Rapid Interface Reliability Testing of Flip Chip Encapsulants

A rapid interface reliability test for underfill materials in real flip chip on board assemblies consisting of autoclave aging at 121degC and 100% relative humidity, followed by vibrational stressing at room temperature has been developed. The moisture concentrations in the flip chip assembly have been calculated by finite element simulations to determine an adequate autoclave aging period. An extended three parameter diffusion model based on Fick´s law has been used. It considers an additional first order binding of the moisture and enables the modeling of a linear moisture absorption beyond the fickian saturation level, as observed for epoxies and polyimides. The diffusion properties and the adhesion of the underfiller are tested by the subsequent vibrational stressing. The results of linear-elastic finite element simulations show, that the vibrational test induces the same shear stresses and strains in the underfill material as those induced by the mismatch of the coefficient of thermal expansion (CTE) between board and chip in temperature cycling. However the CTE mismatch between underfill and its adjacent materials causes additional tensile stress and strain during temperature cycling, which are not induced by the vibrational test. Thus the rapid interface test is independent of the thermo-mechanical properties of the underfill material. It enables a fast selection of underfill materials with good adhesion in a humid environment, which suit applications in harsh environments. By using a defined test setup, in which only the underfill material is varied, the assessment period is reduced to less than a week. The advantage compared to standard material testing procedures is the use of a real flip chip assembly. Thus the materials are tested in the relevant dimensions and combinations processed in a manufacturing assembly flow.