温度循环条件下镀层表面的Sn须生长行为

Behavior of whisker growth from the pure Sn coating under the temperature cycling condition

采用TO220封装的电子元件,研究了在温度循环条件下,元件Cu引脚上纯Sn镀层的Sn须生长行为。研究发现,Sn须在温度循环条件下的生长呈现出较高速率、较高密度、较一致长度的特点。随着温度循环次数的增加,Sn须密度和长度不断增加;与此同时,镀层表面Sn须的附近区域出现凹坑。研究表明,Cu-Sn金属间化合物的生长速率很快,使得镀层内部压应力增大,Sn须生长驱动力增加;同时,不同材料热失配产生的低周疲劳热应力,为Sn须的生长提供了额外的驱动力。此外,交变的热应力更容易破坏表面氧化膜促进Sn须的生长。满足应力条件和取向条件的晶粒首先发生Sn须生长,高密度的Sn须生长诱发部分晶粒发生向内的变形,在镀层表面形成大量的凹坑。

In this study, some Cu leads with pure Sn coating in TO220 packaging were executed the temperature cycling （TC） tests to research their behaviors of whisker growth. Sn whisker growth in TC conditions had in a high rate, a high density and a uniform length. And a large amount of collapses were found on the coating relating to Sn whisker growth in high density. Due to the high temperature in per cycle, IMC growth provided a larger driving force for whisker formation. Moreover, the thermal stress caused by the mismatch of CTE in dif- ferent materials provided an additional driving force. Sn whiskers initiated from some grains in special orientations throughout the cracks on the surface generated by thermal stress. As a result, numerous collapses appeared on the surface of coating to supply Sn whisker growth sufficiently.