Effect of sample size on the solidification temperature and microstructure of SnAgCu near eutectic alloys
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The degree of undercooling of Sn in near eutectic, SnAgCu solder balls upon cooling at a rate of 1 °C/s from the melt was examined and found to increase linearly with inverse nominal sample diameter (for balls of radius between 100 and 1000 m). The mean undercooling for SnAgCu solder balls in a flip chip assembly was 62 °C. The microstructures of these different samples were examined by means of scanning electron microscopy. The Sn dendrite arm width was observed to monotonically increase with ball diameter, indicating a possible dependence of the mechanical response of such solder balls upon size.
I. INTRODUCTION
Rapid solidification occurs in molten alloys that have been significantly undercooled, for instance by rapid cooling from the melt. Reducing the number of nucleation sites in a given sample is another means to attain significant undercooling. This can effectively be achieved for a given alloy matrix by decreasing sample size.1–9 A number of experiments have shown that the degree of undercooling increases monotonically with decreasing sample size.8–12 In general, an increase in the degree of undercooling results in more rapid solidification and in profound changes in the microstructure of the resulting solid.1–25 Depending on the system and degree of undercooling, effects such as fine grains, supersaturated solid solutions, and anomalous eutectic structures are observed.8–10,13 The rapid solidification of Pb–Sn samples has been investigated previously, revealing a change from a lameller eutectic to an anomalous eutectic structure for undercoolings of 6 and 23 K, respectively.13 In some samples, the dendrite arm size was found to decrease with increasing degree of undercooling,8,9,11,25 with systematic decreases in dendrite arm size observed with increased degree of undercooling.25 The SnAgCu eutectic phase mixture (Sn–Ag 3.5 ± 0.3-Cu 0.9 ± 0.2, weight percentages) includes Ag3Sn, Cu6Sn5, and Sn.24 In comparison to Sn, the nucleation of Ag3Sn and Cu6Sn5 is relatively easy.20,21 In fact, Sn is well known for its propensity for undercooling from the melt.2,22 Furthermore, previous study has shown that a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0361 2914
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 11, Nov 2005 Downloaded: 14 Mar 2015
Ag–Sn and Cu–Sn alloys are not effective in nucleating Sn.12 A different investigation found that for near eutectic compositions of SnAgCu where Ag3Sn is the primary crystallization product, Ag3Sn crystallites can nucleate and grow to millimeter length scales upon cooling from the melt at rates of 0.3 to 1 °C/s before the solidification of Sn.21 A correlation was observed between the presence of Ag and Cu in Sn with a cyclic twinning nucleation and growth mechanism of Sn from the melt (after substantial, 25 K, undercooling of Sn in relatively large SnAgCu samples). In summary, existing evidence indicates that the marked supercooling of Sn observed in Sn–Ag–Cu alloys cooled at approximately 1 °C/s from the
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