signs of complete PCB photo-resist development

Question

Currently. I'm making a PCB using the photolithographic method (print on transparency, expose with UV which works, then develop under a bright red LED as the only light source), but half the time, the developing stage is not perfect. I find this out when etching the board is about 5% complete. I then re-develop the board. This is even after I sanded the photo-resist layer off of a small corner of the board (before exposure) to give me an idea of how the background should be after the removal of the unwanted photo-resist.

I'm using 1 part standard sodium hydroxide developer to about 10 parts water and the liquid is greenish since I use the same mix for multiple boards, and it takes two minutes to development to (roughly) complete, but I cannot use time as a factor to development completion.

Rather than having to re-develop the board to get it perfect, what other signs should I look for on the board to indicate completion of photo-resist removal? and would it be a benefit if I used a different colour light source to make the signs I need to look for more visible?

I want to avoid colours which would make the photoresist react (such as the exposure light color purple).

Answer 1

The problem of layouts printed on transparency is the poor density of the black areas. Some UV light is getting through the black color and the resist is exposed partially and thus not fully removed during developing. Try to get better print density by multiple printing or printing with black, magenta, cyan and yellow together. If you cover a unused part of the PCB with a really dense black material you will see if this part behaves better during development and etching.

Answer 2

It depends entirely on which specific photoresist (negative vs positive, chemistry, brand) you use, but I have found that the board is fully developed once the photoresist visibly stops dissolving. I however suspect that your problem isn't actually caused by the development step itself, with the preceding exposure being the real culprit.

The exposure step is often the hardest part to get right, as the illumination has to be uniform, the alignment of the board has to be precise and the timing has to be correct. You can try to cover a PCB + mask with a piece of opaque material (like aluminum foil) during exposure; uncovering a new area at e.g. 20 s increments by moving the foil will allow you to test multiple exposure times at the same time.