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Note the difference in color between this old commodore board and this newer board. It might seem like lighting, but after looking at my own collection of scrap boards, the older ones tend to have a much darker color than the new. I thought it might be aging on the mask, but all the old ones essentially have the same color, so I thought it unlikely.

Does anyone know the reason for this color change?

Is it a change in board material, masking chemicals, etc?

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It is superficial, the color is determined by the place of manufacture. Earlier on a darker green was the most used and often the only color available at most board houses. After a while, there were other colors available but often at an added cost. Today you can even order dark green or light green from some manufacturers with no additional cost. Most of the boards I order look the same color as the commodore board.

The color does not change unless maybe the PCB has been exposed to UV (sunlight) or a chemical and I would expect that would make the color lighter not darker. Exposing the PCB to heat can make the solder mask darker, but usually this is not uniform across the PCB.

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There isn't a tight specification of the color for PCBs. So it mainly depends on the mask materials the manufacturer actually uses.

Even if the PCBs were manufactured in the same location, there is no guarantee that the color would be the same between two batches.

The pictures that you shared are not for the same PCB so we can't determine much from there. One picture has filled vias, while the other does not, one has chemical gold while the other doesn't.

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We can also observe color change due to thermal or thermomechanical aging of PCBAs. So having a darker color for the older PCB could be due to an aging effect as well.

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