- Not sure about exposure time (for my lamp).
- Not sure about UV light position and height required.
With these you just have to experiment, I'd suggest putting the board fairly close to the lamp (20-50mm or so).
I suggest you take one longish strip of photoresist PCB and put something on top of it that blocks UV light, then expose the strip while moving the UV blocker away at predefined intervals (say, every 10 seconds). What you will end up with is a PCB exposed in steps for different exposure times (10s, 20s, 30s, 40s, 50s etc.), then just pick the exposure time that gives off the best result.
- Not sure if my mask pattern is opaque enough (to UV light).
If it's a normal transparency sheet it should be fine. Printing to these with a laser printer works but this depends on your printer. Large areas of black might get overexposed but this depends on your printer. If your quality of black is bad you can print two sheets and align them on top of eachother.
- Not sure about correct proportions of caustic soda to water
I have been using a 3% solution of NaOH (KOH works too) which seems to work well.
- Not sure about time to leave the board in the caustic soda - whether I'm going too far developing.
You just basically move it around in the solution until it seems developed (black stuff comes off the board), usually it develops quite quickly (under 30s) but this depends on your solution. If your solution is too strong or you develop it too long it will strip off all the photoresist if you keep it in the solution too long. A solution not concentrated enough won't successfully develop the board (again, 3% solution seems to work well). Washing the board under a faucet afterwards works well. You can continue developing after taking it away from the solution. You can actually even continue developing after etching the board a bit, just remember to wash the board. Putting the board in the etching solution shows you pretty well where the board has photoresist and where it doesn't, the copper exposed to the etching solution goes to this "dull" color/texture in under a minute in the etching solution.
- I also dont know if a developed pre-sensitized board stays so, and wont fade as its left in daylight over time. How long before normal daylight affects a pre-sensitized developed board?
A board with the plastic shield SHOULDNT be affected by daylight but to be sure I'd keep them in a place shielded from light (like a drawer).
Btw. I'd suggest you try to expose your boards with a 11W fluorescent table lamp (the ones with a "U"-shaped lamp, they should be pretty common and cost like 10e from Ikea). Put the lamp quite close to the board (like 50mm close, and have a thin plate of glass over the board to keep the mask close to the board). You can get suitable plate of glass from picture frames (again, Ikea is a good place to get these), just make sure its real glass and not plastic. Expose for 12-15min (I've used 13,5min for my boards and setup). I have been using this method succesfully for a long time. Won't work for large boards due to the lamp being so narrow but for small boards it works well.
I've used the muriatic acid/hydrogen peroxide solution. It works great when it's fresh, but it does NOT keep for me. If I go back and use it a couple of weeks later, it just doesn't work. I haven't pursued proper disposal, yet, so I've accrued a bit of the stuff in mason jars (whose lids are rusting from the acid, by the way). Once I figure out how to get rid of the acid/peroxide mixture, I think I'm going to try FeCl.
Best Answer
Summary:
I would probably not rinse the board off in the sink but collect rinse water water in a separate container. After that follow MG chemicals recommendation for ferric chloride used in PCB etching below. The containers could be rinsed with water, the water collected then reduced and the sludge collected (if you even got any because with a large quantity of rinsed wastewater there probably would not be any sludge due to low concentration of echants/copper )
Another Idea would be to wash the containers and then let them dry, if there is any residual copper\iron residues they could be scrapped off and sent to the local disposal facility.
The recommendation from MG chemicals for Ferric Chloride used in PCB etching is this:
Reaction and Reducing
Source: https://www.sciencephoto.com/media/780534/view/ferric-chloride-reacts-with-sodium-carbonate
The sludge will be a combination of ferrite, salt and copper chloride, the copper chloride will be the biggest problem as it is toxic to marine life in high concentrations, this should be properly disposed of. In low concentrations (1mg/L) of copper is allowed in drinking water.
You would get a reaction like this:
\$\text{FeCl$_3$ + Na$_2$CO$_3$ + H$_2$O $\rightarrow$ Fe(OH)$_3$ + NaCl + CO$_2$}\$
If you use sodium hydroxide to neutralize ferric chloride you get this:
\$\text{Fe(III)Cl + 3NaOH = Fe(OH)$_3$ + 3NaCl}\$
I'd imagine that Ferrite peroxide and salt are pretty inert. The other problem is the residual copper in these reactions but copper and ferrite and salt are probably not going to be a huge deal and are hazardous substances to marine life, ferrite chloride is not listed as a hazardous substance but it is toxic to marine and corrosive which is why it must be reduced. Copper (II) chloride is a hazardous substance.
The copper (II) chloride is the biggest reason to not pour your etched copper down the drain, but there should not be much of that left over after the solution is reduced.
The copper could be a problem if there were some other residues that had copper/copper salts in them.
Source: Quora: Copper reaction
Do not pour anything with a high concentration of copper (II) chloride down the drain, but reduce\collect it and contact your local disposal site. It is important to contact your local wast facility because the rules vary from country state and local municipalities, they know what to do with the material.
Throwing it out in the backyard? legally I'd have to say contact your local municipalities for their recommendation. I don't feel like having a bit of copper salt and iron in my backyard would be a big deal. Copper Chloride is a one of the ingredients for many fungicides. It is found in the environment in certain minerals. In high concentrations, it is toxic. In low concentrations (1mg/L) of copper is allowed in drinking water and 1ppm of copper from corrosion in pipes is allowed by law so that gives you an idea of what is safe.
I contacted my local hazardous waste company and they said take the ferric chloride/copper etchant down to the local municipal hazardous waste pickup place.
Source: www.arch.ox.ac.uk/files/rlaha_intranet/safety/msds/FeCl3.pdf
MSDS Info
Ferric chloride is not listed as a hazardous waste under RCRA in US (which means its not specifically regulated) but you do still need to contact your local waste facility. It may be that there are other restrictions in other countries. The other problem is it is listed as corrosive substance (it is an etchant after all) with a code of HP8 Copper (II) Chloride is listed as a hazardous substance. And is the reason why you need to return the PCB enchant waste to your local municipality, however if you are washing out containers I would think that the concentration would be too low to worry about after reducing.