The last few months have seen lots of etching attempts with plenty of trial and error, PCBs in the bin and lots of broken drill bits but I think I’ve finally cracked it. I found it difficult to find a decent “how to” article online so hopefully this might prove useful for anyone where I was 8 weeks ago.
It’s worth mentioning up front that there are other ways to do this, especially if you have access to a laser printer. I don’t, and only have an inkjet printer which rules out all of the various toner-transfer methods.
Obviously, there’s a lot of factors at play – types of chemical, chemical concentration, photo-sensitivity of boards, exposure brightness, etc. – but this should serve as a good starting point.
I’ve been using:
- Bungard FR4 photo-resist copper clad board
- Universal photo-resist developer
- Electroless tinning solution
Step 1 – Artwork
I’ve been using a combination of DIYLC and graphics applications to prepare my artwork. DIYLC lets me get the basic layout done, snapping pads to the default 0.1 inch grid, using perf board to double check component sizes and leg spacing.
Once I get things almost finished I’ll export the image to use for later component placement and launch my graphics application.
I have a template set up, again with a 0.1 inch grid, that has all of my pads, lines and outlines which I use to replicate the original design I created in DIYLC. I add any additional white areas to minimize fiddly lines and, if there’s space, I’ll add some branding or text.
I then trim things down and export the image at 100% size and 600dpi.
Step 2 – Print
I first print my design on standard copier paper using my inkjet printer. I have things set to use as much blank ink as possible and I configure my printer to print two copies per page at exactly 100%.
If this is the first time etching the design I’ll use the paper print out to check things like size of the board in my intended enclosure, pin spacing and component placement, just to be double sure before I go any further.
I then cut small pieces of inkjet transparency and stick them onto my paper print, each piece covering a print. I use sticky tape to stick them down and do this just on the leading edge – the edge that goes through the printer first – and then do the same print again.
This gives me two identical copies on the transparencies which I then leave to dry for a few minutes. Once dry I line them up exactly and stick them together.
Doing this means I get double the coverage of black – any small problems in each one gets eradicated as it gets doubled up.
Step 3 – Prepare the board
Using one of the paper prints I trim some board to size. I use a sharp Stanley knife and metal rule given each side of the board multiple scores. I’ve never actually cut all the way through the board but instead score it enough so that I can snap it. A quick sand on the snapped edges, and a check to make sure I haven’t accidently exposed the photo-sensitive side, and it’s good.
Step 4 – Expose the board
I expose my boards in our downstairs bathroom – it’s small and lets in very little natural light. I’ve built a simple darkroom light with an old USB charger and a bunch of red resistors – it’s not much but it’s enough to help me see what I’m doing in my makeshift darkroom.
I peel the protective film off the board and place my transparency on top of it, with the printed side touching the photo-sensitive copper of the board. I then sandwich this between some glass and hardboard (taken from an old picture frame) before carefully sliding it into my light. A bit of sticky tape can help keep things perfectly lined up and a bit of finger pressure on the glass during exposure helps keep the print in full contact with the board.
Exposure time will vary based on the board and light you’re using but I’ve found that 150 seconds exposure is right for me.
Step 5 – Develop
Still in my darkroom I take the board, peel off any sticky tape, and put it into my developer solution. It’ll stay in there for 7 minutes (again, this works best for my chemicals and board) where I’ll gently jiggle it every now and then. If I see any floaty bits lingering on the surface they get a gentle wipe away with a soft paintbrush.
After the time’s up I give it a quick check over under my red light – if it looks good with decent contrast and clear lines between the exposed and protected areas then I give it a rinse in water and a pat dry with a towel.
Step 6 – Check and adjust
Back under a decent light I inspect the resist.
If there’s any parts of the resist that look a bit pale or blurry then I’ll touch those up with a fine line etch-resist pen, just to be safe.
Step 7 – Etch
Etching has been a big area of trial and error. I’ll document what works for me but you should be prepared to find what works best for you with a few attempts.
I use Ferric Chloride and like to sit my plastic tub inside a larger tub of warm/hot water as the etching process seems more effective when everything’s nice and warm.
I drop the board into the etchant and jiggle it around every now and then to prevent the copper residue from lingering on the board too much.
I check the board quite frequently to get a feel for when it’s done. Once the etchant has worn down the bulk of the copper then things start to move quickly.
Beware – Ferric Chloride gives off some fumes when it’s warm and it will stain EVERYTHING it comes into contact with bright orange, including your skin!
When things are getting close and I can see clean, board where the etch has worked, I take it out and give it a quick rinse. Holding the board up to the light makes it easy to see where more time is needed.
I’ll do this a few times before I’m completely happy – I just keep an eye on the traces and pads to make sure I’m not over-etching those areas. If it looks like they’re etching away too much then I pull out the trusty etch-resist pen and give those areas a top up.
Step 8 – Clean and test
Once it’s etched I give the board a good rinse in water and then a clean with some nail varnish remover and paper towel.
Now that all of the photo-resist is gone I use a multimeter on continuity mode to test there’s no shorts where bits of almost-invisible copper remain.
Very rarely have I had a board come out with no sorts and a sharp knife is usually all that’s needed to clean up the gaps between the copper areas.
Step 9 – Tin
When I’m happy the board’s nice and clean still I give it a 30 minute dunk in my liquid tin solution. This gives the board a nice silver coating that makes soldering easier and prevents oxidation.
Once it’s done I rinse in hot water and then cold water before repeating my checks with the multimeter.
Step 10 – Drill
Last step before components is to drill. This is time consuming and took a bit of practice to get right – angle the drill bits too much through the board and they’ll snap.
I start with a small hand drill to get some pilot holes done. I remove just enough material to create a divot in the centre of each pad.
I then step things up with an electric handheld screwdriver with a drill bit attachment. Work my round all of the holes using the divots as a guide. I drill through with the screwdriver set on forwards and then set it on backwards to remove the drill from the material.