Identification and counting procedure for tin whiskers, hillocks, and other surface defects

The purpose of this study was to develop and validate a new surface defect counting procedure to be used in research applications to study the mechanisms responsible for tin whisker growth. The current JEDEC whisker counting standard limits analysis to areas of high whisker density. The JEDEC counting procedure is not representative of the sample as a whole, but it instead, identifies areas of high shorting risk. Although the risk assessment of tin whiskers is relevant in high reliability applications, an understanding of the growth mechanisms can lead to standard manufacturing techniques that prevent whisker nucleation, essentially eliminating the risk of circuit shorting. The proposed counting procedure was developed to allow for scientific comparisons, in order to identify how manufacturing variables interact with stress relaxation mechanisms, including but not limited to whisker formation. In order to theorize based on relaxation mechanisms, this study first isolates the source of stress as a constant so differences in relaxation mechanisms can be independently analyzed. Sample analysis includes comparing defect volumes as volumetric strains and grain diameters before and after storage. The proposed counting procedure was used in a current iNEMI whisker acceleration study. The resulting analysis is included in this study to demonstrate the capabilities of the procedure. The results show a unique dependence of stress relaxation mechanisms on the investigated variables of plating finish, substrate, and storing condition