NXP Semiconductors, Inc.
Austin, TX, USA
This paper describes experimental work to improve the solder-joint reliability (SJR) of a 0.8mm pitch, 25mm body BGA (ball grid array) package used in automotive underthe-hood applications. Testing was TCoB (temperature cycle on board) in AATS (air-to-air thermal shock) between -40 Celsius and +125 Celsius. The target requirement was to pass 3000 cycles before the first failure.
A six-cell experimental matrix was run to study the impact of two variables: substrate dielectric material and package pad design type. Two-parameter Weibull curves were fit to the data from each cell, and regression analyses performed on the characteristic lives derived from those fits. Results showed that lowering the substrate dielectric CTE (coefficient of thermal expansion) improved characteristic life 22%. Also, a package design hybrid of SMD (soldermask defined) and NSMD (non-soldermask defined) pads improved characteristic life 30% compared to a pure SMD design.
The improvement observed by lowering substrate CTE has been attributed to the reduction in package warpage by more closely matching the substrate and mold compound CTEs. The hybrid design improvements resulted by strategically choosing the NSMD sites, altering the distribution of solderjoint cracking within the BGA array.
Either improvement was able to delay first failure beyond 4000 cycles, meeting the target requirements. Together, these changes drove the first failure to 5000 cycles.
The conclusions are:
1) A hybrid design, mixing SMD and NSMD pads on the same package is feasible for manufacturing, and was demonstrated to improve the solder-joint characteristic life of a 512TEPBGA package by 30%.
2) Lowering the package substrate dielectric CTE from 16ppm/Celsius to 11ppm/Celsius was demonstrated to improve the solder-joint characteristic life of a 512TEPBGA package by 22%
3) Greater than 3000 cycles to first failures for the 512TEPBGA package solder-joint lifetime in -40 Celsius to +125 Celsius TCoB testing.
Initially Published in the SMTA Proceedings