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

Abstract

The undercooling of flip-chip Pb-free solder bumps was investigated by differential scanning calorimetry (DSC) to understand the effects of solder composition, solder volume, minor alloying elements, presence of an Under Bump Metallurgy (UBM), and cooling rate. The undercooling is defined as the temperature difference between the melting temperature of a solder during heating and the solidification temperature during cooling. Although the undercooling of a bulk sample can be easily measured by DSC, it is difficult to measure the undercooling of flip-chip solder bumps because an individual solder bump is too tiny to be handled and the amount of heat associated with melting or solidification is also too small to be detected. In this study, 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 on a Si chip. In addition, the solidification of an array of individual solder bumps in a glass mold was monitored in-situ by video imaging technique during both heating up and cooling down cycles. Random solidification of an array of bumps was demonstrated during cooling, which also spans a wide temperature range of 40-80°C. Comparing the amount of undercooling observed in flip-chip vs. BGA-size bumps or bulk solders, it is evident that the solder volume factor is more dominant than the metallurgical factor such as Sn vs. Pb. It is also demonstrated that a small addition of minor alloying elements such as Co, Zn, Ni and Fe was effective in reducing the amount of undercooling of Pb-free solders. ©2007 IEEE.