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The Etching of a Printed Circuit Board
Steps followed to etch at home with chloric acid and oxygen peroxide circuit board for electronic circuits
Project based on Tom Gootee's instructions
The circuit I am etching is a microphone preamp, based on the scheme available on Elliott Sound Products.
To etch this board I used hydrochloric acid (32% concentration), hydrogen peroxide (30% concentration), 250 ml becker, a Petri capsule (15 cm diameter), a Beck's beer bottle (33 cl).
Circuit drawing
The circuit has been planned using Eagle Cad free version, available on Cadsoft USA.
The circuit has been modified compared to the original scheme to allow the use of a 9 V supply, better suited to cheap electret capsules rated to 10 V maximum.
Secondary goal of the modification is the adaptation of the size of the board to a Frisk mints box. I theefore used 1206 SMD components (about 3 mm x 1.5 mm x 0.9 mm).
The project files are the following ones: scheme, drawing, file for print.
The circuit was printed on a sheet of HP Photo paper Glossy, on which I printed two times the same file to obtain a thicker layer of toner. This produces slight blur due to not optimal alignment and circles are slightly oval. Text is also thicker.
Toner transfer to copper
First I cleaned and brushed with a sponge the bare copper, to make the surface rougher. Acetone is a good way to remove finger fat, but dish soap works very well too. When ready and dry, I heated the copper directly with the iron set at the maximum temperature and then I placed the printed photpaper on it, printed layer in contact with the copper. I used the hot iron to press the sheet on the copper and I ensured all areas were kept squeezed and warm.
Once ready (the photopaper was sticked to the copper) I placed the board on hot water and waited for the paper to soften. At that point, layer after layer, I removed the paper, leaving the toner and the superficial layer of paper (the glossy layer) on the copper. I had to rub my finger to remove the glossy layer.
Etching
Tools: plastic gloves, plastic covering for the body (plastic is resistant to acids), glasses or safety glasses.
I worked on a desk close to the window to have a good lighting.
I prepared the mixture in the becker using the Petri capsule to collect drops. I used first 100 ml distilled water, I added 20 ml hydrogen peroxide, 30 ml distilled water. The hydrochloric acid must be added at the end to avoid splashing of concentrated drops and excessive heating. I should have added 100 ml of it but I stopped at 60 ml due to the mixing effect that was worrying me. The final composition is therefore 60 ml HCl @ 32%, 20 ml H2O2 @ 30%, 130 ml distilled water.
The mixing of acid and water was very visible but actually not dangerous, but the gas released was worse: I could feel it even if I was working at arm distance. I kept the window open to dissipate it.
The board was small so I put it directly in the becker, so that the board was also left vertical.
I used wooden clips to handle the board and to place it in the water. The metal spring briefly touched the liquid and it turned immediately red due to oxidation.
The etching was very fast, about one minute. The previous time I used iron chloride it took 40 minutes.
It was possible to monitor the process and to see the copper first oxidate (due to hydrogen peroxide) and then the copper oxide was eaten by the hydrochloric acid. I moved the board from time to time to uniform the process and to remove reacted metal.
The process was clean, without any splash or droplets around. The liquid was just getting greener due to the dissolved copper.
Soldering
At the end of the etching I washed the board and checked the results: the etching was very clean and accurate. Some copper was left only where the clip was holding the board. That part will be cut away.
I prepared a brown bottle of beer (33 cl) and I put the spent solution in it. It should be good for some more etching.
I noticed that the solution was bubbling also without board, both in the becker and in the bottle. The effect inside the bottle almost disappeared now, a day later, after putting the bottle in the fridge. The cap is still holding.
The next step was taking the toner away from the copper, I used a rough sponge.
I soldered immediately the SMD components I had ready, so that the copper was still free from oxide. The result was good after removing excess solder.
Conclusion
Tracks 12 mil wide can be etched with 100% success, 8 mil tracks are reasonably possible and 6 mil tracks may be possible after careful choice of photpaper and etching time.
This seems definitely THE way to etch boards at home, very clean and accurate. I may even etch a visit card.
Thanks to the people of Brumbrum who helped me on the choice of the solutions!
Update (15 March 2007)
I made some other boards using a Kyocera printer and HP Photopaper. The master was visibly lighter, also because I wasn't able to print twice on the same paper sheet. The copper tracks are less defined so I decided to correct the master with a water resistant marker, after transfer of the toner to the copper. The result improved significantly.
Autore: Olaf Marzocchi
First revision: 8 Ottobre 2006. Last revision: 2013-11-10.