Study of the undercooling of Pb-free, flip-chip solder bumps and in situ observation of solidification process
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Study of the undercooling of Pb-free, flip-chip solder bumps and in situ observation of solidification process Sung K. Kang,a) M.G. Cho, P. Lauro, and Da-Yuan Shih IBM T.J. Watson Research Center, Yorktown Heights, New York 10598 (Received 24 July 2006; accepted 30 October 2006)
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.
Sn-rich, near-eutectic binary and ternary solder alloys, such as Sn–0.7Cu, Sn–3.5Ag, and Sn–3.8Ag–0.7Cu, are the leading Pb-free solder candidates for replacing Pbcontaining solders in electronic assembly applications. To ensure a successful transition and minimize the impacts on solder joint integrity, reliability research studies have been performed to investigate the various issues.1,2 Because the compositions of most Pb-free solders are typically more than 90% Sn, their physical, chemical, and mechanical properties are heavily influenced by the properties of pure Sn, as opposed to eutectic Sn–Pb, which consists of a mixture of Sn-rich and Pb-rich phases. One of the distinct properties of Sn-rich solders is a propensity for a large amount of undercooling of –Sn phase during solidification. 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 differential scanning calorimetry (DSC), it is difficult to measure the undercooling of flip-chip solder bumps because the individual solder bump is too tiny to be handled, and the
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0071 J. Mater. Res., Vol. 22, No. 3, Mar 2007
amount of heat associated with melting or solidification is also too small to be detected. The undercooling observed with near-ternary Sn–Ag–Cu solder spheres of a few hundred micrometers in diameter [such as ball grid array (BGA) or chip scale package (CSP) solder joints] was much greater than high Pb solders or Sn–Pb eutectic solders.3,4 The large undercooling is also attributed to the growth of large primary phase such as Ag3Sn in nearternary Sn–Ag–Cu
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