Global-local finite element optimization study to minimize BGA damage under thermal cycling

One of the most critical failure modes in a BGA package is the solder interconnect failure (2^nd level). Conventionally, the solder damage is due to the mismatch of coefficient of thermal expansion (CTE) between the various package components and the PCB. It is absolutely critical to identify and minimize the BGA damage under thermal cycling to improve the package reliability. In the past, the critical design parameters have been investigated and ANSYS APDL code has been leveraged with a built in optimization tool for design optimization, thereby improving the solder joint fatigue life [1]. In this work, a multi-level FE model (sub-modeling approach) is implemented in ANSYS Workbench. Multi-variable-design-optimization (MVDO) is performed to minimize BGA fatigue damage. Damage in the BGA is quantified through strain energy density (SED) in the solder joints. The critical material/design parameters that significantly affect the solder joint strain energy density are identified and subsequently optimized to minimize the damage. For enhanced accuracy of the results and to have efficient computational time, sub-modeling technique is leveraged. The analysis is conducted in two steps- 1) “Global model” (coarse) is solved with the full BGA array and the critical joint is identified, 2) the critical joint (detailed far corner joint with fine mesh) is analyzed using sub-modeling.