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I had some PCBs manufactured prior to ordering all of my components :-(

It turns out that the surface mounted 1W LEDs I'm going to use have the heat slug connected internally to the LED's +ve terminal. Unfortunately I have a copper pour over the entire top of my boards, and it's part of the GND net.

Can I rely on the solder mask to insulate the +ve voltage (~3V ish) from GND? Long term?

Edit following comment:-

  • Simple home project to run in my man cave.
  • Once built, will not really be moved.
  • The led will only run @ 100 mA. I just like their size and spread.
  • If insulation fails, it will be a direct short across a modern 5.1V/2.5A wall brick switcher. Not really a biggie as there's a dropper resistor.

I'm just curious for the future...

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No, I would not rely on solder mask as an insulator. It is very thin and can be nicked easily.

I think the bigger issue is going to be how to properly heat sink the LED without a good thermal connection to the heat slug. High power LEDs usually want the heat slug soldered or thermally epoxied to a heatsink. This can be PCB copper, but since you stated that the pcb copper is connected to the GND net you can't connect to it.

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  • \$\begingroup\$ More specifically, it can be relied on if thickens controll requested and tested. \$\endgroup\$
    – crasic
    Commented Jan 5, 2021 at 23:58
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What do you mean by "rely on"? What happens if the solder mask fails? How badly do you want to avoid that? What sort of longevity are you looking for, and what sort of environment will the pcb operate in?

If you just want to make a home project and are willing to troubleshoot any possible failure, you can use the resist as an insulator, making sure that you put a little bit of thermal compound under the slug area.

You'd do much better to mask the resist from under the slug so the ground plane is bare, and use a Silpad or equivalent between the slug and the ground plane.

Keep in mind that, whichever route you go, the LED will not be quite as well heat-sunk as it would if the slug were soldered to a metal plane, and plan accordingly. The fact that you Ve plane is probably a good deal larger (in area) than the minimum specified will help. On the other hand, you need to check data sheet for your LED, since the spec may be for an array of plated-through holes under the LED to act as a heat sink.

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An orthogonal answer (not (only) for next time, but perhaps for this time):

How many boards are we talking about here, 5 or 5.000? If the former, you could route/etch/scratch/separate a part of the surrounding copper surface to isolate it from ground and serve as a heat dissipator.

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Once it is in place and if you didn't scratch it, it is very likely that soldermask itself will be sufficient in this case.

However if you want to play it safe, apply some polyimide tape (one brand is Kapton) over the affected area. It has very good insulation properties, is much more durable than solder mask and is thin enough that it doesn't pose a problem for soldering the components.

Kapton roll

If you don't have a roll of it and don't want to order, you can often find small pieces of brown polyimide tape in commercial equipment, especially near battery packs.

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