Ultra Low Alpha Emission Lead Free Solder for Flip Chip Bumps

摘要

Recently the change to flip chips is proceeding with the growth of high pin count integrated circuits. The flip chip forms a direct solder bump contact between the chip and the substrate, so soft errors can easily occur if the solder contains alpha emitters. A reduction of alpha emission from the bumps is needed more and more as integrated circuits become more sensitive to soft errors. Mitsubishi Materials developed low alpha emission solders over 20 years ago and now manufactures materials for all bumping methods: ball mounting, plating, printing, vacuum evaporation, etc. These have usually been lead-tin solder alloys. The alpha level is typically 0.02 cph/cm~2 and is trending to 0.01 cph/cm~2. Lead free solders are the latest industry trend. For use in flip chip bumps, they will need to be made from materials free of alpha emitters and demonstrating very low alpha flux levels. Even though the solder is "lead free," these alloys are not alpha free and the main cause of alpha emission is still ~(210)Po, the decay product of ~(210)Pb, present as raw material impurity in tiny amounts. It would seem that these impurities can be easily removed by chemical refining and, indeed, this can lead to a low initial alpha measurement immediately after refining. In fact, the alpha ray flux of alpha free solder just after refining is low ,but it can exhibit a time dependant alpha flux that grows over time to unacceptably high levels and to soft error problems. In this paper we describe lead free, tin based solders that have an ultra low alpha ray level at the detection limit (about 0.001 cph/cm~2) and no time variation of alpha ray flux.