Abstract
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 densities of tin, copper and IMCs are different. Therefore, the IMCs formation reaction can cause volume changes and voids were observed in the coatings. Moreover, 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.Atomic diffusion at room temperature and stress formation are necessary conditions for spontaneous growth of tin whiskers. Therefore, a barrier film deposited on the copper / tin interface can reduce the formation of tin whiskers. In this study, organic films (Nafion and PTFE) were coated on the surface of copper substrates as a barrier layer before the immersion plating, and decreased the rate of atomic diffusion. This had the effect of limiting the amount of copper in the immersion tin coating, and the irregular formation of IMCs was inhibited. In turn, the stresses caused by IMCs, and hence the formation of tin whiskers, were reduced.Immersion tin is used as etching resist and is selectively applied to the metal layer to leave areas of coated and uncoated metal followed by etching the metal not coated with the resist. Porous coating structure should be avoided due to etching solution can attack copper through these pores. Moreover, Nafion modified immersion tin coatings were used to improve its performance. A modified pattern plating process is disclosed for manufacturing a circuit board having a metal layer in which a portion of the metal layer is removed by etching.In the manufacture of PCB's, a metal conductive foil such as copper is bonded to the circuit board. The immersion tin coatings could be used in the manufacture of circuit boards as bonding medium. A method of using immersion tin coatings as bonding medium for improving the adhesion of PCBs in a multilayer board and a new process to make rough copper surfaces were described in this chapter. Coating a tin layer in the copper surface can improve its adhesion to the circuit board. Moreover, stripping the tin coating can make a rough copper surface with higher peel strengths.