IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY—PART B, VOL. 20, NO. 4, NOVEMBER 1997
463
Effect of Intermetallic Compounds on the Shear
Fatigue of Cu/63Sn–37Pb Solder Joints
Y. C. Chan, Senior Member, IEEE, P. L. Tu, A. C. K. So, and J. K. L. Lai
Index Terms— Intermetallic, reliability, shear fatigue, SMT,
solder joint.
prolonged storage and long term operation of the electronic
assembly even at room temperature [6]. Because of its brittle
nature and microstructural mismatch with Sn–Pb solder and
copper, too thick an IMC layer causes the interface in the
solder joint to be more sensitive to stress [2]–[6], especially
under the action of cyclic loading. The effect is much more
marked in a surface mount solder joint of small dimension
(e.g., a ball grid array solder joint).
Numerous investigations of the growth of Cu–Sn intermetallic compounds during isothermal aging have been done
previously in bulk samples and real surface mount solder joints
[3], [6], [8]. Investigation of the effect of intermetallic growth
on the fracture toughness of solder joints has also been done
[2]. Actually, fatigue due to temperature fluctuations either
from internally generated heating or from the external operating environment is a more common solder joint failure mode in
electronic assemblies [9]. This investigation aims at exploring
the effect of growth of intermetallic compound layers on the
shear fatigue failure of eutectic solder joints, by shear cyclic
testing using real surface mount LCCC/PCB assemblies after
isothermal aging. It also focuses on elucidating the reasons
for failure of solder joints. The shear cycling test is one of
the more effective methods of examining the reliability of real
surface mounted assemblies. It can give an important basis for
predicting solder joint lifetime.
I. INTRODUCTION
II. BACKGROUND
HE intermetallic compounds (IMC) microstructural feature in solder joints critically affects the reliability of
surface mounted assemblies (e.g., in telecommunications and
aerospace) since the stresses experienced by the joints during
service can lead to premature failure. These stresses are a
result of the thermal strains generated by the different thermal
expansion coefficients of the many materials used in a typical
assembly [1].
Cu–Sn IMC, which serves as the mechanical bonding between tin–lead solder and the copper pad in a surface mount
solder joint, forms instantaneously when Sn–Pb solder melts
on the Cu pads [5], [7]. A thick Cu–Sn IMC layer in surface
mount solder joints may not only be created by the long reflow
time and high reflow temperature during soldering, but also by
Formally, the reliability is the probability of a product
(here, a solder joint) performing its intended purpose without
failure for a specified period under a given operating condition.
Numerically, the reliability is the fraction of survivors, i.e.,
[10]
Abstract—The effect of Cu–Sn intermetallic compounds (IMC)
on the fatigue failure of solder joints has been studied by means of
shear cycling. The samples consist of leadless ceramic chip carriers (LCCC) soldered onto FR-4 printed circuit boards (PCB), and
are prepared by conventional reflow soldering using a 63Sn–37Pb
solder paste and then aged at 150 C for 1, 4, 9, 16, 25, 36, and
49 days. The specimens are subjected to low cycle fatigue shear
tests controlled by the displacement. The results indicate that
the fatigue lifetime of the solder joints depends on the thickness
of the IMC layer between the Cu-pad and bulk solder, and the
quantitative relationship between the lifetime and thickness can
be described as a monotonically decreasing curve. The greatest
decrease is over the thickness range up to 2.8 m, when the
IMC/bulk solder interface becomes flat, corresponding to a lifetime decrease to 62% of the as assembled value. For further
increase in IMC thickness the lifetime decreases more slowly.
Evidently, the effect of the Cu–Sn intermetallic compounds on
the joint fatigue lifetime is not only concerned with the IMC
thickness but also the interface morphology. A thick and flat IMC
layer has a deleterious effect. The results of X-ray diffraction and
metallographic analysis show that cracks initiate underneath the
component metallization, and propagate along the IMC/solder
interface, then toward the fillet. The Cu3 Sn ("-phase) is formed
between the Cu-pad and -phase, and grows more quickly than
the - phase during storage and long term operation or aging
tests. However, the Cu3 Sn makes only a small direct contribution
toward fatigue failure.
T
Manuscript received November 13, 1996; revised July 22, 1997. This paper
was supported in part by the Universities Grants Council of Hong Kong,
CERG Project 904109.
The authors are with the City University of Hong Kong, Kowloon, Hong
Kong (e-mail: [email protected]; [email protected]).
Publisher Item Identifier S 1070-9894(97)07570-1.
(1)
is the lifetime distribution. In this paper, the
where
lifetime distribution of solder joints is modeled by the twoparameters Weibull cumulative distribution function, which
has the following form [2], [10]:
(2)
In (2), is the value of the random variable—number of cycles
to failure ( ) in the present study,
is the shape parameter
(Weibull slope), and
is the scale parameter (characteristic
value). The “best fit” Weibull parameters (for the median
rank)
and
can be obtained by using the principles
of least squares and ranking. The effect of the IMC layer
1070–9894/97$10.00  1997 IEEE
464
IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY—PART B, VOL. 20, NO. 4, NOVEMBER 1997
Fig. 2. Shear load profile under low cycle fatigue testing controlled by
displacement.
Fig. 1. Schematic of the surface mount solder joint sample used in shear
cycling.
microstructure on joint fatigue lifetime may be expressed as
a function of Weibull parameters
and . Especially, the
bigger is , the longer is the probable lifetime. The value
corresponding to
0.5 is called the number of cycles to
failure at 50% failure rate (
).
is also an important
parameter to express fatigue lifetime. In this study, the fatigue
lifetime of solder joints is characterized using the parameters
and
for the study of the effect of intermetallic growth
on the shear cycling failure of eutectic solder joints. Samples
with varying IMC thickness are obtained by isothermal aging
for varying lengths of time.
III. EXPERIMENTAL PROCEDURE AND METHOD
Surface mount passive components (LCCC: 1206, Resistor:
10 ) were assembled on FR-4 PCB by means of standard
infrared reflow using 63Sn–37Pb eutectic alloy “MULTICORE
SN63 ABS90” solder paste, see Fig. 1. Care was taken to keep
the quantity of solder paste printed on each copper pad fairly
constant so that the effect of solder thickness on the fatigue of
solder joints could be minimized. The thickness of the paste
was about 150 m, as measured by laser section microscopy.
The assemblies were preheated to 100 C for 100 s, and
then reflowed at 230 C for 100 s. in a three-zone infrared
oven (PRECISOLD PS-3000). As suggested by our previous
research work, such soldering conditions can minimize the
formation of pores [11].
The surface mount assemblies were then aged isothermally
in oven at 150 C for 1, 2, 4, 6, 9, 12, 16, 25, 36, and
49 days. These samples were then again reflowed at 230
C for 20 s, to remove grain coarsening in the solder joint.
The metallographic preparation of the solder joints was done
according to the method described in our previous work [7].
The mean thickness of the intermetallic layer in aged joints
was measured using a powerful image processing system, OP-
Fig. 3. Peak load drop versus cycle number during shear cycling.
TIMAS, in conjunction with a Nikon optical microscope. The
micro-structural details of the solder/Cu interface IMC layer
were observed by a JSM-820 scanning electron microscope
(SEM).
The shear cyclic fatigue testing was performed using an
“INSTRON—mini 44” tension tester, with the sample configuration in Fig. 1. A cyclic shear load
is exerted on the
LCCC to move the component up and down relative to the
PCB, with displacement controlled to be from 0.1 to 0.1 mm,
giving a maximum shear strain
1.28, at a speed of 8
mm/min and cycling frequency 1/3 Hz. The displacement and
the speed were kept constant. The load change was sampled
continuously by a computer during testing. A typical recorded
load cycle is shown in Fig. 2. The peak load was about 40–44
N, giving a shear stress of about 16 MPa. When the solder
joint fails under test, the peak load drops quickly as shown
in Fig. 3. Failure was defined as a 50% drop in load [2], [9],
[12], and the cycle number at 50% load drop was regarded as
the fatigue lifetime of the solder joint.
The Nikon optical microscope was also used to observe
the micro-structural details of the solder joints in order to
study the crack morphology formed during shear cycling.
The phase and crystal structure of the bottom fracture surface
were identified by means of X-ray diffraction using a Siemens
D-500 diffractometer with a Cu target.
IV. RESULTS
AND
DISCUSSION
A. Microstructure Morphology
High magnification SEM micrographs of cross-sections of
the solder/Cu interface in LCCC 1206 surface mount solder
CHAN et al.: EFFECT OF INTERMETALLIC COMPOUNDS
465
Fig. 5. The total IMC thickness growth in 1206 LCCC joints at 150 C for
various aging times.
Fig. 6. Weibull lifetime distribution of solder joints for various IMC thickness.
layer can be seen in all the aged samples. The mean thickness
of the duplex phase IMC layer in joints aged for various times
was measured to study its effect on shear fatigue life. The
results are shown in Figs. 4 and 5. The line in Fig. 5 was
obtained by data fitting with “AXUM” software. The IMC
thickness increases linearly with the square root of aging time,
from 1.16 m of -phase in as-solidified solder joint to 6 m
-phase and 3.3 m -phase formed by consuming the -phase
[5] during aging. Simultaneously, the solder/ -phase boundary
flattens as the aging time increases.
Fig. 4. SEM micrographs of solder joints showing the Cu–Sn IMC layers:
(a) reflowed at 230 C for 100 s, no aging, (b) after aging at 150 C for one
day, (c) after aging nine days, (d) after aging 25 days, and (e) after aging
49 days.
joints aged at 150 C for various times are illustrated in Fig. 4.
The single structure of -phase Cu Sn IMC in an as-solidified
surface mount solder joint, Fig. 4(a), and its effect on thermal
fatigue were reported and explained in our previous work [5].
A duplex structure, i.e.; -phase next to the solder and -phase
Cu Sn at the -phase/copper interface, of the intermetallic
B. Shear Fatigue Testing
Samples aged at 150 C for 0, 1, 4, 9, 16, 25, 36, and 49
days yielding various IMC thicknesses were subjected to shear
cycling in order to evaluate the influence of IMC on solder
joint failure. The number of cycles to failure of different sets of
samples are summarized in Table I. The reliability of the solder
joints is appraised with the aid of the Weibull distribution
method. Applying the principles of least squares and ranking
to the test results as shown in Table I, the “best fit” Weibull
parameters (for the median rank) and are calculated, and
466
IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY—PART B, VOL. 20, NO. 4, NOVEMBER 1997
Fig. 7. The shear fatigue lifetime defined as
Q, N50% , and average number of cycles to failure V , as a function of the IMC thickness.
summarized in Table II. The lifetime distributions calculated
using these values and (2) are shown in Fig. 6. As can be
seen, the curves shift from right to left as the IMC layer
in the solder joints thickens, that is, the distributed lifetimes
become shorter. Fig. 7 displays the quantitative relationship
between the shear fatigue statistical lifetime defined as ,
and average number of cycles to failure. Fig. 7 again
shows that the thinner the IMC layer, the greater the probable
number of cycles to failure. The total decrease in
observed was from an unaged initial magnitude of 7732 to
1194 cycles for 49 days aging, corresponding to an IMC layer
thickness increase from the initial 1.16 to 9.26 m. The rate
of decrease of lifetime slows with increasing aging time. The
lifetime drops to 68% (
4887) of the initial value for
change in thickness to 2.8 m. The lifetime then decreases
more slowly for further increase in thickness. This critical
value of Cu–Sn IMC thickness is similar to that obtained
elsewhere [3] in work on the fracture toughness. Obviously,
thickening of the intermetallic layer reduces the reliability of
surface mounted assemblies. Knowledge of the quantitative
relationship between the shear fatigue lifetime and the IMC
thickness of solder joints is useful for fatigue failure prediction.
In conjunction with our previous result that the longer the
reflow time, the thicker the IMC layer but the more uneven the
IMC/solder interface [5], the results of this paper can be used
to optimize the reflow soldering process for the fabrication of
robust solder joints.
C. Failure Analysis of Solder Joints
The question of how the solder joints fail is also an
important problem. The following conclusions are drawn from
optical micrographs of failed joints.
TABLE I
SHEAR CYCLIC TEST RESULTS
TABLE II
WEIBULL’ PARAMETERS FROM STATISTICAL ANALYSIS
First, the load drop during shear cycling is a quick process,
as may be seen in Figs. 3 and 8. Once the peak load begins
to drop, the solder joint will quickly fracture, and the whole
load drop process is completed in about 100 cycles. A 50%
cracked area of the solder joint corresponds to a shear peak
load drop of up to 20%, Fig. 8(a). When the load drop reaches
CHAN et al.: EFFECT OF INTERMETALLIC COMPOUNDS
467
Fig. 9. Crack initiation and growth underneath a surface mounted LCCC.
Fig. 8. Cross-sectional view of cracked solder joints after (a) 20% load drop,
(b) 40% load drop, and (c) 60% load drop.
60%, the cracked area covers up to 90%, see Fig. 8(b). The
fracture behavior appears to be brittle in nature.
Second, the vast majority of failure cracks initiate and grow
under the component metallization as shown in Fig. 9, where
the bulk solder is the thinnest (about 30 m), and the shear
strain is the biggest. The cracks propagate into the fillet at the
same speed as under the component metallization until fracture
as shown in Fig. 8(b). This is different behavior to that seen
in fatigue tests of leaded chips [13].
Last, metallography of failed solder joints and X-ray diffraction of the fracture surface further reveal that the vast majority
of cracks formed during shear cycling initiate and propagate
along the IMC -phase/bulk solder interface as shown in
Figs. 10 and 11. No evidence of the fracture was found at
the -phase/ -phase and -phase/Cu interfaces in failed solder
joints. This result differs from that obtained with fracture
toughness testing [3] and pull-off testing [8] but coincides
with thermal fatigue tests [5]. Thus it seems that the phase only makes a small direct contribution directly toward
the fatigue failure of surface mount solder joints, unless the
growth process results in the -phase forming the intermetallic
for all the tin in the solder alloy underneath the component
metallization and not the -phase. The effect of the -phase
may be only to change the stress distribution in a solder
joint and accelerate indirectly the failure of the -phase/bulk
Fig. 10. Micrographic section of a cracked solder joint: (a) crack propagation
on IMC surface and (b) enlargement of (a).
solder interface since the microstructures of the two phases
are different.
D. Discussion
The aging experiment is an accelerated model for surface
mount assembly aging during operation. In this experiment,
the effect of the IMC thickness on shear cycling lifetime
reflects the effect of the IMC thickness on real operational
lifetime. Stress is produced due to the development of cyclic
468
Fig. 11.
IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY—PART B, VOL. 20, NO. 4, NOVEMBER 1997
X-ray diffraction pattern of the bottom fracture surface of a solder joint.
strains induced by temperature fluctuations and a mismatch in
thermal expansion coefficients between component and PCB.
For the case of thin solder joints underneath the component
metallization, intermetallic growth reduces the joint thickness
causing a local increase in shear stress. Intermetallic growth
also causes localized tin depletion, resulting in a lead-rich layer
adjacent to the intermetallic, and even consumption of all tin in
the solder alloy by the intermetallic. The tin and lead regions
separate out and localized stress concentrations appear which
affect the overall joint properties due to the different moduli
and creep properties of the two materials [1]. These factors
affect the lifetime and failure mode of joints, especially as
the intermetallic forms a highly brittle layer along the joint
boundary. The thicker the intermetallic layer in the joint, the
shorter the fatigue lifetime of surface mounted assemblies. The
greatest decrease rate of lifetime occurs for growth in the IMC
up to a thickness of 2.8 m. The rate of decrease is smaller
for larger thicknesses. This is also the thickness at which the
IMC/solder interface becomes flat. Thus the effect of the IMC
layer on the lifetime is a double action of the thickness and
IMC/bulk interface morphology. A smooth IMC/bulk interface
degrades the solder joint lifetime more than an the uneven
interface, due to the poorer ability of the smooth interface to
resist shearing.
The intermetallic layer and highly brittle region formed
along the solder/IMC interface causes cracks to initiate and
propagate along the IMC -phase/bulk solder interface during
shear cycling. A another possible mechanism is related to the
mode of shear deformation. It is well known that the superplastic eutectic Sn–Pb alloy deforms by grain rotation and
grain boundary sliding [14]. During shear cycling, when the
PCB displaces relative to the component (LCCC), the strain is
mainly due to the bulk solder. The IMC layer is harder than
378, Hv
100) [9], so the
the bulk solder (Hv -phase
shear stress is concentrated at the IMC/solder interface. The
dislocations and vacancies that exist at the smooth boundary
of the IMC gather gradually, then the crack forms, grows, and
gives rise to joint failure.
According to the above results and discussions, the intermetallic compounds can greatly affect the fatigue lifetime of
solder joints, and the solder joint lifetime is related to the
its thickness. Thus, the assembly lifetime can be predicted by
monitoring the thickness growth inside solder joints during
operation of the electronic assembly.
V. CONCLUSION
The effects of Cu–Sn IMC on solder joint failure during
shear cycling are summarized as follows:
A thick IMC layer in surface mount solder joints is not only
caused by a long reflow time and high reflow temperature
during soldering, but also by aging, prolonged storage, and
long term operation of the electronic assembly even at room
temperature. The IMC thickness increases linearly with the
square root of aging time, and the IMC/bulk solder interface
becomes gradually flatter. The -phase is formed between
Cu-pad and -phase, and grows more quickly than the -phase.
The intermetallic reduces the solder joint lifetime, the
reduction being greater with greater IMC thickness and, hence,
aging time. At a thickness of around 2.8 m, corresponding
to a lifetime of 62% of an unaged sample, the IMC/solder
interface becomes as flat, and the rate of decrease of lifetime
with increase of IMC thickness slows. Failure causing cracks
initiate mainly underneath the component metallization and
grow along the IMC/solder interface. Thus for shear cycling,
solder joint fatigue lifetime is affected by the morphology
of the Cu–Sn intermetallic compounds/bulk solder interface
CHAN et al.: EFFECT OF INTERMETALLIC COMPOUNDS
as well as by the IMC layer thickness. A flat IMC/solder
boundary is deleterious for the fatigue lifetime.
The Cu Sn -phase, formed during storage and long term
operation, has only a small influence on the lifetime, unless
the -phase consumes all the tin in the solder underneath the
component metallization.
ACKNOWLEDGMENT
The authors would like to thank D. P. Webb.
REFERENCES
[1] Winter and E. R. Wallach, “Electronic interconnect reliability modeling,”
Solder. Surface Mount Technol., no. 22, pp. 55–58, Feb. 1996.
[2] J. H. Lau, Solder Joint Reliability. New York: Van Nostrand Reinhold,
1991, pp. 406–449.
[3] R. E. Pratt, E. I. Stromswold, and D. J. Quesnel, “Effect of solid-state
intermetallic growth on the fracture toughness of Cu/63Sn–37Pb solder
joints,” IEEE Trans. Comp., Packag., Manufact. Technol., vol. 19, pp.
134–141, Mar. 1995.
[4] Plumbridge, “The mechanical behavior of solders,” Solder. Surface
Mount Technol., no. 22, pp. 27–31, Feb. 1996.
[5] P. L. Tu, Y. C. Chan, and J. K. L. Lai, “Effect of intermetallic
compounds on the thermal fatigue of surface mount solder joints,” IEEE
Trans. Comp., Packag., Manufact. Technol., vol. 20, pp. 87–93, 1997.
[6] A. C. K. So and Y. C. Chan “Aging studies of Cu–Sn intermetallic
compounds in annealed surface mount solder joints,” in Proc. 46th
Electron. Comp. Conf., Orlando, FL, May 1996, pp. 1164–1171.
[7]
, “Reliability studies of surface mount solder joints—effect of
Cu–Sn intermetallic compounds,” IEEE Trans. Comp., Packag., Manufact. Technol., vol. 19, pp. 661–668, Aug. 1996.
[8] B. S. Chiou, J. H. Change, and J. G. Duh, “Metallurgical reactions at the
interface of Sn–Pb solder and electroless copper-plated AIN substrate,”
IEEE Trans. Comp., Packag.,Manufact. Technol., vol. 18, pp. 537–542,
Aug. 1995.
[9] D. Solomon, “Isothermal fatigue of LCC/PWB interconnections,” J.
Electron. Packag., vol. 114, pp. 161–168, June 1992.
[10] D. Fear, H. S. Morgan, S. N. Burcheit, and J. H. Lau, The Mechanics of
Solider Alloy Interconnects. New York: Van Nostrand Reinhold, 1994,
pp. 42–86, 361–378.
[11] 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
solder joints,” in Proc. 6th Int. Symp. Non-Destructive Characterization
Mater., 1993, pp. 683–690.
[12] Y. C. Chan, D. J. Xie, J. K. L. Lai, and I. K. Hui, “Application of direct
strain measurement to fatigue studies in surface solder joints,” IEEE
Trans. Comp. Packag., Manufact. Technol., vol. 18, pp. 715–719, 1995.
[13] H. D. Solomon, V. Brzozowski, and D. G. Thompson, “Prediction of
solder joint fatigue life,” 40th ECTC ’90, 1990, pp. 351–360.
[14] D. Frear, J. W. Grivas, and J. R. Morris, “A microstructure study of
the thermal fatigue failures of 60Sn–40Pb solder joints,” J. Electron.
Mater., vol. 17, no. 2, pp. 171–180, 1988.
469
Y. C. Chan (M’85–SM’95) received the B.S. degree in electrical engineering,
the M.S. degree in materials science, and the Ph.D. degree in electrical
engineering, all from the Imperial College of Science and Technology,
University of London, London, U.K., in 1977, 1978, and 1983, respectively.
He joined the Advanced Technology Department, Fairchild Semiconductor,
as a Senior Engineer. In 1985, he was appointed to a Lectureship in Electronics
at the Chinese University of Hong Kong. From 1987 to 1991, he worked in
various senior operations and engineering management functions in electronics
manufacturing [including SAE Magnetic (Hong Kong) Ltd. and Seagate
Technology]. He set up the Failure Analysis and Reliability Engineering
Laboratory, SMT PCBA, Seagate Technology, Singapore, prior to joining the
City University of Hong Kong as a Senior Lecturer in electronic engineering,
in 1991, and was promoted to University Senior Lecturer, in 1993. His current
technical interests include surface mount technology, electronic materials, and
component reliability.
Dr. Chan is Chairman of the IEEE Hong Kong Center.
P. L. Tu received the B.Eng. degree from the Beijing University of Aerospace
and Astrophysics, Beijing, China, in 1982, and the M.Eng. degree from
Nanjing University of Aerospace and Astrophysics, Nanjing, China, in 1988.
From 1989 to 1995, he was employed as a Lecturer at the Department
of Materials Science, Nanjing University. Since 1995, he has worked as
a Research Assistant in the Department of Electronic Engineering, City
University of Hong Kong. His current research focuses on fatigue life analysis,
solder joint defect studies, and reflow soldering.
A. C. K. So, for a photograph and biography, see p. 166 of the May 1997
issue of this TRANSACTIONS.
J. K. L. Lai received the Ph.D. degree (with first class honors) from Keble
College, Oxford University, Oxford, U.K., in 1971.
From 1974 to 1985, he was employed as a Research Officer at the Central
Electricity Research Laboratories, Leadherhead, Surrey, U.K. In 1984, he was
appointed Project Leader of the Remaining Life Study Group and a member
of the Remnant Life Task Force of the Central Electricity Generating Board,
U.K. He is now a Professor in the Department of Physics and Materials
Science and Head of the Materials Research Centre, City University of Hong
Kong. He holds one patent and has published more than 50 technical papers
in refereed international journals and conference proceedings. He is also the
author of more than twenty industrial reports on failure analysis and materials
for engineering design, issued by the Central Electricity Generating Board,
U.K. He has performed more than 40 cases of consultancy in the area of
failure analysis of metallic components in Hong Kong.
Dr. Lai is a member of the following committees: the International
Institute of Weld Working Group on Creep; the Pressure Equipment Advisory
Committee of the Labor Department; the Plastics Training Board of the
Vocational Training Council and the Consumer Council of Hong Kong.