Prediction of Yield for Flip-Chip Solder Assemblies

A model has been developed to predict the yield of solder flip-chip assemblies. Yield was found in relation to the mean and the standard deviation of the solder volume distribution, the assembly warpage, and the chip size. In this study, solder volume was represented by the diameters of the solder balls. The model identified failed joints using the Surface Evolver and a regression model. The results showed that the volume variation corresponding to a diameter variance of 25μm for solder balls produced very low yields. Warpage greatly decreased the maximum variance allowed for 100% yields. For example, the diameter variance is +/11μm for 150μm solder balls; it is reduced to 7.6μm with a warpage of 6.6μm across the diagonal of a 1x1cm chip. In addition to volume variation and warpage, the pad radius also had an effect on the assembly yield. However, this effect is non-linear. In some cases, larger solder balls could result in lower yields than smaller solder ball sizes with the same pad radius. Finally, larger chip sizes would not decrease yields significantly if warpage level were below8μm. This model is a good guide to use for designing flip-chip assemblies, although it is not complete. Further development will include reliability modeling.