Non-linear analyses of strain in flip chip packages improved by the measurement using the digital image correlation method

Abstract

Numerical methods like the finite element (FE) method are often used to evaluate the reliability of electronic packages. However, the accuracy of non-linear numerical analyses should be confirmed by experimental measurements. In this study, we evaluated the strain distribution in flip chip (FC) packages with multi-layered printed circuit boards (PCBs) by combining the digital image correlation method (DICM) and the non-linear FE method, considering the viscoelasticity of resins and the elastoplasticity and creep of solder alloy. Four types of FC package consisting of two types of buildup (BU) resin and two types of underfill (UF) resin were evaluated. The distributions of strain on the cutting sections of FC packages were measured using the DICM with microphotographs obtained by a confocal laser scanning microscope (CLSM). The strain measurements showed that the UF resin with the low coefficient of thermal expansion (CTE) reduced thermal strain around a solder bump, and the BU resin with the low CTE reduced the strain concentration along the interface between a Si chip and a solder bump. We performed the non-linear FE analyses while taking into account the viscoelastic Poisson’s ratio of the UF resin and the constant instantaneous Poisson’s ratio. The result of the FE analyses with the constant instantaneous Poisson’s ratio did not correspond with the strain measurements using the DICM. The normal strain in a solder bump was less than that obtained by the measurement, and the direction of a shear strain band in a solder bump was different from that measured using the DICM. On the other hand, the FE analyses considering the viscoelastic Poisson’s ratio showed good agreement with the strain measurements using the DICM. The strain measurement using the DICM improved the accuracy of the non-linear FE analysis of microelectronic packages effectively.