The surface of a SSR or a heatsink is never entirely flat. If you mount a Solid State Relay directly on a heatsink, there will be invisible gaps between the two parts. Since air conducts heat poorly, these gaps have a very negative effect on the heat transfer. Therefore, a thermal interface material with a high thermal conductivity is needed to fill these gaps to improve heat conductivity between SSR and heatsink.
You can decide between using thermal paste (or grease) and thermal pads.
When should you use which solution? What are their differences and what are their advantages ? We will try to give you some basic information.
celduc’s ref. 5TH15000
It is a paste which should be applied directly on the heatsink. Thermal grease provides the best possible performance. Unfortunately, thermal paste is not easy to apply. It should be used for Power Dissipation (Pd) in the range of 25<Pd<500W.
Thermal Pads are much easier to install than thermal grease. Unfortunately, they aren’t as effective as a thin layer of thermal paste.
For three-phase applications, for example, this is better to use thermal grease as thermal pad is limited in terms of power.
Power Dissipation (Pd) of thermal pads is in the range of 25<Pd<75W
celduc recommends Aluminium materials thermal pads with very good thermal performances and no change of state in the long terms. On the market you can find thermal pads with carbon materials (named SILPAD) but they are not good in terms of thermal performances.
For a product dissipating less than 20/25W (1 x 25 or 2 x 12,5 or 3 x 8,3A) the Thermal Interface Material (TIM) does not bring enough benefit regarding the cooling or the lifetime. As its use represents an additional cost, the best is to use it only when it is absolutely necessary !