The Mechanism of Residual Stress Relief for Various Tin Grain Structures

Abstract

A pure tin deposition process was developed with various tin grain structures to study tin whisker formation. Samples were tested for 4000 h to examine whisker formation, grain structures, and intermetallic formation using a focused ion beam (FIB). The lateral side of the FIB-cut cavity displayed tin protrusions after 6 days. These phenomena, along with the growth of tin whiskers and/or hillocks, could illustrate the residual stress relief behavior of various tin grain structures. In full columnar structures, whiskers formed normal to the deposition surface and relieved most of the stress. In contrast, stress relaxation in semicolumnar and random structures is highly likely to occur, and proceeds rapidly in the direction parallel to the deposition surface after only a few days. In comparing mixed grain structures, it is apparent that stress is more likely to be rapidly relieved within structures with fewer grain boundaries.