FCOB (Flip Chip on Board) reliability study for mobile applications

This paper presents Flip Chip on Board (FCOB) reliability evaluation for solder bump and ACF/ACP interconnections. Flip chip design rule, IC assembly location, interconnection methods, underfill or adhesive material selection parameters are studied relating to SMT (Surface Mount Technology) process. Four test dies were designed and assembled on cell phone dimensional organic substrates using three flip chip interconnection methods - solder bumps with underfill, Au bumps with ACF, and Au bumps with ACP adhesives-. Each 3 types of snap cure underfills, ACF, and ACP adhesives were compared in regard to board level reliability. Mechanical drop test as well as temperature cycle, high temperature storage, high temperature humidity, pressure cooker tests were performed to assess design and material parameters. CAE (Computer Aided Engineering) was employed to analyze the global and local stress effect on the FCOB reliability during temperature cycling and drop tests. For solder bump FCOB, a proper underfill selection was the key factor to determine solder joint lifetime. In case of ACF and ACP joints, good adhesion strength between interfaces was the most pre-required material property.