Reliability Study of a New PFAS-Free Passivation Material for WLCSP

The characteristics of passivation materials play a crucial role in the performance of Wafer Level Chip Scale Package (WLCSP). This study evaluates a new PFAS-free polymer (PSV-A) against a conventional PFAS-containing polymer (PSV-B) in WLCSP applications. Accelerated aging tests, including preconditioning and unbiased Highly Accelerated Stress Test (uHAST), revealed distinct failure modes: Ti-Cu interface delamination in PSV-A samples and Ti-PSV-B interface delamination in PSV-B samples, respectively. Finite element thermomechanical simulation identified high stress concentration at the via sidewall, with PSV-A exhibiting peak stress near the via bottom, which correlated well with the failure location. Differences in via sidewall angle result in thinner Ti deposition thereby increasing outgassing effects in PSV-A, which weakens Ti-Cu adhesion. A design of experiments (DOE) approach with thicker Ti and optimized curing temperature effectively mitigated outgassing and improved TiCu adhesion. Further process optimization is recommended to enhance PSV-A reliability in WLCSP.