Materials Science and Engineering B106 (2004) 126–131
Dissolution kinetics of BGA Sn–Pb and Sn–Ag solders
with Cu substrates during reflow
Ahmed Sharif, Y.C. Chan∗
Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong
Received 23 April 2003; accepted 2 September 2003
Abstract
An investigation has been carried out to compare the dissolution kinetics of the Cu pad of the ball grid array (BGA) substrate into the molten
conventional Sn–Pb solder and Sn–3.5Ag solder. A fixed volume of the BGA solder ball (760 ␮m diameter) was used on the 15–18 ␮m thick
Cu pad having a circular area with a diameter of 650 ␮m. The dissolution measurement was carried out by measuring the change of Cu pad
thickness as a function of time and temperature. A scanning electron microscope (SEM) was used to observe the microstructure of the solder
joint and to measure the consumed thickness of Cu. A fast dissolution of the substrate occurred in the beginning for the molten solder/solid
Cu reaction couple. But the dissolution in Sn–Ag was much more higher than that in Sn–Pb solder. The rates of Cu dissolution were measured
for different soldering temperatures ranging from 225 to 240 ◦ C and activation energies of 54 and 116 kJ/mol were found for the dissolution
reaction in Sn–Ag and Sn–Pb solder, respectively.
© 2003 Published by Elsevier B.V.
Keywords: Sn–3.5Ag solder; Cu substrate; Dissolution; Cu6 Sn5 phase; Activation energy
1. Introduction
Lead–tin solders still play an important role in electronic
packaging, such as flip-chip, solder-ball connections in
ball grid array (BGA), and integrated circuit assembly to
a printed circuit board [1]. The ban of lead in electronic
products will occur in most industrialized countries before
the end of this decade. Latest reports from the legislation
parliament of the European Community indicate that, for
the EU members, lead has to be replaced by July 2006 [2].
Hence, how to cut down the use of Pb in soldering and
how to develop Pb-free solders are urgent materials issues
in advanced microelectronics technology. Although some
Pb-free solders have been in use for years, there are no
obvious replacements for the Sn–Pb alloys that are most
common in microelectronics industry. Also many of the
fundamental phenomena that determine solder behavior
are yet to be understood. The successful introduction of
lead-free solder joints in the electronic industry must be
∗
Corresponding author. Tel.: +852-2788-7130; fax: +852-2788-7579.
E-mail address: eeycchan@cityu.edu.hk (Y.C. Chan).
0921-5107/$ – see front matter © 2003 Published by Elsevier B.V.
doi:10.1016/j.mseb.2003.09.004
based on the past experience in the use of Sn–Pb solders.
Because of the lack of the information regarding reliability in practice, reliability tests are needed before lead-free
solder can replace traditional tin–lead solder.
As a consequence of the announced lead ban, lead-free
solders are subject to numerous investigations worldwide.
All alternatives to the standard eutectic tin–lead solder investigated so far are based on tin alloys with a tin content
significantly over 90 wt.% in combination with copper, silver, antimony, bismuth, or zinc. A key issue affecting the
integrity and reliability of solder joints for such Sn-based alloys is the fast interfacial reactions between the molten solder and the under bump metallization (UBM). Among the
binary alloys, Sn/0.5–0.8%Cu and Sn/3–4%Ag play a dominant role. The Sn/Ag solders exhibit melting points in the
range of 220–221 ◦ C, which is more than 30 ◦ C beyond the
melting point of the standard eutectic tin–lead solder. A previous study [3] demonstrated a fair wettability of the solder
to Cu substrates.
It is well known that soldering involves a reaction between molten solder and substrate, which forms some sort
of intermetallic layer and which dissolves some of the substrate [4]. The interfacial chemical reactions enhance the
A. Sharif, Y.C. Chan / Materials Science and Engineering B106 (2004) 126–131
wettability between the solder and the substrate. Intermetallic growth kinetics for solid–liquid couples is significantly
faster compared to growth kinetics for solid–solid couples.
Many investigators have studied the intermetallic formation
at liquid solder/solid substrates interfaces [5–13]. Very little
information is available for the substrate dissolution behavior [12,14,15].
Copper is widely used in the under bump metallurgy and
substrate metallization for flip-chip and BGA application.
At the liquid solder/Cu substrate interface, the formation
and growth of intermetallic compounds occur by the dissolution of Cu into the molten solder. The previous studies
have shown that the amount of dissolved Cu influences the
rate at which the intermetallic forms [13]. Ward and Taylor [14] and Ishida [15] proposed equations to describe the
dissolution rates of the substrate. Very little is known about
the dissolution kinetics of Cu into molten solder joints on
a Si chip; the typical thickness of Cu is about 1 ␮m. Since
the amount of Cu is limited and some of the Cu must remain intact through all the reflows and subsequent reworks
to avoid dewetting of the solder, the understanding of the
consumption rate of Cu is very important. This was a strong
motivation for us, to have a closer look at the consumption
rate of Cu in the soldering reaction with Sn–Pb and Sn–Ag
solders.
2. Experimental
Flexible substrates with 15–18 ␮m thick 99.9 wt.% Cu pad
with a diameter of 650 ␮m, surrounded by solder mask were
used to attach the solder ball. All substrates were cleaned in a
dilute solution of hydrochloric acid and then rinsed in water
prior to soldering to remove surface oxidation. The commercial name of the flux used in this work was CLEANLINETM
LR721H2 BGW no-clean flux. Eutectic 63Pb–37Sn solder
balls and eutectic 96.5Sn–3.5Ag BGA solder balls with a
diameter of 0.76 mm were placed on the prefluxed Cu substrate as shown in Fig. 1 and then soldered at four different
temperatures: 225, 230, 235, and 240 ◦ C for times (t) of 1, 2,
5, and 10 min in a convection reflow oven (BTU VIP-70N).
Solder
ball
Solder mask
760µm
15-18µm
Cu pad
Cu area to solder volume ratio = 1.44 mm2/mm3
Fig. 1. Schematic diagram of solder ball attachment on Cu pad (before
reflow).
127
After reflowing the samples were mounted in epoxy molding
compound. For interfacial microstructure examination, the
samples were ground and polished very carefully. The chemical and microstructural analysis of the gold-coated samples
were obtained by using a Philips XL 40 FEG scanning electron microscope (SEM) equipped with an energy dispersive
X-ray (EDX) spectrometer.
3. Results and discussion
Fig. 2 represents the backscattered electron micrographs
for the interfaces of Sn–Pb and Sn–Ag solder balls on Cu
substrate after reflow at 240 ◦ C for various times. By measuring the remaining Cu thickness and by subtracting it from
the initial thickness, the consumed Cu thickness is deduced.
In the flexible substrate, there is a portion of Cu pad under
the solder mask that remained unreacted after the reflow.
From this unreacted part, the initial thickness of the Cu pad
is measured. Only the Cu6 Sn5 intermetallics were observed
in the interfaces using the SEM and no Cu3 Sn was observed.
According to EDX analysis also no Ag or Pb was detected
in the interfacial layer, so Ag and Pb were not directly involved in the interfacial reactions. The dissolution thickness
increases with time. In this figure, the consumed thickness
difference would be more pronounced for the two solders for
various time fractions if there was no variation in the initial
thickness of Cu pad of the substrate. However, from this figure it is apparent that the dissolution is much higher in the
case of Sn–Ag solder. Since there are channels between the
IMCs, the dissolution of Cu into the liquid solder occurs simultaneously with the ripening of IMCs. These channels remain more or less open for the dissolution in Sn–Ag solder.
But in the case of Sn–Pb alloy, these channels between the
IMCs are closed after a certain period. The dissolution rate
in Sn–Pb solder reaches a steady state after a certain time. In
the early period, diffusion of Cu for the dissolution reaction
in the Sn–Pb chiefly occurs by volume diffusion, but in the
later stage is mainly by grain boundary diffusion. For the
Sn–Ag solder, the dissolution rate is very high even at 10 min
and the ripening action of the IMCs reduces the channels for
volume diffusion of Cu, so that the dissolution rate decreases
gradually.
At the solder interface, Cu6 Sn5 grows as scallop-like
grains and results in a rough interfacial morphology between the IMCs and the solder [12]. By examining the
growth of scallops during constant temperature reflowing,
it has been found that they increase in size but decrease
in number. According to Kim and Tu [12], the flux of the
ripening is much larger than that of the interfacial reaction by almost a factor of 10 in the liquid–solid reaction
because of the larger solubility and diffusivity in the liquid state. From Fig. 2 it is also observed that the ripening
action for Sn–Ag solder is much more pronounced. The
size of each scallop grain after reflow at 240 ◦ C for 10 min
for Sn–Ag solder is much larger than that of Sn–Pb sol-
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A. Sharif, Y.C. Chan / Materials Science and Engineering B106 (2004) 126–131
Fig. 2. SEM micrographs illustrating the dissolution of Cu substrate reflowed for (i) 1 min, (ii) 2 min, (iii) 5 min, and (iv) 10 min at 240 ◦ C for (a) Sn–Ag
solder and (b) Sn–Pb solder.
der. So it may be concluded that the interfacial energy
per unit area between Cu6 Sn5 and molten Sn–Ag solder
is higher than that between Cu6 Sn5 and molten Sn–Pb
solder. For this reason, Cu6 Sn5 that forms in the interface
between the liquid Sn–Ag solder and Cu substrate, coalesces more quickly to possess less free energy thermodynamically.
The dissolution of Cu during reflow at the four different temperatures is plotted in Fig. 3. For the Sn–Ag alloy,
dissolution curves are much steeper in the early stage of
reflow. So the consumption rate is relatively high during initial reflow and becomes lower with time. Other researchers
[11,16] also observed that a fast dissolution of the substrate occurred in the beginning for the molten Sn/solid Cu
A. Sharif, Y.C. Chan / Materials Science and Engineering B106 (2004) 126–131
20
Consumed Cu thickness, µm
Consumed Cu thickness x, µm
20
129
16
12
225 ˚C
8
230 ˚C
235 ˚C
4
225 ˚C
16
230 ˚C
235 ˚C
12
240 ˚C
8
4
240 ˚C
0
0
0
(a)
2
4
6
8
0
10
Time, min.
(b)
2
4
6
8
10
Time, min.
Fig. 3. The consumed thickness of Cu vs. reflow time at four different temperatures for (a) Sn–Ag solder and (b) Sn–Pb solder.
reaction couple. From this graph, it is also evident that the
initial consumption rate is higher for the highest temperature. At high temperature, the diffusion rate is much higher
so that Cu atoms can move faster into the liquid solder. A
large flux flowed into the unsaturated liquid phase in the
beginning of the reaction, and the dissolution effect reduced
when a less unsaturated liquid phase is used. The consumption occurs by dissolution through the channels between the
scallop grains. With time these channel areas reduces and
the dissolution of Cu also reduces.
Here for Sn–Pb solder, the dissolution rate is also high
at the initial stage but not as high as that for Sn–Ag solder
alloy. The melting temperatures of Sn–Pb and Sn–Ag solder
are around 183 and 221 ◦ C, respectively. So the working
temperatures for Sn–Pb solders are more than 40 ◦ C than its
melting point. However, the dissolution rate of Sn–Pb solder
alloy is much smaller in each condition than that of Sn–Ag
solder. In the later stage of reflow, the channels between
the IMC grains become closed. So the consumption rate
becomes much slower.
Even after 10 min of reflow at 240 ◦ C for Sn–3.5Ag solder,
there is still some Cu substrate that remains unreacted when
the substrate thickness is more than 17 ␮m (Fig. 2a (iv)),
but if the thickness of the substrate is less then 17 ␮m, after
10 min of reflow, the substrate is fully consumed from the
interface and the IMCs appear in the bulk of the solder
(Fig. 4). Both solder balls, with a volume of 2.29×107 ␮m3 ,
can consume much Cu from the substrate. From the Cu–Pb
binary diagram, it can be seen that the solubilty of Cu in Pb
is extremely low. The eutectic point in the Cu–Pb system
is at 0.06 wt.% Cu. The solid solubility (at T < 327 ◦ C) of
Cu in Pb is less than 0.007 wt.%. However, the solubility of
Cu in liquid Pb is a fraction greater than 0.06 wt.%, though
not appreciably. Also from the Cu–Sn binary diagram, it can
be seen that the concentration of Cu in Cu–Sn eutectic at
227 ◦ C is 0.7 wt.% and the solid solubility of Cu in Sn is
practically close to zero. So the Cu pick up by Sn–Pb and
Sn–Ag can therefore be attributed to the Sn content in the
solders. At 240 ◦ C the maximum solubility of Cu in Sn is
around 1.1 wt.%. For Sn–Ag solder after reflowing at 240 ◦ C
for 10 min, about 15 ␮m of Cu substrate is consumed by the
solder and it is observed that the solder dissolves around
2.7 wt.% of Cu. So it is interesting how the Sn–Ag solder
dissolves more than 1.1 wt.% of Cu at 240 ◦ C. Intermetallic
compound formation by Cu/Sn reaction is not fast enough to
consume all the Cu atoms that have entered the liquid solder.
The unreacted portion will diffuse out to enter the bulk of
the molten solder. When the percentage of Cu reaches the
threshold limit of Cu6 Sn5 compound formation, Cu–Sn IMC
also nucleates in the bulk solder. As a result the Cu content
in liquid solder also decreases, and for that the liquid solder
can dissolve more Cu from the substrate. By this process,
Sn–Ag solder can absorb such a large amount of Cu in the
liquid state. During solidification, as the solubility of Cu in
the bulk solder decreases, Cu and Sn react to form Cu6 Sn5
IMC that will deposit on the existing IMCs in the bulk solder.
Fig. 4. SEM micrograph showing no Cu at the interface in the Sn–Ag
solder after soldering for 10 min at 240 ◦ C.
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A. Sharif, Y.C. Chan / Materials Science and Engineering B106 (2004) 126–131
Fig. 5. SEM micrograph illustrating Cu–Sn IMCS in the top edge of the
Sn–Ag bulk solder after soldering for 2 min at 240 ◦ C.
Fig. 5 shows such kind of IMC in the bulk of the Sn–Ag
solder. Sn–Pb solder also exhibits IMCs in the bulk (Fig. 6),
but the amount is not as much as in the case of Sn–Ag
solder.
A thermally activated parabolic model was used to calculate the dissolution rate constants at each temperature using
x = (kt)1/2 , where x is the dissoluted thickness (␮m), k is
the dissolution rate constant (␮m2 /min), and t is the reflow
time (min). A simple Arrhenius relationship was used to determine the activation energy for the dissolution of the Cu
substrate:
k = A−E/RT
(1)
or, logarithmically
E 1
ln k = ln A −
,
R T
(2)
where k is the rate constant, A is the pre-exponential factor,
E is the activation energy, R is the universal gas constant,
and T is the temperature in Kelvin (K).
2
250
225 ˚C
225 ˚C
230 ˚C
230 ˚C
235 ˚C
235 ˚C
240 ˚C
240 ˚C
40
2
200
x ,µ m
150
2
2
A plot of the square of the consumed thickness versus
time is shown in Fig. 7. The slope of these lines gives
the rate constant, k. An Arrhenius plot of the natural logarithm of k versus 1/T, shown in Fig. 8, gives the activation energy for diffusion. From this plot, E was found
to be 116 and 54 kJ/mol for Sn–Pb and Sn–Ag, respectively for the above condition. The dissolution of Cu in
Sn–Pb solder is much less than Sn–Ag solder. For that
reason, activation energy for Cu dissolution into Sn–Pb
solder is higher than that of Sn–Ag solder. As mentioned previously, published liquid state diffusion data
are very limited. Activation energy for the diffusion of
Cu in liquid Sn is 17.56 kJ/mol [17], which is very low
compared to the both 116 and 54 kJ/mol. The higher activation energy observed in this work may be due to
the lower tendency to absorb Cu as compared to that of
pure tin.
60
300
x ,µ m
Fig. 6. SEM micrograph illustrating Cu–Sn IMCS in the Sn–Ag bulk
solder after soldering for 2 min at 240 ◦ C.
100
20
50
0
0
0
(a)
2
4
6
Time, min.
8
0
10
(b)
2
4
6
Time, min.
Fig. 7. Plot of x2 vs. t for the consumption of Cu substrate with (a) Sn–Ag solder and (b) Sn–Pb solder.
8
10
A. Sharif, Y.C. Chan / Materials Science and Engineering B106 (2004) 126–131
131
2.0
3.5
1.8
3.4
E = 54 kJ/mol
E = 116 kJ/mol
1.6
lnk
lnk
3.3
3.2
1.4
1.2
3.1
1.0
0.8
3.0
1.96
1.98
2.00
-1
(a)
1.96
2.02
1/T(k )X1000
(b)
1.98
2.00
2.02
-1
1/T(k )X1000
Fig. 8. ln k vs. 1/T for the Cu dissolution to determine the activation energy for (a) Sn–Ag solder and (b) Sn–Pb solder.
4. Conclusions
The effects of reflow at four different temperatures (225,
230, 235, and 240 ◦ C) on the dissolution of Cu substrate
in Sn–Ag and Sn–Pb BGA solder ball are presented in this
paper. A very fast dissolution of the substrate is observed in
the early stages of the molten solder/solid Cu reaction for
both the solder alloys. The dissolution in Sn–Ag is much
higher than that in Sn–Pb solder. The consumption occurs by
dissolution through the channels between the scallop grains.
For Sn–Ag solders, these channels remain open more or less,
but for Sn–Pb solder, they become closed in the later stage
of the dissolution. Many Cu–Sn IMCs are observed in the
bulk of the Sn–Ag solder. Sn–Pb solder also exhibits IMCs
in the bulk, but the amount is not as much as in the case of
Sn–Ag solder. The apparent activation energy calculated for
the dissolution of Cu in the Sn–Pb and Sn–Ag solder alloy
are found to be 116 and 54 kJ/mol, respectively.
Acknowledgements
The authors would like to acknowledge the financial support provided by CERG project no. CityU 1187/01E (City
U Project no. 9040621) of the Research Grant Council of
Hong Kong and the research studentship of City University
of Hong Kong. The technical support from Mr. M.O. Alam
is gratefully acknowledged.
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