Electromigration Reliability of Advanced High-Density Fan-Out Packaging with Fine-Pitch 2-/2-μm L/S Cu Redistribution Lines

The novel fan-out (FO) packaging incorporating fine-pitch small linewidth Cu redistribution line (RDL) technology was designed for achieving high-density packaging. However, the downsizing of Cu RDLs gives rise to increasing current densities and raises the electromigration reliability concerns. The failure mechanism involved under electromigration in the advanced fine-pitch package still remains unclear at the present stage. This article investigated the electromigration reliability and the failure mechanism of an advanced FO packaging with fine-pitch 2-/2- $\mu \text{m}$ line/spacing Cu RDLs, $3~\mu \text{m}$ in thickness and 20 or $600~\mu \text{m}$ in length, embedded in polyimide dielectric layers. The Cu RDLs were stressed with the electric current at $8.8\times 10^{5}$ A/cm2 under an ambient 180 °C. The geometry effects of the Cu RDL on the electromigration performance were also elaborated, and the design rule was proposed for reliability optimization. The Cu RDL incorporating a bent-line (45°) structure was found to exhibit a better electromigration resistance compared with the one incorporating a straight-line structure. The in situ resistance analysis revealed a steady resistance increase followed by a rapid resistance jump until open-circuit failure for both 20- and 600- $\mu \text{m}$ -long Cu RDLs. The fracture microstructures further revealed the failure occurrence in the middle region of the Cu RDL rather than on the cathode side, showing atypical failure mechanism under electromigration. The local heat accumulation in the middle region of the Cu RDL produced a large thermal gradient and further triggered Cu thermomigration, which were responsible for the electromigration-induced failure in the fine-pitch Cu RDLs.