Studies on the electroplated flip chip solder bumping processes and Solder/UBM (Under Bump Metallization) interfacial reactions

Abstract

The demand for Pb-free and high-density interconnection technology is rapidly growing. Electroplating bumping method is a good approach to meet fine pitch requirements especially for high volume production because the volume changes of patterned solder bumps during reflow is not so large compared with stencil printing method. Another important issue for future flip chip interconnects is to optimize the Under Bump Metallization (UBM) system for high-density, high power, and Pb-free solder bumps. This thesis is composed of three main parts: bumping process development, UBM interface reactions, and flip chip electromigration. The electroplated bumping process developments are described. The UBM deposition, thick photoresist film pattern, electroplating, and UBM layer-etching processes are optimized for eutectic Pb/63Sn solder bumping process. For Pb-free solders, Bi/43Sn and Sn/3.5Ag solders are selected as alternatives to replace the eutectic Pb/63Sn. The Bi/43Sn is for the low temperature application. And the eutectic Sn/3.5Ag has been reported to have good ductility and solderability. It was found that plated Sn/Ag bump becomes Sn/Ag/Cu by reflowing when Cu containing UBM (Under Bump Metallurgy) is used. Therefore, the composition of small size solder bumps can be determined only after the whole assembly process ?bumping reflow, flip-chip bonding reflow, underfill process, and following assemblies - has been finished. For the UBM interface reaction study, the traditional Cr/CrCu/Cu based UBM systems were compared with Pb/63Sn solder bumps in Chapter 4, and this UBM study was extended to four different types of UBM systems also with Pb-free solder bumps in Chapter 5. In Chapter 4, the intermediate CrCu layer is modified using various sputtering techniques, and the underlying Cr adhesion layer is compared with TiW. The results demonstrate that the final Cu layer should have a minimum thickness, more than 0.8 mm, for interface stability on CrCu base...