This paper investigates the thermal cross-talk between the powered microbumps under one chip and the un-powered microbumps under the neighboring chip. Both chips were on a Si interposer for 2.5D IC. The Joule heating from the powered chip was found to be transferred laterally along the interposer to the unpowered chip and produced a temperature gradient in the microbumps in the unpowered chip. Void formation is observed in both the powered and the unpowered microbumps. The latter is due to thermomigration (TM), and the former is due to electromigration (EM). The amount of voids is bigger by TM than by EM. The void nucleation and growing is studied by examining the un-powered microbumps at different stages during electromigration tests. The nucleation of voids at the cold end in TM is observed, which indicates that Sn atoms diffuse from cold end to hot end. The current-enhanced surface electromigration of Sn along the side walls of Cu pillars to form intermetallic compound is observed in the powered microbumps that were subjected to a 5.3 × 104 A/cm2 current density at 150 °C for a period of time. The depletion of Sn will cause serious void formation in these powered microbumps.