Author Topic: Why are power supplies' PCBs always yellow?  (Read 1395 times)

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Offline anfang

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Why are power supplies' PCBs always yellow?
« on: October 14, 2016, 05:54:17 am »
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?
 

Online Towger

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Re: Why are power supplies' PCBs always yellow?
« Reply #1 on: October 14, 2016, 05:56:46 am »
Cheap...
 

Offline chris_leyson

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Re: Why are power supplies' PCBs always yellow?
« Reply #2 on: October 14, 2016, 06:13:11 am »
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
 

Offline blueskull

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Re: Why are power supplies' PCBs always yellow?
« Reply #3 on: October 14, 2016, 06:22:09 am »
In power supplies they do not use FR4 for cost reasons. It is more common to use CEM1 or CEM3 boards, which is cheaper to punch holes, and the material is also cheaper as fiber glass is not as dense as FR4, and fiber glass does not have to be woven.
CEM boards can not be economically laminated into 4 layer or more PCB, therefore they are only good for single or two layer simple designs.
Another benefit is that FR4 has a rough surface which attracts and traps dust, reducing its CTI, which is not good for HV isolation. CEM series are very smooth.
 
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Offline T3sl4co1l

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Re: Why are power supplies' PCBs always yellow?
« Reply #4 on: October 14, 2016, 05:44:04 pm »
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
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