Charred traces on PCB or carbon coating?

Hello everyone.

We have a especialized PCB here (multipoint control unit, videoconference), and it showed us some alerts of overheating.
Opening it up, we found some "carbonized" traces, showed in the pics below. Our doubt is, is it sign of overheating or is it come coating or insulation?

Thanks in advance.

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They are not Charred as in carbon but they were overheated. Copper turns that nice black color when stored in clean (non-acidic) air for 50 years. Like the wire-wound potentiometers on an old vacuum tube signal generator you may find at an estate sale. Generally a light dusting that wipes away fairly easily or removed with a spray cleaner specifically designed for this (attacks bare copper, not coated copper). Overheating makes the dust faster.

If you have a thick coating, then you likely had a part with medium heat for a long time and someone forgot (decided not to) add a solder mask. If the solder paste was there, then you have another datapoint for your temperature.

See Copper II oxide
https://en.m.wikipedia.org/wiki/Copper(II)_oxide

Note: if it was used in a slightly humid/acidic environment, you would have seen green or bluish-green carbonates, chlorides or sulphates of copper instead (assuming the oxidation happened slowly near room temperature). It will usually be black oxide if it happened at higher temps.

I see a lot of burnt tracks in my work and I've never seen one like that.

Is that board held by its edge in a slot somehow ... and the heat from the track interacted with the frame material?

Interesting how it's so even along the length ... not the black coating, but the lack of colour on the board.

I have seen a discussion of this effect somewhere recently, I will have a look for it.
JimB

Edit

Found it, here it is:

https://www.electro-tech-online.com...osion-deposit-on-the-pcb.152576/#post-1311980

Looks like the same effect - very interesting. Although sadly we didn't really reach a conclusion in the previous thread!
Personally, I've never seen anything like it, but I would argee with the other posters that it doesn't look like overheating. There was a suggestion that the dust might be conductive, so that could be causing some trouble,