Purpose

The purpose of this paper is to carry out a study of the evolution of the microstructure and the microhardness of Sn-Cu-Ag alloys from as-cast condition and under artificial isothermal aging at different temperatures (100ºC and 180ºC) for a treatment time up to 500 h. A comparison with Sn-37% Pb eutectic solder samples was also made.

Design/methodology/approach

Sn-3.5%Ag, Sn-0.7%Cu and Sn-3.5%Ag-0.9%Cu were poured in two different cooling rate conditions and then aged at 100ºC (373ºK) and 180 °C (453ºK) during 500 h. Microstructural changes were observed by optical microscopy, scanning electron micrograph and energy dispersive X-ray microanalysis. Differential scanning calorimetry technique (DSC) was also used to confirm the obtained results.

Findings

A decrease up to 20% in microhardness respect to the value of the as-cast alloy was observed for both aging temperatures. These changes can be explained considering the coarsening and recrystallization of Sn dendrites present in the microstructures of all the systems studied.

Originality/value

There is no evidence of dissolution or precipitation of new phases in the range of studied temperatures that could be detected by DSC calorimetry technique. The acting mechanisms must be the result of coarsening of Sn dendrites and the residual stresses relaxation during the first stages of the isothermal aging.

中文翻译：

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时效对无铅焊料合金的组织和硬度的影响

目的

本文的目的是研究在不同的温度（100ºC和180ºC）下，铸态条件和人工等温老化条件下，Sn-Cu-Ag合金的显微组织和显微硬度的演变。到500小时 还与Sn-37％Pb共晶焊料样品进行了比较。

设计/方法/方法

将Sn-3.5％Ag，Sn-0.7％Cu和Sn-3.5％Ag-0.9％Cu倒入两种不同的冷却速率条件下，然后在100ºC（373ºK）和180ºC（453ºK）下老化500小时。通过光学显微镜，扫描电子显微镜和能量色散X射线显微分析观察到微观结构的变化。差示扫描量热技术（DSC）也用于确认获得的结果。

发现

在两个时效温度下均观察到相对于铸态合金值的显微硬度降低了20％。考虑到所有研究系统的微观结构中存在的锡树枝状晶体的粗化和重结晶，可以解释这些变化。

创意/价值

在DSC量热技术可以检测到的温度范围内，没有新相溶解或沉淀的证据。作用机理必须是锡枝晶粗化和等温时效第一阶段残余应力松弛的结果。