Thermo-mechanical Design of Resilient Contact Systems for Wafer Level Packaging

Wafer Level Packaging (WLP) technologies are cost effective packaging solutions which are used increasingly. Second level reliability, i.e. mainly the thermomechanical reliability during thermal cycling, is a major concern of WLP. To avoid excessive solder straining, solder balls have been replaced by resilient interconnects, which can adopt the main part of the thermal mismatch deformation. One solution combining an increased reliability on module level with advantages in processing and the capability of full wafer level test and burn-in is ELASTec® (ELASTec Elastic-bump on Silicon Technology), particularily developed for memory products. The new failure risks are mainly related to fatigue of the metallic redistribution layer (RDL). Parametric studies using finite element analyses (FEA) were performed to avoid excessive straining of the metal lines. A balance of metal straining and solder straining had to be achieved. Comparisons were made for different soft bump layouts and RDL patterns. Optimal solutions figured out by FEA were also investigated experimentally by thermal cycle tests. However, the thermo-mechanical characteristics like stress-strain behaviour and fatigue resistance of the metallic films are the most important parameters for reliability predictions. In particular, the elastic-plastic properties of thin metallic Cu and Ni films are shown to depend on features like film thickness, grain size and orientation, resulting in a thin film strength exceeding the bulk strength of the same metal by several hundred percent.