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38 SMT007 MAGAZINE I APRIL 2018 Feature by Burton Carpenter NXP SEMICONDUCTORS INC. The automotive industry continues to drive increased solder joint reliability (SJR) for under- the-hood applications. One aspect of SJR, temperature cycle on board (TCoB) assesses thermal fatigue resistance of solder intercon- nection between component and PCB during temperature excursions. In some instances, requirements on number of cycles to first fail- ure have increased 2x over previous product generations. It has been long established that packages using NSMD BGA pads were more resilient than ones with SMD pads to fatigue induced solder joint cracks [1, 2] . However, NSMD pads in our previous investigations on 292MAPBGA and 416PBGA packages failed sooner in AATS test- ing due to an alternate failure mode: substrate Cu trace cracks [3] . Detailed failure analysis revealed that these cracks occurred exclusively on BGA pads in the die shadow. This led to the idea that a mixed design—NSMD pads outside the die shadow, while maintaining SMD pads under the die— could perform better than a pure SMD design. Separately, lower CTE substrate dielec- tric materials were under investigation as a means to reduce package warpage. Below Tg, the mold compound CTE is 9ppm/°C. The standard substrate dielectric CTE is 16ppm/°C, resulting in considerable package warpage at lower temperatures. It was hypothesized that lowering the substrate dielectric material CTE to 11ppm/°C would reduce package warpage which in turn should reduce solder joint strain thereby increasing solder joint lifetime. A six-cell experimental matrix was run to study the impact of these two variables (substrate dielectric material and package pad design type.) These experiments used standard daisy- chain temperature cycle testing methodology. Assemblies were monitored in situ to detect failures as they occurred, and 2-parameter Weibull failure distributions were fit to the data. Various metrics derived from the Weibull fits were regressed against the DOE variables to determine which had significant impact on solder joint lifetime, and to what degree. A look into the effects of substrate material and package pad design on solder joint reliability of 0.8 mm pitch BGA Under the Hood: Solder Joint Reliability Under the Hood: Solder Joint Reliability

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