Effects of Underfill Stiffness on Thermal Deformation of Solder Joint

Abstract

The recent trend in circuit assemblies is to bond larger IC packages to PCB (Printed-circuit Board) by BGA (Ball Grid Array) solder joint and underfills. This would induce more intricate CTE (Coefficient of Thermal Expansion) mismatches among the PCB, the IC package, the solder joint and the underfill. The underfill, which fills the gap between solder balls, is usually used to increase mechanical strength, and to improve the reliability of the solder joint. In this research, 1) The effects of the stiffness of underfills on stresses and strains on solder joint in which CTE mismatch and thermal deformation are involved, are discussed based on finite element analysis. 2) The effects of size of packages on stresses and strains on solder joint are presented. By the procedures, about 50% lower strains of solder joint are obtained, as compared to those of solder joints in which the underfill has 2.5 times larger elastic modulus, or the package has about 28% smaller size. 3) An optimum design method is applied to minimize the thermal deformation of solder joint taking properties (Young's modulus, Poisson's ratio and CTE) of underfill as design variables. The optimization method developed here can be used to optimize the underfill properties, and to improve effectively the reliability of solder joint. The method is useful to evaluate underfill materials for circuit assemblies.