温度循环条件下镀锡引脚的晶须生长

The Whisker Formation on Pure Sn Coating of Leadframe under Temperature Cycling Condition

在热循环条件下,对电镀纯Sn覆层的器件引脚的锡须生长进行了评估.当器件经历500与1 000次温度循环后,纯Sn镀层表面会产生致密的热疲劳裂纹,伴随生长出许多锡须.由于镀层表面的SnO2氧化膜与镀层中Sn的热膨胀系数(CTE)存在较大的差异,在温度循环条件下发生SnO2与Sn的热失配,进而在其界面处产生了极大的交变应力,最终导致SnO2氧化膜破裂,为锡须生长提供了条件.此外,在Sn镀层与Cu基板界面处形成的Sn-Cu金属间化合物(IMC),提供了锡须生长的驱动力.镀层中的Sn原子在压应力的驱动下,沿着氧化物表面裂纹位置挤出,从而释放了压应力形成锡须.在锡须生长的初始阶段,阻力较大,故而速率较慢,在锡须突破表面氧化物层后,阻力较小,故加速了锡须的生长.

Temperature cycling tests were executed to assess the reliability of pure Sn coating.After 500 and 1 000 thermal cycles,Sn whiskers were found to grow initially from the thermal fatigue cracks on the surface with a length in several microns.The mismatch of the coefficient of expansion(CTE) between Sn and SnO_2 produced a very large field of alternative stress in the interface of them.As a result it sheared the SnO_2 and created many cracks on the surface of Sn coating.At the same time,the formation of intermetallic compound(IMC) at the interface between Sn coating and Cu substrate generated a compressive stress to some Sn grains in the coating.This compressive stress was a driving force which promoted Sn atoms out of the surface from the cracks to form Sn whiskers.At the initial stage,the growth rate of Sn whisker was slow because of the retard of oxide.By contrast,when Sn whisker broke the oxide,the growth rate of it was faster.