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July 2014 • SMT Magazine 25 TIN WHISKERS: CAPSulIzATIoN continues The above parameters are "known knowns." Nonetheless, the "known unknowns," such as specific external conditions, application envi- ronment either during service or during testing, the uncertainty of tin whiskers remains to be inevitable. Real-life stresses either introduced at or subsequent to the tin plating or during service life may lead a different tin whisker behavior as in accelerated tests (e.g., temperature cy- cling, elevated temperature storage). Alloy- ing-making process to achieve homogeneity needs to be taken into consideration. For an impurity system, how the process that adds elements into tin could also affect the whisker propensity. Testing tin whisker propensity, due to its underlying mechanisms, is a more challenging endeavor than testing solder joint reliability. Not to over-test nor under-test is the gist of the effort. For both theoretical and practical rea- sons, a reference material incorporated in the test scheme is a must. Indeed, testing such intricate phenomena of tin whisker formation and growth is not straightforward, not to mention its laborious and costly nature. Nonetheless, a well-thought- out test plan including the properly selected parameters is the prerequisite in order to draw a viable conclusion, positive or negative, from the test results. As selecting testing parameters that are in sync with the intrinsic properties of the system is a critical step, it is plausible to choose the test parameters based on the anticipated underlying process and/or a postulated theory so that the tests can capture the action. Prevention and Mitigation Measures Prevention and mitigation start at the un- derstanding of the causations of tin whiskers. It is indicative that tin whisker phenomenon is both thermodynamically and kinetically controlled process. Based on the test data, field experience, and the material crystal growth theory, a smorgasbord of tactics is list- ed below, which serves as a guide to prevent or retard tin whisker growth. Discussion will appear in a future column in my series on tin whiskers. fEATURE Impact of Testing Conditions JEDEC Solid State Technology Association (formerly known as the Joint Electron Device Engineering Council) has published several documents that directly address and/or are re- lated to the testing of tin whiskers, which are good guidelines to start from. • JEDEC Standard No. 201: Environmental Acceptance requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes, JESD201 • JEDEC Standard No. 22A12: Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes, JESD22A121 • JEDEC Standard No. 22-A104D, Temperature cycling, JESD22-A104D Primarily three sets of testing conditions are included in the JEDEC documents: ambient temperature storage, elevated temperature stor- age and temperature cycling. In contrast to testing the mechanical behav- ior of solder joints (e.g., thermal fatigue, me- chanical shock), the test parameters should be set to monitor the nucleation and growth pat- tern of tin whiskers or lack thereof. More impor- tantly, the tests for the intended purpose are to gauge the relative susceptibility to whiskering. Testing the absence of whiskers is as meaning- ful as the presence of whiskers. The end game is to secure a tin-whisker-resistant system or to dis- cern between the tin-whisker-resistant and tin- whisker-prone systems. To this end, one has to define what is deemed to be tin-whisker-resistant in a practical sense (SMT Magazine, May 2014). Tests should monitor: • First appearance of whisker, if feasible • Max length of whisker at high T • Max length of whisker at low T • Density of whiskers • Overall pattern and appearance Desirably: • Rate of formation over a range of temperature • Activation energy Ideally: • Accelerated test vs. real-life phenomena