Page 48 - ChipScale_Sep-Oct_2020-digital

of the lead-free package. It can be seen 5. What is the MTTF of the lead-free important. The fan-out lead-free package
from Figure 3c that the Weibull slope fan-out package? MTTF = 2382Г(1 + and the PCB are the same as those [5,6]
(β) and characteristic life (θ) of the lead- 1/1.8) = 2382 x 0.97084 = 2313 cycles. for the thermal cycling test. The drop test
free fan-out package are, respectively, 1.8 setup is according to JEDEC Standard
and 2,382 cycles. Once the parameters The failure locations occur at the JESD22 - B111 as shown in Figure 5.
of the life distribution for the fan-out package corner solder joints (see Figure After more than 20 tries, the correct height
lead-free package have been estimated 4a), and at the corner solder joints right of the drop table is obtained, which yields
by a reliability test, important reliability underneath the chip corners. The failure the drop spectrum with 1500G/ms as
questions such as the following can be mode is the cracking of the solder interface shown in Figure 6.
readily answered: between the RDL3 of the fan-out package The drop condition for the test
and the bulk solder as shown in Figure 4b. described above is 1,000 drops. There are
1. What is the probability that the Drop test example. For mobile 24 samples. The ones without underfill
lead-free fan-out package will fail products, completing a drop test is very failed very early and the failure mode was
by 400 cycles? F(400) = 1 – exp[-
1.8
(400/2382) ] = 0.039, i.e., 3.9% of
the lead-free fan-out package will
fail at 400 cycles.
2. If we use 1000 units of them, how
many do we expect to fail in the
first 400 cycles? We will expect
1000 x 0.039 = 39 units will fail in
the first 400 cycles.
3. What is the probability that the
lead-free fan-out package will
survive 300 cycles? R(300) = exp[-
(300/2382) ] = 0.976, i.e., 97.6% of
1.8
the lead-free fan-out package will
survive at 300 cycles.
4. What is the failure rate of the lead-
free package at 365x24 cycles?
0.8
h(8760) = (1.8/2382)(8760/2382) =
2141808 x 10 per cycle = 2141808
-9
FITs (ppm/k).

Figure 5: Drop test setup.

Figure 4: a) Failure location, and b) failure mode of
the fan-out package with lead-free solder joints on a Figure 6: Drop spectrum and Weibull plot of the drop test results for the fan-out package with lead-free solder
PCB subjected to thermal cycling. joints on a PCB with underfill.

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46 Chip Scale Review September • October • 2020 [ChipScaleReview.com]