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

Balázs Illés,Tamás Hurtony,David Bušek,Olivér Krammer,Bálint Medgyes,Karel Dušek

doi:10.3390/ma12213609

Balázs Illés, Tamás Hurtony + Show 4 more

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https://doi.org/10.3390/ma12213609

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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.

Highlights

Tin whiskers are mono- or polycrystalline surface eruptions [1]
The whisker development is the stress release of the Sn layer against internal stresses, which can originate from direct mechanical load [6], from residual stress during deposition of the Sn layer [2], from volumetric expansion inside the layer structure [7], and from thermomechanical effects [8]
Some nodule type whiskers were found at the 7th days, but most of the developed still filament type

Summary

Introduction

Tin whiskers are mono- or polycrystalline surface eruptions [1]. They can develop from Sn surface coatings [2], solder joints [3] or thin-film layers [4]. A recent trend in microelectronics is to use low silver content micro-alloyed solders (in which the tin content can reach 98–99 wt%) and pure tin surface finishes. In these material systems, whisker formation is even more likely than it was at tradition SnAgCu and SnCu alloys [3]. The whisker development is the stress release of the Sn layer against internal stresses, which can originate from direct mechanical load (by test needles, connectors, etc.) [6], from residual stress during deposition of the Sn layer [2], from volumetric expansion inside the layer structure (like oxide formation, intermetallic layer growth) [7], and from thermomechanical effects [8]

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