Study of the undercooling of Pb-free, flip-chip solder bumps and in situ observation of solidification process

• Published in: Journal of materials research Vol. 22; no. 3; pp. 557 - 560
• Main Authors: Kang, Sung K., Cho, M.G., Lauro, P., Shih, Da-Yuan
• Format: Journal Article
• Language: English
• Published: New York, USA Cambridge University Press 01-03-2007
• Subjects: Packaging; Rapid Communications; Soldering; Packaging; Soldering; Sn
• ISSN: 0884-2914; 2044-5326
• DOI: 10.1557/jmr.2007.0071

The undercooling of flip-chip Pb-free solder bumps was investigated by differential scanning calorimetry (DSC) to understand the effects of solder composition and volume, with and without the presence of an under bump metallurgy (UBM). A large amount of the undercooling (as large as 90 °C) was observed with Sn-rich, flip-chip size solder bumps sitting in a glass mold, while the corresponding undercooling was significantly reduced in the presence of a wettable UBM surface. In addition, the solidification of an array of individual solder bumps was monitored in situ by a video imaging technique during both heating-up and cooling-down cycles. Data obtained by the optical imaging method were used to complement the DSC thermal measurements. A random solidification of the array of bumps was demonstrated during cooling, which also spans a wide temperature range of 40–80 °C. In contrast, an almost simultaneous melting of the bumps was observed during heating.