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→7.4 Hot (-Dipped) Tin Coated Strip Materials
==7.4 Hot (-Dipped) Tin Coated Strip Materials==
During hot-dip tinning pre-treated strip materials are coated with pure tin or tin alloys from a liquid solder metal. During overall (or all-around) tinning the stripsthrough a liquid metal melt. For strip tinning rotating rolls are partially immersed into a liquid tin melt and transport the liquid onto the strip which is guided above them. Through special wiping and gas blowing procedures the deposited tin layer can be held within tight tolerances. Hot tinning is performed directly onto the base substrate material without any pre-coating with either copper or nickel. Special cast-on processes or the melting of solder foils onto the carrier strip allow also the production of thicker solder layers (> 15 μm).
The main advantage of hot tinning of copper and copper alloys as compared to tin electroplating is the formation of an inter-metallic copper-tin phase (Cu<sub>3</sub>Sn, Cu<sub>6</sub>Sn<sub>5</sub>) at the boundary between the carrier material and the tin layer. This thin (0.3 – 0.5 μm) intermediate layer, which is formed during the thermal tinning process, is rather hard and reduces in connectors the frictional force and mechanical wear. Tin coatings produced by hot tinning have a good adhesion to the substrate material and do not tend to tin whisker formation.