Processing, properties, and reliability of electroplated lead-free solder bumps

Eutectic tin-silver (Sn-Ag), tin-copper (Sn-Cu), and tin-silver-copper (Sn-Ag-Cu) solder alloys are potential lead-free materials for low-temperature processing of solder bumps on wafers. But, before they can be adopted to replace the existing lead-free materials, processes have to be developed to provide controls over composition, height and shape uniformity, and defect formation, such as micro-voids, which are detrimental to bump reliability. Over the last six years, we have developed an electroplating technology using a dip-plating machine for processing the three types of eutectic lead-free solder bumps on silicon wafer. Our process is suitable for mass production with well-controlled bump geometry and composition, and uniformity within 10% over the entire wafer. In this paper, we will describe our process and present our results on the bump properties, such as composition, melting point, and microstructure of the bumps.