SMT007 Magazine

SMT-Feb2015

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30 SMT Magazine • February 2015 As a brief survey of relevant data, it should be noted that whiskers grow up to ~1–10 mm in length and vary from ~100 nm to ~30 μm in thickness. Their parameters are characterized by broad statistical distributions: side by side with fast-growing whiskers there can be oth- ers, on the same surface, whose growth is much slower or completely stalled. The metal surface conditions play a significant role. In particular, oxide structure and various contaminations are important factors determining whisker con- centration, growth rate and dimensions. The metal grain size appears to be less significant for small grains (nanometers to few microns), while whiskers are unlikely for very large grains, recrystallization can be of importance. Various additives can have significant effects on whis- ker growth, such as Pb strongly suppressing tin whiskering. Electric bias was reported to expo- nentially increase whisker growth rate, which was attributed to the effects of electric current, although other publications reported no bias ef- fect on whisker growth and even the negative effect of bias suppressed whiskering. A common observation is that whiskers grow from the root rather than from the tip, and the material re- quired for their growth is supplied from large distances through long range surface diffusion rather than from a narrow neighboring proxim- ity; there is no surrounding dent formed in the course of whisker growth. More appealing is an informal list of ob- servations given below with the permission of its author, Dr. Gordon Davy [2] . It reflects the perspectives and the spirit of the live whisker research, shared by many in the community of Tin Whisker Group teleconferences. It has proved extremely useful for the author of this paper allowing multiple comparisons between the theory and the experiment. • There are no "tin whisker experts." Work- ers in the field differ only in their degree of perplexity in the face of so many inconsis- tencies. • Nominally identical specimens may dem- onstrate drastically different densities and growth rates. • Density may differ greatly from one region of a specimen to another; on a finer scale, there is a whisker growing here, but not there. • Growth is at the base (i.e., the film), not the tip. • Growth may be from the tin-substrate in- terface or from near the tin surface. • Growth rate is often not constant. A whis- ker may stop growing for a while, then start growing again. • Growth rate is zero at low and high tem- peratures, and seems to peak at about 25– 50°C. eLeCTrOSTaTIC MeCHaNISM OF NuCLeaTION aNd GrOWTH OF MeTaL WHISKerS continues Feature Figure 1: SeM pictures of tin (left) and zinc (right) whiskers (courtesy of the nASA electronic parts and packaging [nepp] program).

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