Abstract
The influence of underfill material properties on the fatigue life of a Ball Grid Array (BGA) package in the presence of thermal cycling is investigated in this study. The underfill material properties that are varied include Young’s modulus, Poisson’s ratio, and the coefficient of thermal expansion. The range in values are in accordance with typical underfills used in packages today. A finite element model is created using general purpose Ansys code by assuming that there exists an infinite array of solder interconnects, cylindrical in shape, surrounded by underfill material. The finite element geometry generated consists of a unit cell of concentric cylinders, with the inner being solder material and the outer being underfill material. The interconnect material is modeled as eutectic solder that behaves elastically-perfectly plastic. The Mode I cyclic stresses in the solder are determined as a temperature loading is applied. These stresses are then compared to the residual compressive stresses that are induced as a result of underfill shrinkage upon curing. Results suggest that Mode I stresses induced in the interconnects upon thermal cycling are not negligible in comparison to the beneficial compression that they provide upon curing. Even in the presence of this residual compression, for several material combinations a substantial amount of tension is induced in the connections while being cycled.
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