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Creating Printed Circuit Boards - Easy (Part II, Etching)
Previously in Part I we looked at applying the resist layer to our PCB blank using laser printer toner transfer. In this article we'll look at using Ammonium persulphate to etch our board.
What you're going to need...
- Safety glasses, lab coat and gloves; If the etchant gets into your eyes or on your skin it can be potentially extremely painful. Be safe, be aware, wear protective gear.
- Etchant (Ammonium persulphate); There are many different types of etchant available to etch boards with, Ferric Chloride, Cuperic Chloride, Hydrogen peroxide + Hydrochloric acid (a precursor combination that forms Cuperic Chloride) and several others. We've chosen Ammonium Persulphate because it's fairly low cost, lasts a reasonable period of time, isn't too toxic and doesn't stain as horrendously as FeCl3. The downside to Ammonium persulphate is that it has to operate at an elevated temperature, however this tutorial shows you how to effectively solve that problem.
- Water jug; Used to boil water for our etchant.
- Etching container; A tall, clear plastic container is best. Ideally your board should be able to stand upright in the container.
- Foam box/container with a lid; Because ammonium persulphate requires a temperature of 60~70'C in order to effectively operate, we need to prevent heatloss from the system.
- Aquarium air pump ; Available at any pet store or even supermarkets. For most PCB work we only need a $10~$20 unit.
- Plastic utensils; Tongs to handle the PCB and stir the crystals. NOTE do not use these utensils for anything else - ever.
The process...
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Measure out your etchant crystals, READ THE INSTRUCTIONS supplied with your etchant as they may vary from what is stated here.
For the crystals used in this article, they state to mix 400g of crystals with 1500mL of hot water. Since that'd be an excessive amount of etchant for this project we'll actually go with 40g of crystals and 150mL of hot water (note that the ratio is the same). You could also use 80g of crystals with 300mL of hot water. The crystals should fully dissolve to form your etchant solution which will appear clear and colourless.
Safety tip
Always add acids, crystals or other concentrated chemicals to water.
(If you add water to acid the process can generate enough heat to cause the mix to react violently and spit acid out of the container)
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Pour enough hot water into the foam box to only reach to the level of the etchant in the etching container. The reason why we don't fill beyond this point is because otherwise the etchant container will start floating around (unless it's weighed down).
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Place the etchant container into the foam box with the hot water in it. The outside of the foam box won't be very hot to touch (it'll feel warm). The etchant container should be resting on the base of the foam container, not floating around.
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Place a small notch into the side of the foam container and run the airpump + airstone into the etchant container. If you haven't done so already, put on some plastic safety glasses to protect your eyes. Turn on the airpump. Avoid breathing any vapors or getting close to the bubbles coming from the etching container, a single popping bubble can project a droplet of etchant far enough to reach your face/eyes.
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Place the PCB into the etchant solution
Ideally, it's better to use a tall-narrow container so that the PCB can stand upright, the bubbles from the pump can create a much better flow of etchant over the board.
Put the lid onto the container to hold the heat in
... wait 3 minutes ...
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After a couple of minutes you'll want to lift the lid on the foam box to inspect the etching process. WARNING - when lifting the foam lid there will be an escape of hot steam vapor and potentially vapor from the etching process. Avoid being in the direct path or breathing this vapor.
... wait 2~3 minutes ...
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After about 6~7 minutes almost all the excess copper should be removed from the board and the etchant should have a distinctive blue tinge to it. If there's some small areas on the board which still haven't etched, you can encourage them a bit by gently rocking the etchant tank.
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Completed
When all the exposed copper has been removed, use the plastic tongs to remove the board from the etching tank and wash under tap water or soak in a bucket of water for several minutes.
Inspect your board for pitting, over-etched areas or any excess copper that didn't get etched ( if there's still copper left, you can put it back into the etching tank )
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Probably the easiest way to remove the toner-resist from the etched board is to use acetone (personally I dislike acetone a lot - wear good gloves and do this in a very well ventilated area).
Use 3 or 4 paper napkins folded up and apply acetone. Make a couple of clean wipes of the board and fold over the napkins again, making sure you keep fresh napkin exposed to prevent the napkin clogging up with toner (which just causes smearing).
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The final result...
When you're done, your again need to inspect the board for bridges, pitting, broken tracks etc. All going well you should have a great appearing PCB in your hands ready now to be tinned, masked or simply used directly.
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