Eutectic Solidification Morphologies in Rapidly Solidified Hypereutectic Sn–Ag Solder Alloy

Huamao Rao, Andrew Mullis, Robert Cochrane
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Abstract

The effect of rapid solidification upon hypereutectic Sn–Ag solder alloy has been investigated using a 6.5 m drop tube. Powder sizes ranging from > 850 to < 38 μm were produced, with equivalent cooling rates of 250 to 14,800 K s−1 for 850 and 38 μm droplets, respectively. At all cooling rates investigated, dendritic β-Sn was observed as the primary solidification phase, not proeutectic Ag3Sn as predicted by the phase diagram. The volume fraction of interdendritic eutectic was observed to decrease with increasing cooling rate, with the Ag concentration in the residual interdendritic liquid estimated at 12.5–15 wt pct Ag, far in excess of the eutectic concentration of 3.8 wt pct. Much of the Ag3Sn observed within the eutectic had a blocky, divorced eutectic appearance. A model is proposed which can explain these observations in terms of sluggish nucleation of the Ag3Sn intermetallic, coupled with a metastable phase diagram that permits significant supersaturation of Ag at modest undercooling.

Abstract Image

快速凝固超共晶锡银焊料合金中的共晶凝固形态
使用 6.5 米滴管研究了快速凝固对超共晶锡铍焊料合金的影响。产生的粉末大小从 850 微米到 38 微米不等,850 微米和 38 微米液滴的等效冷却速率分别为 250 至 14,800 K s-1。在所有研究的冷却速率下,都观察到树枝状的β-Sn是主要的凝固相,而不是相图预测的原共晶Ag3Sn。根据观察,树枝状晶间共晶的体积分数随着冷却速率的增加而降低,残留树枝状晶间液体中的Ag浓度估计为12.5-15 wt pct Ag,远远超过3.8 wt pct的共晶浓度。在共晶中观察到的大部分 Ag3Sn 具有块状、分离共晶的外观。我们提出了一个模型,该模型可以从 Ag3Sn 金属间化合物的缓慢成核,以及允许在适度过冷的情况下显著过饱和 Ag 的可转移相图来解释这些观察结果。
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