Effects of design, structure and material on thermal-mechanical reliability of large array wafer level packages

In this paper, thermo-mechanical reliability of a variety of state-of-art wafer level packaging (WLP) technologies is studied from a structural design point of view. Various WLP technologies, such as Ball on I/O with and without redistribution layer (RDL), Ball on Polymer with and without under bump metallurgy (UBM) process, and encapsulated Copper Post WLPs, are investigated for their structural characteristics and reliability performance. Ball on I/O WLP, in which solder balls are attached directly to the metal pads on silicon wafer, is used as a benchmark for the analysis. 3-D finite element modeling is performed to investigate the effects of WLP structures, UBM process, polymer film material properties (in Ball on Polymer), and encapsulated epoxy material properties (in Copper Post WLP). Fundamentals underlying thermomechanical reliability mechanisms are uncovered through detailed parametric studies. Experimental tests with various parameters were conducted to validate simulation results. Both Ball on Polymer and Copper Post WLPs have shown great reliability improvement in thermal cycling. Encapsulated copper post WLP showed the best performance.