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I'm etching a double sided circuit board for my project in electrical engineering but somehow the PCB gets these weird marks and turns out bad. I tried different things like washing the PCB before hand, being really careful not to touch it etc. but it still turns out like this. And it's usually one side.

The process I use is to have regular printer paper, front and back on two A4 pages then tape them together around the PCB. UV light for 300 seconds and then remove the film with NaOH. At this point the PCB looks OK but it gets these marks. I then wash it carefully in water before I etch it. I'm baffled what happens here, does anyone have a good answer for what I am doing wrong?

Edit: The problem occurs before etching. When applying NaOH the PCB gets weird marks that kind of resembles water drying on a surface. The UV light is a dedicated machine with UV light tubes that is tested with these PCBs to work on 280-300s with normal paper at my university. It's highly unlikely this is the source of the issue.

enter image description here

enter image description here

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  • \$\begingroup\$ Your weird marks are incompletely etched areas which need more time or agitation. You could also try cleaning them before continuing or sand them off, that said why not leave all the border copper. It looks like you have porosity in the resist on planes, you could try manually protecting them with paint or packing tape before etching... Keep in mind prototype services are cheap and get you plated through holes... \$\endgroup\$ – Chris Stratton Apr 15 at 17:13
  • \$\begingroup\$ Are you sure? it etched away the traces on one side, I don't see how it would need more etching. I like the tape idea though! \$\endgroup\$ – Marius Gulbrandsen Apr 15 at 17:35
  • \$\begingroup\$ Your process sounds a little problematic. Often folks print onto plastic film, or paper, then iron the copier toner onto board, and etch from there. The toner blocks the keep area from being etched, leaving the traces. It appears your protected area aren't being protected enough. \$\endgroup\$ – CrossRoads Apr 15 at 17:38
  • \$\begingroup\$ That you have unetched copper, as well as etching through the resist in other places is overwhelmingly obvious. This is a very tricky process to perform consistently, and you haven't even gotten to the point of trying to solder headers to both sides of the foil because the lack of hole plating means it will not wick through. Given the low complexity you probably should have just point to point wired the modules on perfboard. \$\endgroup\$ – Chris Stratton Apr 15 at 17:40
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    \$\begingroup\$ Correction: you have not yet discovered the difficulty lack of plated through holes will cause when soldering the top side of your board. In comparison, the issues you are presently worried about are more easily repaired and ignored. \$\endgroup\$ – Chris Stratton Apr 15 at 18:19
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I found the answer to the problem. It was not the etching or UV process that was the issue. It turns out that the solution of NaOH was more concentrated than it should have been, so that it took off more photoresist and etching through parts of the copper. This made the copper oxidize so that the board got oxidation marks and the traces was etched away because of the missing photoresist film.

Below is a newly etched board with a lower concentrated NaOH solution. It turned out great! enter image description here

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  • \$\begingroup\$ It looks perfect. With translucent paper you can reduce the exposure time. \$\endgroup\$ – Decapod Apr 16 at 7:59
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Well adjust your procedure a little bit.

Prepairing the print

Using printer paper works well but need some attention. After the the prints are made make the paper translucent with handy oil wipe the excess of and let them dry. (oven at 80C) Do not use vegatable oil because of the yellow color.

Exposing the pcb

I do not know your light source but I use an array of 150 UV leds. 5 mm plexiglass at a distance of 50 mm exposure time 180 sec.

Make sure the prints are placed with the printed side in contact whith the PCB. Make sure that the prints are in firm contact with the pcb thereby preventing exposure failures. There is where I do not like your taping of the prints.

If you can not expose both sides simultaniously then make some markers on the prints drill them beforehand through the pcb. This ensures perfect lineup.

Developing

Develop the prints in NAOH and rinse them well afterwards. Do not let them dry.

Etching

Etch them with Fe2O3 or your own etchant. Fe2O3 should be about 40 C. Make sure the agitation is perfect.For that you can use an aquarium pump with many holes in the end of the closed tube. 15 min etching should do it.

I you can afford some time you could make use of the many cheap prototype services available with excellent quality.

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  • \$\begingroup\$ I am currently trying to make sure the paper is securely, for sure, entirely in contact with the PCB. I have narrowed the issue down to UV or when removing the photoresist before etching. To make the paper translucent I'm not so sure I'm able to do. Wouldn't this make the paper shrink? \$\endgroup\$ – Marius Gulbrandsen Apr 15 at 21:25
  • \$\begingroup\$ @MariusGulbrandsen. With oil it works perfect. With something else you are in trouble \$\endgroup\$ – Decapod Apr 16 at 7:58

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