How to choose a good surface treatment for PCB?
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Re: How to choose a good surface treatment for PCB?

by adafruit_support_mike on Tue Oct 23, 2018 4:19 am

Specific answers need specific requirements. What kind of board do you want to make, what quantity do you want to make, what kind of environment will the board have to survive, etc?

Surface treatments generally do two things: make it easier to solder parts to the board, and keep the copper from corroding.

Tin spraying is the simplest, and usually cheapest. It basically means coating the copper with a thin layer of solder. The specific machine has a pool of molten solder and pumps that make fountains maybe 1/2" above the surface. A conveyor carries the board across the fountains so all the exposed copper gets covered, then there are compressed air jets that blow off the excess. The air is hot so the drips of tin don't just solidify on the surface of the board.

The next simplest is immersion tin plating, where a layer of tin forms on the surface of the copper chemically. The layer of tin is thinner and more uniform than a mechanically sprayed layer, but is a bit more fragile and has some long-term chemical issues.

All the other metal-based methods end up putting a layer of gold on the exposed copper, they just do it in different ways. The nickel, silver, and palladium processes put a thin (a few atoms thick) layer of some other metal between the copper and the gold, which is chemically desirable. Copper and gold in direct contact form an 'intermetallic' alloy layer that causes problems electrically. Silver and nickel plate onto copper easily and coexist with copper well, and both of them also work and play nicely with gold. Palladium is an exceptionally good base for metal plating, but is more expensive than the gold itself.

The major advantage of gold plating is shelf life. Tin oxidizes when exposed to air, so it's best to use tin-coated PCBs within about six months of fabrication. If you let them sit longer than that, it becomes hard to solder to them, and you need stronger flux to cut through the oxide layer. Gold doesn't oxidize in air (it doesn't like to form compounds with oxygen at all), so the pads remain easy to work indefinitely.

The organic coatings also exist to keep the exposed copper from corroding, and if you'll pardon some editorializing, are more hand-waving than useful. Europe has established a law to keep toxic chemicals like lead and cadmium out of electronics, called the Reduction of Hazardous Substances (RoHS) directive. Unfortunately, it bans certain chemicals without suggesting any useful replacement, and that vacuum has been filled by chemical labs with marketing departments.

The simplest organic coatings are thin layers of rosin (dried flux), wax, or shellac. Those do a reasonably good job of keeping copper from oxidizing, but don't last as long as tin. The new coatings are engineered to keep oxygen away from the copper longer, while still melting away with heat or dissolving into the flux when you start soldering. If you don't have a specific reason to use one of them (i.e.: a manager who got their hands on a trade journal somewhere), you can safely ignore them.

All the rest of the terms like 'microetching' and 'activation' are technical details of the electroplating process. Microetching means cutting microscopic pits in the surface of the exposed metal so the layer of stuff being applied has a large surface to hang onto. Activation means stripping away any compounds on the surface of a piece of metal so the raw metal atoms are exposed. Raw metal surfaces really like to form chemical bonds with something else, which creates good conditions for electroplating or chemical coating. You don't need to deal with any of those issues yourself if you're having boards made by a board house. You're paying them to know what to do.

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Please be positive and constructive with your questions and comments.