Effect of Cu Substrate Roughness and Sn Layer Thickness on Whisker Development from Sn Thin-Films

Abstract

The effect of copper substrate roughness and tin layer thickness were investigated on whisker development in the case of Sn thin-films. Sn was vacuum-evaporated onto both unpolished and mechanically polished Cu substrates with 1 µm and 2 μm average layer thicknesses. The samples were stored in room conditions for 60 days. The considerable stress—developed by the rapid intermetallic layer formation—resulted in intensive whisker formation, even in some days after the layer deposition. The developed whiskers and the layer structure underneath them were investigated with both scanning electron microscopy and ion microscopy. The Sn thin-film deposited onto unpolished Cu substrate produced less but longer whiskers than that deposited onto polished Cu substrate. This phenomenon might be explained by the dependence of IML formation on the surface roughness of substrates. The formation of IML wedges is more likely on rougher Cu substrates than on polished ones. Furthermore, it was found that with the decrease of layer thickness, the development of nodule type whiskers increases due to the easier diffusion of other atoms into the whisker bodies.

Kinetics of Sn whisker growth from Sn thin-films on Cu substrate

“…Basically, it is accepted that an uneven IMC layer can generate higher mechanical stress and more Sn whiskers than a uniform one [38,39] due to the larger active surface of the IMC layer. Compared to 2-μm-thick Sn film-layers-where almost only filament-type whiskers were found [40]-, the amount of the non-filament-type whiskers (mainly the hillocks) were very high in the present study (over 90% from the total amount of whiskers). The reasons for this considerable difference could be seen in the FIB-SIM micrographs.…”

“…The Cu contaminants might twist the whisker body and result in the formation of nodule-type whiskers [41]. In the case of 2-μm-thick Sn layer [40], the Cu atoms needed more time to reach the root of the whiskers, and it delayed the distortion of the regular geometry of the formed whiskers.…”

Whisker Development from SAC0307-Mn07 Solder Alloy

“…Comparing the results with previous researches, the number of developed whiskers is extremely high [14,15], which was probably caused by the very thin Sn layer. Here only 100-150 nm was applied contrary to the usual 0.5-2 µm.…”

“…Here only 100-150 nm was applied contrary to the usual 0.5-2 µm. In the case of 1 and 2 µm thick Sn thin films, the average whisker densities were only 2900 and 70 pcs/mm 2 , respectively, after 4 weeks on ambient condition [14]. Probably here, the thinner Sn layer could relax less the stress caused by the IML growth.…”

“…The root cause of the relatively high number of hillock and nodule type whisker could be the thin Sn layer, which ensures the easy diffusion of Cu into the whisker bodies. The contamination can cause the formation of nodule type whiskers [14].…”