2008 International Conference on Electronic Packaging Technology & High Density Packaging (ICEPT-HDP 2008)
978-1-4244-2740-6/08/$25.00 ©2008 IEEE
Computer Simulation of Crack Propagation in Power Electronics Module Solder Joints

Hua Lua†, Steve Ridouta, Chris Baileya, Wei Sun Lohb, Agyakwa Pearlc, and Mark Johnsonc
a School of Computing and Mathematical Sciences, University of Greenwich, 30 Park Row, London SE10 9LS, UK
b Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield, S1 3JD, U.K
c University of Nottingham, School of Electrical and Electronic Engineering, University of Nottingham, Park, Nottingham,
NG7 2RD, U.K.
†Telephone: +44(0)2083318536, Email address: h.lu@gre.ac.uk

Abstract
A numerical modelling method for the analysis of solder
joint damage and crack propagation has been described in this
paper. The method is based on the disturbed state concept.
Under cyclic thermal-mechanical loading conditions, the level
of damage that occurs in solder joints is assumed to be a
simple monotonic scalar function of the accumulated
equivalent plastic strain. The increase of damage leads to
crack initiation and propagation. By tracking the evolution of
the damage level in solder joints, crack propagation path and
rate can be simulated using Finite Element Analysis method.
The discussions are focused on issues in the implementation
of the method. The technique of speeding up the simulation
and the mesh dependency issues are analysed. As an example
of the application of this method, crack propagation in solder
joints of power electronics modules under cyclic thermal-
mechanical loading conditions has been analyzed and the
predicted cracked area size after 3000 loading cycles is
consistent with experimental results.
Introduction
Power electronic modules (PEM) are widely used in the
industry and consumer products for the control and
conversion of electric power. A typical power module consists
of several layers of insulator, conductor and semiconductor
plus some metal wires, encapsulations, electric terminals and
the casing components [1]. The materials in power modules
are assembled together in the packaging process to form
power electronic circuits and the mechanical structure. Since
PEMs contain different materials, they are highly
inhomogeneous devices and there are many interfaces in
them. Furthermore, PEMs are often used in extreme
conditions and this poses a great challenge for the designers
of highly reliable PEMs.
Of the several possible failure mechanisms of a PEM,
solder joint fatigue is one of the most important one and it is
this mechanism that will be discussed in this work. In
particular, the focus of this paper will be on the substrate
solder interconnect which is the solder layer connecting the
isolation substrate to the baseplate (see Fig. 1).
Under cyclic thermal mechanical loading, materials in PEM
expand and contract at different rate and this causes the solder
joint to deform and suffer from damage over time. Fatigue
cracks initially appear at the most stressed locations and then
propagate. In Fig. 2, the scanning acoustic micrograph shows
that after 8000 cycles under a cyclic thermal loading
condition, much of the solder interconnect of a substrate
solder joint has cracked. In this experiment, four dies are
soldered to a ceramic substrate. In the picture the light part in
the middle of each solder joint is the intact area.




Figure 1. The cross section of the substrate mountdown solder
interconnect.




Figure 2. Scanning Acoustic Micrograph images of the
isolation substrate solder joints after 7000 cycles.

The solder interconnect functions as the mechanical
support of the substrate as well as the heat conduction path. In
service conditions, if the cracked area is too large compared to
the total solder-substrate interfacial area, or if the cracked area
is directly under the active die which is the heat source the
thermal resistance may increase significantly and the
temperature of the components on the substrate will be too
high so that the semiconductor device cannot work reliably.
Therefore, in order to design a highly reliable PEM, it is
important to be able to predict the growth of solder joint crack
propagation and the time to failure of the solder joint. In the
copper baseplate
solder
ceramic
copper
copper