Page 49 - ChipScale_Sep-Oct_2020-digital

The most important factor in reliability tests
is the failure criteria.
During reliability tests, we continuously
perform resistance measurements.
The failure criterion is defined as the
resistance increases to certain (e.g., 1 to
∞) percentages of the original resistance.
Usually daisy chains, which connect the
solder interconnects of the chip/package and
substrate/PCB, allow the measurements.
Because most solder joints under fatigue
loadings will go through crack initiation,
crack propagation, and crack rupture sooner
or later, the daisy chains’ resistance will
increase accordingly, that is, from small to
large, and become infinite when the solder
joint is totally cracked (opened). (Most
people unintentionally pick the last one.)
Because the exact “resistance vs. crack
length” relations of various lead-free solder
joints don’t exist today, people just randomly
pick a number, which is from 1 to an infinite
percentage of the initial resistance. That’s
one of the reasons why there are so many
different Weibull plots in the research
Figure 7: Failure modes of the fan-out lead-free package PCB assembly subjected to a drop test (>500 drops):
a) The fan-out package PCB assembly; b) Cross section of the assembly; c) and d) Close-up of a cracked RDL; a literature, even with the same package, PCB,
cracked EMC; and a cracked underfill (but no crack in the solder joint). solder paste, sample size, test condition, and
test period.
the breaking of the Cu conductor wiring
of the RDL in the fan-out package near the
solder joint. Another 24 packages and their
PCB assemblies (samples) were made and
underfilled. The material properties of the
underfill are: the filler content = 25%, the
maximum filler size = 5μm, the average
filler size = 1-2μm, the curing time and
curing temperature = 8 minutes @ 135ºC,
or 5 minutes @ 150ºC. The Young’s
modulus, Poisson’s ratio, and coefficient of
thermal expansion (CTE) are respectively,
-6
4-5GPa, 0.35, and 50-52x10 /ºC. The
drop condition is also 1,000 drops. The
results (with a Weibull distribution) are
shown in Figure 6. It can be seen that the
Weibull slope is 2.8 and the characteristic
life is 1,271 drops, and all 24 samples
passed 480 drops without failure. The
first failure occurs after 500 drops and the
failure modes are shown in Figure 7. It
can be seen that the RDLs of the fan-out
package are broken. In this case, the EMC
of the fan-out package has cracks and the
underfill between the package and PCB
also has cracks. However, the solder joint
did not fail during the drop test.
Failure criteria
As mentioned earlier, the one and only E-Tec Interconnect AG, Mr. Pablo Rodriguez, Lengnau Switzerland
way to determine the reliability, failure rate, Phone : +41 32 654 15 50, E-mail: p.rodriguez@e-tec.com
characteristic life, mean life, etc., of lead-free
solder joints is by conducting reliability tests.

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