Lets assume the GFK is 0.1mm thick and lambda is 1W/m.K . Lets assume the area is limited to the area surrounded by the srews, which is 1.75"x3.25" =5.6875sqinch=0.0036693475m^2.
So the thermal resistance of the part is l/(lambda x A) = 0.0001m/(1W/mK x 0.0036693475m^2)= 0.02725 K/W. That is very low. Even at 250W it would only add 7K.
I assume a Kapton tape would add the same or even less, but its dependent on parameters i do not know. I think the highest resistance comes from the thermal grease and the unevenness of the PCB and the fact the thermal conductivity of the PCB is not evenly distributed.
I just had a look at the roughness of the PCB including the kapton tape and it is around 0.05mm. That would fit to EYG-S091210DP
I had to go my own way regarding the mounting of Nicks VESC6. I have the aluminium cooler outside the box, and i bored it through and made threads for M3. I am using Nylon screw stubs, i just glued them in, for safety reasons. I follow a rule, that there may be only one loose end, one point of electrical leakage, and that is the motor. So the other side, the battery box must be isolated under any circumstance, even if it has water ingress. So it is not an option to mount the PCB to an aluminium cooler with metal screws, because the screw heads sit right beneath the electrics and you would need a single drop of (salt) water to make a bridge to the cooler. As the motor is driven with voltage between 50 and 0 volts, any leakage from plus or minus of the battery voltage to the cooler would cause an alternating or direct voltage differential of different frequency. I got zipped by this voltage before by leaking dead man switch and i definitely do not want it. So i have to keep the barrier tight. So i have to use isolating (plastic) screws.
I also experimented with 3DP cap nuts, but there remains a slot. Maybe its small enough to limit the current by some amount.
I tried other approaches as well, like a frame which spans the complete VESC and has screws to press the board to the cooler without perforating it. It is also promising, maybe i get back to it, 3DP parts can be used. But its also complex and i love simplicity. One needs to mount a lot of screws, if its precise it could work.
What do you think. Which way should i go?