PCB copper layer without solder resist gives copper oxidisation risk?

Hello,

I just noticed on a competitor product today, that they have three 500mA LEDs in series on a ~0.8mm PCB with numerous thermal vias to the back copper layer, which is thermal padded to the aluminium heatsink.

The strange thing was that the back copper layer of the PCB was *bare* copper, with no solder resist over it.

-I thought as well as repelling solder, the solder resist layer was there to protect the copper from oxidising?

-So why has this company got bare copper on the back of the PCB?......surely it may react with the chemical of the thermal pad?

-It seems probable that they have neglected to add solder resist in order to get better thermal transfer from the LEDs to the heatsink, but surely that thin, skinny layer of solder resist doesn't hamper thermal transfer that much?

Bare copper is fairly chemical stable, as long as no larger amounts of hydrogen sulphide are present in the atmosphere. Many electrical parts, e.g. bus bars have a bare copper surface. It's not so good for electronical parts because oxidized copper can't be soldered easily.

For better chemical stability, you can apply e.g. electrolytical tin plating. But if the copper surface is glued to the heatsink, I don't see any problem with bare copper surfaces.

A regular PCB doesn't expose bare copper unless the manufacturer omits the standard surface finish. May be available on special order, but I don't see why you won't want to use standard surfaces.

FvM

But if the copper surface is glued to the heatsink, I don't see any problem with bare copper surfaces.

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.....so would you say that for a PCB thats going to be glued to an aluminium heatsink, it is completely pointless to get the bottom copper layer covered in solder resist?
(the bottom copper layer being the layer thats going to be glued to the heatsink)

(By the way , the bottom copper layer is electrically isolated from any electrical power or signals in the circuit)

for a PCB thats going to be glued to an aluminium heatsink, it is completely pointless to get the bottom copper layer covered in solder resist?

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Yes. In addition, solder mask is slightly increasing the thermal resistance.