A processing failure of void formation has been observed in 3D IC microbumps due to small solder volume. We prepared the sandwiched Ni/Sn2.3Ag/Ni microbumps with 4 μm and 11 μm thick solders and reflowed them at 260°C to study the mechanism of void formation in the processing. Due to the thin solder, intermetallic compound formation of Ni3Sn4 from the two interfaces of the solder joint can physically bridge each other. When that happens, the degree of freedom of motion in the direction normal to the interfaces is removed. Consequently, when the remaining molten solder is drained by side wall reaction, large voids form in the joint. This is a unique mode of processing failure because of the smaller and smaller volume of solder joints in the trend of miniaturization.