The photoresist method involves creating a template to block out light, placing it over a photosensitized PCB and exposing the board to UV light. You then develop the board and etch it.
It requires more equipment than toner transfer, but it tends to be much easier and more accurate, with good repeatable accuracy once you get your exposure times dialed in.
I am using positive presensitized boards from MG Chemicals (Positive means you block out the UV light where you want copper to remain after etching).
This is my third attempt and I am extremely impressed with the results.
- Presensitized PCBs (Positive)
- Ferric Chloride etchant
- MG Chemicals positive developer
- A UV Exposure box
- Plastic tubs for the etchant, developer and water to dip the board in to stop either reaction
- A red “Safelight” bulb to align the pcb with the artwork without exposing it
- The artwork: I am using three transparency sheets printed on a laser printer on top of each other
- A sheet of glass to place over the PCB and artwork. I use a 5×7 picture frame off Amazon
- Foam paint brushes
The first step is to print out the artwork on to transparency sheets. You want two or three printouts to stack on top of each other, as laser printers tend to leave lots of pinholes on transparencies, and for this to work the artwork must be completely opaque when held up to a bright light. I had to use three sheets.
Note that you want to mirror the artwork before printing it, as you want to lay the toner side so it is facing the PCB to prevent light from getting around the transparency.
You then prepare the etchant and developer tub. Note they must be glass or plastic, the etchant will usually eat any metal container. I also wouldn’t plan on ever eating out of these containers again.
The etchant is used without diluting, the developer I used needs to be diluted 10 parts water to one part developer, so be sure to read the directions. The etchant works better when heated, I just placed the etchant tub in my bathroom sink, closed the drain and poured boiling water in, letting it sit for awhile.
It’s time to expose the board, so turn off all light sources except the red “safe light” bulb and remove the PCB from the protective wrapper.
I have heard a red “safe light” is not necessary and you can just use an incandescent bulb, but I didn’t want to take any chances.
Peel off the protective paper and align it with the artwork, put the glass on top and insert into the exposure box.
The glass is to flatten the artwork against the board. I used a 5×7 glass picture frame with a spring-loaded backing plate.
The UV exposure box I used is actually a UV fingernail paint curing box. Commercial PCB exposure boxes cost hundreds of dollars, this one was about $28 and works just as well, plus the girlfriend can use it to do her nails.
In case this link goes dead, the box contains 4 9-watt fluorescent UV bulbs, so 36 watts in total.
You might want to look around for a different box as this one doesn’t fit an entire 4×6 PCB, about an inch sticks out the front. I’ve been considering tearing it apart to make a bigger housing for the bulbs and associated electronics to power them.
Now you get to turn on the UV light box and expose the PCB. I found that one minute of exposure is about perfect for my light source, but this can depend a lot on the quality of the transparency sheet/stencil.
After it has been exposed, you then insert the PCB into the developer solution. Wear gloves, this is caustic shit man.
At this point it is okay to turn on the normal lights. The developer takes a minute or two, depending heavily on exposure time. You’re going to want to be brushing the board with a foam brush the entire time.
You want to develop until the parts that have been exposed to UV light are copper. Be sure to keep it in for enough time, you don’t want a thin (Nearly invisible) layer of photo resist to remain otherwise etching will not go very well. It should look similar to this:
After developing you need to rinse the board in water to stop the reaction, then dry it.
The final step is to etch it. I use ferric chloride.
You need to be cautious around this stuff, it will turn your skin yellow and even eats stainless steel, so wear some fucking gloves.
Place the board into the etchant tub and constantly agitate it with another foam brush, the areas you rub the most will be the first areas to have the copper eaten away.
I ruined my second board by not agitating it enough, leading to a longer etching time, leading to the etchant eating the photo resist coating and causing massive pinholes and pitting throughout the resist.
Keep it in the etchant for as short a time as possible, always rubbing and wiping it with the foam brush. You shouldn’t have to worry about damaging the photo resist with the foam brush, all you’re doing is wiping away the gunk so the etchant can do its work.
For this board I started on the inner area with the traces until the unwanted copper was gone, then worked around the perimeter and less critical areas. When you think it’s done wash it in the water bath and make sure all the unwanted copper is gone.
Remember you can always put it back into the etchant it to remove more copper, it’s much easier to take it slow than to accidentally over etch the damn thing and have to start all over again.
This is what my board looked like after etching:
There are a small amount of pinholes in the ground plane when held up to a light, but this doesn’t really matter, this board is totally usable so I’m going to keep it and start populating it.
That’s about it. I’m sure I left some stuff out, I just learned how to do this myself.
If there’s something blatantly wrong or fucked up let me know and I’ll edit it, or if there are any questions I’d be happy to go into more detail.
Remember this is my third attempt ever at this. Anyone can do this. Don’t make the mistake of thinking you aren’t capable, you’ll be surprised at what you can do.
Artwork: It is extremely important to get the black to be as opaque as possible, this is why I had to use three sheets aligned on top of each other.
I’ve heard of people also using tracing paper, at the cost of a longer exposure time.
I’ve also heard of people using inkjet transparency sheets with good results.
UV Light Sources: I’ve seen people use normal “daylight” CFL bulbs for exposing the board, I’ve even seen people place it in the sunlight on a nice day. I wouldn’t attempt this with an incandescent bulb though, they tend to put out very little UV.
The traces on this board are 16mil thick with a minimum of 16mil separation. I’ve seen people get down to 8mil traces with 8mil separation, but I have no need to get into that level of detail yet.
This board is destined to become a tachometer for my car, I’m replacing all the stock gauges and custom making my own to fit in the stock dash.