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

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 degrees 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 degrees C. In contrast, an almost simultaneous melting of the bumps was observed during heating.