isw wrote in message
]...
In article , "N_Cook"
wrote:

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 cock-up?

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

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