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36 SMT Magazine • May 2014 feATuRe we concluded that the interaction of underfill and solder mask is the main impact to the over- all drop performance. As shown in this analysis, the main influ- ence factor for this DOE was the underfill (with or without), which caused performance chang- es up to 100 times. Surprising was the prevalent failure mode which was independent of under- fill, solder mask or surface finish. Differences in the pad design seemed to create the only im- pact, resulting in either "solder crack close to PCB" in the case of solder mask defined pads, or "via/prepreg crack" in the case of copper de- fined pads (Figure 6). The different DOE parameters have been compared by two-parameter Weibull analysis. For below comparison of the different DOE fac- tors, an improvement factor was calculated by using the same slope parameter (β) for all Weibull curves (Table 4). It should be mentioned that due to the complexity of the interaction of all parameters, these test results are only valid for mECHaNICaL RELIaBILITy continues Figure 3: Doe output for enig surface finish. high phosphorous (9% – 12%) vs middle phosphorous (6% – 9%). Table 4: Impact of Doe factors.