The cause for the short circuit was a thermal paste claiming to be not conductive. Unfortunately the dielectric strength of the thermal paste was not enough. It broke down eventually, creating a conductive path around the PET-Foil, where the screw hole was.

Using Silicone Pads as thermal compund was the solution, because of its high dielectric strength.

Thanks to everybodys input on this topic.

The cause for the short circuit was a thermal paste claiming to be not conductive. Unfortunately the dielectric strength of the thermal paste was not enough. It broke down eventually, creating a conductive path around the PET-foil, where the screw hole was.

Using silicone pads as thermal compound was the solution, because of its high dielectric strength.

Thanks to everybodys input on this topic.