Parametric finite element analysis of solder joint reliability of flip chip on board

Numerous studies have indicated that by encapsulating the solder joint with underfill material, the reliability of flip chip on board (FCOB) assemblies can be effectively enhanced. Typical manufacturing process for FCOB assembly with underfill, however, involves long throughput time and additional equipment sets which are undesirable for high volume manufacturing environments. Hence, design/process simplification if not total elimination of underfill from the conventional FCOB assemblies that can directly result in productivity gain should be considered. A comprehensive parametric finite element analysis has been conducted to assess the feasibility of FCOB structures with partial underfill (i.e. only the peripheral joints are encapsulated with underfill material.) The effects of some critical design parameters such as die size, joint height, joint diameter, joint pitch, printed circuit board (PCB) thickness and material properties of underfill on the solder joint reliability of FCOB structure were investigated in this study. Two-dimensional nonlinear plane strain finite element models of FCOB package are employed. Moire and IR Fizeau interferometry technique are used to measure the thermal deformation of the FCOB for model validation. Elasto-plastic deformation behaviors of solder were simulated under thermal cyclic loading from -55 degrees C to 125 degrees C. Maximum effective elastic and plastic strains of the solder joint were calculated and used as the indicator for determining the solder joint reliability of the structures.