The thermal compound is there to fill in all the microscopic air pockets between the heatsink or device and the pad (air is a VERY poor conductor of heat). The silpads are a special phase change substance which becomes more like a liquid when hot to fill in the same gaps, so doesn't require thermal compound.
Originally Posted by srserl /img/forum/go_quote.gif The thermal compound is there to fill in all the microscopic air pockets between the heatsink or device and the pad (air is a VERY poor conductor of heat).
This, and it seems to me to be the better safe than sorry approach and I find it actually helps hold things in place during assembly which is a bonus.
Unfortunately I was not able to use the ceramic insulators n_maher recommended, they were a little too thick and would have left the FET legs too short to be solderable to the underside of the boards it’s a very tight fit, but definitely a consideration for my next project. I found these instead:
The material is boron nitride loaded silicon elastomer, and they are at least twice as thick as the standard pads at 0.38mm. I felt a lot more comfortable with these pads, although one did tear quite easily from over tightening, as n_maher stated a snug fit is all that’s needed.
The old FET’s were removed, although I hadn’t blown any, I felt I had traumatized the legs several times and wanted to wipe the slate clean for a totally new installation. I applied a liberal dose of thermal paste to the underside of each FET and pad with a clean makeup brush for an even layer.
The two boards now measure no shorts and power up just fine, thanks again for your help!
EDIT: The underside of the board has a nylon washer and M3 nut followed by a standoff, I’ll use some loctite on the standoff’s to limit any chance of assembly becoming loose.
It’s a combination of the pad bending and distorting from tightening as they are quite squishy, and the angle of that particular photo, the FETs are in good contact with the sink. BTW they were all tightened first, before soldering