This study examines the mechanisms governing sustained tin plasma formation in vacuum. The authors have experimentally demonstrated that sustained tin plasmas can form in vacuum at dc power supply voltages as low as 4 V, and present a qualitative model for the observed voltage and current signatures associated with tin plasma formation. Engineering estimates were developed to help quantify tin whisker risk as a function of power supply voltage. Implications for space applications are also discussed. (c) 2007 American Vacuum Society.