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In this installment of the tin whisker series, we'll take a look at causes and factors that have been found or are considered to contribute to the appearance of tin whiskers. In the remaining installments, we will address “Tin Whiskers - Plausible Theory,” “Tin Whiskers - Impact of Testing Conditions,” and “Tin Whiskers - Preventive and Mitigating Measures.”
As all-encompassing tests to confirm or deny the culprits that cause tin whiskers are prohibitively costly and time-consuming, my thoughts focus on the logical causes and contributors. Fundamentally, the tin whisker follows the basic physical metallurgy in its principles on nucleation and crystal growth through the classic theories of dislocation dynamics and of other lattice defects in tin crystal structure. Thus, for whiskers to appear from the tin-plated (or tin-coated) surface, the causes and contributing factors should be intimately related to the nucleation sites creation and the subsequent growth paths after the coating process. However, for tin whisker due to tin’s intrinsic characteristics, the actual processes of nucleation and grain growth are dauntingly complex.
Nucleation and growth can be encouraged by stresses introduced during and after the plating process. The sources of these stresses come from multi-fronts. This includes residual stresses caused by electroplating and/or additional stresses imposed after plating, and/or the induced stresses by foreign elements, and/or thermally-induced stresses. Specific causes and contributing factors are outlined below.
Read the full column here.
Editor's Note: This column originally appeared in the March 2014 issue of SMT Magazine.