Fine-Pitch Cu-Sn Transient-Liquid-Phase Bonding Based on Reflow and Pre-Bonding

Among the various metal bonding techniques, Cu-Sn transient-liquid-phase (TLP) bonding has received extensive attention in recent years because the melting of Sn allows low bonding temperature and ease of operation. In this paper, a Cu-Sn TLP bonding method is proposed to achieve fine-pitch Cu-Sn bonding to avoid the problem of Sn extending by using reflow and pre-bonding. By elaborating control of reflow, the remaining Sn in the bonding interface is sufficient for wettability and compensation for the height variations of Cu bumps, but is insufficient for lateral expansion. By avoiding Sn extrusion, Cu-Sn bumps with a diameter of5µm and a pitch of 25µm can be bonded successfully with a bonding strength higher than the adhesion strength of sputtering.