Flex on cap-solder paste bumping

Soldered flip chips require the formation of a solder bump on the semiconductor device. Traditional solder deposition methods of evaporation and plating have limitations both from a cost structure as well as in capability to meet product requirements. Presented in this paper are the capabilities and reliability results of the FOC (Flex on Cap) solder paste bumping process developed by Delco Electronics and practised by flip chip technologies. This paper describes the fine pitch capabilities, solder alloys, bump height uniformity, alloy control and reliability data for the FOC process. The solder paste process permits cost effective bumping of low alpha solders either W/Pb or Pb free alloys. The importance of the UBM (Under Bump Metalization) for reliable eutectic Pb/Sn and other high Sn content solders that are important for the DCA (Direct Chip Attach) market is shown. The system level cost savings of having all the necessary solder present on the IC, eliminating the deposition of solder on the board, are given.