Self-patterning, pre-applied underfilling technology for stack-die packaging

Die stacking is one of the next-generation 3D IC packaging methods, but its stringent material requirements are unlikely to be met by traditional underfills. Moreover, filler trapping is becoming an increasingly serious issue in no-flow and wafer-level underfills. In this report, we demonstrate a novel underfilling technology for the reduction of filler trapping in fine-pitch interconnects. In our method, we fabricate superhydrophobic bond pads, and control the flow of the underfill material by the surface energy difference between the bond pads and the Si₃N₄ substrate. The superhydrophobic bond pads are shown to have no effect on the bonding of soldering materials to the pads.