1. 1 23
Journal of Electronic Materials
ISSN 0361-5235
Journal of Elec Materi
DOI 10.1007/s11664-013-2786-6
Interfacial Reactions Between Columnar or
Layered Ni(P) Layers and Sn-Ag-Cu Solder
Y.J. Hu, Y.C. Hsu, C.T. Lu, T.S. Huang,
C.Y. Chen, W.N. Chuang, C.Y. Hsiao,
C.P. Lin & C.Y. Liu

2. 1 23
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3. Interfacial Reactions Between Columnar or Layered Ni(P)
Layers and Sn-Ag-Cu Solder
Y.J. HU,1,3
Y.C. HSU,1
C.T. LU,1
T.S. HUANG,1
C.Y. CHEN,1,4
W.N. CHUANG,1
C.Y. HSIAO,2
C.P. LIN,2
and C.Y. LIU1
1.—Department of Chemical Engineering and Materials Engineering, National Central Univer-
sity, Jhongli 32001, Taiwan, ROC. 2.—Taiwan Uyemura Co., LTD., Taoyuan, Taiwan. 3.—e-mail:
hu771015@hotmail.com. 4.—e-mail: chengyi@cc.ncu.edu.tw
Comparative study on the interfacial reactions between lead-free Sn-Ag-Cu
solder and Ni(P) bond pads (with columnar or layered microstructure) has
been performed. The microstructure of the columnar Ni(P) is vertical, while
the microstructure of the layered Ni(P) tends to be parallel to the solder/Ni(P)
interface. The consumption rate of the columnar Ni(P, 7 at.%) layer is larger
than that of the layered Ni(P, 7 at.%). We believe that the faster Ni(P) con-
sumption rate of the columnar Ni(P, 7 at.%) layer is due to the orientation of
the grains. Spalling of the interfacial (Cu,Ni)6Sn5 compound can be seen at the
reaction interfaces for the columnar Ni(P, 7 at.%) layer. On the contrary, no
spalling can be seen for the case of the reacted layered Ni(P).
Key words: Electronic package, interfacial reaction, Pb-free solder, Ni(P)
INTRODUCTION
Electroless Ni(P) plating has been widely used as
a diffusion barrier layer on Cu substrates for flip-
chip and ball grid array solder bumps.1–4
The Ni(P)
layer has been proved to have numerous advanta-
ges, for example, a mature and low-cost plating
process, and high resistance to corrosion. However,
one of the major concerns regarding the use of Ni(P)
bond pads is the solder-reaction-induced formation
of the Ni3P compound between the Ni3Sn4 com-
pound and the Ni(P) layer.3,4
Ni3P formation has
been reported to cause (1) serious voids inside the
Ni3P layer and a gap underneath the Ni3P layer,
and (2) spalling of the Ni3Sn4 compound layer.
These two phenomena degrade the reliability of the
solder joints.5,6
In the current Ni(P) electroless plating process,
the Ni(P) usually has a layered crystalline struc-
ture. This means that the grains of Ni(P) appear to
have an elongated shape. Owing to this layered
shape, the Ni(P) layer is quite brittle. Bending can
easily cause the Ni(P) layer to crack. Nowadays,
flexible substrates are widely used in handheld
devices. The Ni(P) layer plated on such flexible
substrates needs to be more flexible and ductile.
One approach to produce a ductile Ni(P) layer is to
manipulate the microstructure of the Ni(P) layer. In
a private communication from Taiwan Uyemura
Corporation, their internal results show that a Ni(P)
layer with columnar crystalline structure would
have improved ductility. In this study, firstly, we
investigate the interfacial reactions between lead-
free SAC305 solder and a Ni(P) bond pad with
columnar microstructure. It is known that the
phosphorous (P) in the electroless Ni(P) would
greatly influence the reliability of the solder joints.
So, in this paper, we also study the effect of the P
content on the interfacial reaction with the same
reflow processes.
EXPERIMENTAL PROCEDURES
Figure 1 illustrates the studied bump joint
structure. Sn-3Ag-0.5Cu (wt.%) solder bumps were
reflowed on electroless Ni immersion gold bond pads
on the Cu pads of a FR4 printed circuit board. The
thickness of the Ni(P) layer was about 4.75 lm to
5 lm, and the thickness of the immersion Au layer
was about 0.06 lm. The opening in the passivation
layer was about 500 lm. Two kinds of Ni(P) layer(Received May 2, 2013; accepted September 7, 2013)
Journal of ELECTRONIC MATERIALS
DOI: 10.1007/s11664-013-2786-6
Ó 2013 TMS
Author's personal copy

4. were prepared by electroless plating processes,
having columnar and layered crystalline structure,
respectively. Figure 2a and b show cleaved cross-
sectional images of the columnar and layered Ni(P)
layers, respectively. One can clearly see that the
grain microstructure of the columnar Ni(P) layer
tends to be vertical to the Ni(P) layer, whereas the
grain microstructure of the layered Ni(P) layer
tends to be parallel to the Ni(P) layer. The P content
in both Ni(P) layers was about 7 at.%.
Then, the prepared Au/Ni(P)/Cu bond pads were
reflowed with 760-lm-diameter Sn-Ag-Cu solder
balls. Note that the Au/Ni(P)/Cu bond pads were
prefluxed with a thin rosin mildly activated (RMA)-
type flux. The reflow temperature was 250°C, and
the duration of each reflow was 60 s. Samples were
subjected to one, three, or five reflows. After a cer-
tain number of reflows, the samples were removed
from the hotplate and air-cooled at room tempera-
ture. The reflowed samples were mounted with
epoxy resin and polished with sandpaper and pol-
ishing cloths. The fine-polished samples were
examined by scanning electron microscopy (SEM).
The compositions of the interfacial phases were
analyzed by field-emission electron probe micro-
analysis (FE-EPMA). The analyzer electron beam
size used to verify the compound phases was
0.5 lm 9 0.5 lm.
The second part of this work studied the effect of
the P content on the soldering reaction. By varying
the electroless plating condition, the P content in
the Ni(P) layers can be increased to 10 at.% with
a layered microstructure. All the reflow processes
were the same as the previous experimental
conditions.
RESULTS AND DISCUSSION
Interfacial Reaction of Layered and Columnar
Ni(P) Layers
Figure 3a, b, and c show SEM cross-sectional
images at the interfaces between layered Ni(P,
7 at.%) and Sn-Ag-Cu solder after one, three, and
five reflows, respectively. The reactions between
Sn-rich Pb-free solders and Ni(P) bond pads have
been studied by many researchers.7–10
Typically, it
is known that the top Au finish layer quickly dis-
solves into the molten solder. In this study, by using
FE-EPMA analysis, we also verified that the Au
finish layer on the Ni(P) metal bond pad dissolved
into the molten SAC305 solder; no Au-containing
compound phases could be detected at the interface
or inside the solder matrix. We believe that the
entire Au layer on the Ni(P) layer quickly dissolved
into the molten Sn-Ag-Cu solder, and no Au element
could be detected in the interfacial compound pha-
ses. Thus, the Au layer on the Ni(P) layer did not
play a role in the Sn-Ag-Cu/Ni(P) interfacial reac-
tion.
As seen in Fig. 3, the interfacial intermetallic com-
pound layer shows the common scallop-type morphol-
ogy, being found to be the ternary (Cu,Ni)6Sn5
compound phase by FE-EPMA (electron-probe x-ray
microanalysis). The ternary (Cu,Ni)6Sn5 intermetallic
compound phase has been observed in many previous
works on the reactions between Cu-containing Sn-rich
solders and Ni(P) metal bond pads.3–5,7,8,10–12
Also, a
layer of the Ni3P compound phase can be seen under-
neath the ternary (Cu,Ni)6Sn5 compound layer, as
indicated by arrows in Fig. 3d–f. The formation of a
Ni3P layer has been proven to be caused by the mech-
anism of ‘‘solder-reaction-assisted crystallization.’’11,12
AstheNi3Playergrowswiththenumberofreflows,the
P content in the Ni(P) layer would be enriched and
promote the growth of the Ni3P layer. Typically,
workers agree that the growth of the interfacial
(Cu,Ni)6Sn5 compound layer consumes Ni from the
Ni(P) layer and enhances the growth of the Ni3P layer.
Figure 4a, b, and c show cross-sectional images at
the interfaces between columnar Ni(P, 7 at.%) and
Sn-Ag-Cu solder after one, three, and five reflows,
respectively. As for the reaction of the layered Ni(P)
layer, the ternary (Cu,Ni)6Sn5 compound is also
observed at the interface. However, we found that
the morphology of the interfacial (Cu,Ni)6Sn5 com-
pound in the case of the columnar Ni(P, 7 at.%)
layer has a facet-like appearance, which is closer to
the morphology of the Ni3Sn4 compound formed at
the Ni/Sn reaction interface. On the other hand, the
morphology of the interfacial (Cu,Ni)6Sn5 compound
in the case of the layered Ni(P, 7 at.%) layer shows a
more scallop shape, which is commonly observed in
the Cu6Sn5 compound at the Cu/Sn reaction inter-
face.13,14
Thus, we believe that the interfacial
(Cu,Ni)6Sn5 compound in the case of the columnar
Ni(P, 7 at.%) layer could contain more Ni content
than the interfacial (Cu,Ni)6Sn5 compound in the case
of the layered Ni(P, 7 at.%) layer. This expectation
also agrees with the data presented in Table I show-
ing that the columnar Ni(P, 7 at.%) layer contains
more Ni content than the layered Ni(P, 7 at.%) layer.
Figure 5a shows the consumed thickness of the
Ni(P) layer versus the number of reflows. It is found
that the Ni(P) consumption rate of the columnar
Ni(P, 7 at.%) layer is larger than that of the layered
Fig. 1. Schematic of a solder joint with a Ni(P) layer.
Hu, Hsu, Lu, Huang, Chen, Chuang, Hsiao, Lin, and Liu
Author's personal copy

5. Ni(P, 7 at.%) layer. We believe that the faster Ni(P)
consumption rate of the columnar Ni(P, 7 at.%)
layer is due to its grain microstructure. For the
columnar Ni(P, 7 at.%) layer, the grain boundaries
tend to orient vertically to the joint interface. Thus,
it can be expected that the Ni atomic diffusion flux
(Ni supply to the interfacial reaction) from the
columnar Ni(P, 7 at.%) layer toward the reaction
interface would be larger than that from the layered
Ni(P, 7 at.%) layer. Thus, owing to the larger Ni
diffusion flux to the reaction interface, faster inter-
facial compound formation and Ni(P) consumption
should result for the case of the columnar Ni(P,
7 at.%) layer. In addition, spalling of the ternary
(Cu,Ni)6Sn5 compound layer would be another
important factor causing the faster Ni(P) consumption
Fig. 2. Cross-section images of (a) columnar and (b) layered Ni(P) layers.
Fig. 3. SEM images of SAC305 solder joints with layered Ni(P, 7 at.%) after (a) one, (b) three, and (c) five reflows, and magnified views after
(d) one, (e) three, (f) and five reflows.
Interfacial Reactions Between Columnar or Layered Ni(P) Layers and Sn-Ag-Cu Solder
Author's personal copy

6. for the case of the columnar Ni(P, 7 at.%) layer.
As shown in Fig. 4, spalling of the interfacial
(Cu,Ni)6Sn5 compound can be seen at the Sn-Ag-Cu/
Ni(P, 7 at.%) reaction interfaces. After one reflow, a
small amount of compound grains can be seen spalled
above the reaction interface. After three reflows, two
kinds of spalled-off compound grains are observed.
Round-shaped compound grains are found spalled
away from the reaction interface, whereas small
plate-like compound grains are spalled off at the
reaction interface. After five reflows, more plate-like
compound grains are spalled off at the reaction
interface. As the interfacial (Cu,Ni)6Sn5 compound
spalled away from the Sn-Ag-Cu/Ni(P) reaction
interface, the molten Sn-Ag-Cu solder would likely
Fig. 4. SEM images of SAC305 solder joints with columnar Ni(P, 7 at.%) after (a) one, (b) three, and (c) five reflows, and magnified views after
(d) one, (e) three, and (f) five reflows.
Table I. EPMA analysis of Ni content at middle and
outer region for different substrates
Middle Region
(Ni wt.%)
Outer Region
(Ni wt.%)
7% P layered 89.063 88.545
7% P columnar 90.667 89.201
10% P layered 84.037 82.152
11% P layered 85.942 84.539
Fig. 5. Thickness of Ni(P) consumption versus number of reflows at
250°C.
Hu, Hsu, Lu, Huang, Chen, Chuang, Hsiao, Lin, and Liu
Author's personal copy

7. react with the Ni(P) layer directly. Consequently, the
Ni(P) consumption would be enhanced in the case of
the columnar Ni(P, 7 at.%) layer. It is well accepted
that the left-over P content owing to Ni(P) consump-
tion is the key to the formation of the Ni3P layer.
Thus, the degree of Ni consumption would be pro-
portional to the formation of the Ni3P layer. In the
current results, we observe that the Ni(P) consump-
tion in the columnar Ni(P) case is much greater than
in the layered Ni(P) case after five reflows. However,
the Ni3P thickness in the columnar Ni(P) case is not
very different from that in the layered Ni(P) case. We
tend to believe that part of the Ni3P layer in the
columnar Ni(P) case reacted with the solder to formed
a Sn-Ni-P layer, which is why we do not observe more
Ni3P forming in the case of the columnar Ni(P) layer.
Another important finding is that a continuous
Ni–Sn-P layer is observed between the interfacial
(Cu,Ni)6Sn5 compound layer and the Ni3P layer in
the case of the columnar Ni(P, 7 at.%) layer, as
indicated by arrows in Fig. 4e, f. Such a continuous
Ni-Sn-P layer has been observed by many
researchers.15–17
Also, we note that, in the case of
the layered Ni(P, 7 at.%) layer, we can hardly
observe the Ni-Sn-P layer at the reaction interface.
We believe that spalling of the interfacial (Cu,
Ni)6Sn5 compound at the reaction is the key to the
formation of this Ni-Sn-P layer, because it causes a
direct reaction between the molten Sn-Ag-Cu solder
and the Ni(P) layer. There are several reasons for
the spalling of the interfacial (Cu,Ni)6Sn5 compound
on the Ni-Sn-P layer. Firstly, the interfacial energy
between the interfacial (Cu,Ni)6Sn5 compound and
the Ni-Sn-P layer could be very high, thus the
interfacial (Cu,Ni)6Sn5 compound tends to spall off
the Ni-Sn-P layer to reduce the total energy of
the system. As seen in Fig. 4f, after five reflows, the
Ni-Sn-P layer at the interface with the spalled-off
(Cu,Ni)6Sn5 compound is much thicker than that at
the interface without spalling. Another possible
reason for the spalling of the interfacial (Cu,Ni)6Sn5
compound on the Ni-Sn-P layer could be the higher
Ni diffusion into the interfacial (Cu,Ni)6Sn5 com-
pound. As mentioned above, the greatest difference
between the layered and columnar Ni(P, 7 at.%)
layers lies in the grain microstructure and the grain
boundary orientation. For the columnar Ni(P,
7 at.%) layer, the grain boundaries tend to be ver-
tical to the joint interface. Thus, it can be expected
that Ni in the columnar Ni(P, 7 at.%) layer could
have a greater tendency to diffuse to the reaction
interface and form an interfacial compound. Such a
large amount of Ni incorporated into the interfacial
compounds could cause a change in the morphology
(to become the more facet-like Ni3Sn4 compound).
As a result, a change in the morphology could be
Fig. 6. SEM cross-sectional images at interfaces between layered Ni(10 at.% P) and Sn-Ag-Cu solder after (a, d) one, (b, e) three, and (c, f) five
reflows.
Interfacial Reactions Between Columnar or Layered Ni(P) Layers and Sn-Ag-Cu Solder
Author's personal copy

8. another possible cause for the spalling of the inter-
facial (Cu, Ni)6Sn5 compound on the Ni-Sn-P layer.
High-P Layered Ni(P, 10 at.%) Reacted
with Sn-Ag-Cu Solder
Figure 6a, b, and c show SEM cross-sectional
images of interfaces between layered Ni(10 at.% P)
and Sn-Ag-Cu solder after one, three, and five
reflows, respectively. The interfacial (Cu,Ni)6Sn5
intermetallic compound grew thicker with increas-
ing number of reflows. Figure 6d–f shows SEM
cross-sectional images at the edge of the solder joint
for the layered Ni(10 at.% P). The interfacial
(Cu,Ni)6Sn5 compound layer near the edge of the
solder joint is found to spall into the Sn-Ag-Cu sol-
der matrix completely. The dotted lines in Fig. 6d–f
indicate the boundary between the spalling region
and the nonspalling region. The spalling region
increases with the number of reflows. Also, large
chunks of spalled (Cu,Ni)6Sn5 compound grains can
be observed at the interface with increasing number
of reflows.
To clearly observe the spalling of the interfacial
ternary compound at the reaction interfaces, the
Sn-Ag-Cu solder bumps were completely etched away
using dilute HCl acid, revealing top views of the
spalled-off reaction interfaces. Figure 7a–c shows
top-view SEM images of the reaction interfaces with
increasing number of reflows at 250°C. It can be seen
that spalling of the interfacial (Cu,Ni)6Sn5 starts at
the edge of the interfacial (Cu,Ni)6Sn5 compound
layer and moves toward the inner region with
increasing number of reflows. The interfacial
(Cu,Ni)6Sn5 compound layer in the spalling region
completely spalled off from the reaction interface.
Many huge hexagonal (Cu,Ni)6Sn5 compound grains
are present around the boundary between the spall-
ing region and the nonspalling (inner) region.
Figure 7d shows a SEM top-view image for the case of
the layered Ni(P, 7 at.%) layer after five reflows. We
observe that no spalling is found at the reaction
interface and large (Cu,Ni)6Sn5 grains are formed in
the reaction interface. So, it can be concluded that the
P content in the Ni(P) layer would greatly affect the
spalling of the interfacialcompound layer.The results
of this work suggest that a higher P content in the
Ni(P) layer leads to a greater tendency for spalling of
the interfacial compound layer. We tend to believe
that, for higher P content in the Ni(P) layer, the for-
mation of the Sn-Ni-P layer would be enhanced. Thus,
faster growth of the Sn-Ni-P layer would result in
serious spalling of the interfacial compound layer.
CONCLUSIONS
The grain microstructure of columnar Ni(P) tends
to be vertical to the Ni(P) layer, whereas the grain
microstructure of a layered Ni(P) layer tends to be
Fig. 7. Top views of SAC305 solder joints with layered Ni(P,
10 at.%) after (a) one, (b) three, and (c) five reflows, and (d) layered
Ni(P, 7 at.%) after five reflows.
Hu, Hsu, Lu, Huang, Chen, Chuang, Hsiao, Lin, and Liu
Author's personal copy

9. parallel to the Ni(P) layer. The present results show
that the Ni(P) consumption rate of the columnar
Ni(P, 7 at.%) layer is larger than that of the layered
Ni(P, 7 at.%) layer. We believe that the faster Ni(P)
consumption rate of the columnar Ni(P, 7 at.%)
layer is due to the orientation of the grain structure.
Spalling of the interfacial (Cu,Ni)6Sn5 compound
can be seen at the reaction interfaces of the colum-
nar Ni(P, 7 at.%) layer. On the contrary, no spalling
can be seen for the case of the layered Ni(P) layer.
Spalling of the ternary (Cu,Ni)6Sn5 compound layer
is an important factor causing the faster Ni(P)
consumption rate for the case of the columnar Ni(P,
7 at.%) layer.
In addition, we investigated the effect of the P
content on the interfacial reaction with Sn-Ag-Cu
solder. The results of this work show that higher P
content in the Ni(P) layer leads to a greater ten-
dency for spalling of the interfacial compound layer.
We tend to believe that, for higher P content, the
formation of the Sn-Ni-P layer would be enhanced.
Thus, the faster growth of the Sn-Ni-P layer would
result in serious spalling of the interfacial com-
pound layer.
ACKNOWLEDGEMENTS
This study was supported by Taiwan Uyemura
Co., Ltd. and in part by the National Central Uni-
versity’s Plan to Develop First-class Universities,
top-level Research Centers Grants 100G903-2, and
National Science Council Grant Projects NSC 98-
2221-E-008-027-MY3 and NSC 100-3113-E-008-00.
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Interfacial Reactions Between Columnar or Layered Ni(P) Layers and Sn-Ag-Cu Solder
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