The failure mechanism of WLCSP interconnect under board level bending stress

The polymer ball interconnect has been evaluated in ball grid array type IC package for many years either in small form factor WLCSP or large form factor CBGA devices. The innovative material design is aimed at to enhance the board level reliability compared with current metal alloy interconnect in the portable electronics in terms of mechanical and thermal fatigue endurance due to its unique spherical polymeric core able to absorb the strain energy generated during the service life. FBGA and WLCSP are usually used in portable devices to meet the small form factors requirement. With the fast growth of smart phone market, the solder interconnect with highly cracking resistance between IC package and PCB is also required in order to sustain its reliability during the service life, which will raise the opportunity to introduce the polymer ball interconnect in the fine pitch package application. In the study, the WLCSP with polymer ball interconnect is discussed its failure mechanism under various board level bending stress. Various failure modes are observed over the bending strain by way of cross section and dye and pry with SEM and OM to understand the weakest structure in the package design for further improvement. Moreover, one example with SnAgCu ball attached WLCSP is referenced for the failure mode evolution as comparison.