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I am trying to remove most of the solder mask on my bottom layer so that I can mount a large heat sink on the PCB. I don't have any traces or any components. So what I did was I did a polygon pour on the entire layer and changed the layer to bottom solder and have it as a GND net.

However, for some reason Altium will not tent VIAs that are not GND that are in the polygon pour.

enter image description here

enter image description here

As you can see all VIAS don't have tents any more. So when I install my heatsink I will short.

I tried to do a solid region but that didn't work either. I even tried to play around with the keepout but had no luck.

What is also weird is that when I check the box to remove dead copper on the polygon I created for the solder mask, it removes all of the solder mask. It is like the solder mask is not connected to my GND. Which doesn't make sense to me.

I don't want to manually route the solder mask layer because this is a pretty big board and I am just showing a subset of it.

Any ideas? Thanks

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    \$\begingroup\$ Solder mask is not a replacement for proper insulation. Even with tenting you will get shorts was solder mask is not sufficiently scratch resistant. Add some Kapton foil! \$\endgroup\$
    – asdfex
    Aug 19, 2019 at 17:57
  • \$\begingroup\$ In your screenshots, there is no solder mask at all, the second one shows plain copper. \$\endgroup\$
    – asdfex
    Aug 19, 2019 at 17:58

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You should re-think your design.

  1. Solder mask is not reliable enough to trust to prevent shorts when there's otherwise a mechanical contact between two surfaces.

  2. If you add solder mask above these vias, that will stop your heatsink from making good contact with the surrounding copper (that you're clearing of solder mask to allow close contact to).

If you can do it, the best solution is to move all the vias on other nets away from the region where you want your heat sink to contact the ground polygon.

In comments you said,

I am not concerned about the 2nd point since I have plenty of room in the rest of the board to make contact,

Your heat sink is presumably a rigid object, and its contact surface is flat. If you have 3 areas of solder mask (or one that is big enough) you will prevent the heat sink from making metal to metal contact anywhere. There will be gap between the pcb copper and the heat sink surface equal to the thickness of your solder mask.

I will be using some sort of thermal compound to bridge any gaps.

Compare the thermal conductivity of your thermal compound to that of copper and do some thermal resistance calculations before you choose to use a layer of thermal compound to do more than fill in the air gaps caused by the inescapable roughness of your two nominally contacting surfaces.

Depending on your thermal requirements, you may get away with it, but it's going to be noticeably worse than metal-to-metal contact.

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  • \$\begingroup\$ I am not concerned about the 2nd point since I have plenty of room in the rest of the board to make contact, and I will be using some sort of thermal compound to bridge any gaps. You do bring up a good point regarding the first part. I was under the impression solder mask is sufficient to keep me from shorting. \$\endgroup\$
    – J. Jones
    Aug 19, 2019 at 18:06
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    \$\begingroup\$ @J.Jones, thermal compound really isn't that great a thermal conductor. It's a lot better than air, but you still want to lay it on as thin as possible, with as much metal-to-metal contact as possible, to get good thermal conductivity. \$\endgroup\$
    – The Photon
    Aug 19, 2019 at 18:09
  • \$\begingroup\$ What about blind Vias? I think that should do the trick right? I know it is a cost increase. Mechanicals really want the entire board for a heatsink. \$\endgroup\$
    – J. Jones
    Aug 19, 2019 at 20:30
  • \$\begingroup\$ @J.Jones, do you want the board to be a heat sink, or to contact a heat sink? It's pretty common to just use airflow over the outer layer copper to cool a board, without using any separate heat sink part, but it's not clear if that's what you are actually trying to do. \$\endgroup\$
    – The Photon
    Aug 19, 2019 at 20:59
  • \$\begingroup\$ I need to attach a heat sink to the board. I am trying to dissipate about 3.5W of heat. I have a bit of air flow but trying to leave myself outs to be able to mount a heatsink in case I need to. I took your suggestion of not relaying on the solder mask and changed my VIAs to blind. This did increase my board price by about 25%. \$\endgroup\$
    – J. Jones
    Aug 20, 2019 at 14:59

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