I am doing a flow analysis study with a fan pushing air across an oil can with some number of watts being dissipated inside and a fin array on the outside to be used as a heat exchanger. The  oil can is mounted to an aluminum base, which is mounted to some structure at the customers site; for lack of better information, I assumed it was a flat plate. The can needs to operate at 90°C, and the junction/film temperatures of the components inside the can must stay below 125°C, which is proving very difficult. My question is regarding contact resistances between different surfaces. When selecting surfaces for contact resistance values, do you have to select BOTH surfaces, or just one of them? I.e., if I know there is going to be a thermal interface material of some thermal resistance, if I were to select both the surface the TIM is applied to, and the mating surface, am I "adding another layer" of material? I realize often times, these values are so small that they are negligible, but even in this setting, every single degree is important. I can provide more information if need be.

Thanks,

Tim