Why Is Copper Coated on High-Quality PCBs? Solder Mask, Tin, and Oxidation Prevention

Professionally manufactured printed circuit boards almost never have bare copper exposed, but rather coat it with some other material. Sometimes the coating is tin, while other times the solder mask serves as a coating. Can you explain why it is important to coat the copper on a high-quality PCB?

There are two coatings on most printed circuit boards. Solder resist is the (usually) green coating thcat covers all parts of the board that will not be used to solder to surface mount or through-hole parts. This is what gives printed circuit boards their characteristics colors (the FR4 substrate is usually a tan-ish color).

The solder resist helps keep solder from bridging between adjacent pads during the reflow process and protects the traces from chemical attack..

On standard boards, in the places where copper is not covered by the resist, a tin plating is added (usually by covering the board and removing the excess with a hot air "blade". This tin coating provides for better solder wetting during the reflow process and also is less reactive than the copper itself so it doesn't immediately tarnish or corrode preventing good solder joints.

There are coatings other than tin are available including ENG gold and others.

Some PCBs with precise requirements for trace inductance so not use solder resist as the solder resist does have a non-zero effect on trace characteristics.

Our local PCB manufacturers use gold, silver, or hot air leveled solder to finish the bare copper. Gold gives a very even, untarnishable finish, which means that if the PCB is left lying around for a long time, you can still solder it without resorting to cleaning the lands. It is also useful at VHF frequencies on things like PCB aerials. Silver finishing is also good and level, but may tarnish within months in air. If the boards are to be loaded later, then they should remain wrapped in shrink wrap until ready. HALS is similar to silver, but does not have as good a finish since it is not a plating process. There is not a great price differential between the three off them.
Cheers,
Richard

Copper is very chemically reactive, so leaving it exposed invites corrosion, which will cause a variety of electrical problems. Tin is a much less reactive metal, and solder mask is a polymer which cannot corrode. So it is important to coat the copper on a high-quality PCB
Anyone who has ever etched their own board at home (and not coated it) either has seen or will see what happens to bare, unprotected copper over time!

Normally, the copper plane and the copper track would be covered with solder mask, the pads would be covered with tin or gold or other surface finish. The copper is very easy to be corroded if exposed into the air. And if no solder mask, many uncertain issues would happen during assembly process, e.g. short circuit.

The copper coating is very important on PCB due to following reasons:
* Low Cost
* Widely Available
* Re-workable
* Excellent Shelf Life
It is important to select the appropriate surface finish for your project by considering the various options while factoring in performance requirements and material costs.
Unsure of what you will need? Consult with a PCB fabricator prior to you making a selection. This will ensure that the combination of the surface finish and material will result in a high-yielding, cost-effective design that will perform as expected.