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Whisker Formation in Pb-Free Surface Finishes



Gery R. Stafford, Maureen E. Williams, C E. Johnson, Kil-Won Moon, Ugo Bertocci, William J. Boettinger


High purity bright Sn, Sn-Cu and Sn-Pb layers measuring 3,7 and 16 microns thick were electrodeposited onto phosphor bronze cantilever beams in a rotating disk apparatus. Beam deflection measurements within 15 min. of plating proved that all electrodeposits had in-plane compressive stress. In several days, the surfaces of the Sn-Cu deposits, which have the highest compressive stress, develop 50 microns contorted hillocks and 200 micron whiskers, pure Sn deposits develop 20 micron compact conical hillocks, and Sn-Pb deposits, which have the lowest compressive stress, remain unchanged. The differences between the initial compressive stresses for each alloy and pure Sn is due to the rapid precipitation of Cu_{6}Sn_{5} or Pb particles, respectively, within supersaturated Sn grains produced by electrodeposition. Over longer time, analysis of beam deflection measurements indicates that the compressive stress is augmented by the formation of Cu_{6}Sn_{5} on the bronze/Sn interface, while creep of the electrodeposit tends to decrease the compressive stress. Uniform creep occurs for Sn-Pb because it has an equi-axed grain structure. Localized creep in the form of hillocks and whiskers occurs for Sn and Sn-Cu because both have columnar structures. Compact hillocks form for the Sn deposits because the columnar grain boundaries are mobile. Contorted hillocks and whiskers forms for the Sn-Cu deposits because the columnar grain boundary motion is impeded.
Journal of the Electrochemical Society


Stafford, G. , Williams, M. , Johnson, C. , Moon, K. , Bertocci, U. and Boettinger, W. (2006), Whisker Formation in Pb-Free Surface Finishes, Journal of the Electrochemical Society (Accessed December 11, 2023)
Created September 11, 2006, Updated February 17, 2017