Thermal fatigue reliability optimisation of flip chip assemblies

The thermal cycling life of flip chip assemblies is often limited by solder joint fatigue. The reliability can be increased by a factor 10 using an underfill material. However, the solder joint reliability is still very dependent on the choice of the underfill material. Using thermo-mechanical simulations, based on non linear finite element simulations, the induced inelastic strains in the solder joints are calculated, and give a value for the expected thermal fatigue life. Combing these FE simulations with an optimisation tool, the optimal underfill material is proposed. The optimisation is based on a parameter sensitivity analysis using DOE techniques. The optimal properties for the underfill are a high elastic modulus and a CTE closely matched to the CTE of the solder joint.