Influence of Indium Addition on Whisker Mitigation in Electroplated Tin Coatings on Copper Substrates

Among many factors that influence whisker nucleation and growth in electroplated tin, it is now well established that small additions of Pb leads to whisker mitigation. To date, a good non-toxic elemental alternative to Pb that would mitigate whiskers remains elusive. In this work, a 50-100 nm In electroplated layer was incorporated into a 1-mu m-thick electroplated Sn on a pure Cu substrate. In order to permit diffusion of In into Sn, heat treatments (HTs) between 125A degrees C and 160A degrees C were performed. The diffusion profile of In was altered by varying the dwell times of the HT and by utilizing two variants of In layer deposition, namely, (1) electroplating In at the top of the Sn plating, and (2) by sandwiching the In plating between two Sn layers, each approximately 500 nm thick. Appropriate control samples of pure Sn were utilized to permit valid data on the influence of In on whisker mitigation. Indium additions reduced whisker growth by at least two orders of magnitude following the 160A degrees C treatment, independent of the location of the In layer. X-ray microanalysis of a focused ion beam cross section of the sandwich plating confirmed that In had indeed diffused into the Sn through the 160A degrees C HT and was a likely reason for the mitigation of Sn whiskers.