A MODELING STUDY OF THE EFFECT OF UNDERFILL MATERIALS ON SOLDER JOINT THERMAL FATIGUE OF BALL GRID ARRAY PACKAGE

Flip Chip Ball Grid Array packages (FCBGA) have been widely used in microelectronic industry in integrated circuit (IC) packages. Due to the intrinsic mismatch of the coefficient of thermal expansion (CTE) between silicon chip and Printed Circuit Board (PCB) material, solder joint fatigue failure due to thermal cycling becomes the most important concern for this technology. Underfill materials have been widely used as a solution to improving solder joint fatigue life. It is of importance to understand the effect of underfill material properties on the solder joint fatigue life. In this study, finite element method (FEM) was employed to study the effect of underfill materials on solder joint low cycle fatigue life in thermal cycling. ANSYS code was used to calculate the inelastic energy density generated in temperature cycling. The viscoplastic model was used for the solder to consider the inelastic and time dependent behavior under thermal cycling. By using the FEM model, the underfill material properties, the Young's modulus and CTE were examined to study their effects on the solder joint fatigue life. It was found that the improvement of solder fatigue life could be achieved only when the CTE was low. This improvement could be strengthened by large Young's modulus to increase the solder strength. In contrast, a large CTE underfill material could deepen the solder joint fatigue damage. This worsening effect became more significant as the Young's modulus became larger. This study could serve as a foundation for understanding the mechanism of solder joint fatigue in the presence of underfill materials and provide guidance to choose appropriate underfill materials to improve BGA solder joint thermal fatigue in temperature cycling.