Solder bulging is detected on the exposed paddle of Device A after burn-in causing the affected units to fail the coplanarity criteria. The affected units show up at random burn-in board socket locations and occur with varying frequency. Potential causes are plotted through an Ishikawa diagram which reveal fusion and creep as the potential mechanisms behind the solder bulging phenomenon. This paper seeks to determine the mechanism behind the solder bulging phenomenon via a 2-step metallographic investigation through (i) material deformation characterization and (ii) deformation mechanism simulation. In material deformation characterization, visual inspection on affected units show that the solder bulge is generally circular and is located on the center of the exposed paddle. Moreover, SEM/EDX analysis reveal that the solder bulge is not caused by a foreign contaminant or a compositional anomaly in the solder plating. On the other hand, deformation mechanism simulation involves the metallographic comparison between controlled simulations of fusion and creep versus the actual unit with solder bulge. Metallographic inspection reveal that the grain size and grain shape of the solder bulge possess the characteristics of creep phenomenon. Additionally, investigation on the burn-in (BI) process conditions also supports creep over fusion as the mechanism behind the solder bulging phenomenon. The static stress induced by the socket on the package at elevated temperature caused the solder plating to creep towards the free area which is the hole on the bottom of the socket.