Microstructural characteristics of immersion tin coatings on copper circuitries in circuit boards

Immersion tin coating, which is used as a lead-free surface finish, is deposited on the surface of copper circuitries on circuit boards by a replacement reaction. The characteristics of immersion tin coatings and the formation of tin whiskers and the intermetallic compound (IMC) are described. A Cu6Sn5 phase forms at the beginning of the immersion-plating process and expands until all of the tin has been transformed into a copper–tin alloy. The IMC layer becomes thicker and the volume of the pure tin layer decreases during storage. The tin involved in the formation of whiskers must originate from the tin layer of the immersion tin coating. Therefore, the formation of tin whiskers stops when all of the tin has been expended. Moreover, relatively larger whiskers grow on thicker coating layers, which contain more tin.