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 practiced by Flip Chip Technologies. This paper will describe the fine pitch capabilities, solder alloys, bump height uniformity, alloy control and reliability data for the FOC process. The ability of the solder paste process to meet the Pb free requirements of alpha particle sensitive devices and future environmental requirements will be discussed. The importance of the UBM (Under Bump Metallization) for reliable eutectic Pb/Sn and other high Sn content solders that are important for the DCA (Direct Chip Attach) market will be shown. The system level cost savings of having all the necessary solder present on the IC eliminating the deposition of solder on the board will be given. Examples will be provided on the cost of wafer bumping as related to both DCA applications and IC packaging