IEEE TRANSACTIONS ON COMPONENTS. PACKAGING, AND MANUFACTURING TECHNOLOGY-PART
B, VOL. 19, NO. 3, AUGUST 1996
66 I
Reliability Studies of Surface Mount Solder JointsEffect of Cu-Sn Intermetallic Compounds
Alex C. K. So and Yan C. Chan, Senior Member, IEEE
Abstract-CuSn intermetallic compounds (IMC's), formed at
the interface between the solder and the copper substrate are
found to play a dominant role in determining the thermal fatigue
life of surface mount solder joints fabricated from a conventional
infrared reflow process. In order to predict the growth of this
IMC layer during the operating life of the solder joint and its
effect on the thermal fatigue life, the formation characteristics of
the IMC's in 0805 and 1206LCCC solder joints are systematically
studied in this investigation. Only the stable CucSns'X rpphase
intermetallic compound was observed in all as-solidified solder
joints as confirmed by scanning electron microscopy (SEM) and
energy dispersive x-ray (EDX). The mean layer thickness was
found to increase almost linearly with reflow time up to about
200 s. The thickness of the interfacial IMC layer increased with
increasing reflow temperature for 0805-type solder joints up to
around 250 "C and reached a saturated thickness of 2.5 pm
beyond this temperature. Additional intermetallic formation due
to higher reflow temperature or longer reflow time would appear
as C u S n whiskers in the bulk solder of the joint. The copper land
pad size and quality of component lead metallization were also
found to greatly affect the formation of C u S n IMC in surface
mount solder joints, and hence its reliability in terms of thermal
fatigue life and mechanical properties.
I. INTRODUCTION
0
NE of the crucial factors in the reliability of surface
mount solder joints is the formation of an copper-tin
intermetallic compound (IMC) between the solder and copper
land pad of a solder joint. When molten Sn-Pb solder comes
in contact with the copper pad surface, a layer of Cu-Sn IMC,
consisting of the C u ~ S n s7-phase adjacent to the solder and
the CuySn &-phase next to the copper land pad surface, is
formed in-between and serve as the bonding material for the
solder joint. However, due to its brittle nature and thermal
mismatch with the solder and the printed circuit board (PCB),
excessive IMC formation at the solder-copper interface of
the solder joint will potentially cause weakening in joint
strength and eventually fatigue failure. In order to systematically study the formation of intermetallic compound formation
in surface mount solder joints, the effect of various infraredreflow process parameters (such as the reflow time, reflow
temperature, and size of copper land pad) were examined in
this work.
Manuscript received September 14, 1995; revised March 12, 1996. This
work was supported by Research Grants from the Council of Hong Kong
under CERG Project 904 109.
The authors are with the Department of Electronic Engineering, City
University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong.
Publisher Item Identifier S l070-9894(96)05016-5.
11. BACKGROUND
Tin-lead solder alloy is usually the main constituent material
in the fabrication of solder joints in surface mount PCB
assemblies. When molten Sn-Pb solder comes in contact with
the copper pad surface, intermetallic compounds formed are
entirely of the Cu-Sn intermetallic species (with an eutectic
composition of about 0.7 wt.% Cu). The formation of Cu-Pb
intermetallic compounds is negligible due to the low solubility
of Cu in Pb (the eutectic composition of the Pb-Cu binary
system is 0.06 wt.% Cu).
Various works [1]-[7], [9], [lo], [12], [13] have been done
on the formation and growth of the Cu-Sn IMC's in different
binary metal systems. It is generally observed that a thin
layer of 7-phase (Cu~Sns)intermetallic compound is formed
instantaneously as soon as eutectic Sn-Pb solder is melt on
a copper substrate. Tu and Thompson [l] found that C u ~ S n s
compound was formed once Cu and Sn thin film came in
contact and its thickeness grew fairly linearly in a bimetallic
Cu-Sn thin film at room temperature but no Cu3Sn IMC was
detected. Dimfeld and Ramon [2] melted 30Sn-70Pb solder
on plug-and-ring copper specimens at 310 "C for a prolonged
period of time (up to 64 s). It was observed that the mean
thickness of the interfacial intermetallic layer formed increased
parabolically with the raction time of molten solder with
copper. Sunwoo er al. (31 studied the intermetallic formation
and growth in a double shear specimen structure with eutectic
solder between a stack of three copper plates and concluded
that only the CuGSns 7-phase intermetallic was formed in assolidified eutectic solder joints. Grivas et al. [4] submerged
copper plates in a molten bath of 95Pb-5Sn solder at 400
O C , where it was seen that only &-phase Cu3Sn intermetallic
compound was present and the interfacial IMC layer thickness
increased almost linearly with the time for which the solder
was molten.
However, most of the previous works were either performed
under nonstandard industrial reflow conditions or are not of
direct relevance to practical surface mounted solder joints
fabricated from a conventional infrared reflow process. In
view of the solder joint miniaturization in advanced surface
mount technology, the applicability of these previous data is
rather limited for small-geometry solder joints. Therefore, the
present studies will focus on 0805 and 1206 LCCC solder
joints that are close to those found in practical surface mount
PCB assemblies. The effects of various process variables such
as reflow time and temperature on the formation kinetics of
Cu-Sn IMC's are investigated. It is hoped that such a study
1070-9894/96$05.00 0 1996 IEEE
662
IEEE TRANSACTIONS ON COMPONENTS, PACKAGING. AND MANUFACTURING TECHNOLOGY-PART B, VOL. 19, NO. 3, AUGUST 1996
14 FR-4 PCB
-0805 LCCC resistor x 4
-1206 LCCC resistor x 8
----,
(-)
Stencil printing
i
Optical microscopy
Grinding
SEM & EDX
Polishing
Thickness
Measurement
Selective
etching
Fig. 1.
Basic process flow of experiments.
Fig. 2.
A typical IR-reflow temperature profile.
Pick-and-place
can throw light on some of the critical reliability problems
encountered in practical surface mount PCB assemblies.
111. EXPERIMENTAL
PROCEDURE
The basic process flow of the experiments carried out is
shown in Fig. 1. Four 0805 LCCC and eight 1206 LCCC
resistors were soldered on FR-4 PCB’s using a standard
infrared reflow process: MULTICORE SN63 RM92 solder
paste of 63Sn-37Pb were applied onto the PCB using stencil
printing. The amount of solder paste applied on each copper
land pad was carefully controlled in order to minimize the
\ /r
Reflow: 230 C
Various times
Reflow : 100 s
Various temperatures
effect of different amounts of tin available for Cu-Sn intermetallic formation in the soldered joint. The solder paste
height was measured to be within 160-180 ,um Components
were then placed onto the copper pads of the PCB using a
semi-automatic pick-and-place machine. The surface mount
assembly was subsequently passed through a PRECISOLD
PS-3000 IR reflow soldering system.
To study the effect of reflow time and temperature on the
formation of Cu-Sn intermetallic compounds, different IRreflow temperature profiles were selected (a typical IR-reflow
temperature profile is shown in Fig. 2). Seven specimens were
SO AND CHAN: RELIABILITY STUDIES OF SURFACE MOUNT SOLDER JOINTS
663
Fig. 3. Cross-sectional view of the solder/Cu interface in an as-solidified 0805 surface mounted solder joint showing the Cu6Sn5 intermetallic layer formed
in between: (a) secondary electron image, and (b) back-scattered electron image.
Fig. 4. Equilibrium phase diagram of Cu-Sn binary system [9].
reflowed at 230 "C for various times, whereas the other seven
were reflowed at different temperatures for 100 s. The preheat
temperature and time were kept constant at 100 "C and 100 s.
From our previous work [14], such a preheat condition could
minimize the formation of porosity in surface mount solder
joints.
For metallographic observations, specimens of single component were first mounted in Klarmount and cross-sectioned
perpendicular to the solder-copper interface of the solder joint.
They were then successively grounded down to 1000 grit on
a silicon carbide paper under water cooling. Polishing was
performed using 5 pm A1203 suspension followed by 0.25 pm
diamond compound in Aerosol. The specimens were etched
in a freshly prepared mixture of 4 ml concentrated Hz02,
6 ml concentrated NH40H, and 6 ml HzO for 30-60 s to
reveal the Cu3Sn phase. This was followed by deep etching
664
IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART
B, VOL. 19, NO. 3, AUGUST 1996
(b)
(b)
Fig 5 Interfacial Cu-Sn intermetallic morphology in the same 0805 LCCC
solder joint reflowed at 230 O C for 175 s (a) under the bulk solder and (b)
under the component metallization
Fig. 6. High magnification SEM micrographs of the interfacial Cu-Sn
intermetallic layer in solder joints reflowed at 230 OC for (a) 25 s and (b)
175 s.
in a dilute solution of 2% concentrated HC1, 6% concentrated
HN03 and 92% H20, as this process would blacken the Sn-Pb
solder for 30 s to contrast the boundary between Cu6Sn~and
by means of EDX and ZAF-4 analyzes. Such an IMC layer
has a fine globular structure. No evidence of the €-phase
CuSSn IMC was found in the as-solidified solder joint. This
can simply be explained by the equilibrium phase diagram
of Cu-Sn binary system [9] shown in Fig. 4. From this, Ephase can only be formed when the eutectic solder joint is
reflowed at a temperature greater than 415 OC, the peritectic
temperature of the r)-phase. Other researchers [5]-[SI found
that &-phase IMC was formed at the copper/r)-phase interface only after prolonged ageing of that solid soldedsolid
copper substrate interface. Boundaries of the 7-phase CusSn:,
interfacial intermetallic layer were more easily observable in
the back-scattered electron image shown in Fig. 3(b). Such a
closely matched boundary of the IMC layer in the as-solidified
solder joint indicates that tin is the dominant diffusion species
through the 7-phase intermetallic compound for the formation
Cu-Sn intermetallic compound during infrared reflow, in good
agreement with the results of Mei et al. [9].
Besides, it was generally observed in all solder joints that
the Cu-Sn interfacial intermetallic layer formed underneath
the component terminal metallization was thinner than that
formed below the bulk solder (Fig. 5). In the present study,
the solder. Both optical and scanning electron microscopy
(SEM) were used to study the microstructural morphology
of Cu-Sn intermetallic compounds in the as- soldered solder
joint and energy dispersive x-ray (EDX) and ZAF-4 analyzes
deployed to characterize the composition of the intermetallic
compounds. A powerful image processing system incorporating a NIKON optical microscope and OPTIMAS software was
used to observe the microsection of the sample and measure
the mean IMC thickness.
Iv.
mSULTS AND
DISCUSSION
A. Microstructural Morphology in As-Solidified
IR-ReJowed Solder Joint
Fig. 3(a) is a high magnification SEM micrograph of the
solder-copper substrate interface in an as-solidified 0805 surface mount solder joint reflowed at 230 OC for 25 s. A
continuous layer of 7-phase CuGSn:, intermetallic compound
lcoated at the solder-copper substrate interface was identified
SO AND CHAN: RELIABILITY STUDIES OF SURFACE MOUNT SOLDER JOINTS
665
A1 1 e 1a t s ana 1ysed
ELNT
ZRF
,KORNBL I SED
ZELHT
BT0H.Z
CuK : 1 1.033
CuL : 1 ,525
24.879 31.687
30.244 38.520
SnL : 1 ,887 42.104 28.708
Pbli : 1 .769
2.777 1.025
TOTRL
100.004 100.000
(a)
(b)
Fig. 7 (a) EDX and (h) ZAF-4 results showing the presence of only CusSns in the interfacial IMC layer in SMT solder joint even after prolonged
period of reflow.
3.0-
2.5
T
-
2.0 1.51.0
-
4
o o " " " ~ ' ~ ' ' ' " '
0
20
40
60
80
100
120
140
160
180
Reflow Time (s)
Fig. 8. Mean interfacial Cu-Sn intermetallic layer thickness in both 0805 and 1206 surface mount solder joints as a function of reflow time.
the metallization of both 0805 and 1206 leadless components
was solder plated nickel layer. Although the dissolution rate
of Ni in liquid solder is much slower than that of Cu [lo],
[ 111, Ni-Sn intermetallic compound could indeed be formed
as well. The competition for tin due to the Ni-Sn and Cu-Sn
intermetallic formation in the solder joint would eventually
tend to lower the formation rate of the latter. This undesirable
effect is expected to be more serious and can cause reliability
problems in fine pitched surface mount solder joints where the
solder thickness between component lead and copper substrate
is very small.
B. Effect of Refow Time
0805-type and 1206-type "Ider joints
at
230 OC for various times were studied. AS shown in Fig. 6,
Fig. 9. Optical micrographs of 0805 solder joint reflowed at 230 OC for 150
S. Only trace amount of small Cu-Sn intermetallic whiskers (bright spots)
the interfacial CuGSn5 intermetallic was thicker for longer
were
Observed
in lhe
(X200).
666
IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART B, VOL. 19, NO. 3, AUGUST 1996
3.0
25
-e
G!
2.0
ul
(0
m
c
%
z
c-
1.5
0
3
1 .0
9
0.5
c
m
0.0
1
1
1
/
1
1
1
1
190 200 210 220
1
/
1
1
1
/
/
1
1
,
1
230 240 250 260 270 280
Reflow Temperature (C)
Fig. 10. Mean thickness of the interfacial Cu-Sn intermetallic layer in 0805 solder joint versus reflow temperature.
reflow time and again no CuaSn a-phase was revealed by EDX
and ZAF-4 analyzes (Fig. 7) even after prolonged period of
infrared reflow. The mean thickness of the interfacial Cu-Sn
intermetallic in both 0805 and 1206 solder joints as a function
of reflow time is shown in Fig. 8.
It can be seen that the formation rate of the interfacial
intermetallic layers in both 0805 and 1206 solder joints is
fairly linear. According to classical kinetic analysis, a linear
growth rate indicates that the formation of the interfacial
Cu-Sn intermetallic layers is an interfacial-reaction-controlled
process. The formation of the interfacial IMC layer in a liquid
solderlsolid rpphasdsolid Cu substrate matrix involves two
rate-determing steps: 1) diffusion of tin through the formed qphase to the rpphaselcopper substrate and 2) reaction between
tin and copper at the interface to form the IMC layer. Since
at such a high reflow temperature, tin in liquid solder may
move around freely and the diffusion of tin through the already
formed rj-phase CusSn:, intermetallic layer is fast, makes the
whole process of the interfacial intermetallic formation to be
reaction-controlled. Moreover, only trace amounts of Cu-Sn
IMC whiskers were observed even after 150 s of IR-reflow
(Fig. 9).
In addition, it was found that the formation rate of the interfacial IMC layer in 0805 solder joints was slightly higher than
that in the 1206 solder joint. This can be explained by the fact
that the heat supply rate from the panel heater of the IR-reflow
machine is constant during soldering. The larger 1206 land pad
size means more reactions between copper and tin are required
to form one crystal layer of the interfacial Cu-Sn intermetallic
compound. Thus, the thickening of intermetallic layer between
the copper substrate and solder may be suppressed by a larger
copper land pad.
C. Effect of Refow Temperature
The mean thickness of the interfacial Cu-Sn intermetallic
layer in 0805 solder joints reflowed at various temperatures
for 100 s versus the reflow temperature is shown in Fig. 10.
The intermetallic layer increases fairly linearly with reflow
temperature from 190 "C to 250 "C and attains a peak
thickness of about 2.5 pm. It then drops slightly for higher
reflow temperature. This small drop in layer thickness is
attributed to the development of Cu-Sn whiskers in the bulk
solder of solder joints reflowed at higher temperature, as
illustrated in series of optical micrographs in Fig. 10.
At lower reflow temperature, the formation rate of Cu-Sn
intermetallic is so slow that its formation is confined to
the solder-copper interface during the reflow time (100 s)
in this study. Hence no C u S n whisker was observed in
the bulk solder [Fig. Il(a)]. At higher reflow temperature,
the formation rate is faster and the IMC intermetallic layer
thickness increases with reflow temperature for a given reflow
time. The thickness of the interfacial IMC layer reaches a
saturated value of about 2.5 pm at a reflow temperature of
250 "C within the designated reflow time. Further formation
of Cu-Sn intermetallic appears as faceted rod-like crystal
growing out, most likely by a screw dislocation mechanism,
from the interfacial intermetallic layer into the bulk solder.
The rods then break off by turbulence in the molten solder
[12] and reappear as whiskers in bulk solder of the solder joint
[Fig. 1l(c)]. The random breaking point of the faceted CusSns
rods leaves behind different amounts of Cu-Sn intermetallic
compounds in the interfacial layer. As a result, the mean
thickness of the interfacial intermetallic layer tends to fluctuate
around the saturated thickness.
V. CONCLUSION
From a systematic study of the Cu-Sn intermetallic formation in both 0805 and 1206 LCCC solder joints fabricated
through a conventional infrared reflow process, it is clearly
shown that a continuous layer of rpphase CusSn:, intermetallic
compound, as confirmed by SEM and EDX analyzes, is
present at the solder-copper interface in all the solder joints.
SO AND CHAN RELIABILITY STUDIES OF SURFACE MOUNT SOLDER JOINTS
661
(c)
Fig. 11. Optical micrographs of cross-sectional view of 0805 LCCC solder joints reflowed for 100 s at (a) 190 O C , (h) 250 O C , and (c) 280 OC. More
and longer Cu-Sn intermetallic whiskers are found in bulk solder in joints reflowed at higher temperature.
No evidence of &-phase CusSn intermetallic is found. The
formation rate of the interfacial Cu-Sn intermetallic compound
beneath the component terminal metallization is also found
to be suppressed by Sn-metal (Sn-Ni in the present case)
intermetallic compound formed at the solderkomponent terminal metallization interface. However, the precise quantitative
effect on the mechanical properties or reliability of solder
joint requires further investigation for a better understanding.
The mean thickness of the 7-phase interfacial intermetallic
layers in both 0805 and 1206 solder joints increases almost
linearly with reflow time up to 200 s and the formation rate
in a 0805 solder joint is faster than that in a 1206 solder
joint under similar reflow conditions. This is most likely due
to the larger land pad size of the 1206 LCCC solder joint.
Moreover, the thickness of the interfacial Cu-Sn IMC layer
in 0805 LCCC solder joints that have been infrared-reflowed
for 100 s increases with increasing reflow temperature up to
around 250 OC and reaches a saturated thickness of 2.5 pm.
Above this temperature, additional growth of the IMC is found
to be minimal. Instead, the additional formation of Cu-Sn
IMC’s simply appears as Cu-Sn whiskers in the solder bulk
of the joint. Generally, it is observed that the copper land pad
size and quality of component lead metallization greatly affect
the formation of Cu-Sn IMC in surface mount solder joints,
and hence its reliability in terms of thermal fatigue life and
mechanical properties.
ACKNOWLEDGMENT
A. C. K. So is grateful for a research studentship in support
of his study for a Ph.D. degree in the Department of Electronic
Engineering at City University of Hong Kong.
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IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY-PART
[7] D. R. Frear, F. M. Hosking, and P. T. Vianco, “Mechanical behavior of solder joint interfacial intermetallics, ” in Proc. Mater. Dev.
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[I41 Y. C. Chan, D. J. Xie, J. K. L. Lai, F. Yeung, and H. Wong, “Applications of X-ray radiography to the study of porosities in surface mount
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[15] J. S . Hwang, Solder Paste in Electronics Packaging. New York Van
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B, VOL. 19, NO. 3, AUGUST 1996
[17] H. H. Manko, Solders and Soldering: Materials, Design, Production,
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__
~
~~
.
Alex C. K. So received the B.Sc. degree in applied
physics with First Class Honors from the City
Polytechnic of Hong Kong, in 1993.
He joined the Department of Electronic Engineering, City University of Hong Kong as a Ph.D.
research student in October 1993, and has been
working on the reliability studies of surface mount
solder joints. His research‘interests are in advanced
electronics manufacturing technology and reliability
issues.
Mr. So is a Committee Member of the YMSLEE
(Hong Kong Centre).
Yan C . Chan (SM’95) , for a photograph and biography, see p. 153 of the
February 1996 issue of this TRANSACTIONS.