Investigation of Polyimide Cracking Effects in Wafer-Level Chip Scale Package on Substrate (WLCSPoS) During Component-Level and Board-Level Reliability Testing

Polyimide cracking is a significant reliability challenge when packing a Wafer-Level Chip Packaging (WLCSP) on substrate with normal flip chip process. In this study, 3-layer WLCSPs were fabricated using a 1P2M (One Polyimide, Two Metal) redistribution layer (RDL) structure. WLCSPs were subsequently mounted onto an organic substrate by a flip chip process, followed by under filling and molding processes to provide 6-side protection. This repackaging methodology is widely adopted to enhance board-level reliability. Reducing the stress on RDL is a common topic as defects in the PSV layer can lead to functional failures. With polyimide (PI) serving as a buffer layer, the endurance against internal and external stresses can be enhanced. These stresses typically arise from mismatches in the Coefficients of Thermal Expansion (CTE) between materials, such as solder bumps, under filling glue and mold compounds. This study reveals that under filling glue and solder bumps are the primary sources of stress that inducing PI or PSV cracking. Thermal cycling test were applied to exam how and by which level that CTE and Glass Transition Temperature (Tg) of under filling glue impact on PI and PSV cracking in this study. The main purpose of under filling glue is to increase the adhesion at die surface comparing to molding compound. Three types of underfill were selected based on their material properties to identify the main factor of stress improvement through simulations. According to stress simulations, high Tg and lower CTE of under filling glue are identified to be preferred properties. A reliability Design of Experiment (DOE) was arranged to validate the simulation results. The results demonstrate that under filling glue with higher Tg and lower CTE can significantly reduce polyimide cracking ratios, thereby enhancing overall package reliability performance. This finding provides valuable insights for material selection in advanced electronic packaging applications or products with a critical reliability requirement.