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

Wafer Level Packages (WLPs) are mainly used in handheld electronic devices. During operating, shipping and handling these devices are often subjected to high and low temperature conditions. Therefore, thermo-mechanical reliability performance of WLPs becomes a critical concern.;In this study, thermo-mechanical reliability of a variety of state-of-the-art wafer level packaging (WLP) technologies is studied from a structural design point of view. Various WLP technologies, such as Ball on Input/Output (1/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 thermo-mechanical reliability mechanisms are uncovered through the detailed parametric studies including wafer bumping process, bump structural design, material selection, ball shape optimization, PCB design and PCB material selection.;Results show that in Ball on Polymer WLP structures, polymer film between silicon and solder balls creates a 'cushion' effect to reduce the stresses in solder joints. Such cushion effect can be achieved either by an extremely compliant film or a 'hard' film with a large coefficient of thermal expansion. UBM layer shows slightly beneficial to thermal cycling reliability in Ball on Polymer structure. For large array WLPs, thermal cycling performance can also be improved by excluding corner solder balls from being electrically connected. Finite element analysis demonstrated that making corner balls not electrically connected can improve the WLP thermo-mechanical reliability by 20%. In addition, using low CTE PCB can also improve solder ball reliability significantly. Both Ball on Polymer and Copper Post WLPs have shown great reliability improvement in thermal cycling. Encapsulated copper post WLP shows the best performance.;Finally, the simulation results are compared to experimental tests with various parameters. Finite element modeling predictions are consisted with the observations from failure analysis.