Reliability of high I/O count wafer level packages

Thermomechanical fatigue and drop performance of wafer level packages (WLP) are two main reliability requirements that prevent the WLPs from scaling up. A comprehensive investigation into the methods that would enable such transition was undertaken. The current qualification involved a package with 5.2 mm times 5.2 mm die and 144 l/Os. The packages were supplied by four vendors who are the leading suppliers of WLPs. One package design was based on the Cu post technology and the other three utilized double polymer layer for enhanced compliance. In addition to standard WLP designs that have been used on smaller packages, a design of experiments (DOE) also included novel designs which aimed at enhancing the package reliability. The main package modifications included a redesigned under bump metallurgy (UBM) structure with changes made to the polymer opening and the shape of the UBM pad. The packages were manufactured with lead-free SAC105 and LF35 solder balls. The second level reliability was assessed by thermal cycling, drop testing and cyclic bend testing, all according to governing JEDEC specifications. The results obtained from the reliability testing allowed for direct comparison between different vendors and also between different package designs from the same vendors. The failures were examined using a dye stain penetrant method and cross-sectional SEM/EDX. The failure modes and Weibull failure statistics are presented with the emphasis on the qualitative understanding of the results. Recommendations as to the preferred package type, solder ball alloy and package design are also included.