Injection Molded Solder IMS for Fluxless Lead-free Solder Bumping of Decoupling Capacitor Package

Abstract

A new solder processing technology called Injection Molded Solder, or IMS, has been used for lead-free bumping of a high performance decoupling capacitor package. Several challenges addressed were the inability to use plating; prohibitively long processing times for evaporation, and the need to bump lead-free alloys without the use of flux. Already demonstrated for semiconductor wafer bumping, this application shows that the IMS transfer process can also be effectively used to bump hundreds of decoupling packages simultaneously. Lead-free solder in shot or wire form is melted in the solder reservoir of an IMS head which then scans over a mold sheet to fill cavities therein with molten solder. The mold sheet is then cooled to solidify the solder. Advantages of this process are: 1. The ability to inspect the mold sheet before transfer, assuring excellent fill yields. 2. Efficient usage of solder (compared to evaporation which wastes over 95%). 3. No tool cleaning of excess solder as with evaporation. 4. No flux required for transfer. For transfer, a fluxless "pressure variation method" was used. The solder filled cavities in the mold sheet are aligned to wetting pads of the capacitor array in an abutting contact. In a fixture, the assembly passes through a vacuum oven to transfer the solder from the mold sheet cavities to the capacitors. After cooling, the assembly is separated to reveal an array of bumped capacitors. The mold sheet may be refilled indefinitely without any intervening flux cleaning steps, thus reducing process steps and cost. Economical and environmental advantages are examined in light of key characteristics of the process, namely the absence of solder waste and wet processing, flexibility of alloys and simplicity. Initial manufacturing concepts for low-cost package bumping with IMS are also discussed.