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Tinning the Board (Optional)
To give your board a professional look, prevent future oxidation of the copper, and make soldering easier, you can "tin" your board all over with solder before you install any components. To do this you will first need to coat the raw copper with a thin film of rosin-based liquid soldering flux, which you can apply with a disposable tissue. (NEVER use acid-based plumbing-type flux.) Rosin flux is very sticky stuff, so use an old magazine or newspaper to protect your work area.
Then hold your soldering iron tip flat against the board and apply normal rosin-core electronic solder. It will spread as it melts, and you can "paint" it back and forth across the board quite easily to plate all the copper.
Hold the board up to the light and view from the top side to locate any holes you have plugged with solder... they can be easily unplugged by passing the iron over them again.
You may sometimes get an imperfect etch, where there are small "freckles" of copper left between the traces. Now is the time to remove these: They are so small that the soldering iron tip can easily overheat them to the point where they unbond from the board, since there is no surrounding copper to act as a heat sink. Just "scrub" them gently with the iron until they lift, and sweep them away.
Since solder smoke may contain trace metals and other things that it would be better not to breathe, you should not only work in the proverbial "well-ventilated area" but also use a small table fan to pull smoke away from the work area. Don't blow the fan onto the board: It will cool down and make soldering very difficult. Ideally, you should have a carbon filter to pull the smoke into before running it up a vent hood, if you are going to do go into circuit production.
When the board is all tinned, there will be patches of dark hardened rosin on the solder, and sticky rosin on the bare board. Ordinary rubbing alcohol will remove these. One trick you may want to try is to keep a whole bottle just for this purpose. Put the board in a tray and pour the alcohol over it, then scrub with an old toothbrush. Return the alcohol to the bottle afterward and you can reuse it many times.
Rub the board dry, and it will have a dazzling chromed appearance. Even if tinning didn't have any other advantages, it might be worth it just for aesthetics alone!
CAUTION: You may see "electroless tin" solutions for sale that promise to plate the board by simple immersion. These do NOT work for this purpose: They leave a dull gray film over the copper that is just about impossible to solder to, and they tarnish just about as easily as bare copper!
See also Printed Circuit Construction
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