Solder Bump on Copper Stud (SBC) Method of Forming the Solder Joint in Flip Chip

摘要

There are several processes in growing solder bumps in a wafer intended for Flipchip applications. To mention a few are "Electroplated", "Ball Placed" and "Screen-printed" bumps. Upon Flipchip Attach, some manufacturing assemblies require a solder dispensing process on the leadframe to complete the interconnection between the bumps and the leadframe substrate, while others re-melt the bump on the chip to directly solder onto the substrate. For small-pitch bumping, i.e. <150μm, with several number of = 150μm-sized bumps, solder dispensing on the leadframe substrate prior to Flipchip Attach is a major source of yield and reliability problem on the product if the dispensing results to non-uniform volume for the solder dots, not withstanding the productivity loss during dispensing due to mandatory vision system monitors to ensure completeness and volume consistency of solder dispensed on the leadframe. It is the reason why some assemblies are trying to find ways to mitigate or eliminate these limitations. On the other hand, directly re-melting the bumps to form the joint is a faster process, but has its share of disadvantages in maintaining the stand-off height after reflow, which results to poor performance during Temperature and Power cycling tests. The Solder Bump on Copper stud (SBC) method defined in this paper resolved the problems mentioned and is a better solution to the manufacturability of assemblies requiring flip chip attach. This paper outlines a new method of forming a solder joint between a metal substrate and the bumps attached to the silicon in flip chip applications by solder bump on copper stud (SBC) process. This new method makes use of copper stud bumping technology coupled with solder screen-printing on wafer-level to pre-form the bumps on the silicon, This is a cost-effective alternative to electroplated bumps. The process eliminates solder dispensing process during flip chip assembly to form the final interconnection between substrate and silicon. The target applications are those that require stand-off control after flip chip attach for better reliability performance and maintain a highly conductive path for products requiring very low Rds(on) applications. Another merit of the SBC process is for applications where leadframes require deep downset or step-down and screen-printing is not an option. The process also addresses the low productivity of the solder dispensing process, consequently decreasing the overall product cost as a result of the improved equipment UPH.