Whisker formation in Sn and Pb–Sn coatings: Role of intermetallic growth, stress evolution, and plastic deformation processes

Abstract

We have simultaneously measured the evolution of intermetallic volume, stress, and whisker density in Sn and Pb-Sn alloy layers on Cu to study the fundamental mechanisms controlling whisker formation. For pure Sn, the stress becomes increasingly compressive and then saturates, corresponding to a plastically deformed region spreading away from the growing intermetallic particles. Whisker nucleation begins after the stress saturates. Pb-Sn layers have similar intermetallic growth kinetics but the resulting stress and whisker density are much less. Measurements after sputtering demonstrate the important role of the surface oxide in inhibiting stress relaxation.

Finite Element Modeling of Stress Evolution in Sn Films due to Growth of the Cu6Sn5 Intermetallic Compound

“…The relationship between IMC growth and film stress has been investigated through a series of experiments which allow simultaneous measurement of IMC volume and stress in the Sn for the same samples (presented previously by Chason et al 13 ). Here, we briefly review those results, calling attention to several key features.…”

“…This conclusion is also consistent with reported stress measurements within the Sn layer which are very near the nominal yield stress for Sn (14.5 MPa 15 ) and remain relatively constant over time. 7,11,13,16 In addition, self-diffusion along grain boundaries within Sn is assumed to provide a fast pathway for long-range transport. 5,17 The role played by these plastic deformation processes in the development of stress within Sn films, however, has not been quantitatively assessed.…”

“…[5][6][7][8][9][10][11][12][13][14] In order to understand how stress develops in response to IMC growth, it is necessary to identify the mechanical deformation processes operating in the surrounding Sn film. Recent experimental work provides evidence of extensive dislocation activity within the Sn film, especially in the neighborhood of IMC grains, 13,14 indicating that the Sn grains yield plastically. This conclusion is also consistent with reported stress measurements within the Sn layer which are very near the nominal yield stress for Sn (14.5 MPa 15 ) and remain relatively constant over time.…”

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.…”