16 Processing and Reliability of Solder Interconnections …
507
for each of the three levels of solder joints. The corresponding 2P Weibull parameters
are listed below:
Top solder joints: η = 2600 ± 200 cycles; β = 8.7 ± 4.0
Middle solder joints: η = 2500 ± 500 cycles; β = 6.6 ± 3.4
Bottom solder joints: η = 2600 ± 200 cycles; β = 7.6 ± 3.5
The mixed Sn-Pb/SAC305 (bottom) solder joints experienced a statistically higher
characteristic lifetime, albeit, just barely so. The β value was also higher for the mixed
alloy interconnections, but still within the statistical error versus the 100% SAC305
solder joints. The 2P Weibull parameters were not statistically different between the
behaviors shown in Figs. 16.13 and 16.14 with respect to the top and middle, secondlevel interconnections. This observation was anticipated—that is, TMF would occur
independently at the three levels of PoPoP interconnections (in the absence of an
underfill).
The effect of non-homogeneous, mixed Sn–Pb/SAC305 solder joints could
not be evaluated, quantitatively, because the single-pass, assembly process caused
homogeneous, second-level interconnections in all but one test vehicle. Therefore,
that one test vehicle had metallographic cross sections performed on individual
packages extracted from the test vehicle at these “stopping points” during thermal
cycling: 0 (as-fabricated), 600, 1000, 1500, 2500, 5000, and 7000 cycles.
The results of the microstructure analyses are shown by the optical micrographs
in Fig. 16.15. Figure 16.15a is an image the bottom solder joint microstructure at 0
cycles (as-fabricated). The micrograph in Fig. 16.15b confirmed the stability of the
microstructure through 600 cycles, which is the typical, accelerated aging limit used
to establish the long-term reliability of military and space electronics. In addition,
the hour-glass geometry of the joint did not accelerate TMF deformation or crack
initiation. After 2500 cycles (Fig. 16.15c), the mixed Sn–Pb/SAC305 microstructure
grew into the SAC305 solder ball. In fact, the latter microstructure was coarsening,
the endpoint of which, is show in Fig. 16.16d after 5000 cycles. Therefore, neither the
microstructural changes nor the retention of the hour-glass geometry caused the nonhomogeneous, mixed Sn-Pb/SAC305 solder joints to be prone to accelerated TMF
degradation. These interconnections are suitable for high-reliability applications.
Temperature Cycling—Effect of Conformal Coating
The TMF of PoP and PoPoP solder joints was also evaluated when conformal
coatings or underfill were added to the assembly [38]. Conformal coating was
predicted to have only a negligible effect because metallographic cross section images
indicated that the coating it did not bridge the gaps between either the individual
packages or that between the bottom package and the PCB of either PoP or PoPoP
configuration. The same images determined that conformal coating flowed on the
substrate (interposer) surfaces and collected around the solder balls. The Weibull
507
for each of the three levels of solder joints. The corresponding 2P Weibull parameters
are listed below:
Top solder joints: η = 2600 ± 200 cycles; β = 8.7 ± 4.0
Middle solder joints: η = 2500 ± 500 cycles; β = 6.6 ± 3.4
Bottom solder joints: η = 2600 ± 200 cycles; β = 7.6 ± 3.5
The mixed Sn-Pb/SAC305 (bottom) solder joints experienced a statistically higher
characteristic lifetime, albeit, just barely so. The β value was also higher for the mixed
alloy interconnections, but still within the statistical error versus the 100% SAC305
solder joints. The 2P Weibull parameters were not statistically different between the
behaviors shown in Figs. 16.13 and 16.14 with respect to the top and middle, secondlevel interconnections. This observation was anticipated—that is, TMF would occur
independently at the three levels of PoPoP interconnections (in the absence of an
underfill).
The effect of non-homogeneous, mixed Sn–Pb/SAC305 solder joints could
not be evaluated, quantitatively, because the single-pass, assembly process caused
homogeneous, second-level interconnections in all but one test vehicle. Therefore,
that one test vehicle had metallographic cross sections performed on individual
packages extracted from the test vehicle at these “stopping points” during thermal
cycling: 0 (as-fabricated), 600, 1000, 1500, 2500, 5000, and 7000 cycles.
The results of the microstructure analyses are shown by the optical micrographs
in Fig. 16.15. Figure 16.15a is an image the bottom solder joint microstructure at 0
cycles (as-fabricated). The micrograph in Fig. 16.15b confirmed the stability of the
microstructure through 600 cycles, which is the typical, accelerated aging limit used
to establish the long-term reliability of military and space electronics. In addition,
the hour-glass geometry of the joint did not accelerate TMF deformation or crack
initiation. After 2500 cycles (Fig. 16.15c), the mixed Sn–Pb/SAC305 microstructure
grew into the SAC305 solder ball. In fact, the latter microstructure was coarsening,
the endpoint of which, is show in Fig. 16.16d after 5000 cycles. Therefore, neither the
microstructural changes nor the retention of the hour-glass geometry caused the nonhomogeneous, mixed Sn-Pb/SAC305 solder joints to be prone to accelerated TMF
degradation. These interconnections are suitable for high-reliability applications.
Temperature Cycling—Effect of Conformal Coating
The TMF of PoP and PoPoP solder joints was also evaluated when conformal
coatings or underfill were added to the assembly [38]. Conformal coating was
predicted to have only a negligible effect because metallographic cross section images
indicated that the coating it did not bridge the gaps between either the individual
packages or that between the bottom package and the PCB of either PoP or PoPoP
configuration. The same images determined that conformal coating flowed on the
substrate (interposer) surfaces and collected around the solder balls. The Weibull