A Parametric Study of Flip Chip Reliability Based on Solder Fatigue Modelling: Part II – Flip Chip on Organic
A solder fatigue model for the 63Sn/Pb solder alloy has been previously introduced which characterizes the creep fatigue phenomena of the solder by combining nonlinear finite element modelling with experimental thermal fatigue lives of various flip chip assemblies. The model correlates the amount of creep strain energy dissipated per thermal cycle with the characteristic Weibull life of the critical flip chip solder joint. The model has been validated for various die sizes, bump geometries, board materials and thermal profiles. Furthermore, the model has accurately predicted fatigue life for flip chip assemblies with and without underfill.
The solder fatigue model has been previously employed to study the effect of design parameters on the reliability of flip chip on ceramic assemblies subjected to thermal cycling (i.e., Part I). In particular, the parametric study showed the effect of die size, die thickness, solder joint geometry and underfill properties on predicted solder fatigue lives. A similar study has now been performed for flip chips assembled to organic printed circuit boards (Part II). This study investigates the parameters listed above, as well as specific solder joint parameters such as conductor thickness, total stand-off and net solder joint height (stand-off minus conductor thickness). A comparison of fatigue lives for ceramic and organic assembled flip chips is also made. Results show that the coefficient of thermal expansion of the underfill is the most significant parameter affecting fatigue life, with no significant difference seen between fatigue lives of underfilled ceramic and organic assembled devices. The effect of solder joint geometry is shown to be solely dependent on the ratio of net solder joint height to chip stand-off.
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