Page 47 - ChipScale_Sep-Oct_2020-digital

during testing) after they are assembled.
There is no underfill between the
package and the PCB.
Fif teen boards (each with four
packages) are used for the temperature
cycling tests. The sample size is 60
fan-out packages. Thermal cycling
is performed in a Votsch 7027-15
environmental chamber as shown in
Figure 3a. The temperature input to
the chamber (measured in the air of the
chamber) goes from room temperature to
85ºC and stays there for 15 minutes; the
temperature then ramps down to -40ºC
and stays there for 15 minutes and then
ramps up again to 85ºC and stays there
for 15 minutes, and so forth. The ramp
up and ramp down times are 15 minutes
each. The cycle time is one hour, i.e., one
hour per cycle. The acquisition system is
an Agilent 30970A data logger as shown
in Figure 3b.
The Weibull cumulative distribution Figure 3: a) Thermal cycling chamber and samples; b) Data acquisition system; c) Weibull plot of the thermal
function F(x), reliability function cycling results for the fan-out package with lead-free solder joints on a PCB without underfill.
R(x), failure rate h(x), and mean- early failure at 58 cycles). The failure assembly increases by 50%. The cycle at
time-to-failure (MTTF) are given by, criterion is when the resistance of the which the first solder joint of the package
respectively [5]: daisy-chain of the PCB fan-out package failed is considered as the cycle-to-failure

and

where x is the random variable (e.g., life
or cycles), γ is the expected minimum
value of x (it is also referred to as the
location parameter), θ is the characteristic
value (at 63.2% failures) or the scale
parameter, which could be used to
represent the quality of a product, and β is
the Weibull slope, or the shape parameter,
which is a measure of the uniformity of a
product (the larger the slope, the more
uniform the product). It is frequently
reasonable to assume that the expected
minimum value of life (γ) of the
population, i.e., the lower bound of life of
the population, is equal to zero, then we
have the two-parameter (θ, β) Weibull
distribution. Г is the gamma function.
The thermal cycling test of the lead-
free fan-out package stops at 1,100 cycles
and there are 14 failures (including one

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