The effects of underfill and its material models on thermomechanical behaviors of flip chip package

In this paper, underfill effects on thermomechanical behavior of two types (B and D) of flip chip packages with different bump sizes and stand-off heights were studied under thermal cycling by experiments and finite element analysis. Continuous electrical detection, intermittent C-SAM inspection and final metallographic analysis were used in the experiments. The material inelasticity, i.e. viscoelasticity of underfill U8437-3 and viscoplasticity of 60Sn40Pb solder, were considered in the simulations. Results show that use of underfill encapsulant greatly increases (~20 times) SnPb solder joint thermal fatigue lifetime, weakens stand-off height effects on reliability, and changes the package deformation mode. It was found that thermal fatigue cracks occur in the maximum plastic strain range region, and a Coffin-Manson type equation could then be used for packages with or without underfill. It was observed that solder joint crack initiation occurred before delamination when using underfill with high adhesion (75 MPa), and underfill delamination may be not a dominant failure mode in this study. Moreover, the effects of underfill material models, i.e. constant elasticity (EC), temperature dependent elasticity (ET) and viscoelasticity (VE), on the thermomechanical behaviors of flip chip packages were also studied in the simulation. The VE model gives comparatively large plastic strain range, big displacements in the shear direction, and sequentially low solder joint lifetime. The ET model gives close results to the VE model and could be used instead of VE in simulations for the purpose of simplicity