Kinetics of Sn whisker growth from Sn thin-films on Cu substrate
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Kinetics of Sn whisker growth from Sn thin‑films on Cu substrate Balázs Illés1,2 · Olivér Krammer1,2 · Tamás Hurtony1 · Karel Dušek2 · David Bušek2 · Agata Skwarek3 Received: 13 June 2020 / Accepted: 5 August 2020 © The Author(s) 2020
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.
1 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]. According to the shape and size of the whisker, three main whisker types are distinguished: hillock, nodule, and filament [4] The most dangerous, filament-type whiskers can reach even millimeters in length. Therefore, the whisker phenomenon exhibits a serious reliability risk in microelectronics due to the risk of short circuit formation. 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. Usually, the following primary stress sources are distinguished: direct mechanical load (e.g., by connectors or test probes) [5], remaining residual stresses after Sn layer deposition [2], volumetric expansion/ * Balázs Illés [email protected] 1
Department of Electronics Technology, Budapest University of Technology and Economics, Budapest, Hungary
2
Department of Electrotechnology, Czech Technical University in Prague, Prague, Czech Republic
3
Łukasiewicz Research Network - Institute of Electron Technology, Kraków, Poland
shrinkage in the Sn-content item (e.g., oxide and intermetallic growth) [6], and thermomechanical stresses [7]. The physical properties of the Sn grains,—size, shape, and crystallographic structure [8, 9]—influence the propensity of whisker growth. A fine grain stru
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