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

Balázs Illés,Karel Dušek,Tamás Hurtony,Agata Skwarek,Olivér Krammer,David Bušek

doi:10.1007/s10854-020-04180-2

Abstract

The kinetics of Sn whisker growth was investigated on vacuum-evaporated Sn thin-films. Sn film layers were deposited on a Cu substrate with 0.5 and 1 µm thicknesses. The samples were stored in room conditions (22 ± 1 °C/50 ± 5RH%) for 60 days. The Sn whiskers and the Cu–Sn layer structure underneath them were investigated with both scanning electron and ion microscopy. Fast Cu–Sn intermetallic formation resulted in considerable mechanical stress in the Sn layer, which initiated intensive whisker growth right after the layer deposition. The thinner Sn layer produced twice many whiskers compared to the thicker one. The lengths of the filament-type whiskers were similar, but the growth characteristics differed. The thinner Sn layer performed the highest whisker growth rates during the first 7 days, while the thicker Sn layer increased the growth rate only after 7 days. This phenomenon was explained by the cross-correlation of the stress relaxation ability of Sn layers and the amount of Sn atoms for whisker growth. The very high filament whisker growth rates might be caused by the interface flow mechanism, which could be initiated by the intermetallic layer growth itself. Furthermore, a correlation was found between the type of the whiskers and the morphology of the intermetallic layer underneath.

Highlights

Sn whiskers are surface eruptions [1], which grow out spontaneously from high Sn content items applied in microelectronics like different surface coating layers [2] and solder joints [3]
One day after layer deposition, both samples were full of hillocks (Fig. 2a, b), and the first filament whisker already appeared on 0.5 μm thick Sn layer (Fig. 2a)
The kinetics of Sn whisker growth was investigated in the case of Sn thin-films on Cu substrates

Summary

Introduction

Sn whiskers are surface eruptions [1], which grow out spontaneously from high Sn content items applied in microelectronics like different surface coating layers [2] and solder joints [3]. Sn whisker growth is always caused by strain-induced mechanical stress acting on the Sn-content item, and that can relax this stress via whisker growth. The higher number of grain boundaries (in a fine grain structure) increases the stress relaxation ability of the Sn-content item against mechanical stresses. The shape of the Sn grains has a similar effect: globular/ horizontal grains have better stress relaxation ability than the columnar ones [13]. Yu et al [14] reported that the wedge-type IMCs (which can cause significant mechanical stresses) form more frequently between the columnar grains between horizontal ones. The crystallographic structure of the Sn grains can influence on the whisker growth; it is more likely from Sn grains with a lower index than from higher ones [15, 16]

Methods

Results

Discussion

Conclusion