THERMOMECHANICAL FATIGUE OF SOLDER
JOINTS: A NEW COMPREHENSIVE TEST METHOD
D. R. Frear
Division 1832
Sandia National Laboratories
P. 0. BOX 5800
Albuquerque, NM 87185
ABSTRACT
The thermomechanical fatigue behavior of solder joints is a
critical reliability issue in electronic packaging. A need exists for a
thorough metallurgical understanding of solder joints in conditions
of thermd fatigue. A review of current methods to test solder joints
reveals that each method lacks some important facet that would lead
to a fundamental understanding of the thermal fatigue process. This
paper presents a new comprehensive method to test solder joint in
thermomechanical fatigue. The method involves simultaneous
imposition of temperature cycles and strain on discrete solder joints
in a shear orientation. The stress, microstructure, and number of
cycles to failure were monitored. Cycles to failure were determined
by a continuous electrical detection method. 60Sn-40Pb and 40Sn-
40111-20Pb solder joints were tested using this new method at 20%
shear strain. The 60Sn-40Pb alloy has a shorter fatigue lifetime than
did 40Sn-40In-20Pb. This is attributed to heterogeneous coarsening
that concentrates strain into a small area of the 60Sn-40Pb
microstructure. In contrast the 40Sn-401n-20Pb microstructure
becomes refined. The heterogeneous coarsening also results in
cyclic softening in 60Sn-40Pb. which was not observed in 40Sn-
40111-20Pb. Failures initiated within the coarsened band in 60Sn-
40Pb at Sn-Sn grain boundaries or phase boundaries. In contrast,
failures initiated at the surface of 40Sn-40In-20Pb joints and
propagated through both phases of the microstructure.
JlYTRoDUCTIoN Renewed interest in solder has been driven by reliability
concerns of solder joints in electronic packages. One packaging
scheme where solder joints are critical is Surface Mount
Technology (SMT). In SMT the solder joint alone bonds the chip
carrier to the Printed Circuit Board (PCB). The advantage SMT
has over conventional plated-through-hole technology is the
reduction in PCB size, easier automated assembly procedures,
lower overall cost, and better electrical performancel. However,
one major drawback of SMT is that the solder joint reliability
becomes increasingly critical as the joints must act both as an
electrical connector and a mechanical bond. The failure of a single
joint could render a device, or an entire machine, inoperable.
The primary failure mechanism of SMT joints arises from
the imposition of mechanical strain on the joints while in service.
The strain is produced by the combination of fluctuations in
temperature acting on the materials of differing thermal
expansivities used in the electronic package. Thermal fluctuations
arise due to environmental temperature changes and/or temperature
fluctuations from the Joule heating of the devices. The imposed
strain is primarily in a shear orientation although a tensile
component may be present2. The pliable nature of solders allows
for some relief of the shear strain, but upon repeated cycling solder
joints have been observed to fai13-6. Therefore to increase joint
reliability there exists a need to understand the metallurgical aspects
that lead to failures in solder joints. This need can be met by the
experimental testing of solder joints under conditions of
thermomechanical fatigue.
In order to more accurately study the thermomechanical
fatigue of solder joints a new comprehensive test method has been
developed. The method involves simultaneously imposing
temperature and strain to a number of solder joints in a shear
orientation while monitoring stress, the number of cycles to failure,
and solder joint microsnuctm. In this manner it is possible to gain
significant information on solder joints during thermomechanical
fatigue. Furthermore, it is also possible to readily compare the
relative behavior and lifetimes of solders in thermomechanical
fatigue in order to determine, quantitatively, the best solder alloy
for a given application. This paper discusses why this new method
of testing is the most representative and informative test available.
Details of the test method a . ~ presented as well as results from two
29
solder alloys tested, 60Sn-40Pb and 40Sn-40In-20Pb. These
alloys have a relatively low melting point and are commonly used
in the surface mount of chip carriers to PCBs. -
In order to develop a reliable and accurate
thermomechanical fatigue test it was necessary to examine the
methods currently in use. A great deal of research has been
performed to study solder joints in conditions of thermomechanical
fatigue. This work can be divided up into three types of testing
termed here as: 1) thermal cycling of components, 2) isothermal
fatigue, and 3) thermal fatigue of simplified test specimens. Each
of these test procedures were examined, and rejected, as the best
means to explore the thermomechanical fatigue of solder joints.
Their advantages and disadvantages are discussed below. The end
result of this review was the design of a new test method which
combines the best elements from the three previous tests into a
single more comprehensive test methd
Thermal cvclinc of commnents
This method involves imposing thermal cycles on actual
components soldered to PCBs. Strain is imposed by the difference
in thermal expansivity between the component and the board as the
temperature is cycled. The main advantage to this type of testing is
that actual solder joints on components of interest are fatigued.
With this method it is also possible to monitor the number of cycles
to failure as well as examine the resultant microstructure.
There are three means by which thermal cycles may be
imposed: 1) thermal shock, 2) temperature cycling, and 3) power
cycling. Thermal shock involves cycling components between two
thermal baths at opposite temperature extremes. Temperature
cycling involves imposing strain in a similar manner as thermal
shock but at a significantly slower rate. Turning power
components on and off creates Joule heating resulting in power
cycling. Thermal shock is the least conservative of the three as
found by Kubik and Li7 where thermal shock yielded a longer life
than thermal cycling. Englemaier38 emphasizes that power cycling
is important, especially for consumer applications, where
components are turned on and off in an ambient environment. The
area that the most research has been devoted to is thermal cycling9-
l6 which mimics environmental cycling. These tests model the
most severe cycling solder joints may encounter in service, and are
especially important for aerospace applications.
Determining the number of cycles to failure is one of the
beneficial aspects of components thermal cycle testing but can be a
shortcoming if performed incorrectly. The four most common
methods of determining joint failure are: I) Visud inspection79.15,
2) Electrical discontinuity7.11.12, 3) Electrical resistance
increase16.17 and, 4) Continuous continuity monitoring1. The
inherent problem with the electrical detection of a failure in a shear
deformation mode is that after a crack has formed throughout the
joint there still may be physical contact. Visual inspection is
unsuitable because cracks are difficult to observe, and once a crack
is visible on the surface of the solder the joint is well beyond
failure. Continuous electrical continuity monitoring is the
recommended method for failure detection'. (This method was
used in the work performed in this paper and will be discussed
later).
Unfortunately there are many disadvantages to this testing
procedure for developing a fundamental understanding of solder
joints in thermomechanical fatigue. Although deformation is
primarily in shear for a SMT joint the overall strain state is
complicated. Lau, Rice, and Averyls performed a 2D and 3D
Finite Element analysis on SMT solder joints and quantitatively
found the joint to be in a complex state of strain due to the large
'3 0569550318910293 $1.00 0 IEEE