Energy measurement: The main metric for high strain rate bond testing of solder balls
testing is that the incidence of brittle
fracture failures increases as the test speed
is raised. All tests are accompanied by
force displacement curves, the shapes of
which are characteristic for different failure
modes (Figure 1). This example shows two
curves, one for a brittle failure (Test 1) and
the other for a ductile failure (Test 2). Note
their distinctive shapes and different areas
under the graph representing distinct bond
energies.
energy as a main metric
There is good reason to select energy as the
main metric of solder joint reliability; after
all, the solder joints must survive the stress-
es and strains which the devices experience
through manufacture and use, absorbing
the energy of high strain rate impacts and
bending moments. Therefore, maximiz-
ing the energy that the bond interface can
Figure 2. Ductile, quasi-ductile, quasi-brittle and brittle failure modes versus transition speed.
absorb reduces the possibility of bond
failures. Thus, high strain rate bond testing
using energy as the main metric is a valu-
Force (g) Total energy Pre-peak Post-peak
able tool in rapid screening of new bond
(mJ) (mJ) (mJ)
materials such as different solder alloys and
Ductile 1,091 1.82 0.53 1.29 surface finishes, as well as the refinement
and monitoring of production processes in
Brittle 995 0.71 0.30 0.41
BGA and flip-chip manufacture.
Quasi-brittle 1,054 1.17 0.41 0.77
The use of energy
Quasi-ductile 1,045 1.59 0.47 1.12
Determining transition speed allows selec-
All failures 1,063 1.57 0.47 1.10
tion of the optimum test speed at which
energy measurements can be used in the
Table 1. Peak force and total, pre-peak and post-peak energy values for different failure modes.
ways already described including product
design and production monitoring. Find-
Total energy (mJ) Pre-peak (mJ) Post-peak (mJ)
ing the optimum transition speed for pull
or shear testing consists of carrying out a
Baseline mean 1.57 0.47 1.10
series of tests at different speeds and de-
Batch 1 1.36 0.39 0.97
termining the proportions of each failure
mode within each set. The test speed at
Batch 2 1.56 0.56 1.00
which roughly equal proportions of ductile
Batch 3 1.59 0.46 1.13
and brittle failure modes occur is termed
the transition or optimum speed. This
Batch 4 1.50 0.46 1.04
method is used to generate a histogram
Batch 5 1.49 0.41 1.08
such as that shown below for shear testing
(Figure 2).
All batches 1.50 0.46 1.05
In this example the transition speed
Table 2. Batch testing readings compared to baseline mean.
used is between 500mm/sec and 1000mm/
sec and further testing at a range of other
speeds would be needed to determine it
provides a credible alternative to the other the main concerns on the introduction of
more accurately. The next step is to carry
methods cited above. Traditional bond test- Pb-free solder was the increased incidence
out multiple tests at the transition speed in
ing at low test speeds of less than 5mm/s of brittle fracture failures—rupture of the
order to obtain force and energy values for
for pull, or less than 1mm/s for shear uses bond at the bond to pad interface induced
each failure mode. Mean values from such
accurate force measurement as the princi- by impact or other high strain rate events.
tests measured in grams (g) and millijoules
pal quality metric of bond strength. Bond testing at the slow speeds mentioned
(mJ) are summarized in Table 1.
Strictly speaking, this type of bond above is inadequate to detect brittle frac-
Once again, it should be noted that
testing is more a measure of solder strength ture failures. Even using high strain rate
force measurement alone provides poor
than the bond-to-pad interface, the true testing and peak force as the metric, differ-
discrimination between the various failure
bond. The principle of high strain rate ent failure modes cannot be distinguished.
modes. In sharp contrast there is excel-
bond testing is that the load is transmitted But different failure modes such as brittle
lent correlation between failure mode and
to the bond interface, thereby providing or solder (ductile) can readily be distin-
either total or post-peak energy values.
a truer measure of bond strength itself
1
. guished if the failure energy is measured.
In previous studies it has been shown
Many readers will be aware that one of A feature of high strain rate bond
that high strain rate bond testing strongly
16 – Global SMT & Packaging – June 2009 www.globalsmt.net
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