Electromigration behavior in Cu/Ni–P/Sn–Cu based joint system with low current density

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Although electromigration in solder joints has great influence on reliability, few study has been reported on the Cu/Ni–P/Sn–Cu based joint system electromigration with realistic current density range lower than 10kA/cm2. We investigated a Cu/Ni–P/Sn–0.7Cu/Ni–P/Cu joint with current densities of 5.0 and 7.5kA/cm2 at 423K. Solder joint breakdown at the cathode side was detected for both stress conditions. Ni–P plating disappeared completely at the cathode side and a Cu–Sn intermetallic compound (IMC) formed at the interface. Cu–P IMC formed on the solder breakdown interface. Ni diffusion in Ni–P plating at the cathode was accelerated and the P-rich layer grew thicker than at the anode side before breaking down under electromigration stress. The P-rich layer reached the Cu electrode resulting in cracking along the interface between solder layer and Cu. Sn was diffused from the Ni3SnP IMC to the P-rich layer cracks and formed Cu3Sn IMC with the Cu electrode. Thus, the electromigration mechanism in an electroless Ni–P plating/Sn–Cu based joint system with low current density was clarified.