Joule Heating Enhanced Electromigration Failure in Redistribution Layer in 2.5D IC

In system level electromigration test of 2.5D IC, Joule heating enhanced electromigration failure has been found to occur in redistribution layer in the interposer. In our test samples, there are two redistribution layers (RDL), each between every two levels of solder joints, so there are three levels of solder joints. First, the microbumps connect a Si chip on top and an interposer chip in the middle. Second, the flip chip solders connect the interposer and a polymer substrate on the bottom. Third, the BGA solder balls on the back of the substrate as I/O connections to the outside. In our system level electromigration tests, a BGA ball served as the cathode and allowed electrons to flow through the circuit of path-Through-holes, flip chip solder joint, TSV, and microbumps, and then to return in the opposite direction to another BGA ball as the anode. We found that the RDL between the flip chip solder joints and micro-bumps is the weak-link and failed easily by burn-out in electromigration tests. The failure is time-dependent with sudden resistance increase. Preliminary simulation results show that because of poor heat dissipation, Joule heating will accumulate, leading to temperature increase. The increased temperature has a positive feedback to enhance electromigration in the RDL and caused the thermal run-away failure.