One of the large backplane boards had 49 pin D-sub connectors that were soldered to the board, but also had screws at each end of the connector, that went through the connector and the board with nuts on the bottom (board) side. Every time they had a new assembler putting these particular boards together we would start seeing shorts from the floating ground plane to the chassis ground when testing them. The build plan called for a lock washer (split type washer) and a flat washer under the nut on the bottom side. Invariably the new person would randomly swap the positions of the lock washer and the flat washer, placing the lock washer against the board surface instead of the flat washer. The lock washer would cut through the solder mask and into the top copper layer when it was tightened down, shorting that layer (the floating ground) to the screw and metal portion of the connector, which was a chassis ground.
To my understanding a flat washer and a lock washer are still electrically conductive and not really an "isolation barrier" as
conversekidz is recommending. Like a rubber/nylon washer.
Also that is pretty stupid since one of the main functions of a lock washer is to puncture and grip the surface. Which is why they are also used for earthing.
Simple lesson - if a screw or a nut is against a circuit board, place a flat washer under it (smaller the better). And if you want to use a lock washer, place it on top of a flat washer, never place it against the board surface.
Totally agree and glad you have recommended this as I don't see people doing it,
Maybe it is because people are used to Samsung strips, Bridgelux strips, Citizen cobs, Cree cobs, Veros, Luminus cobs etc, all having something in common, they all lack live copper around the mounting holes.
The original point of my post was to let everyone know that these boards require some extra safety attention.
Aka did you spec the aluminum for the board? if not, great its softer than other stuff you have worked with....again what is your point?
I was talking about the silkscreen not the aluminium.
Specifying aluminium grade from a Chinese factory for a small order, good luck with that.
For your form factor board....strip users can use thermal tape
Well you originally implied that I should have used thermal paste to avoid over tightening but now you are saying it is not necessary for my form factor, I'm confused.
Sure strip users can use thermal tape they can also use thermal paste to avoid over tightening if screws are used right?
same...poor assembly practice...a dot of blue on the threads..no retighten
If you are trying to prevent loosening of screws during vibration or thermal cycling then it is actually better practice to use self-locking fasteners than thread-locking sealants due to the unknown chemical nature of various sealants.
But I was not implying re-tightening for the purpose above but rather because heating/cooling causes the thermal paste to flow, spread, fill the tiny gaps and set. this reduces the tension of boards to heat-sink so it is ''good practice" to tighten them after the first heat cycle.
And overall even if it where poor practice to not use "a dot of blue" it shouldn't be life threatening to re-tighten the board right?