Key metrics for the electromigration performance for solder and copper-based package interconnects

Abstract

This paper presents electromigration results over a wide spectrum of far backend interconnects, including microbump, copper pillar, thermal compression flip chip bump, lead free bump and solder ball, in order to rank their performances and identify key metrics for robust electromigration reliability. A compilation of results show two regimes of performance, or current-density-carrying capability, according to the structure's solder-to-Cu ratio. That is, relatively low performance was measured for structures with solder-to-Cu ratios higher than 3 such as solder balls, lead-free bumps, and Cu pillars on narrow trace. Here, to local Cu is consumed by CuSn formation, and voids formed along the interface of the CuSn and solder in the bump or ball, or in the cathode trace connection. Relatively high performance was measured for structures with solder-to-Cu ratios less than 3 such as Cu pillars and TCFC bumps on wider traces or pads as well as microbump. Here, the Cu connection remains intact despite CuSn formation. A steady-state or near steady-state situation develops no EM voids develop and the resistance is stable. The microbump, which had lowest solder-to-Cu ratio, was measured to be immortal since its solder had totally transformed at time zero and subsequent EM testing did not alter it.