A reliable technique for characterizing the hygroscopic swelling of materials has been developed and used to characterize a number of packaging materials. Using these data, hygroscopic stress modeling were performed. The hygroscopic stress induced through moisture conditioning was found to be significant compared to the thermal stress during solder reflow. Hygroscopic stress in over-molded wire bond PBGA and molded Flip Chip PBGA was found to be 1.3 times to 1.5 times that of thermal stress. Hygroscopic swelling of the underfill in FCPBGA was found to be the main failure driver during autoclave test. Autoclave performance of FCPBGA package assembled with different underfills and chips were analyzed. Excellent correlation was found between autoclave performance and the hygroscopic swelling characteristics of the underfills.

This paper presents a summary of the fundamental theories behind board level drop impact covering the dynamics of drop impact assembly, dynamics of PCB, as well as interconnection stress. This is followed by a comprehensive study of the fracture characteristics of solder interconnections under high-speed impact using a newly developed Micro Impactor which provides both the fracture strength as well as fracture energy of impact. The test matrix consists of 5 solder alloys, 4 pad finishing, 3 thermal histories, and 2 solder mask designs forming a total of 120 combinations. The test has highlighted weakness in NSMD design and caution on SnAgCu solder when used in drop impact application.

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