This paper reports the effect of corrosion caused by high temperature and humidity on pure tin and tin-copper alloy coatings. A new phenomenon was observed; the development of copper-oxide whiskers on tin-copper alloys plated on copper substrates (1-5 % copper content stored at 105 A degrees C/100 % relative humidity). The copper-oxide whiskers showed similar growth properties to tin whiskers. We have made a model to understand the development of copper-oxide whiskers. Localized corrosion of the tin coating reaches the Cu6Sn5 intermetallic layer, and copper oxide accumulates after the corrosion of Cu6Sn5. The dilating SnO (x) compresses and extrudes out the copper oxide in a whisker form.