Effects of Ni and Cu Additive on Electromigration in Sn Solder Lines
S. C. Hsu and C. Y. Liu
Dept. of Chemical Engineering and Materials Engineering,
National Central University, Chung-Li, Taiwan, R. O. C.
Abstract:
The effect of Cu content in Sn(Cu) alloys on the interfacial reaction between Ni
thin film and Sn(Cu) alloys has investigated.
We have found that the variation of Cu
content has a strong influence on the spalling of the Ni thin film.
With small Cu
additives in the Sn, spalling was deferred to longer reflowing time. When the Cu
content increased to about 1.0wt.%, a layer of Cu-Sn compound formed on the Ni thin
film and no spalling was observed after 20 minutes reflowing.
mechanism of spalling deferring is proposed.
The possible
A Cu flux from the solder to the
interface compensated the ripening flux of the semi-spherical compound grains,
therefore, spalling was retarded. The driving force of the Cu flux was attributed to
the reduction of Cu solubility due to the presence of Ni at the interface of the Ni thin
film.
The Cu flux from solder to the interface is calculated to be in the same order
with the ripening flux of Cu6Sn5 compound grains, which confirms the proposed
mechanism of spalling deferring.
For the Sn(Cu) alloys having Cu content over
1.0wt.%, the Cu-Sn compound layer grew so fast that the surface of the interfacial
compound layer was free of Ni. There was no Cu flux to compensate the ripening
flux, therefore, the ripening flux dominated and spalling occurred after short
reflowing time.
Key words: Pb-free solder, Thin film UBM, interfacial reaction, Cu diffusivity.
The corresponding author's e-mail address is chengyi@cc.ncu.edu.tw .
Introduction:
1
In the advanced IC (Integrated Circuits) packaging, the thin-film metallization is
commonly used as UBM (Under Bump Metallization) on the chip side to bond small
C4 solders balls. [1]
Yet, the fast consumption of the limited metal thin layer due to
the reaction with Sn-bearing solders is always a concern for the reliability of solder
joints.[2,3,4]
Tri-layer Cu/Ni(V)/Al UBM has successfully used as flip-chip UBM
for many years. No spalling was observed after 40 minutes reflowing, when it
reacted with the eutectic SnPb solder.[5]
However, when the eutectic SnPb solder
was replaced with Sn-rich Pb-free solders, spalling in tri-layer Cu/Ni(V)/Al UBM was
found after several reflows.[6]
Therefore, it is a challenging task to develop a thin
film UBM without spalling for flip-chip Pb-free solder bumps.
In previous study by Wang, the spalling of Ni thin film was found to be
prevented, if a Cu reservoir was introduced into the structure of C4 (Controlled
Collapse Chip Connections) solder joints during the soldering reaction.[7]
The Cu
reservoir could be the incorporated Cu particles inside the solder or the Cu pad in the
package side. The major role of Cu reservoir was a source to provide Cu atoms to
form a layer Cu-Sn compound on Ni thin film, which was believed to be the key for
the prevention of spalling.
Some reports have demonstrated that the small amount of Cu additives can
improve the Pb-free solders’ mechanical properties, such as, fatigue, creep, and tensile
strength.[8,9,10]
So, it seems inviting us to understand the role of Cu additives in
the interfacial reaction between Cu-bearing Pb-free solders and Ni thin film
metallization. It will also be interesting to study that will the limited Cu additives in
Cu-bearing Pb-free solders have the same effect as the Cu reservoir.
In this study,
nine dilute Sn(Cu) were used to study the effect of the variation of Cu content on the
interfacial reaction between Sn(Cu) alloys and the Ni thin film.
Experimental Procedures:
2
The Ni/Ti metallization was deposited on an oxidized Si wafer by E-beam
evaporation process.
The Ti layer is for the adhesion purpose and the Ni layer is
simulated to the wettable metal bond pad. The thickness of Ti and Ni are 500 Å and
2000 Å, respectively. 99.99% purity of Sn and Cu purchased from Alfa AESAR Inc.
were used to prepare nine SnCu alloys, which range 0, 0.2, 0.6, 1, 1.4, 1.8, 2.2, 2.6,
and 3.0. To produce accurate composition of SnCu alloys, weight Sn and Cu in
desired weight percentage by a digital balance (SCALTEC SBC31), then, place them
into evacuated and sealed quartz tubes. Heat quartz tubes to a constant temperature
of 900C in a furnace for 150 hours. During the heat treatment, swirled quartz tubes
to evenly mix the metal elements. After the heat treatment, quartz tubes were quickly
quenched by water cooling. Cut 2mg of each of nine alloys and melt them under the
flux ambient to produce a spherical solder ball.
The reflowing process steps are in following. Right amount of flux was poured
into a stainless beaker.
Put the beaker on a hot plate and maintain the flux at a
constant temperature of 250C. The Si wafer coated with Ni/Ti metallization was
cut into 2x2 cm square pieces. Those Si pieces were immersed into the heated flux
for 30 seconds to clean the Ni surface. Then, 2mg solder balls were placed on the
top of Si pieces.
As solder ball touched Ni surface, the molten solder started
spreading and reacting with Ni/Ti metallization. Five different reaction times were
carried out for each composition, which are 30sec, 1min, 5min, 10min, and 20min.
After certain reflowing time, Si chips were removed from flux and cooling in
the air for one minute, then they were cleaned by alcohol in an ultrasonic machine.
To perform the SEM examination, mount reflowed-samples by the epoxy resin.
First,
the epoxy mounted samples were cut by a diamond saw and abraded by coarse
sandpapers.
Then, samples were finished by 0.3μm Al2O3 powder polishing.
Before SEM observation, samples were slightly etched by light acid (10%H2SO4) for
3
10 seconds to delineate the morphology of the compound layer.
Results:
Fig.1 shows the cross-sectional SEM images of interfacial reactions between
low Cu-content Sn(Cu) alloys and Ni thin film.
As seen in Fig. 1(a) and (b), the
original Ni thin film could not be observed at the interface after 30 seconds reflowing.
We believe that the Ni thin film was completely consumed by the formation of the
Ni-Sn compound layer and, then, spalled into the molten solder.
It is striking that, as
shown in Fig.1(c), a layer of interfacial compound was found to reside at the interface,
as Ni thin film reacted with Sn0.6Cu for 30 seconds.
It implied that 0.6wt.% of Cu
content could defer the on-set time of spalling to longer reaction time. Using EDX
analysis, the compound layer was determined to be the Ni3Sn4 compound phase
dissolved with Cu. The Cu in (Ni,Cu)3Sn4 intermetallic compound layer should
attribute to the Cu content in the molten solder. Yet, we found that spalling occurred
after 5 minutes reflowing, as shown in Fig.1(d).
The big separated Ni-Sn grains on
the Ti surface indicated the occurrence of the ripening process.
As the Cu content in Sn(Cu) alloy increased to 1.0 wt.%, remarkably, we found
that spalling could not be observed even after 20 minutes reflowing!
Fig.2 is the
SEM cross-sectional images of interfacial reaction between Sn1.0Cu and Ni thin film
for different reflowing times. The EDX analysis shows that a scallop-like Cu6Sn5
compound layer covered the original Ni thin film after 30 seconds reflowing, as seen
in Fig.2 (a). The substantial portion of the initial Ni thin film remained un-reacted.
With prolonged reflowing, the growth of the interfacial compound layer was very
sluggish.
After 20 minute reflowing, EDX results show that a thin Ni thin film layer
still can be observed and covered by a (Cu, Ni)6Sn5 intermetallic compound layer.
Interestingly, the morphology and size of the scallop-like compound grains show no
4
big change during the reflowing process, as seen in Fig.2.
It indicates that ripening
did not occur in compound grains.
Fig.3 is the cross-sectional SEM images of interfaces between Sn1.8Cu/ Ni thin
film for different reflowing times. Spalling did not occur in the initial 5 minutes
reflowing.
As the reflowing was prolonged to 10 minutes, the ripening process
occurred and the resultant big compound grains started departing from the Ti surface,
as seen in Fig.3 (c).
Fig.4 is the SEM cross-sectional images of the interfacial
reaction between Sn3.0Cu and Ni thin film.
Unlike the scallop-type compound layer
of other Sn(Cu) alloys after 30 seconds reflowing, the morphology of interfacial layer
of Sn3.0Cu is relatively flat.
In addition, the primary Cu6Sn5 compound phase was
observed in the solder, as indicated by black arrows in Fig.4 (a) and (b).
After one
minute reflowing, the primary Cu6Sn5 compound precipitates inside the solder were
greatly reduced. We believe that the primary Cu6Sn5 compound particles in the
solder were quickly expensed by the growth of the compound layer. Coalescence or
ripening between primary Cu-Sn compound particles and interfacial compound layer
were the two possible processes for the consumption of the primary Cu6Sn5 particles.
After 5 minutes reflowing, big chunk of compound grains were found at the interface
and about departing from the surface of Ti layer, as seen in Fig.4 (c).
thickness of compound layer is estimated to be about 4.2μm.
The average
In the Fig.4 (d),
clearly, we can observe that the chunky Cu6Sn5 compound grains are floating upward
in the solder after 20 minutes of reflowing.
Table one is a summary for the on-set time of spalling versus different Sn(Cu)
alloys. The on-set time of spalling strongly depends on the Cu concentration in the
Sn(Cu) alloys. The initial small amount of Cu additives deferred the on-set time of
spalling to longer reflowing time. Amazingly, no spalling can be found for the
Sn1.0Cu alloy. As Cu content in the Sn(Cu) alloys are over 1.0 wt. %, the spalling
5
backed to occur.
Note that in region of high Cu-content alloys, the more Cu
additives, the earlier spalling occurred.
Discussions:
(1) Review of spalling phenomenon
The typical picture of the spalling process in the Cu thin film is illustrated in
Fig.5.
First, the Cu thin film was completely converted to a scallop-appearing
compound layer after a short soldering reaction, as shown in Fig.5 (a).
Then, the
ripening process occurred among scallop-like compound grains; the bigger
scallop-like compound grains grew at the expense of smaller scallop-like compound
grains. [2]
Due to the mass conservation, the incremental of the bigger grain in
radius, dr1, was smaller than the decrease of smaller compound grain in radius, dr2.
Thus, a small portion of the Ti surface was exposed to the molten solder, as shown in
Fig.5 (b). As a result, unstable triple junctions of three interfacial energies, i.e.,
γTi/Solder, γTi/compound, and γSolder/compound, occurred, as indicated by three arrows in Fig.5
(c).
The unbalanced interfacial energies at the triple junctions led to the shape
transformation from semi-spherical grains to spherical grains, as seen in the dash
circle in Fig.5 (c). The spherical compound grains had very little contact with the Ti
surface, so, they tend to float into the molten solder due to the gravity effect.
The
resultant high interfacial energy between the Ti surface and the molten solder caused
the dewetting of the molten solder on the Ti surface, as shown in Fig. 5 (d).
From the review above, we recognize that the ripening process, which caused
the exposure of the Ti surface to the molten solder, is the key step for the occurrence
of spalling.
In the present study, we found that the Cu additives in Sn could defer or
prevent the spalling of the Ni thin film during the soldering reaction. So, we believe
that the Cu additives in solders should have a strong influence on the repining process
6
in the interfacial compound layer.
In the following, we will first discuss the
influence of Cu additives on the interfacial reaction between Sn(Cu) alloys and Ni
thin film and, then, lead to the possible mechanism of the spalling deferring.
(2) Formation of interfacial compounds on Ni thin film
SEM cross-sectional results have shown that a layer of Cu-Sn compound formed
on the Ni thin film, as the Ni thin film reacted with Sn(Cu) alloys having Cu content
over 0.6wt.%.
A similar finding on the bulk Ni substrate was reported by Kao.[11,12]
It is plausible that Kao explained this finding well by using the diffusion paths in the
Sn-Cu-Ni isotherm. Here, we found that it also can be interpreted by the concept of
the Cu solubility limit in Sn. Fig.6 is the enlarged Sn corner of the Sn-Cu-Ni ternary
phase diagram calculated by Zeng et. al. [13,14]. According to this diagram, the
solubility limit of Cu in Sn is estimated to be about 1.1 wt. %, which is close to the
experimental value, 1.5 wt. %, reported from Steen.[15]
The slight discrepancy
between experimental and calculation values could be due to the difference in
temperature.
It is of interesting that with little additional Ni in Sn, as indicated by a
black dot in Fig.6, the Cu solubility limit is greatly reduced to about 0.6 wt%.
In the very early stage of the soldering reaction, Ni would dissolve into the local
region of the molten solder near the interface of Ni thin film before the formation of
the Ni-Sn compound.
As pointed out earlier, the presence of little Ni dissolution in
the liquid Sn would cause the reduction of the Cu solubility limit to be around 0.6
wt.% at 250 C. Therefore, for Sn(Cu) alloys having the Cu content over 0.6 wt.%,
the Cu concentration near the Ni surface became over-saturated. The access Cu
would instantaneously precipitate out as a Cu-Sn compound layer on the Ni interface
to meet the Cu solubility limit under the presence of Ni.
Consequently, an instant Cu
concentration difference would be established between the Ni interface and the bulk
7
of molten Sn(Cu) alloys and generate a Cu flux from solder to Ni interface. As long
as sufficient Ni dissolution could be maintained near the interface of the Ni side, the
Cu atomic flux would exist and the Cu-Sn compound layer would grow. Yet, during
the formation of Cu-Sn compound layer on Ni side, the Ni dissolution could be
incorporated into the formation of the Cu-Sn compound and formed a Cu-Ni-Sn
ternary compound.
The decreasing Ni dissolution at the Ni surface could diminish
the Cu atomic flux toward the Ni interface. The Cu flux would resume, if the Ni
dissolution could be replenished by the diffusion of Ni through the Cu-Sn compound
layer from Ni thin film.
So, we believe that Cu flux from solder to Ni interface
could likely to be an on-and-off process rather than a continuous process.
It really
depended on the Ni diffusion rate in the Cu-Sn compound and the thickness of Cu-Sn
compound on the Ni side.
Fig.7 is a plot of thickness of interfacial compound layers versus the square root
of reflowing time for Sn(Cu) alloys with Cu content over 0.6wt.%.
stages are observed.
Two growth
In the first minute reflowing, the compound growth rate did not
show much difference for all alloys.
To interpret it, first, we realize that for
Sn(Cu)alloys having Cu content over the Cu solubility limit in Sn at 250 C, the
primary Cu6Sn5 solid phase will coexist with the Cu-saturated liquid Sn(Cu) alloy at
250 C. The ratio between the liquid and solid phases depends on the composition.
So, during the early soldering reactions at 250 C, the interfacial reactions mainly
occurred between the Cu-saturated Sn(Cu) liquid and Ni thin film.
Therefore, a
similar reaction behavior could be expected for Sn(Cu) alloys having Cu content over
10 wt.%. After one minute of reflowing, beside Sn1.0Cu, the thickness of Cu-Sn
compound layers increased dramatically, because the primary Cu6Sn5 compound
particles started participating the growth of the interfacial Cu-Sn compound layer.
It
is worthy of note that an incubation time was needed for the primary Cu6Sn5
8
compound particles to participate the interfacial compound growth.
It is not
understood yet.
Another interesting point is that we found that the compound growth of Sn1.0Cu
was very slow in the late reflowing stage.
The sluggish Cu-Sn compound
formation could be because that no primary Cu-Sn would exist in the molten solder of
Sn1.0Cu during the soldering reaction. After one minute reflowing, 1.1 μm Cu-Sn
compound layer consumed about 0.3wt.% -0.4 wt.% of Cu content in the molten
solder. The Cu content in the liquid solder was reduced to be very close with the Cu
solubility limit under the presence of Ni. Hence, the formation of Cu-Sn compound
on the Ni thin film was very slow in the late stage of reflowing.
(3) Kinetics of Cu-Sn formation on Ni thin film in the initial reflowing stage
In Fig.7, the compound growth showed a linear relation with the square root of
reflowing time in the initial reflowing.
It implied that the formation of Cu-Sn
compound might be controlled by a diffusion process in the initial reflowing stage.
We realize that the Cu in the Sn(Cu) alloys was the only source for the formation of
the Cu-Sn compound layer on Ni thin film. Hence, two sequential processes were
indispensable for the formation of Cu-Sn compound layer; they were the precipitation
of Cu-Sn compound on the Ni interface and the transportation of Cu to the Ni
interface by the liquid diffusion. Assuming the precipitation of Cu-Sn compound on
the Ni interface was a fast and a constant-rate process, the formation of Cu-Sn
compound on the Ni thin film could be limited by the Cu liquid diffusion process in
the molten solder.
The initial Cu flux from the molten solder toward the Ni interface can be simply
obtained by calculating the growth rate of the Cu-Sn compound layer on the Ni
interface. So, the Cu flux can be expressed as:
9
JCu= L·ρ·fCu·N0 / t·MCu
Where, L is the thickness of compound layer, ρis the density of Cu6Sn5 (8.27 g/cm3),
fCu is the Cu weight fraction in Cu6Sn5, N0 is Avogadro number, t is the reflowing
period, and MCu is the molecular weight of Cu.
In the initial growth period, 0.8
micrometers of compound layer formed in the first minute of reflowing.
Plug in the
compound thickness and the reflowing time, the Cu flux for the initial growth period
is about 2.4x1017 atoms/cm2·s.
Using the quasi-steady-state approximation, the Cu flux can also be expressed as
J=D
dC
.
dX
As discussed previously, the Cu concentration difference, 0.5 wt.%
(1.1-0.6 wt%), established between the molten solder and the Ni interface was
considered to be the driving force for the Cu flux in the initial reflowing stage.
To
obtain the concentration gradient, the distance for the Cu concentration difference
should be determined.
Since the exact distance for the Cu concentration difference
is unknown, here, we estimate the average Cu concentration gradient by dividing the
Cu concentration difference with the height of the solder cap, 200 µm.
Plug in the
number of the Cu concentration gradient and Cu diffusivity in Sn, 10-5cm2/s [16], into
the flux equation, we can obtain the magnitude of Cu flux from solder to the Ni
interface to be about 1.6 x1017 atoms/cm2·s, which is very close with the experimental
number of Cu flux calculated previously. So, we believe that the kinetic behavior of
Cu condensing flux in the initial reflowing period could be described by the Cu
solubility difference between the molten solder and the Ni interface.
In the late
reflowing period, the primary Cu6Sn5 compound phase in the solder was involved in
the growth of the interfacial compound layer.
Thus, the growth behavior of the
interfacial compound layer in the late stage was very different with that in the initial
stage.
10
(4) Spalling deferring mechanism
We have realized that the ripening process among the compound grains played
the key role for the occurrence of spalling. One can postulate that if the ripening
flux of shrinking smaller compound grains could be compensated by the condensing
Cu flux from the molten solder, Ti surface will not expose to the molten solder.
Then, spalling could be prevented. The ripening flux for semi-spherical Cu6Sn5
compound grains was equated by Tu [4]:
J

2DC0
3 LRT
Where, D is the diffusivity of Cu in the molten solder, Ω is the molar volume of
Cu6Sn5, γ is interfacial energy per unit area between Cu6Sn5 and molten solder, C0 is
the equilibrium concentration of Cu in solder, R is the gas constant, T is the
temperature,  is the mean separation between grains, and r is the mean grain radius.
To evaluate the repining flux in the initial reflowing stage, materials properties
are given as Ω=117.87 cm3/mole, reflowing temperature is 523 ºK, and R=8.3
Joule/mole· ºK. We take the Cu equilibrium concentration in Sn about 1.1wt.%.
The Cu diffusivity in Sn is 10-5cm2/s.
μm after 30 seconds reflowing.
The mean grain radius is estimated about 1.5
Also, we use the same approximation of the
interfacial energy per unit area between Cu6Sn5 and molten solder and the separation
length taken by Kim.
Substitution of all parameters into the ripening equation, the
repining flux of the initial reflowing was obtained to be about 3.8x1017 atoms/cm2·s.
Strikingly, we found that it is in the same order with the condensing Cu flux from the
molten solder to the Ni interface in the early stage of reflowing. The correspondence
between the ripening flux and the condensing Cu flux sustains our postulation that the
ripening flux could be compensated the Cu flux from solder.
For Sn(Cu) alloys having Cu content less than 0.6 wt.%, no Cu concentration
11
difference would develop between the molten solder and the Ni interface.
Therefore,
the Cu condensing flux was absent or too mall to compensate the ripening flux among
compound grains.
As a result, the ripening process and spalling would occur in the
interfacial compound grains within one minute reflowing. For Sn(Cu) alloys having
Cu content over 1.0wt.%, spalling did not occur in the first minute of reflowing
because the ripening process was hindered by the Cu flux.
So far, we can consistently explain the mechanism of spalling retarding in the
initial reflowing stage.
Yet, with longer prolonged reflowing, the mechanisms for
the absence of spalling in the case of Sn1.0Cu and the occurrence of spalling in the
high Cu-content alloys, such as, Sn1.8Cu, Sn2.2Cu, Sn3.0Cu, are still not very clear
to us. Here, possible interpretations are provided in the following. For the high
Cu-content alloys, Sn1.8Cu, Sn2.2Cu, Sn3.0Cu, the thickness of the Cu-Sn compound
layer increased dramatically after one minute reflowing, because the primary Cu 6Sn5
compound particles in the solder participated the growth of the interfacial Cu6Sn5
compound layer.
When all the primary Cu6Sn5 compound particles were quickly
consumed by growing the interfacial Cu-Sn compound layer, a quasi-equilibrium state
was maintained between the molten solder and the compound interface.
No net Cu
flux would exist toward the compound interface. So, the ripening process would
proceed and spalling would occur after one minute reflowing.
In the case of Sn1.0Cu, since the Cu content is just about the Cu solubility limit
at 250 C, no primary Cu-Sn compound phase co-existed with the molten solder
during the soldering reaction.
So, in the late soldering reaction, the growth of
interfacial compound layer was very sluggish, which only expensed the Cu content in
the molten solder. Owing to the relatively thin Cu-Sn compound layer, Ni could
diffuse through the Cu-Sn compound layer. The constant presence of Ni on the
surface of interfacial (Cu,Ni)6Sn5 compound layer would have strong effects on the
12
ripening flux among interfacial (Cu,Ni)6Sn5 compound grains in couple aspects.
First, it would persistently induce Cu flux toward the Ni side, therefore, the ripening
process among interfacial (Cu,Ni)6Sn5 compound grains would be retarded constantly.
Secondly, the Ni on the surface of interfacial compound grains would block the
dissolution of Cu into the molten solder, since Ni has a much lower dissolution rate
than Cu has in the solders.
Hence, the ripening flux among the interfacial
(Cu,Ni)6Sn5 compound grains would be diminished. Coupling the two effects above,
we believe that the constant exist of Ni on the surface of the interfacial compound
layer is the key for the absence of spalling.
Summary:
13
We have found that the Cu additives in Sn can defer or prevent the occurrence
of spalling in Ni thin film during the reflowing process.
For the Cu content in Sn(Cu)
alloys below 0.6wt.%, the spalling quickly occurred after 30 seconds of reflowing.
Spalling was not observed in Sn0.1Cu after 20 minutes of reflowing.
mechanism of spalling prevention is proposed.
The
The presence of Ni at the interface
caused the reduction of Cu solubility near the Ni interface. The reduction of Cu
solubility resulted a Cu concentration gradient between solder and Ni interface, which
induced a condensing Cu flux from solder to the Ni thin layer.
The condensing Cu
flux was calcluated to be in the same order with the ripening flux from smaller
compound grains to larger compound grains.
We tend to conclude that the
condensing Cu flux compensated the ripening flux, therefore ripening process in
compound grains was hinderred. Then, spalling was deferred or prevented.
For the high Cu-content Sn(Cu) alloys, a large Cu condensing flux from solder
to Ni interface prevented spalling in the compound layer at the initial one minute of
reflowing. The condensing Cu flux was due to the ripening process between Cu-Sn
compound precipitates and the interfacial Cu-Sn compound layer. After one minute
reflowing, the Cu-Sn particles in solder quickly consumed. The ripening process
proceeded in the compound grains at the interface, then spalling backed to occurr.
Acknowledgements:
Authors would like to thank the support from NSC (Taiwan National Science
Council) and MOE Program for Promoting Academic Excellence of Universities
under the grant number 91-E-FA06-1-4.
References:
14
1. R.R. Tummala, E. J. Rymaszewski, Microelectronics packaging Handbook (Van
Norttrand Reinhold, New York, 1989).
2. Ann A. Liu, H. K. Kim, K. N. Tu, and P. A. Totta, J. Appl. Phys.,80, 2774-2780
(1996).
3. H. K. Kim, and K. N. Tu, Appl. Phys. Lett., 67, 2002 (1995).
4. H. K. Kim and K. N. Tu, Phys. Rev. B, 53 (23), 16027, (1996).
5. C. Y. Liu, K. N. Tu, T. T. Sheng, C. H. Tung, D. R. Frear, and P. Elenius, J. Appl.
Phys. 87, 750-754, (2000).
6. M. Li, F. Zhang, W. T. Chen, K. Zeng, K. N. Tu, H. Balkan, and P. Elenius, J. Mater.
Res., 17, 1612-1621, (2002).
7. C. Y. Liu and S. J. Wang, J. Electronic Materials, 32(1), L1, (2003).
8. D. R. Frear, J. W. Jang, J. K. Lin and C. Zhang, JOM, June, 28 (2001).
9. Technical Reports for the Lead Free Solder Project: Properties Report, National
Center for manufacturing Science (NCMS), (1998).
10. J. D. Sigelko and K. N. Subramanian, Adv. Mat. & Proc., 47-48, March (2000).
11. W. T. Chen, C. E. Ho, and C. R. Kao, J. Mater. Res., 17, 263 (2002).
12. C. Chen, C. E. Ho, A. H. Lin and C. R. Kao, J. Electronic Materials, 29(10), 1200,
(2000).
13. K. Zeng and K. N. Tu, Materials Science and Engineering Reports, R38, 55-105
(2002).
14. K. Zeng and J. K. Kivilahti, J. Electr. Mater., 30, 35 (2001).
15. H. A. H. Steen, Swedish Institute for Metals Research, Report No. IM-1643,
(1982).
16. S. J. Wang, C. Y. Liu, submitted to JEM.
Captions:
15
Figure 1 SEM cross-sectional images of interfacial reactions between low Cu-content
Sn(Cu) alloys and Ni thin film.
Figure 2 SEM cross-sectional images of interfacial reaction between Sn1.0Cu and Ni
thin film for different reflowing times.
Figure 3 SEM cross sectional images of interfacial reaction between Sn1.8Cu and Ni
thin film for 10 minutes and 20 minutes reflowing.
Figure 4 SEM cross-sectional images of interfacial reaction between Sn3.0Cu and Ni
thin film for different reflowing times.
Figure 5 Schematic picture for spalling process.
Figure 6 Enlarged Sn corner of the Sn-Cu-Ni ternary phase diagram.
Figure 7 Plot of thickness of interfacial compound layers versus the square root of
reflowing time for Sn(Cu) alloys with Cu content over 0.6wt.%.
Table one: spalling on-set time for different Sn(Cu) alloys
16