The following are the thermal limitations of plating systems when nickel is used as an underplate:
|Cobalt Hardened Gold:||125°C — Cobalt will diffuse to the surface at temperatures >125C|
|Nickel:||150°C — Nickel will diffuse to the surface at temperatures >150C|
|Tin and its Alloys:||105°C — Softening temperature|
|Gold Flash Palladium Nickel:||125°C|
Relative to the cold side, all of the above materials may be used at -65°C except for tin and its alloys which should be limited to -40°C.
If nickel is not used as an under-plate, copper diffusion can occur quickly when gold is used. The following table illustrates time at temperature which would result in unstable resistance levels being reached.
|100 micro-inch Au over Copper||100 micro-inch Au over Nickel|
|100°C||≈ 30 Days||No Evidence|
|125°C||10 Days||No Evidence|
|150°C||2 Hours||>100 Days|
For tin (alloys), the problem is intermetallic formation. The figure below illustrates time/temperature realtionship and intermatallic thickness. The intermatallic is resistive. Thus if 100 µinch of tin is plated over a substrate, in less than 2 years at 50°C, the total coating will be intermetallic and will cease to function.