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25 can take a very long time and it would be very useful to
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35 the magnitude of cracking within the two Sn–3.5Ag solder
36 joints of a 2512 surface mount resistor (6 mm long, 2.6 mm
37 wide, 0.55 mm high), mounted at the centre of a strip of
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47each of the tests under investigation. Modelling methods
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55the cracks caused by thermal cycling. In a thermal cycling
56test, the specimen will be placed in an oven where the tem-
57perature will be repeatedly cycled between two extremes.
58An example of a commonly used thermal cycle is 55 C
59to +125 C with a period of 45 min, however many varia-
60tions are used – e.g. a shorter period may be chosen in
* Corresponding author. Tel.: +44 20 8331 8761.
E-mail address: s.w.ridout@gre.ac.uk (S. Ridout).
Microelectronics Reliability xx
MR 8422 No. of Pages 9, Model 5+
5 June 2006 Disk Used
ARTICLE IN PRESSNC
Odetermine the amount of damage which has been doneto the joint before it ultimately fails. One method used isto cut the sample in half and inspect its cross-section with
a microscope [1–3]. Another method is to use dye penetra-
tion. 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
are then discussed, before presenting the results of the sim-
ulations. For each of the tests investigated, a prediction is
given of the sensitivity of each test to the different kinds
of cracks which occur.
2. Cracks caused by thermal cycling
In order to model the crack detection tests presented in
this paper, it is first necessary to determine the geometry ofRE
CT1. IntroductionAccelerated life tests are a widely used method in indus-
try to assess the reliability of soldered assemblies. Typi-
cally, a component will be exposed to a number of
environmental loading conditions (thermal, mechanical,
etc.) 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
FR4 PCB (360 mm long, 5 mm wide, 1.3 mm thick). We
envisage these tests being used to periodically test for dam-
age during accelerated testing, this will allow the growth of
a crack though a single specimen to be recorded which is
not possible using destructive methods such a cross-sec-
tioning. The tests are not intended to detect damage on a
PCB with many components.
The paper will first discuss the kinds of cracks which are
caused by thermal cycling, following this is a description ofAssessing the performance of cra
Stephen Ridout a,*, Milos Duse
a School of Computing and Mathematical Sciences, Universi
b NPL Materials Centre, National Physical Laborator
Received 19 May 2005; receive
Abstract
This paper presents both modelling and experimental test dat
focus is on determining the presence and rough magnitude of therm
on a strip of FR4 PCB. The tests all operate by applying mechanic
the resistor. The modelling results show that of the four tests invest
magnitude. Hence these tests show promise in being able to detect c
these results although more validation is required.
 2006 Published by Elsevier Ltd.0026-2714/$ - see front matter  2006 Published by Elsevier Ltd.
doi:10.1016/j.microrel.2006.05.001D
PR
O
O
Ff Greenwich, Park Row, Greenwich, London SE10 9LS, UK
ueens Road, Teddington, Middlesex TW11 0LW, UK
revised form 25 October 2005
characterise the performance of four non-destructive tests. The
fatigue cracks within the solder joints for a surface mount resistor
oads to the PCB and monitoring the strain response at the top of
ed, three are sensitive to the presence of a crack in the joint and its
king caused by accelerated testing. The experimental data supportsdetection tests for solder joints
b, Chris Bailey a, Chris Hunt b
www.elsevier.com/locate/microrel
x (2006) xxx–xxx

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108in the joints since the presence of a crack will affect the way
109the load (and therefore strain) is distributed through the
110specimen.
2 S. Ridout et al. / Microelectronics Reliability xxx (2006) xxx–xxx
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EC
order to damage the joints more rapidly, or a milder tem-
perature range may be chosen to better represent the
intended use conditions.
A number of experiments have been reported in which
thermal cycled joints have been cross-sectioned [1–3], the
results show a number of different directions of crack prop-
agation, three of which are illustrated in Fig. 1. It is possi-
ble that there may be a correlation between the thermal
cycle used and the kind of crack generated. For instance,
recent work at the NPL (National Physical Laboratory,
UK) has indicated that with very fast ramp rates (thermal
shocking), a delamination of the solder from the compo-
nent’s vertical surface in the fillet region is observed. And
with slower ramp rates, cracks tend to form within the bulk
of the solder. However more evidence is required to con-
firm this phenomenon. For the purposes of this work, it
will be assumed that all three of the crack directions shown
in Fig. 1 are possible.
3. Test methods
As discussed above, the traditional manner of testing
solder joints in components during an accelerated test is
to obtain a cross-section for examination. A limitation of
this approach is that the crack is seen only as a 1D line,
not a 2D area. If the crack propagation direction was in
the plane of the SEM images then this would not be a prob-
Fig. 1. Crack propagation directions.UN
COlem, however dye-penetration experiments at the NPL haveshown this not to be the case. FEA simulations performedat Greenwich using a simple damage model support these
results and show the prominent direction of crack propaga-
Fig. 2. Crack propagation in stand-off region (LED
PR
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F
tion to be in the out-of-plane direction with respect to the
SEM images – as shown in Figs. 2 and 3. This implies that
the crack propagation time through the stand-off region as
measured from a cross-section [1] is significantly underesti-
mated. In most cross-sections the cracks are not symmetri-
cal, for example see Fig. 4.
The following tests have been designed so that cross-sec-
tioning is not required. All the tests work in a similar way,
by applying a mechanical loading to the specimen and
monitoring the strain response at the top of the resistor.
The strain in the direction of the longest component side
is measured with a strain gauge which is glued to the resis-
tor as seen in Fig. 5.
3.1. Pull test
The test specimen is subjected to a 100 N tensile load
using the apparatus shown in Fig. 5 and the strain on the
resistors top surface is monitored with a strain gauge. This
measured strain will depend on the magnitude of the cracks
Fig. 3. Result showing damage build-up in a 1206 resistor joint after
thermal cycling.1113.2. 3-point bend test
112In this test, displacements are imposed on the test spec-
113imen at three points causing it to bend as illustrated in
HS – actual shape, RHS – idealised shape).