Near eutectic tin-silver (SnAg) is currently the alloy of choice for electroplated lead-free solder bumping and Cu pillar capping. While lead-tin (PbSn) is still used in some applications, there has been considerable momentum in moving away from the use of lead in semiconductor packaging. Both solders are normally electroplated as alloys with specific compositions to target a desired melting point. Since the deposition potentials of lead and tin are very close together, they plate with similar characteristics. This makes it possible for PbSn plating systems to use a consumable anode system where the anode composition matches the desired deposit composition. Metals are replenished into the bath in much the same ratio they are consumed. In a SnAg plating bath, the deposition potential of Ag is much more positive than Sn, so very low potential is required for Ag deposition. The plating rate of Ag is generally mass transfer limited. The plating rate of Sn is current controlled. While similar in concept to systems using consumable anodes for plating other metals, the SnAg alloy system presents some unique challenges. Because it is more noble, the Ag+ will deposit onto the Sn anode material by displacement reaction, and passivation will occur. Thus, the Sn anodes cannot come in contact with the Ag+ in the bath. Historically this problem is overcome by using an inert anode and metals replenishment by liquid concentrate. This paper outlines a method for plating SnAg using a consumable Sn anode, thereby reducing cost of ownership (CoO) and increasing bath stability compared to conventional SnAg wafer level packaging (WLP) plating.
