Chapter 7: Quantitative Assessment of Stress Relaxation in Tin Films by the Formation of Whiskers, Hillocks, and Other Surface Defects

Published

March 15, 2016

Author(s)

Nicholas Clore, Dennis D. Fritz, Wei-Hsun Chen, Maureen E. Williams, John E. Blendell, Carol A. Handwerker

Abstract

This chapter focuses on a study to develop & validate a surface defect counting procedure to be applied in research on the specific mechanisms responsible for stress relaxation & tin whisker formation in tin films. The current JEDEC standards [1,2] for the evaluation of tin whiskers is focused on identifying high aspect ratio whiskers & limits analysis to areas of high whisker density. The JEDEC counting procedure is, therefore, not representative of the sample as a whole, but instead, identifies areas of high shorting risk. Although the risk assessment of shorting by tin whiskers is important, an understanding of whisker growth mechanisms & competing mechanisms for stress relaxation has the potential to lead to manufacturing techniques that prevent whisker formation, essentially eliminating the risk of circuit shorting. By looking at tin film surfaces, it is clear that stress relaxation occurs not only by the formation of whiskers but also by the formation of other surface defects, including but not limited to hillocks, holes, sunken grains, & irregularly shaped structures & low-aspect ratio grains that protrude from the film surface. If these defects form to relax stresses, tin whisker formation will surely be affected. The question is how to quantify the stress relaxation that can be attributed to the formation of these separate types of defects, each with a lower reliability risk than tin whiskers. In response to this need, a methodology to obtain statistically significant datasets on defect formation was developed to allow for such quantitative comparisons as a function of film characteristics, manufacturing variables, sources of stress, and storage conditions. Using this methodology, differences in defect morphologies, dimensions, densities, & the total volume relaxed by particular types of defects can be assessed. Of particular interest is the comparison of the total defect volumes & the corresponding volumetric strains relaxed by defect formation.

Citation

Mitigating Tin Whisker Risks: Theory and Practice

Publisher Info

John Wiley & Sons, Hoboken, NJ

Pub Type

Book Chapters

Citation

Clore, N. , Fritz, D. , Chen, W. , Williams, M. , Blendell, J. and Handwerker, C. (2016), Chapter 7: Quantitative Assessment of Stress Relaxation in Tin Films by the Formation of Whiskers, Hillocks, and Other Surface Defects, Mitigating Tin Whisker Risks: Theory and Practice, John Wiley & Sons, Hoboken, NJ (Accessed May 7, 2024)