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condition to an operating condition. In
establishing the acceleration models for
lead-free interconnects, their surrounding
materials (e.g., solder, molding plastic,
ceramic, copper, fiber-reinforced glass
epoxy, and silicon), loadings (e.g.,
stress, strain, temperature, humidity,
current density, and voltage), and failure
mechanisms and modes (e.g., overload,
fatigue, corrosion, and electromigration)
must be considered.
Objective of qualification testing.
Unlike reliability tests, the objective
of qualification tests is “PASS” or
“NOT PASS” and the test time is well-
defined ahead of time. As soon as there
is a failure before the agreed test time,
the test will usually stop and failure
analysis is performed to find out why the
failure occurred. After all the changes,
e.g., redesign, a new qualification test
will start again. The sample size of
qualification tests is usually less than
Figure 2: Fan-out package with lead-free solder joints on a PCB. that of reliability tests. In short, the
objective of qualification tests is not
to aggressive environmental conditions are needed to map (transfer) the failure intended to obtain failures nor life
and realistic sample sizes and test probability, reliability function, failure distribution (or reliability).
times. Acceleration models, therefore, rate, and mean time to failure from a test Thermal cycling test example.
Thermal cycling is the most common test
in solder joint reliability. For example, the
solder joint reliability of a fan-out wafer-
level package (FOWLP) of a 10mm x
10mm chip on a PCB, as shown in Figure
2, subjected to thermal cycling has been
reported [5,6]. The package dimensions
are 13.42mm x 13.42mm (see Figure
2b), and there are three redistribution
layers (RDLs) (see Figure 2c) and 908
solder balls at a diameter of 0.2mm on
0.4mm-pitch. The solder balls are made
of Sn3Ag0.5Cu.
The dimensions of the PCB are 103mm
x 52mm x 0.65mm (see Figure 2d). The
PCB has 6 layers and is made of FR-
4, and the pad finishing is an organic
solderability preservative (OSP). It is
NSMD (non-solder mask defined) and the
solder mask opening diameter is 0.28mm.
There are four packages on a PCB as
shown in Figure 2d.
Figure 2e shows the cross section
of one of the fan-out package PCB
assemblies. It can be seen that the solder
joints are properly made (no bridging
and head-in-pillow) and the daisy
chain on the PCB pads is clearly seen.
The pads on both package and PCB
are interconnected in an alternating
pattern so as to provide a daisy chain
connection (for continuous measurement
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44 Chip Scale Review September • October • 2020 [ChipScaleReview.com]
