An Investigation Into the Role of Lead as a Suppressant for Tin Whisker Growth in Electronics

The effect on Sn electrodeposits of up to 10 wt.% codeposited Pb was investigated. The influence of Pb on cathodic polarization of the electrodeposition process, the microstructural and crystallographic characteristics of as-deposited films, the Sn-Cu intermetallic compound (IMC) formation between the deposits and Cu substrate and surface oxidation after ambient storage were examined. During electrodeposition, the plumbous (Pb 2+ ) ions were observed to inhibit stannous (Sn 2+ ) ion reduction. The use of a metallographic cross sectioning approach enabled the observation of a grain structure evolution toward a more equiaxed morphology with increasing Pb content. This was accompanied by a duplex-phase structure consisting of isolated Pb-rich particles at grain boundary intersections together with a continuous Sn-rich phase. X-ray diffraction measurements corroborate the existence of the duplex-phase structure and also indicate an as-deposited texture transition from (112) through (200) and further toward (220) with an increase in Pb content from 0 to 8 wt.%. A marked topological transition of the interfacial Cu-Sn IMCs from wedge-shaped and discretely distributed to relatively even and uniformly distributed was found to occur concurrently. After tin whisker growth had occurred from low-Pb content (up to 5 wt.%) electrodeposits on brass, Pb was generally observed to be strongly associated with actual Sn whiskers themselves, thus the spontaneous redistribution of Pb after electrodeposition has been evidenced. X-ray photoelectron spectroscopy sputter profiling indicates the absence of Pb in the Sn oxide layer and the enrichment of metallic Pb beneath the Sn oxide from a 90Sn-10Pb deposit ambient stored for 270 days. The whisker-inhibiting effect of Pb codeposition is postulated to emanate from more than one source: not only those reflected in grain structure modification, grain texture transition, and changes in the growth mode of the interfacial Cu-Sn IMCs as prevailingly acknowledged, but also significantly, the universal distribution of the soft Pb-rich phase, providing simultaneous diffusion toward potential whisker sites as well as effective strain dissipation.