PCB etching gone bad

Question

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.

Answer 1

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!

Answer 2

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.

Answer 3

There are a lot of aspects to etching boards and using positive artwork. It really starts with the positive that you are laying on the board to expose to UV. Perfect artwork positive would be absolute black the blocks all UV and perfectly clear transparent that passes UV- essentially this is the contrast. The more you improve this the better. Next the developer, getting the right mix to not be too strong is essential, also having enough volume of developer works Breyer. A lesser volume of this solution means resist that is washed off can settle back onto the board. The cleaning step between develop and etch is important. With all the elements of this process the temperature can play a part and makes troubleshooting difficult. If you are moving from etch to tin plating it is helpful to get all acid and resist off the board. Scrubbing with a pad and baking soda helps to neutralize the acid and help scrub like a fine abrasive. This step will help the tinning solution work better without leaving streaks or oxidized areas that become hard to solder later. Lastly if you are buying pre-coated pcb boards finding a vendor that has a thicker resist layer will be better. But the coating on these boards can also have manufacturing variations that can confuse trying to get consistency. Also the mixture of pool acid and hydrogen peroxide is a much easier acid and it is clearer so you can view the etching progress better. Hope this helps!