Whisker Mitigation Mechanisms in Indium-Doped Tin Thin Films: Role of the Surface

Recent Advances in Mitigation of Whiskers from Electroplated Tin

“…Recent work has demonstrated that addition of 6% to 10% indium prevents growth of long whiskers from electroplated Sn coatings during isothermal aging at room temperature. [74][75][76] Unlike with Pb additions, where whisker growth is mitigated but not eliminated, addition of In at this concentration level completely eliminates whisker growth.…”

“…The efficacy of In for mitigation of whisker growth in 1-lm-, 3-lm-and 6-lm-thick electroplated Sn was assessed by comparing whisker growth rates in Sn doped with (6 wt.% to 10 wt.%) In versus pure Sn processed under identical conditions. [74][75][76] Since an electrolyte was not available initially for codepositing Sn and In, Sn-In samples on Cu substrates were made in three layers (i.e., Sn-In-Sn), followed by a diffusion heat treatment to homogenize the through-thickness composition, for comparison with a control trilayer Sn-Sn-Sn sample and a monolithic Sn coating of the same thickness (1 lm, 3 lm, or 6 lm). As seen in Fig.…”

“…Similar whisker mitigation has been reported for a 1.6-lm Sn-In-Sn sample with 7 wt.% In over a 1.5-year period, where a focused ion beam (FIB) cross section revealed a columnar microstructure. 76 A major problem with Sn-In codeposition is the significant difference in standard electrode potential with respect to the standard hydrogen electrode between Sn/Sn 2+ and In/In 3+ , the difference being much larger compared with Pb/Pb 2+ . Nevertheless, electrolytic techniques have now been developed to codeposit Sn-In alloys.…”

“…Based on the whisker growth mechanisms outlined above, the influence of dopants on whisker kinetics may be grouped into the following four categories: (i) change in grain boundary self-diffusivity of Sn, which changes the diffusion of Sn through the coating (in both lateral and thickness directions) to feed a potential whisker; (ii) change in the compressive stress within the film, which is governed by the reaction kinetics between the substrate and coating; (iii) change in the microstructure to an equiaxed one where vacancies can be created at horizontal grain boundaries or where stress relaxation can occur via highly mobile grain boundaries; and (iv) alteration of the composition and/or morphology of the surface passivation layer (i.e., surface oxide), potentially reducing its stiffness, 75 strength and tenacity. 76 The effect of In addition on each of the above mechanisms is briefly presented below:…”

“…Influence on Microstructure Whisker studies on Sn-In alloys have been conducted over a range of plating thicknesses from 1.5 lm to 6 lm. [74][75][76] The microstructure changed from columnar to mixed columnar equiaxed to fully equiaxed morphology as the thickness was increased. Since whiskers were eliminated for all the thicknesses, one may conclude that whisker mitigation due to In did not occur due to the microstructure.…”