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Once I etch my PCB from copper-clad FR4, do I need to tin the board with solder?

If I do not tin the traces, what are the possible issues besides greening over time?

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    \$\begingroup\$ Unprotected copper will probably corrode, eventually making your PCB useless. If you don't want to tin for some reason, you might try spaying (or dipping) a non-conductive coating over the copper after you have soldered your components. \$\endgroup\$ Feb 13, 2021 at 23:49

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For fully assembled boards, it will probably work OK in a nice clean dry environment unless the corrosion eats up so much of the copper that the traces do not conduct anymore. The "greening" is an oxide of copper that is not conductive, and which consumes the conductive copper, although usually it is self arresting after a thin layer forms.

For boards which have NOT been assembled, in other words, for bare boards with no parts installed, the solderability of the copper pads will decline rapidly if they sit on the shelf. To assemble the boards after they have sat unprotected, you will have to use an aggressive flux or pre-clean them somehow prior to soldering. The aggressive flux will require washing thoroughly with warm water after soldering.

It is pretty easy to tin a board compared to the difficulty of etching one in the first place. I don't see why you wouldn't tin all your boards. But maybe you were curious what are the tradeoffs, which is fine. It is good to be curious.

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    \$\begingroup\$ As an intermediate solution there is a spray lacquer that protect from oxidation and comes out when soldering. Just spray it on \$\endgroup\$ Feb 14, 2021 at 13:41
  • \$\begingroup\$ You might underline ""The "greening" is an oxide of copper that is not conductive, and which consumes the conductive copper."", it is really what I was wondering when I asked the question. \$\endgroup\$
    – KJ7LNW
    Feb 19, 2021 at 0:55
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The copper corrodes and becomes difficult to solder. If you have not populated the boards, you can simply clean it with a product such as Brasso (image from Amazon):

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.. and it will be perfectly solderable once more.

If you assemble the boards and there are a few spots where you need to attach terminals or whatever it will be more difficult because you'll have to selectively clean it. A fiberglass brush-tipped pen may help.

The corrosion is self-limiting in reasonable circumstances and will not affect the board in normal operation in a normal environment, so tinning is unnecessary.

It's possible to coat the board after polishing with a thin layer of organic lacquer-like compound which will protect it for some time. It's called Organic Solderability Preservative (OSP). It's essentially dilute rosin flux with some antifungal agents for humid countries.

P.S. Many years ago I made some etched board prototypes (using a silk screen printing process and etchant, as is done commercially today for mass-produced single-sided boards), and literally tinned them with an electroless Sn plating solution (rather expensive). They looked fine, but when they corroded over a month or so they were actually worse to solder than corroded bare copper.

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  • \$\begingroup\$ Some PCB fabs give you the option of having your boards finished with OSP instead of HASL or ENIG. I'm not really sure why anyone would ever want that, but if the professionals do it, and it's easy enough for the amateur to do, it's probably worth a look at least! \$\endgroup\$
    – Hearth
    Feb 14, 2021 at 0:05
  • \$\begingroup\$ @Hearth I think the main advantage of OSP is that it leaves a PCB surface that is extremely flat (so, more suitable for BGA, QFN etc) without the disadvantages of ENIG (gold). \$\endgroup\$ Feb 14, 2021 at 0:09
  • \$\begingroup\$ That makes sense. Of course, the same fabs that offer OSP also seem (in my experience) to offer immersion silver, which as far as I'm aware also has an extremely flat surface, and a longer shelf-life than OSP... Unless you're in a heavily polluted area with lots of sulfur in the air, I suppose. \$\endgroup\$
    – Hearth
    Feb 14, 2021 at 0:12

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