TO3 mounting query

Curiouser and curioser. There is a black "washer" between tag and pcb pad. Undoing a bolt this was a standard black nylon TO3 mounting insulated washer but on the underside. So instead of bolt head, insulator washer, TO3 flange, mica, heatsink, pcb it is bolt head, flange , mica, heatsink, pcb, pcb and trace, TO3 insulator upside down, solder tag, star washer, nut. Flange is same thickness as pcb and nylon insulator is only long enough to half penetrate the pcb so what advantage ? or c*ck-up?

Some times if you were mating with the board copper, they used different types of material to prevent erosion.

Jamie

pad.

washer

flange,

and

is

But an insulator ? I reckon cockup , myself included. I should have said plain cylinder insulator set solely in the heatsink hole rather than necked washer under the head, or there would be no electrical connection to the TO3 flange .

I think the star washer used alone is not a good idea. True, it may dig in and make a good electrical contact, but thermal cycling may cause it to loosen. There should also (or instead of) be a split washer in the stack, too.

Isaac

pad.

washer

flange,

and

is

ISTM the main problem with this arrangement of bolting through pcb is compression of the polyester/glass of the pcb. Another amp in yesterday had a nut bouncing around inside causing mayhem. Just a 3mm nut with star washer for holding pcb via standoff spacer to chassis. Or was, until pcb compression and vibration. Personally ,in non hot areas, I prefer star washer and nut with nail varnish , or a nylock nut. I'm not convinced that the radial edges of spring washers is enough to stop vibration induced undoing , combined with plastic pcb material