Micro-Impact Test on Lead-Free BGA Balls on Au/Electrolytic Ni/Cu Bond Pad
Shengquan Ou*, Yuhuan Xu and K. N. Tu
Department of Materials Science and Engineering, UCLA, Los Angeles, CA, 90095-1595
M. O. Alam, and Y. C. Chan
Department of Electrical Engineering, City University of Hong Kong, Kowloon, Hong Kong
[email protected]
Abstract
The most frequent failure of wireless, handheld, and
movable consumer electronic products is an accidental drop
to the ground. The impact may cause interfacial fracture of
wire-bonds or solder joints between a Si chip and its
packaging module. Existing metrologies, such as ball shear,
and pull test cannot well represent the shock reliability of the
package. In our study, a micro-impact machine is utilized to
test the impact reliability of three kinds of lead-free solders:
99Sn1Ag, 98.5Sn1Ag0.5Cu and 97.5Sn1Ag0.5Cu1In (in
weight percent, later abbreviated as Sn1Ag, Sn1Ag0.5Cu, and
Sn1Ag0.5Cu1In in the paper). The effect of thermal aging on
the impact toughness is also evaluated in this study. We find
a ductile-to-brittle transition in SnAg(Cu) solder joints after
thermal aging. The impact toughness is enhanced by the
thermal aging. This is a combination effect of the growth of
intermetallic compound (IMC) at the interface provided
strong bonding, and the softening of the solder bulk during
the thermal aging provided more plastic deformation.
Introduction
The drop impact induced solder joint fracture has become
one of the critical system failure modes of interest in the
electronics industry due to the migration of market focus to
portable applications. For example, cellular phones might be
broken due to impact by dropping. Especially, in recent years,
while the weight of the personal digital assistant (PDA)
devices is reduced greatly, ball grid arrays (BGAs) and chip
size packages (CSPs) is adopted to effectively reduce
mounting areas. The major damage of the BGA/CSP solder
joints is not from external temperature changes but short-time
stresses like drop impacts.[1]
To evaluate the joint reliability between BGA ball and
under bump metallization (UBM) pad, industry widely uses
shear test and pull test.[2] Unfortunately, these tests can not
evaluate the impact reliability of solder joints, because the
testing speeds are typically lower than 1 mm/s, well below the
velocity of impact applied to solder joints by dropping. Some
previous study by Chiu et. al.[3] proved that there is a very
strong correlation between drop reliability and voiding at the
UBM/solder interface. However, ball shear testing does not
correlate to drop test performance, and ball pull strength is not
a good indicator of shock reliability either. Therefore, how to
characterize the impact reliability induced by dropping
becomes very crucial.
Recently, the research group from Hitachi Metals, Ltd.,
Japan[4,5] proposed a miniature impact test for solder bumps
by adopting the principle of the classic Charpy impact test.[6]
Based on their work, we have built a micro-impact testing
0-7803-8906-9/05/$20.00 ©2005 IEEE
machine to study quantitatively the bonding strength between
BGA ball and UBM pad (as shown in Fig. 1). The sample is
placed on an XYZ-adjustable positioning stage. The initial
position of the hammer and its final position after impact are
recorded by an angle recorder with accuracy of ±0.5 degree.
Since the measured angle difference in a typical impact test is
larger than 10 degree, the resolution is about 5% of the
measured energy change.
Sn1Ag, Sn1Ag0.5Cu and Sn1Ag0.5Cu1In are chosen for
this study because they have good wettability and mechanical
strength. Adding Indium into SnAgCu solder can lower the
melting temperature by about 20°C, which can reduce the
temperature distribution across the board, especially large
boards. The UBM structure on the bond pad is
Au/electrolytic Ni/Cu metallization. Impact tests are carried
out after reflow and after the thermal aging at 150°C.
Fig. 1 Photograph of the micro-impact test machine.
Experimental
A. Sample preparation
We used 760 µm (in diameter) size solder balls for the
impact test. The opening of the solder resist was 650 µm in
diameter, 30 µm in depth at the Au/electrolytic Ni/Cu bond
pad. Gold thickness over electrolytic Ni was 0.5 µm. The
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Fig. 2 JEDEC reflow profiles for Sn-Pb and Pb-free assemblies.
average thickness of electrolytic Ni was 2.54 µm, and the
thickness of Cu underneath the electrolytic Ni was 13 µm.
The reflow process of the solder ball on bond pad followed
the standard JEDEC reflow profiles for Pb-free assemblies as
shown in Fig.2. The reactive mild active (RMA) flux was
used during reflow, and the peak reflow temperature for
Sn1Ag and Sn1Ag0.5Cu was 260°C, and it was 240°C for
Sn1Ag0.5Cu1In. The duration time at the peak reflow
temperature was 1 minute.
After reflow, some substrates were put into furnace for
thermal aging at 150°C. We took samples out at 100 hour,
500 hour and 1000 hour to test the change of the impact
toughness with the aging time. The fracture surface and crosssectional microstructures of the solder joints were analyzed
by optical microscope, scanning electron microscopy (SEM)
(b)
and energy dispersive X-rays (EDX). The fracture surface
was examined on both the solder ball side and the bond-pad
side for complete understanding of the failure mechanism.
B. Impact test on solder bumps
Solder ball
The way to determine the impact toughness was shown in
No solder
Sample #
Fig. 3(a). The hammer hit one side of the solder ball, and
Total solder ball
created two fracture surfaces, and at the same time, deformed
number = 44
the solder bump. The energy spent was measured by the
gravitational potential energy change of the hammer before
and after the impact. The potential energy change, ∆E, was
measured from the difference between the initial pendulum
height, h0, and the maximum height achieved during the
follow-through, h, or the difference between the height of the
Fig. 3 (a) Impact a solder ball: 1-- start position of
hammer before and after the impact.
hammer, 2 – impact to the ball, and 3 – final position
∆E = mgh0 – mgh = mg∆h
of the hammer. (b) Designed pattern of solder balls
arrays on the substrate.
where m is the mass of the hammer (in units of gram), g is
gravity constant or acceleration of gravity (980 cm/sec2), ∆h
= h0 – h = L ( cosθ0 – cosθ ), where L is the length of the Results & Discussion
hanging hammer (in units of cm), and θ0 and θ are the angles A. Microstructures of solder bumps on Au/Ni/Cu pad
As shown in Fig. 4, we can see, after reflow, a very thin
of the hammer before and after impact. The unit of the
layer of binary Ni3Sn4 IMC formed at the Sn1Ag solder and
potential energy is in mJ, mille Joule.
The size of the substrate we used for the impact test was Au/electrolytic Ni/Cu interface. By adding 0.5 wt.% Cu into
3.5 cm × 3.5 cm. The positions of the solder balls were Sn1Ag solder, the IMC at the interface changed into the
specially designed for the impact test. Fig. 3(b) showed the ternary compound (Cu,Ni)6Sn5 due to the gain in free energy.
designed pattern. On one substrate, there were 44 balls in [7] At the Sn1Ag0.5Cu1In solder and Au/Ni/Cu interface, we
total.
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150°C/1000
As-reflowed
was defined as a brittle fracture. In this study, we also found
a third mode of fracture within the bond pad metallization.
This mode would occur after a long time aging.
(d)
(a)
Ni3Sn4
Ni3Sn4
(e)
(b)
(Cu,Ni)6Sn5
(Cu,Ni)6Sn5
(f)
(c)
Cu-Ni-Sn
Fig. 5 Schematic of the ductile-to-brittle transition
behavior in BBC metals. (after ref. 9)
(Cu,Ni)6Sn5
Fig. 4 SEM pictures of the cross-section of the solder
joints. (a) and (d) Sn1Ag; (b) and (e) Sn1Ag0.5Cu; (c)
and (f) Sn1Ag0.5Cu1In.
found the formation of very tiny needle-like Cu-Ni-Sn
compound. The IMCs at the interface grew slowly during the
thermal aging process. In the solder bulk, the coarsening of
Ag3Sn compound has occurred. It was hard to tell in Fig. 4
because of the chemical etching. But this was verified by the
work done by M. Date. [8] Due to the low Ag concentration
(1 wt.%) in the solder composition, we did not observe large
plate-like Ag3Sn after the thermal aging, which was known to
be detrimental to the joint strength. There was also an
expected grain growth after the long time aging at this high
temperature for solder joints.
B. Impact toughness of the solder joints
One of the primary functions of impact test is to determine
whether or not a material experiences a ductile-to-brittle
transition. It is known that in many body-centered-cubic
metals, including ferretic steels, a ductile-to-brittle transition
temperature (DBTT) exists. The metals are ductile above
DBTT, yet they become brittle below DBTT, as shown in Fig.
5.[9] From the impact test on solder joints, we also found a
ductile-to-brittle transition in the solder joint. This ductile-tobrittle transition found in our experiments, was not due to
application temperature, rather it was due to reflow, or aging
time, or a combination of the two, or the bond pad
metallization. Using SEM and optical microscope, we
examined both solder ball side and bond pad side to evaluate
the mode of fracture. In Fig. 6, we showed all the fracture
modes observed in our impact tests. Mode 1 was the fracture
across the solder bump. We treated this mode as ductile
fracture. Mode 2 was the fracture across an interface, which
Ductile
Fracture
Brittle
Fracture
100µm
Bond-Pad
Fracture
100µm
100µm
Fig. 6 Fracture modes in the impact test on solder joints.
A statistical analysis of the fracture modes of all solder
joints under every condition was summarized and plotted in
Fig. 7. From the plot, we can tell that all the as-reflowed
Sn1Ag solder joints had ductile fracture across the solder
bulk. With thermal aging, the fracture mode shifted to brittle
fracture. A clear ductile-to-brittle transition phenomenon
illustrated in this case. The transition also appeared in
Sn1Ag0.5Cu solder joints, but less obvious. EDX analysis
was employed to get the composition information on the
collected solder balls, which had been knocked out from the
bond-pad.
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Sn1Ag
Sn1Ag0.5Cu
solder joints, the impact toughness increased graduately to the
point of 500 hours aging at 150°C. After 500 hours aging, the
impact toughness started to drop.
In both cases of
Sn1Ag0.5Cu and Sn1Ag0.5Cu1In, we can see an increase of
the impact toughness with the aging time.
Sn1Ag0.5Cu1In
100
Percentage, %
80
60
26
40
24
22
Reflow 100hrs 500hrs 1000hrs Reflow 100hrs 500hrs 1000hrs Reflow 100hrs 500hrs 1000hrs
Thermal Condition
Thermal Condition
Thermal Condition
Fig. 7 Percentage of different fracture mode for Sn1Ag,
Sn1Ag0.5Cu, and Sn1Ag Sn1Ag0.5Cu1In solder joints at
each thermal condition.
The results from EDX analysis (listed in Fig. 8) indicated the
fracture crossed interface was located in the place between
Cu-Ni-Sn compound and the UBM metallization layer.
During aging, the thickness of IMCs at the interface increased
with time. The brittle nature of IMCs resulted in a fracture
across the interface. Lots of studies [7, 10] on the interfacial
reaction between Ni and solders reported Kirkendall voids
formation at aging due to unbalance of out-flux and in-flux.
As a result, Kirkendall voids would cause a poor adhesion
between the metallization layer and the solder joints. All these
factors leaded to the brittle fracture at the interface. In the
case of Sn1Ag0.5Cu1In solder joints, they appeared that all
interface fracture thru all thermal condition. This may related
to the tiny needle-like Cu-Ni-Sn compound formed at the
interface, and higher yield stress compared with Sn1Ag and
Sn1Ag0.5Cu. The needle-like IMC may provide more
channels for fast Ni diffusion. But we need further detailed
study on the cross-section to prove this.
20
18
16
14
12
10
8
4
200
400
600
800
1000
Thermal Anealing Time at 150C, hour
24
22
20
18
16
14
12
10
8
Sn1Ag0.5Cu
6
4
0
200
400
600
800
1000
Thermal Anealing Time at 150C, hour
Element Weight% Atomic%
------------------------------------------P
0.40
1.18
Ni
26.38
41.21
Cu
1.44
2.08
Ag
0.88
0.75
Sn
70.90
54.79
Total
100.00
26
24
22
Impact Toughness, mJ
Solder Ball Side
0
26
Bond-Pad Side
100µm
Sn1Ag
6
Impact Toughness, mJ
0
Impact Toughness, mJ
20
Element Weight% Atomic%
------------------------------------------P
0.00
0.00
Ni
1.09 2.14
Cu
1.87
3.40
Ag
0.00
0.00
Sn
97.05
94.46
Total
100.00
20
18
16
14
12
10
8
Sn1Ag0.5Cu1In
6
4
100µm
0
200
400
600
800
1000
Thermal Anealing Time at 150C, hour
Fig. 8 Typical fracture across the interface in Sn1Ag
solder joints after aging at 150°C for 1000 hours.
The results of the change of the impact toughness with the
aging time were shown in Fig. 9. We found as for Sn1Ag
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Fig. 9 Impact toughness change as a function of
thermal aging time for Sn1Ag, Sn1Ag0.5Cu, and
Sn1Ag0.5Cu1In solder joints.
2005 Electronic Components and Technology Conference
In the impact test, if we assume that the impact energy is
the sum of the energy spent to create the fracture surfaces and
the energy spent to deform the solder ball, we have
∆E total = ∆E fracture surfaces + ∆E deformation of solder ball
We should emphasize that in the ductile fracture of the
solder joint as shown in Fig. 10(a), the solder ball itself,
which was knocked away, should have been heavily
deformed plastically. Therefore, more impact energy might
have been spent to deform the solder ball than to create the
fracture surfaces. On the other hand, when brittle fracture
occurred as shown in Fig. 10(b), we expected that the solder
ball itself might have less plastic deformation than that in
ductile fracture shown in Fig. 10(a). Generally speaking, if
the matrix of solder ball is soft, it absorbs more impact energy
and deforms more heavily. On the other hand, if the matrix of
the solder ball is rigid, it will not absorb much impact energy
and will deform much less.
Bond-Pad Side
Solder Ball Side
(a)
Direction of
Impact
100µm
100µm
(b)
Direction of
Impact
100µm
100µm
(c)
Direction of
Impact
100µm
100µm
Fig. 10 Three kinds of failure shapes in impact test: (a)
fracture in solder bulk; (b) fracture at interface; (c)
fracture at interface and a deformed solder bump
During the long time thermal aging, the IMC growth at the
interface could strengthen the bonding before continuous
Kirkendall voids formed. In addition, the grain growth in the
solder matrix softened the solder bumps by reducing the total
area of grain boundary. Therefore, we observed huge
deformation in the solder bulk as shown in Fig. 10(c).
Combine those factors, the energy needed to create the
interfaces between IMC and solder and deform the solder
bump would increase. In another word, the impact toughness
would expect to increase for longer aging time.
Conclusions
In this study, we find impact test is an effective tool to
detect the ductile-to-brittle transition in the Sn1Ag, and
Sn1Ag0.5Cu solder joints on Au/electrolytic Ni/Cu bond-pad.
The fracture interface was located at the place between UBM
metallization layer and Ni(Cu)-Sn intermetallic compound.
The needle-like Ni-Cu-Sn compound formed at the
Sn1Ag0.5Cu1In and Au/electrolytic Ni/Cu interface might be
the season for the weak interface bonding at as-reflowed
condition. The change impact toughness with the thermal
aging time showed an increasing trend. The continuous IMC
formation at the interface without Kirkendall voids formation
and the soften solder bumps after aging might contribute to
the increase of impact toughness.
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