Driving Force and Mechanism for Spontaneous Metal Whisker Formation

Abstract

The room temperature spontaneous growth of low melting point metal whiskers, such as Sn, poses a serious reliability problem in the semiconducting industry; a problem that has become acute with the introduction of Pb-free technology. To date, this 50+ year old problem has resisted interpretation. Herein we show that the driving force is essentially a reaction between oxygen and the sprouting metal. The resulting volume expansion creates a compressive stress that pushes the whiskers up. The model proposed explains our observations on In and Sn whiskers and many past observations. The solution is in principle simple: diffusion of oxygen into the metal must be prevented or slowed down. This was demonstrated by coating the active surfaces with a polymer coating.

Electrostatic Theory of Metal Whiskers

“…Refs. [21][22][23][24] revolved around the role of surface stresses relived by whisker production, dislocation effects, and oxygen reactions. It was shown 18,19 that stress gradients along with certain assumption about system parameters can explain tin whisker growth rates but not their existence, shapes and statistics.…”

Manipulating Crystallographic Texture of Sn Coatings by Optimization of Electrodeposition Process Conditions to Suppress Growth of Whiskers

“…[12][13][14][15] Many mechanistic models for whisker growth, generally asserting that whiskering is a stress-relaxation process and relying on a self-sustaining source of continuous feed material at the whisker root, have been proposed; 5,[12][13][14][15][16][17][18][19] however, there is still no single universally accepted mechanism which completely explains whisker growth. Much of the debate on the mechanism arises from the lack of understanding of the mechanistic dependence of whiskering on the microstructure and the crystallographic texture of Sn.…”

“…Nevertheless, it is well established that whiskers grow to relieve the compressive stress present in coatings. 5,[12][13][14][15][16][17][18] Stress can be generated in polycrystalline tin coatings as a result of several factors. Stress can broadly be divided into two categories: global stress and localized stress.…”

Effect of Environmental Conditions on Tin (Sn) Whisker Growth

“…It has been claimed that an oxide layer formed on the surface is required for whisker growth [8]. It was also proposed that whiskers can be prevented by the sole removal of oxygen from the tin film [15]. The formation of a tin oxide layer within the grain boundaries also increases internal compressive stress to the tin grain boundaries.…”

“…Some factors behind these driving forces have been considered. These are based on internal residual stresses, externally applied stresses [14], stored energy, surface energy effects [15], formation of intermetallics [8], thermal expansion mismatch, corrosion, and electromigration [16]. There are also other processing variables that can affect whisker growth such as film thickness and plating conditions [3], grain size [7], microstructure and composition [17].…”