This paper presents the results of an ongoing effort to develop a direct solder bumping process for electronics packaging. The proposed process entails delivering molten droplets onto specific locations on electronic devices to form solder bumps. This study is focused on investigating droplet deposition behaviors that affect solder bump characteristics such as final bump volume, shape, and adhesion strength. The occurrence of droplet bouncing has a strong influence on these characteristics. The potential for a droplet to bounce in the absence of solidification was modeled in discrete stages based on energy conservation. Wetting and target surface roughness were identified as the critical parameters affecting bouncing. The experimental results showed that improvements in wetting and decreases in surface roughness retard bouncing. These observations agreed well with the trends predicted by the energy conservation based model. The knowledge acquired in this study is expected to contribute to the development of an efficient solder bumping process. / Singapore-MIT Alliance (SMA)

This article reports the recent progress of re-search made in the Droplet-Based Manufacturing Laboratory at MIT. The study has been focused on obtaining a fundamental understanding of microdroplet deposition and applying the technology to various practical applications. Specific scientific contributions include the development of an analytical model for droplet splashing/recoiling, an in situ droplet size control methodology, and a study of microstructure design for spray forming. The research per-formed in the lab provides both fundamental knowledge base and practical process developments for a range of manufacturing applications, including electronics packaging, spray forming and freeform fabrication. / Singapore-MIT Alliance (SMA)