An RDL UBM Structural Design for Solving Ultralow-K Delamination Problem of Cu Pillar Bump Flip Chip BGA Packaging

Copper (Cu) pillar bumps tend to induce high thermal-mechanical stress during environmental tests and fabrication processes due to the high hardness of Cu, especially when applied with an ultralow-K (ULK) chip. A previous experiment showed that interfacial delamination was often observed in the ULK layers of conventional Cu pillar bump-type flip chip ball grid array (FCBGA) packages under thermal cycling, where under bump metallurgy (UBM) layers directly sit on the metal pads of silicon chips (herein termed ''direct UBM structure''). In this study, a UBM pad relocation scheme through redistribution layer (RDL) technology (herein termed ''RDL UBM structure'') is proposed to relieve the stress or ULK delamination issue. The proposed technique is tested on Cu pillar bump-type FCBGA packages subjected to thermal loading, the effectiveness of which is demonstrated through finite element stress simulation and experimental reliability tests. Simulation results reveal that the RDL UBM structure can greatly reduce the maximum stress in the ULK layers by as much as about 10% to 44%. Besides, it turns out that the Cu pillar bump-type FCBGA packages with the RDL UBM structure show good interconnect reliability performance in terms of thermal cycling, highly accelerated stress, and high-temperature storage.