAC405 (11x reflows)
(d) SACN0.02 (as reflowed)
(e) SACN0.02 (4x reflows)
(f) SACN0.02 (11x reflows)
ICEPT2007 Proceedings 414
Fig. 12 (con’t). 3D-IMC images comparison for as-reflowed, 4
times and 11 times of reflow at 245oC.
Figure 10 and 11 showed the Ag3Sn inside the bulk solders
were pebble or plate-like. Figure 13 shows a typical 3-D
image of Ag3Sn for SAC405 after 4 times of reflow. The
Ag3Sn had a prismatic shape and the sizes were in the ranges
of 6-12um. AuSn4 platelet was detected in the bulk solders
for SACN0.05 after 11 times of reflow (see Figure 11). Also,
a compound of AuxCuyNizSn-x-y-z was found inside the bulk
solder for SACN0.05 after 4 times of reflow (see Figure 10).
Ag3Sn
acicular (Cux,Ni1-x)6Sn5
Residual solder
Fig. 13. Ag3Sn morphology for SAC405 with Ni-Au surface finish
after 4 times of reflow.
During soldering, the Ni from BGA surface finish diffused into
the solder and an IMC formed. If the solder did not contain Cu,
the IMC at the interface was typically Ni3Sn4, however, when Cu
existed as little as 0.6wt.% in the solder, the IMC became
(Cu,Ni)6Sn5 [4]. In the Cu-Ni-Sn ternary system as shown in
Figure 14, (Cu,Ni)6Sn5 is more stable than Ni3Sn4, (Cu,Ni)6Sn5
preferentially formed at the interface with the Cu in the solder
[5]. In this study, only Cu6Sn5 with some solution of Ni exited
at the interface up to 11 times of reflow for all alloys system.
Fig. 14. Isothermal Cu-Sn-Ni phase diagram at 235oC. Redrawn
from [5]
As more reflow cycles were applied, the IMC were
precipitated out in the solder matrix and thereby the amounts
of Cu and Ni were reduced. Interestingly, the available of the
Cu inside the solder matrix next to the IMC is seems to
sustain the substantial growth of (Cu,Ni)6Sn5 after multiples
reflows for SACN0.05. However, this is not the case for the
identical SACN0.02 but with 300ppm less Ni concentration.
Further investigation will be required to understand the
mechanism behind the much higher IMC growth for
SACN0.05 than SACN0.02 after 11 times of reflows.
3.3 Effect of solder mask design In this study, SAC405 solder balls with the SMD pad design
exhibited either bulk solder fracture or IMC fracture while all
those with NSMD pads design showed pad lift phenomenon.
Figure 15 provides a side-by-side comparison of such pad
designs at the same length scale. For SMD pad design, the
bonding between solder balls and copper pads was provided
by the top pad area. The solder resist overlapped the pad area
and enhanced the adhesion between the copper pad and the
substance. This helped to enhance adhesion of the copper pad,
resulting in a ball shear failure mode.
(g) SACN0.05 (as reflowed)
(h) SACN0.05 (4x reflows)
(i) SACN0.05 (11x reflows)
ICEPT2007 Proceedings 415
Fig. 15. Comparison of SMD (top) and NSMD pads (bottom) after
SAC405 balls soldering. In this study, the overplated Cu pad
diameter is 0.33mm.
Figure 16 shows a typical force-displacement diagram for a
sheared single ball with SMD and NSMD pad design. The
slope to failure was produced after removal of the
neighboring balls. In this example with a 0.33mm pad
diameter and a 0.4mm SAC405 ball, SMD pad design
exhibited slightly higher shear force than NSMD pad design.
Also, the large area under the force-displacement diagram for
SMD design indicates its ability to absorb more energy up to
a fracture. A study by Lim et al.[6] has shown that the relative
strength of SMD and NSMD ball pads was determined by the
pad size and substrate thickness. It is also interesting to note
that the diagram shows multiple stress peaks before failure
for NSMD pad design.
0
100
200
300
400
500
600
700
0 50 100 150 200 250 300
NSMD_1
NSMD_2
NSMD_3
SMD_1
SMD_2
SMD_3
Displacement (µm)
Force (g)
Fig. 16. Typical force-displacement graph for single shear test.
Comparison of SMD and NSMD pad design for SAC405 solder. A
constant shear speed of 500um/sec was applied.
4. Conclusions The following conclusions could be drawn for the SAC405,
SACN0.02 and SACN0.05 solders when reacted with Ni-Au
surface finish.
• All three systems gave rise to only (Cu,Ni)6Sn5 IMC
layer at the soldering interface after multiple reflows at
245oC.
• The (Cu,Ni)6Sn5 IMC had a needle-shaped structure after
1 time reflow and the morphology remained the same
after multiple reflows.
• SACN0.05 appeared to be inappropriate for a BGA pad
finish. It exhibited a drastic increase in the IMC layer
thickness after 11 times of reflow and this corresponded
to a significant increase in IMC fracture mode
percentage.
• NSMD pad design led to ball pad lifting because the
bonding between the pad and substrate was weaker than
the bulk solder strength and the IMC-pad interface
bonding.
5. Acknowledgement The authors would like to thank the management teams of
United Test and Assembly Test Center Ltd (UTAC) and
Accurus Scientific Co., Ltd for their support on this project.
6. References 1. J.Y. Tsai, Y.C. Hu, C.M. Tsai and C.R. Kao, “A study on
the reaction between Cu and Sn3.5Ag solder doped with
small amount of Ni”, J. Electronic Materials, Vo1. 32, No.11,
2003, pp12-3-1208.
2. I.E. Anderson, J.C. Foley, B.A. Cook, J. Harringa, R.I.
Terpstra and O. Unal, “Alloy effects in near-eutectic Sn-Ag-
Au solder alloys for improved microstructral stability”, J.
Electronic Materials, Vol.30, No.9, 2001, pp.1050-1059.
3. KY. Lee, M Li, D.R. Olsen and W T. Chen,
“Microstructure, Joint Strength and Failure Mechanism of Sn-
Ag, Sn-Ag-Cu versus Sn-Pb-Ag Solders in BGA Packages”,
2001 ECTC.
4. C.E Ho, R.Y. Tsai, Y.L. Lin and C.R. Kao, “Effect of Cu
Concentration on the Reactions between Sn-Ag-Cu Solders
and Ni”, J. Electronic Materials, Vol. 31, No. 6 2002, pp-584-
590.
5. K. Zeng and K. N. Tu, "Six cases of reliability study of Pb-
free solder joints in electron packaging technology,"
Materials Science and Engineering Reports, R38, 55-105
(2002). (A review paper).
6. A.C.P. Lim, T.K Lee, and Airin Alamsjah, “The Effect of
Ball Pad designs and substrate materials on the performance
of second-level interconnects,” 2003 EPTC, pp.563-568.
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