Finite Element Modelling of Crack Detection Tests

1Stephen Ridout, 2Milos Dusek, 1Chris Bailey, 2Chris Hunt

1Centre of Numerical Modelling and Process Analysis
University of Greenwich
Email: s.w.ridout@gre.ac.uk

2NPL Materials Centre
National Physical Laboratory
Queens Road, Teddington
Middlesex, TW11 0LW
UK

Abstract
Four non-destructive tests for determining the length
of fatigue cracks within the solder joints of a 2512 surface
mount resistor are investigated. The sensitivity of the tests
is obtained using Finite Element Analysis with some
experimental validation.
Three of the tests are mechanically based and one is
thermally based. The mechanical tests all operate by
applying different loads to the PCB and monitoring the
strain response at the top of the resistor. The thermal test
operates by applying a heat source underneath the PCB,
and monitoring the temperature response at the top of the
resistor. From the modelling work done, two of these tests
have shown to be sensitive to cracks. Some experimental
results are presented but further work is required to fully
validate the simulation results.
1. Introduction
Accelerated life tests are a popular method in industry
of comparing the reliability of soldered assemblies.
Typically, a component will be exposed to a number of
thermal and/or mechanical cycles and monitored for
electrical continuity. However, this approach will only
provide the time to complete failure, i.e. when a crack has
grown completely through the joint. This can take a very
long time and it would be very useful to determine the
amount of damage which has been done to the joint
before it ultimately fails. One method used is to cut the
sample in half and inspect its cross section with a
microscope [1]. Another method is to use dye penetration.
In this way, the fatigue cracks can be observed and
measured. However, these methods are destructive to the
component and the solder joints.
The tests presented in this paper are intended to be a
quicker and cheaper, non-destructive method to detect
crack length within the two solder joints of a 2512 surface
mount resistor mounted to a FR4 PCB with Sn-3.5Ag
solder.
1.1 The Nature of the Cracks caused by thermal
cycling
A study has been conducted by Shangguan [1] in
which a newly soldered 2512 surface mount resistor was
cross-sectioned and pictures were taken of the same
sample after different numbers of thermal cycles,
allowing the measurement of crack propagation speed and
direction. These cracks were shown to initiate in the
stand-off region and propagate into the fillet region as
shown in figure 1. The propagation in the fillet region was
found to be far slower than in the stand-off region.

Fig. 1 Diagram of a solder joint with arrows indicating
the directions of crack propagation (not to scale)

It is not clear which side of the stand-off region the
crack initiates during thermal cycling. Shangguan [1]
reports that it starts at the end of the termination closest to
the centre of the component. However cross sections from
the NPL (National Physical Laboratory, UK) have shown
that cracks can initiate at the corner of the component. It
seems likely that micro-cracking occurs at both locations
prior to macro crack initiation. This uncertainty is not a
big problem since the speed of crack propagation is very
fast in the stand-off region so in most specimens there
will either be a complete stand-off crack or none at all.
2. Modelling Procedure
The PHYSICA software was used to run the
simulations presented in this paper. In order to determine
the sensitivity of the tests, many FEA simulations have
been run on models of the test specimen containing
different crack lengths. A picture of the FEA mesh used is
shown in figure 3 – slight variations to this mesh were
required when introducing the different sized cracks. It
was decided to assume the crack starts at the end of the
termination and propagates towards the fillet as reported
in [1]. From there three possible propagation directions
were modelled – horizontally, vertically and at 45°. The
stand-off crack and vertical fillet cracks are represented
by a delamination between the solder and the resistor. All