Recently, lead-free solders have been used in electronic equipments. There are many reports on mechanical properties of lead free solders. It is important to understand the evolution of solidified structure, though, such reports are few.
In order to understand the solidification process, the Sn-Cu alloys, which are one of the candidate alloys for lead-free solders, have been used in this study. Three alloys, hypo-eutectic, eutectic, and hyper-eutectic, have been prepared. The specimen was quenched during solidification. Then the solidified structures of interrupted specimens were compared to thermal histories.
In the case of hypo-eutectic and hyper-eutectic alloys, it was found that the undercooling of approximately 10 degrees was necessary for nucleation of β-Sn as a primary dendrite or as a halo. On the other hand, the undercooling was not necessary for crystallization of primary Cu
6Sn
5. The latter indicates that the primary Cu
6Sn
5 is easy to nucleate and grow even it is a facet. Primary Cu
6Sn
5 may be a starting site of a halo, but it doesn't facilitate nucleation of β-Sn.
In the case of the hypo-eutectic alloy, a small undercooling for nucleation of eutectic structure was observed. Such undercooling was not observed in eutectic and hyper-eutectic alloys. This may be interpreted by the difference in the liquid composition when the eutectic solidification starts.
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