Thermal Considerations

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
Palladium: 200°C
Palladium Nickel: 100°C
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.

Surface Thickness (Angstrom)
100 micro-inch Au over Copper100 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.

Figure 8. Intermetallic Compound Formation Figure 8. Intermetallic Compound Formation