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I have acquired this PCB.

The right side of the PCB handles high voltage AC power (250 V maximum,) while the left side handles low DC voltages (24 V maximum.)

They are separated, not only by board cuts, but also with this yellowish line in the middle. Is it just coloring to differentiate between the areas or is it some sort of protection material?

If it is some sort of protection material, how do I include it in my future designs?

My PCB

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The green areas have been coated in solder-mask.
The yellowish area is simply the base color of the PCB material, which has not been coated.
It's the same base material throughout.

You would typically achieve this in your designs by adding features to the solder-mask layer(s) of your board.
The solder-mask layers are usually 'negative' images, so drawing something on those layers will result in the solder-mask being left out in those areas on your board.

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    \$\begingroup\$ Out of curiosity, can one tell what material the PCB is made of from this? The color looks slightly different than what I expect for FR-4. \$\endgroup\$ – Hearth May 27 '19 at 15:27
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    \$\begingroup\$ @Hearth - "FR-4" doesn't really tell you very much about what the PCB substrate is actually made from. It's supposed to conform to this list of specs, and is probably a glass-reinforced epoxy laminate of some kind. The example in the OP's pics looks reasonable normal, maybe slightly on the pale side of average - but that could be the lighting. \$\endgroup\$ – brhans May 27 '19 at 16:59
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    \$\begingroup\$ @brhans Hmm, and here I was working as an electrical engineer for years thinking FR-4 was one specific material! I guess it doesn't matter exactly what it is if it has all the relevant properties, though. \$\endgroup\$ – Hearth May 27 '19 at 17:02
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    \$\begingroup\$ More important is the fact that the "yellowish area" has had the copper etched off. This is what creates the electrical separation to isolate the HV and LV sides. It is not only missing the soldermask, but the copper. \$\endgroup\$ – khargoosh May 27 '19 at 23:05
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    \$\begingroup\$ @khargoosh It's not only the yellowish area that has had the copper etched off - the dark green area has too. The yellowish area has the solder resist not placed there to clearly mark the separate zones (but that is a purely visual marking). \$\endgroup\$ – Martin Bonner supports Monica May 28 '19 at 10:02
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The yellowish PCB laminate has better electrical insulation than some solder masks, so sometimes air gaps and solder mask free areas are used to separate the high voltage and low voltage side.

I work as CAM/CAM designer in a PCB fab, and recently we made a run of boards with no solder mask at all, because the boards were going in X-ray machines. Extremely high voltages, and the voltages would jump very easily through the solder mask.

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    \$\begingroup\$ When I saw this question I was wondering why the solder mask was removed, this clears it up. \$\endgroup\$ – Redja May 28 '19 at 22:16
  • \$\begingroup\$ Wouldn't proper conformal coating solve this issue ? \$\endgroup\$ – Malek May 31 '19 at 7:19
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The yellowish line in the middle of the board is the true PCB colour.

The green areas either side are solder resist.

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In this case the lack of solder mask won't afford any additional protection that adequate isolation doesn't already provide, so removing it for that purpose is futile. You may want to remove the solder mask for other reasons like adding plated text or artwork, peeling back the mask around pads, while creating custom footprints, and so on. While most programs have different naming conventions for these layers, they generally all have the same approach of applying solder mask anywhere there isn't a line, polygon or text on the layer. For example, in Eagle if you wanted to have a line with no mask separating two halves of the board you would draw a line on the tstop or bstop layers. In KiCAD you would do the same but on the f.mask or b.mask layers

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    \$\begingroup\$ There might be differences in how the solder mask vs the bare pcb material behave regarding moisture absorption, dust adhesion, carbon tracking ... \$\endgroup\$ – rackandboneman May 28 '19 at 23:02

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