Significantly retarded interfacial reaction between an electroless Ni–W–P
metallization and lead-free Sn–3.5Ag solder
Ying Yang a , J.N. Balaraju b , Ser Choong Chong c , Hui Xu d , Changqing Liu d , Vadim V. Silberschmidt d ,
Zhong Chen a,⇑
a School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
b Surface Engineering Division, CSIR National Aerospace Laboratories, Bangalore 560 017, India
c Institute of Microelectronics, A STAR (Agency for Science, Technology and Research), 11 Science Park Road, Singapore Science Park II, Singapore 117685, Singapore
d Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough LE11 3TU, UK
Received 22 January 2013
Received in revised form 21 February 2013
Accepted 21 February 2013
Available online 28 February 2013
Due to health and environmental concerns, implementation of
lead-free soldering has commenced since July 1st, 2006. Over the
past two decades, the interfacial reactions between lead-free solders
and Cu-based UBM during reflow and aging as well as their
mechanical properties have been widely studied [1–5]. However,
lead-free solders have higher melting temperatures and higher
Sn content than the conventional eutectic Sn–Pb solder, which
make their reactions with soldering metallization more rapidly,
leading to reliability problems. Thus, it is important to seek solutions
to effectively slow down the interfacial reaction with leadfree
One way to suppress the interfacial reaction is to change the
solder composition by adding of small quantities of additives into
the solder alloy [6–8], but this method is only applicable to solder
paste. Another approach is to modify the composition of soldering
metallization. Cu-based metallization was found to be unreliable
with lead-free solders as it will be completed reacted away during
reflow process, leaving only intermetallic compound (IMC) at the
⇑ Corresponding author. Tel.: +65 6790 4256.
E-mail address: firstname.lastname@example.org (Z. Chen).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
Journal of Alloys and Compounds 565 (2013) 11–16
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
To address the potential reliability challenges brought by the accelerated reaction with the adoption of
lead-free solders, an electrolessly-plated Ni–W–P alloy (6–7 wt.% of P and 14–16 wt.% of W) was developed
as the soldering metallization in this study. It was found that the electroless Ni–W–P layer was consumed
much more slowly than the binary Ni–P layer after prolonged reaction. Unlike the Ni–P/Sn–3.5Ag
interface where three intermetallic compounds (IMCs) are formed, only two IMCs are found at the Ni–W–
P/Sn–3.5Ag interface. Besides, there is no void formation at the soldering reaction interface with the ternary
metallization. The growth of Ni3Sn4 and (Ni,W)3P layers at the Ni–W–P/Sn–3.5Ag interface is found
to be diffusion-controlled. The activation energies for the growth of Ni3Sn4 and (Ni,W)3P layers are
62.3 kJ/mol and 58.2 kJ/mol, respectively.
Ó 2013 Elsevier B.V. All rights reserved.
interface. This will cause severe degradation of the mechanical
strength of the joint. It is well known that Ni and its alloys have
a slower reaction rate with Sn than Cu and Cu alloys. Ni-based metallizations
therefore receive great attention with lead-free soldering.
Among Ni-based metallizations, electroless Ni–P has been
extensively studied [9–16]. Soldering reaction between lead-free
solders and electroless Ni–P, in terms of IMC morphology and
growth kinetics has been well understood, so does the joint
strength degradation with thermal treatment [17–19]. However,
formation of Ni 3P and a ternary Ni–Sn–P compound layer at the
reaction interface makes the solder joints more prone to brittle
cracking [17,20]. The industry has adopted electroless Ni–P as soldering
metallization in a number of applications. Nevertheless
from material’s point of view, there is still room to further improve
the reliability of such solder joints by exploring the use of new
materials, which is very important for packages undergoing multiple
Incorporation of a third element into electroless Ni–P was
considered as an approach to improve the properties of Ni–P as
a soldering metallization. Ni–W–P alloy is considered as a good
candidate since W is a refractory metal element. Duh’s group
has reported that the crystallization temperature of Ni–P
compounds in electroless Ni–P (8.5 wt.% of P) and electroless
Ni–W–P (7.6 wt.% of P and 10.9 wt.% of W) coatings were 337
12 Y. Yang et al. / Journal of Alloys and Compounds 565 (2013) 11–16
and 406 °C, respectively , indicating the addition of W into
Ni–P can effectively retard the crystallization of Ni–P compounds.
Thus, it is possible to slow down the interfacial reaction with
adoption of Ni–W–P as the soldering metallization by retarding
the formation of fast diffusion path, which is grain boundary of
Ni–P compounds in this case. Electroless Ni–W–P film was first
made by Pearlstein and Weightman  by adding salt of W into
the electroless nickel bath. Attempts to use the Ni–W–P coating
as the soldering metallization in microelectronic packaging were
first made by Chen et al. , and they reported that the
Ni–W–P coating (7 wt.% of P and 14 wt.% of W) with a high W
content have much longer service lifetime than the Ni–P coating
by studying long term interfacial reaction between Ni–W–P and
molten Sn–58Bi solder at 200 °C. After that, the Ni–W–P films
with three different levels of W at two fixed levels of P (5 and
9 wt.%) were prepared by Jang and Yu , and the interfacial
reaction between Sn–3.5Ag solder and these Ni–W–P films, as
well as the drop impact test results of these solder joints were
studied. However, the diffusion mechanism and the growth kinetics
of the IMCs at the Ni–W–P/solder interface have not been
In this work, a Ni–W–P alloy was prepared by electroless plating.
Its interfacial reaction with Sn–3.5Ag solder after reflow and
prolonged aging were studied. The interfacial reaction with a binary
electroless Ni–P under the same reflow and aging conditions
was investigated for comparison. In addition, the growth kinetics
of the Ni 3Sn 4 and (Ni,W) 3P layers was also investigated with their
activation energies reported.
Cu plates (6 mm thick, 99.98 wt.%) were used as the substrate for both Ni–P and
Ni–W–P plating. Prior to the plating, the Cu substrates were polished and etched
with 35 vol.% nitric acid for 30 s, followed by a commercial Ru activation treatment
for surface activation. Electroless Ni–P plating was carried out in a commercial
acidic sodium hypophosphite bath (from MacDermid) with a pH level of 5.3 (adjusted
by ammonium hydroxide) at 88 ± 2 °C for 40 min. Electroless Ni–W–P plating
was conducted in a self-prepared alkaline bath with a pH level of 9.0
(adjusted by 25% sulphuric acid) at 85 ± 2 °C for 2 h. As listed in Table 1, this alkaline
bath contains nickel sulphate and sodium tungstate as nickel and tungsten
sources, respectively, sodium hypophosphite as a reducing agent along with sodium
citrate as the complexing and buffering agent.
After the plating, Ni–P/Sn–3.5Ag and Ni–W–P/Sn–3.5Ag solder joints were
formed for the study of interfacial reactions. Prior to the soldering, a thin layer of
no-clean paste flux was applied on top of the plated surface of Cu to remove oxides.
Commercially obtained Sn–3.5Ag solder wires with flux in core were used for the
soldering. Solder joining was carried out in an IR reflow oven (ESSEMTEC RO-06E)
with a reflow temperature at 260 °C for 60 s. After the reflow, solid-state aging
was carried out at 200 °C for up to 50 h for the Ni–P/Sn–3.5Ag solder joint and up
to 625 h for the Ni–W–P/Sn–3.5Ag solder joint, respectively. In order to study the
growth kinetics of the IMCs formed at the Ni–W–P/Sn–3.5Ag solder interface, solid-state
aging was also carried out at three lower temperatures (140 °C, 160 °C,
and 180 °C) for up to 625 h.
The surface morphologies and the thicknesses of both deposited layers were observed
by SEM (JEOL JSM-6360A). The interfacial microstructures of both types of
solder joints after prolonged aging at all temperatures were examined using SEM
as well. For cross-sectional SEM, samples were cold mounted in epoxy and polished
down to 1 lm finish, followed by etching with 4% hydrochloric acid to reveal the
interfacial microstructure. The compositions of the as-deposited, the as-reflowed
and the aged samples were analyzed using energy dispersive X-ray (EDX) incorporated
in the SEM.
Composition of the plating bath for electroless Ni–W–P.
Constituents of plating bath Concentration (g/L)
NiSO4 6H2O 26
Na2WO4 2H2O 33
NaH2PO2 H2O 12
Na3C6H5O7 2H2O 75
3. Results and discussion
3.1. As-deposited metallizations
The Ni–P coating contains 6–7 wt.% of P. As shown in Fig. 1a, the
surface of the deposited Ni–P layer has smooth nodules with an
uneven size distribution. The thickness was measured to be around
14 lm (Fig. 1b). The Ni–W–P coating contains 6–7 wt.% of P and
14–16 wt.% of W. As shown in Fig. 2a, the surface of the deposited
Ni–W–P layer also contains smooth nodules, but the nodules have
a more even size distribution than the ones in Ni–P. Its thickness
was measured to be around 9.8 lm(Fig. 2b). It was noted that both
coatings have a good adhesion to the Cu substrate (Figs. 1b and 2b).
3.2. Interfacial microstructures after the reflow
The cross-sectional micrograph of as-reflowed Ni–P/Sn–3.5Ag
solder joint is shown in Fig. 3a. A layer of Ni3Sn4 was formed due
to reaction between Sn from the solder and Ni from the metallization.
Some of the faceted Ni3Sn4 particles spalled into the bulk solder.
Beneath the Ni 3Sn 4 layer, there was a very thin layer of Ni 2SnP
[25,26]. Adjacent to the coated Ni–P layer, a dark layer of Ni3P was
present. A number of voids were clearly visible inside the Ni 3P
layer. The formation of these voids has been explained in previous
The cross-sectional micrograph of as-reflowed Ni–W–P/Sn–3.5Ag
solder joint is shown in Fig. 3b. It was observed that a layer of
Ni3Sn4 clearly formed, and there was no spallation of IMC as it
did in the binary Ni–P reaction. Under the Ni 3Sn 4 layer, there
was a thin layer consisting of Ni, W, and P elements. The composition
of this ternary layer was measured to be 62.5 at.% of Ni,
10.5 at.% of W and 27 at.% of P by EDX. Such composition is suggestive
of (Ni,W)3P compound. Jang and Yu  have conducted the
XRD analysis for the heat-treated Ni–W–P films with different W
levels. The XRD results showed that the Ni3P lattice parameter
did not seem to be affected by the W addition, so they concluded
that W solubility in Ni3P is very limited or the (Ni,W)3P stoichiometry
is quite stable. In addition, this (Ni,W) 3P layer was found to be
amorphous by studying its diffraction pattern under TEM . As
shown in Fig. 3b, the interface between the (Ni,W) 3P layer and
the unconsumed Ni–W–P layer was uneven, implying consumption
of Ni–W–P layer is non-uniform during reflow soldering.
3.3. Solid-state interfacial reactions
Fig. 4a shows the growth of various compounds at the Ni–P/Sn–
3.5Ag interface after aging at 200 °C for 50 h. It was found that
upon 50 h of aging, Ni3Sn4 grew much thicker, and some Ag3Sn
particles accumulated inside this layer. The as-deposited Ni–P
layer was fully consumed and transformed into Ni3P layer. The
thickness of this Ni3P layer ( 7.4 lm) is much smaller than that
of the as-deposited Ni–P layer ( 14 lm). Such shrinkage indicates
that, Ni atoms diffuse out from Ni–P to form Ni 3Sn 4 during the
interfacial reaction. It was observed that only a few voids appeared
in the Ni 3P layer after reflow (Fig. 3a). However, as the reaction
continued upon aging, these voids increased in both the size and
number (Fig. 4a).
Fig. 4b shows the growth of various compounds at the Ni–W–P/
Sn–3.5Ag interfaces after aging at 200 °C for 625 h. It was found
that during aging, the (Ni,W)3P layer grew very slowly. The thickness
increase of the Ni 3Sn 4 layer is much slower than that formed
at the Ni–P/Sn–3.5Ag interface (Fig. 4a). Unlike the Ni–P layer
which was fully consumed after 50 h of aging (Fig. 4a), the Ni–
W–P layer after 625 h of aging still had 6.9 lm left, which is
70% of its original thickness. Moreover, it is interesting to note that
even after 625 h aging, no voids were found in the Ni–W–P/Sn–3.5Ag
solder joint. After comparing the two reaction couples, it is
reasonable to attribute the slow IMC growth and consumption rate
of the Ni–W–P metallization pad to the amorphous nature of the
(Ni,W)3P layer which has effectively hindered the diffusion of Ni
from the underneath layer. This point will be elaborated later in
conjunction with diffusion profile analysis.
3.4. IMC growth kinetics at the Ni–W–P/Sn–3.5Ag interface
In the current work, the growth kinetics of both Ni3Sn4 and
(Ni,W) 3P layers was presented. Three aging temperatures (140,
160, and 180 °C) were selected for the study. The average thicknesses
of the Ni 3Sn 4 and (Ni,W) 3P layers were obtained from at
least 10 SEM images of different locations of each sample by measuring
the cross-section area of the layer over a certain length on
the SEM image with the help of an image analyzer. The thickness
of the reaction layer in the solder joint can be generally expressed
Y. Yang et al. / Journal of Alloys and Compounds 565 (2013) 11–16 13
Fig. 1. As-deposited Ni–P layer: (a) surface morphology, (b) cross-sectional micrograph.
Fig. 2. As-deposited Ni–W–P layer: (a) surface morphology, (b) cross-sectional micrograph.
Fig. 3. Back-scattered SEM images showing IMCs formed in the as-reflowed solder joints: (a) Ni–P/Sn–3.5Ag solder joint, and (b) Ni–W–P/Sn–3.5Ag solder joint.
d d0 ¼ kt 1=n
where d and d0 are the thickness of the reaction layer at time t and
zero, respectively, k is the growth rate constant, and n is the time
exponent. Fig. 5 shows the thickness of both the Ni3Sn4 and
(Ni,W) 3P layers as a function of the square root of the aging time
(i.e. assuming n = 2) at different aging temperatures. The thickness
increment of both layers was found to increase linearly with the
square root of aging time, suggesting that the growth of these
two layers at the Ni–W–P/Sn–3.5Ag interface are both controlled
by diffusion. The growth rate constant is calculated from a linear
regression analysis of (d d 0) versus t 0.5 , where the slope is equal
to k, and the values of k for both layers at each aging temperature
are listed in Table 2. The growth rate constants of Ni3Sn4 and
(Ni,W)3P layers increase with increasing aging temperature, indicating
that the growths of both layers were faster at higher aging
To calculate the activation energy for the interfacial compound
growth, the Arrhenius equation is used:
14 Y. Yang et al. / Journal of Alloys and Compounds 565 (2013) 11–16
Ni 3Sn 4
k 2 ¼ A expð Q=RTÞ ð2Þ
where k 2 is the square of the growth rate constant, A is a prefactor, T
is the absolute temperature, R is the gas constant, and Q is the activation
energy. The value for Q is obtained from the slope of the
Arrhenius plot, as shown in Fig. 6. The activation energy for the
Ni3Sn4 growth in the solid-state reaction between the Ni–W–P
and Sn–3.5Ag solder is obtained to be 62.3 kJ/mol, and the prefactor
is 2.75 10 7 cm 2 /s. Table 3 lists the values of the activation
energies for the growth of Ni3Sn4 in the Ni-based UBM/Sn–3.5Ag
solder systems obtained from previous works [9,27,28]. Our result
lies within the range of values obtained by previous works on
Sn–3.5Ag/Ni or Ni–P reactions. However, when comparing the activation
energies, caution has to be taken that since the solubility of
Ni in liquid Sn is rather high, formation of compound layers actually
takes place under conditions of simultaneous dissolution of the solid
in the melt. The rate of dissolution is known to be dependent on
the experiment geometry, the surface area of contact between the
solid and liquid phases and the volume of the liquid phase in particular.
As a result, compound layer growth kinetics proves to be also
Ni 3Sn 4
Fig. 4. Back-scattered SEM images showing IMCs formed in the aged solder joints: (a) Ni–P/Sn–3.5Ag solder joint after aging at 200 °C for 50 h, and (b) Ni–W–P/Sn–3.5Ag
solder joint after aging at 200 °C for 625 h.
of Ni3Sn4 (µm)
of (Ni,W) 3P (µm)
5 10 15 20 25 30
Square root of aging time (hour 1/2 )
5 10 15 20 25 30
Square root of aging time (hour1/2 )
Fig. 5. Thickness of the (a) Ni 3Sn 4 and (b) (Ni,W) 3P layers formed in the Ni–W–P/
Sn–3.5Ag solder joint during aging at 140, 160, and 180 °C up to 625 h.
IMC growth rate constants at the Ni–W–P/Sn–3.5Ag interface at various aging
Temperatures (°C) k of Ni 3Sn 4
( 10 8 cm/s 1/2 )
140 6.13 2.13
160 8.78 2.20
180 13.7 2.30
k of (Ni,W) 3P
( 10 8 cm/s 1/2 )
dependent on the experiment geometry. Since it is usually different
in different works, close values of the activation energy can hardly
be expected. Similarly, the reaction kinetics in solid-state reaction
could be affected by the difference in these experimental conditions
too. The twofold difference observed in its values as listed in Table 3
could be due to this reason.
Despite the possible discrepancy, the activation energy is an
important parameter as it reflects the magnitude of the critical barrier
for the compound growth. The comparable activation energy
value with other reported Sn–3.5Ag/Ni-based soldering systems
indicates that controlling mechanism for the Ni3Sn4 IMC formation
ln (k 2 in cm 2 /s) of Ni 3 Sn 4
ln (k 2 in cm 2 /s) of (Ni,W) 3 P
2.15 2.2 2.25 2.3
2.35 2.4 2.45
2.15 2.2 2.25 2.3 2.35 2.4 2.45
Fig. 6. Arrhenius plot of the growth of (a) Ni3Sn4 and (b) (Ni,W)3P layers in the Ni–
W–P/Sn–3.5Ag solder joint.
Activation energy for the growth of Ni 3Sn 4 in the Ni-based UBM/Sn–3.5Ag solder
systems obtained in different studies.
Solder/substrate Experimental conditions
Q (kJ/mol) Reference
Sn–3.5Ag/Ni–W–P 140–180 °C/up to 625 h 62.3 Present work
Sn–3.5Ag/Ni–P 130–170 °C/up to 625 h 110.0 
Sn–3.5Ag/Ni–P 140–200 °C/up to 400 h 98.9 
Sn–3.5Ag/Ni–P 100–170 °C/up to 60 days 49.0 
Sn–3.5Ag/Au/Ni 70–170 °C/up to 100 days 72.5 
Fig. 7. Elemental distribution at Ni–W–P/Sn–3.5Ag interface after aging at 200 °C
for 225 h.
might remain the same, which is likely to be the diffusion of Sn in
the Ni 3Sn 4 IMC. As cited in a review article by Ho et al. , Ti marker
experiment in a Ni/Sn reaction couple has proven that Sn diffuses
faster than Ni through Ni3Sn4 IMC, therefore the IMC
growth activation energy is likely related to the Sn diffusivities in
Y. Yang et al. / Journal of Alloys and Compounds 565 (2013) 11–16 15
the Ni3Sn4 IMC. Unfortunately, data for Sn diffusivities in Ni3Sn4
are unavailable, which could be an interesting topic for future
The activation energy for the (Ni,W) 3P growth in the solid-state
solder reaction is found to be 58.2 kJ/mol, and the prefactor is
2.46 10 15 cm 2 /s. It is worth noting that the prefactors for
growth of both the Ni3Sn4 and (Ni,W)3P layers are many orders
of magnitude lower than the prefactor of Ni 3Sn 4 formation in soldering
with Ni–P , which is due to the presence of W in the metallization
pad that decreases the availability of Ni at the soldering
interface. Although the activation energy of (Ni,W)3P growth is
similar to that of Ni 3Sn 4 growth, the prefactor of (Ni,W) 3P growth
is much lower, so (Ni,W)3P grew much slower than Ni3Sn4.
3.5. Formation of IMCs at the Ni–W–P/Sn–3.5Ag interface
EDX line scan and element mapping analysis were obtained to
understand the diffusion profile at the Ni–W–P/Sn–3.5Ag interface.
Fig. 7 shows elemental distribution at Ni–W–P/Sn–3.5Ag interface
after aging at 200 °C for 225 h. It was noted that at the interface between
the Ni–W–P layer and the (Ni,W) 3P layer, Ni intensity
started to decrease. This indicates that Ni atoms diffuse outward
from the deposited Ni–W–P layer during the interfacial reaction,
leading to the formation of Ni3Sn4. A sudden drop of Sn intensity
was observed at the (Ni,W) 3P/Ni 3Sn 4 interface, which indicates
there is no Sn in the (Ni,W)3P layer.
From the mapping result of the sample after aging at 200 °C for
625 h (Fig. 8), it is clearly seen that W and P remains in the Ni–W–P
and (Ni,W) 3P layers. In contrast, Ni atoms diffuse out from the
Ni–W–P layer to form Ni3Sn4. The signal from Sn was found to
Fig. 8. EDX element mapping analysis of the Ni–W–P/Sn–3.5Ag interface after aging at 200 °C for 625 h: (a) SEM image, (b) mapping for Ni, (c) mapping for W, (d) mapping
for P, (e) mapping for Sn, (f) mapping for Cu, and (g) mapping for Ag. Elemental concentration decreases with increasing black intensity.
16 Y. Yang et al. / Journal of Alloys and Compounds 565 (2013) 11–16
terminate at the Ni 3Sn 4/(Ni,W) 3P interface, and no signal from Sn
was detected in the (Ni,W)3P layer, indicating that Sn atoms did
not diffuse through the (Ni,W) 3P layer. These are in agreement
with the findings from the line scan (Fig. 7). It was also noted that
after such a prolonged aging, Cu atoms still remain in the substrate,
and no (Cu,Ni)6Sn5 or (Ni,Cu)3Sn4 compounds formed, suggesting
that Ni–W–P is a good diffusion barrier preventing inter-diffusion
between Cu and Sn.
Fig. 9 illustrates the diffusional formation mechanism of the
Ni3Sn4 and (Ni,W)3P layers at the Ni–W–P/Sn–3.5Ag interface
schematically. Based on elemental analysis, P atoms in the Ni–
W–P remained in the deposited layer during soldering reaction.
The deposited Ni–W–P layer contains 82 at.% of Ni, 5 at.% of W
and 13 at.% of P, while the newly-formed (Ni,W)3P layer contains
62.5 at.% of Ni, 10.5 at.% of W and 27 at.% of P. Thus, the ratio of
W to P in the (Ni,W)3P layer ( 2.6) is the same as that in the
deposited Ni–W–P layer, suggesting that Ni was the only element
to diffuse outward from the Ni–W–P coating layer during soldering
reaction. The driving force for the Ni diffusion is the Ni concentration
difference between the Ni–W–P coating layer and the bulk solder,
which contains no Ni. As shown in Fig. 9, since concentration
of Ni is higher in the coating layer, Ni atoms diffuse out from the
Ni–W–P layer towards solder and react with Sn atoms from the
solder to form Ni3Sn4. Meanwhile, the formation of Ni3Sn4 causes
the depletion of Ni from the surface of the Ni–W–P layer, leading
to continued growth of the newly formed ternary (Ni,W)3P layer.
It was reported that the Ni3P layer formed at the Ni–P/Sn–3.5Ag
interface has a fine columnar structure, so this layer acts as a fast
diffusion path for Ni atoms during the interfacial reaction . As
a result, the rapid diffusion of Ni atoms through the Ni 3P layer
leads to the formation of voids inside this layer while there is
not enough compensation from other elements to fill the vacant
sites left by Ni. Jang and Yu have reported that the (Ni,W)3P layer
formed at the Ni–W–P/Sn–3.5Ag interface has an amorphous
structure . Since the amorphous structure of this (Ni,W)3P
layer is free of fast diffusion channel such as grain boundaries, it
is more difficult for Ni atoms to diffuse out through this layer. This
is evidenced by observation that no voids are formed inside this
layer after prolonged reaction (Fig. 4b). As a result, the interfacial
IMC growth rate at the Ni–W–P/Sn–3.5Ag interface was much
slower than that at the Ni–P/Sn–3.5Ag interface. This is also evidenced
by the much slower consumption rate of the Ni–W–P metallization
compared to the binary Ni–P.
Ni 3 Sn 4
(Ni,W) 3 P
Fig. 9. A simplified scheme illustrating the diffusional formation mechanism of
Ni3Sn4 and (Ni,W)3P layers between Ni–W–P metallization and Sn–3.5Ag solder.
In this work, an electrolessly-plated Ni–W–P alloy (6–7 wt.% of
P and 14–16 wt.% of W) was developed as an alternative Ni-based
metallization for lead-free soldering. Interfacial reaction between
Sn–3.5Ag solder and electroless Ni–W–P after reflow and prolonged
aging was investigated, with interfacial reaction between
the same solder and electroless Ni–P (6–7 wt.% of P) as a benchmark.
At the Ni–W–P/Sn–3.5Ag interface, only two interfacial compounds,
Ni 3Sn 4 and (Ni,W) 3P are formed, and the diffusion
mechanism was proposed. After prolonged aging, unlike at the
Ni–P/Sn–3.5Ag interface, no voids were found at the Ni–W–P/Sn–
3.5Ag interface, and the Ni3Sn4 grew much more slowly. These
facts indicate a significantly slow out-diffusion of Ni atoms at the
Ni–W–P/Sn–3.5Ag interface; hence, Ni–W–P layer stands out as
an impressive diffusion barrier for lead-free soldering. In addition,
the growth kinetics of the Ni3Sn4 and (Ni,W)3P layers at the Ni–W–
P/Sn–3.5Ag interface was examined as a consequence of aging at
three temperatures (140, 160 and 180 °C) for up to 625 h. The
growth of these two layers was found to be a diffusion-controlled
process. The activation energies for the growth of Ni3Sn4 and
(Ni,W)3P layers during solid-state reaction are 62.3 kJ/mol and
58.2 kJ/mol, respectively.
The authors very much appreciate technical discussions with
Dr. K. Chen and Prof. K.N. Tu. Financial assistance from MOE Singapore
(Grant RG 19/00, RG 14/03), and UK Department for Innovation,
Universities and Skills (DIUS) through a PMI2 Project (Grant
No. RC 41) is gratefully acknowledged.
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