Mitigation of Long Whisker Growth Based upon the Dynamic Recrystallization Mechanism

The role of an Fe layer under a Sn layer to mitigate long whisker formation by the Fe/Sn interface through modification of the driving force for dynamic recrystallization (DRX) has been studied using a laboratory test vehicle with a Si (wafer) base material and evaporated layers of Cr (adhesion layer), Fe, and Sn. The latter two layers had thickness combinations of 10 nm Fe/0.5 μm Sn and 40 nm Fe/2.0 μm Sn. Aging was carried out for 9 days at temperature of 25°C, 60°C, and 100°C under stress conditions. Applied loads of 0 g or 500 g compression did not affect the results. Industrial test vehicles including electroplated layers of Sn and Fe (0.5 μm to 2.0 μm) were subjected to the same aging treatments. The absence of long whiskers and hillocks from the laboratory test vehicles indicated that the Fe/Sn interface altered the driving force of DRX rather than altering grain boundary mobility. The presence of depleted zones confirmed that the Fe layer did not affect the long-range diffusion. The industrial test samples showed that the Fe/Sn interface prevented the formation of long whiskers on the electroplated Sn films for similar aging conditions. The Fe layer provided a suitable barrier against Cu diffusion into the Sn layer. Copper in the Sn layer, even when only in solid solution, enhanced the DRX driving force for whisker growth.