Formulation of underfill materials for simultaneous underfilling and stack bonding

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. We have previously reported an underfilling technology to reduce filler trapping using no-flow underfills and surface modified Cu bond pads. In such process, underfilling and interconnect bonding are carried out simultaneously, which calls for new underfill materials that would meet process requirements while leaving minimal residue filler on the bonding pads. Here we report a development epoxy composite material that is compatible with the bonding profile for eutectic SnAg solder.