Yellowishing of Tin Coatings at elevated Temperatures

Tin coatings will be used in electromechanical components like connectors or spring contacts as corrosion protection, to stabilize the contact behavior or for example as a coating to improve the solderability. At elevated temperatures several effects occur which change the appearance and the properties of tin. Yellowishing for example can be one effect of oxidation of the tin under special circumstances and is related even to electroplated and hot dip tinned coatings. It can occur during reflow soldering especially with leadfree solder, the reflow process during electroplating (additional thermal treatment inline), the post bake process after plating, the use of tinned connectors at elevated temperatures and other uses. The question is now, if the yellowishing process is only a poor cosmetic appearance or if there is an influence on the contact resistance and the solderability of the coating detectable. The paper gives answers to these questions and gives some ideas to avoid this.