Why are power supplies' PCBs always yellow?

[anfang]

How come the PCBs of power supplies are always yellow?

There is always at least one side that is not coated with the usual (green) layer of soldermask. I suppose the yellow color comes from the exposed FR4 layer.

Do they use a different coating? Are there safety reasons? Are they just being cheap?

[Towger]

Cheap...

[chris_leyson]

I can't say I've ever seen a yellow power supply PCB, do you mean light brown ? FR4 without the solder mask is light green and that is most likely due to the flame retardant chemicals in the resin for UL approval, they used to be clear back in the day before UL but only old timers on the forum would remember this. Paper reinforced phenolic resin PCBs are light brown and they are the cheapest to manufacture, the holes for through hole components are usually stamped out rather than drilled. Does that help or have missed the point ? Regards
Chris

[T3sl4co1l]

And CEM1 tends to be white, and phenolic (designation??) tends to be light brown colored.

The motivation is [almost] only cost.  The power supply in my desktop is black colored (on top), for cosmetic reasons alone I'm sure.  It has no copper on top -- the black solder mask layer is purely cosmetic and added cost -- the bottom solder mask is conventionally green!

Tim