Page 23 - ChipScale_Mar-Apr_2020-digital

mismatch between the FOWLP and PCB.
The primary crack and failure mode can
be seen to be concentrated on the package/
solder ball interface. Some cracks were
observed at the solder ball/PCB interface.
The failure mode stays the same for the
SACA no UBM, and SACB no UBM
cases of BLR-TC. However, Table 4 shows
the BLR-TC characteristic life increases
by ~341% when changing from SACA
to SACB solder alloy. This improvement
in BLR-TC life is because the SACB is
stiffer than SACA, and also because of the
incorporation of an optimized RDL stack
Figure 8: Bump shear strength variation with 150°C high-temperature storage hours for SACB and SACA solder alloys. up for SACB solder alloy that prevents
RDL cracking, which had been reported in
high- temperature storage at 150°C. As ball. The failure mode is again in the bulk a previous study with stiffer solder alloy [6].
expected, both solder alloys show some solder, however, it is at the solder ball/ RDL stress modeling showed that with
degradation in bump shear strength after PCB pad interface. The presence of UBM the appropriate package RDL stack up,
150°C aging for 1000h and 2000h that improved the strength of the solder ball/ RDL stress decreased when using the stiffer
is attributed to intermetallic formation. UBM interface joint so the primary failure SACB solder alloy in comparison to the
However, even after 2000h, SACB solder mode, in turn, shifts to the solder ball/ SACA. The SACB solder alloy legs passed
alloy shows a 50% greater bump shear PCB pad interface. However, some crack all reliability tests including daisy chain-
strength in comparison to the SACA alloy. initiation can also be seen at the UBM/ based BLR-TC and drop tests, as well as
Figure 9a shows a cross section of solder ball interface. Figure 10a shows a functional product-based AL-TC, biased
SACA no UBM leg solder alloy after 475 FA cross section overview of the SACB HAST (BHAST), and high-temperature
cycles of BLR-TC. The first solder ball to no UBM leg after 3,164 cycles of BLR-TC. storage life (HTSL) tests.
fail is the package corner ball. Bulk solder As expected, corner ball failures can be
ball cracking on the package side was the seen with bulk solder ball cracking. Figure Summary
failure mode. Some crack initiation was 10b is a magnified cross section view of FOWLP using traditional fan-out
also observed at the solder ball/PCB pad the outermost corner ball that shows quite package process flows were used to
interface. Figure 9b is a cross section from significant solder ball distortion because develop optimized packages that passed,
the SACA UBM leg after 525 cycles. The of the relatively large number (3,164) of and significantly exceeded product
first solder ball to fail is the package corner thermal cycles and the associated CTE reliability testing requirements of BLR-TC,
drop test, BHAST and AL-TC tests. The
stiffer solder alloy SACB in combination
with an optimized package RDL stack up
led to significant improvement in BLR-TC
characteristic life and drop test reliability.
After 3,164 BLR-TC cycles, the failure
mode for SACB solder alloy was still bulk
solder ball cracking, which is indicative of
excellent chip package integration across
all silicon-package-board interconnect
levels. Drop test reliability was also
significantly improved with SACB solder
alloy with only one failure observed at
1,000 drops.
References
1. M. Brunnbauer, “Embedded wafer-
level ball grid array (eWLB),” Proc. of
8th Elec. Packaging Tech. Conf., 10-
12, Dec. 2006, Singapore, pp. 1-5.
2. G. Sharma, “Design and development
Figure 10: a) (top) Package cross section showing a of multi-die embedded micro wafer-
corner ball fail after BLR-TC 3164 cycles for SACB — level packages with laterally-placed
Figure 9: a) (top) Solder ball cross section after no UBM DOE leg; and b) (bottom) Magnified corner and vertically-stacked thin dies,”
BLR-TC 475 cycles for SACA — no UBM leg; and b) ball cross section showing bulk corner solder ball IEEE Trans. on Adv. Packaging, Vol. 1,
(bottom) Solder ball cross section after BLR-TC 525 cracking primarily at the package side, as well as Issue 1, 2011, pp. 52-59.
cycles for SACA UBM leg. some crack initiation at the PCB side.

21
Chip Scale Review March • April • 2020 [ChipScaleReview.com] 21