Plating film and forming method thereof

Abstract

Tin plating film composed of tin or tin alloy is formed on a front surface and a rear surface of a substrate composing a lead frame. As tin alloy, for example, tin-copper alloy (content of copper: 2 mass %), tin-bismuth alloy (content of bismuth: 2 mass %) and the like can be cited. The substrate is composed of, for example, Cu alloy or the like. Within the tin plating film, plural crystal grains are arranged irregularly. Further, plural gap portions exist within the tin plating film. An external stress is reduced even if a bending process or the like are performed subsequently, because the gap portions exist within the tin plating film. Consequently, growths of whiskers accompanied by the external stress are suppressed.

1 . A forming method of a plating film composed of tin or tin alloy, comprising:
 immersing a substrate in a plating solution, a concentration of a surfactant to be added to said plating solution being set as 10 g/liter or less; and   performing electrolysis of said plating solution while making said substrate a cathode, a current flowing in said cathode being set as 2.5 A/dm 2  or more.