I would need a little help. I have two surfaces, gap between them are 0.160 mm. Normally that gap is filled with solder balls and amout of them are almost 2000. Running a simulation with 2000 small solder ball is out of question. I would want to define thermal resistance between these two parts but only on those soldering pad areas, not whole faces. I have tried to use sketch to define the thermal resistance area but contact set is only valid for faces, not for sketches. How should I do this?