Page 19 - 2019 How-to-Specify E-Book
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The two exceptions to the clad inlay dominant rule are tin (Sn) and pure or soft gold (i.e., gold with a Knoop hardness of 60–85)2. The upper temperature limits for tin are independent of application method, as the failure of tin connectors exposed to elevated temperatures is caused by the consumption of the tin via a chemical reaction with the copper in the spring material that forms an intermetallic compound. The absence of free tin on contact surface precludes metal-to-metal contact across the interface. It is also worth noting that tin on stainless steel has been evaluated at temperatures up to the melting point of tin with no significant degradation in contact resistance because tin does not form intermetallic compounds with stainless steel. Above 175°C, however, electroplated pure gold and soft gold will fully anneal as well, which coarsens the grain structure, decreases the grain boundary area, and reduces sensitivity to diffusion effects.
Diffusion Studies
Several comprehensive diffusion studies have been undertaken to understand the behavior of contact materials at elevated temperatures. Below 250°C, defect-assisted or grain boundary diffusion dominates. Concentration profiles generated after exposure to elevated temperatures depict high concentrations at both the contact surface and the original interface and smaller amounts of material within the contact material.
Above 175°C, the nickel diffusion barrier becomes the source for non-noble contaminants at the surface. If a nickel barrier is specified, then the contact material thickness must be increased to form a physical barrier to nickel diffusion. Both electroplated palladium (Pd) and clad inlay (60Pd40Ag) are effective barriers to nickel diffusion. These materials have been successfully applied between nickel and the contact material to extend the upper temperature limit.
Design Guidelines
Table Ill contains recommended upper temperature limits and contact material thicknesses for a variety of connector contact materials. This data was generated by evaluating contact resistance changes before and after isothermal aging for 3,000 hours at temperature.
 Table III: General guidelines for contact material selection and thickness based on maximum operating temperature.
For application-specific assistance with connector contact materials designed to withstand elevated temperatures and myriad other challenging operating conditions, contact the expert chemists and engineers at your preferred advanced materials supplier.
Visit Materion online. References
1. Wikipedia contributors, “Arrhenius equation,” Wikipedia, The Free Encyclopedia, index.php?title=Arrhenius_equation&oldid=877606370 (accessed February 5, 2019).
2. Wikipedia contributors, “Gold plating,” Wikipedia, The Free Encyclopedia, php?title=Gold_plating&oldid=876079251 (accessed February 5, 2019).

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