Thursday, 24 April 2014

Resistance is not futile

I’ve been playing with toner print transfer for PCB creation for a little while now, with varying degrees of success.

There are a couple of things that I’ve learned from my experimentation and I’d like to save others from the frustration and failed experiments that I’ve had.

In case you are not aware of the process, I’ll give you a brief overview.

  1. Design your circuit in software. Fritzing is my preference, there are several others out there … but after evaluating a few, Fritzing is a good match for my knowledge, experience and the way I design.
  2. Print your circuit in mirror format on a laser printer. This really is the heart of the toner transfer method. Laser toner contains particles of plastic. These particles are “welded” together and onto the paper. When you heat the toner sufficiently, it becomes sticky and will adhere to another surface.
  3. Place the printed image (print side down) onto your copper clad board.
  4. Iron the paper into the board using a hot iron.
  5. Soak the paper away from the board.
  6. Immerse the board into an acid bath.
  7. Clean off the toner.

That’s the process in a nutshell. However, there are a couple of things that you need to keep in mind or do to prepare.

IMPORTANT: ACID IS NOT A TOY. EXERCISE DUE CAUTION AT ALL TIMES. I do not take ANY responsibility for you playing the idiot with acid. If you are a minor, YOU MUST GET YOUR PARENT/GUARDIAN TO ASSIST OR GIVE IT UP NOW!

The paper that you use in your laser printer is important. Some folk swear by using (recycling) magazine paper. This is, typically, a thin, semi-gloss to glossy paper. Others say to use a good quality photographic paper. I’ve experimented with different papers a bit and, for my money, a fair quality A4 laser printing paper (80gsm) is good enough. I get more consistent results using printer paper than anything else.

When you put your printed image on the copper clad board, cut it down to size first. You’ll also have a better result if you use some sticky tape to hold it down. Don’t use much sticky tape, and trim it to make it even less. The tape is only there to hold the paper onto the board until the toner becomes tacky. By then, the adhesive on the sticky tape is likely to have melted off and, for that matter, the sticky tape will be permanently attached to the paper.

If you are ironing the PCB on an ironing board … the ironing board will be your enemy. It has too much give and the PCB will wobble away from the heat with pressure. By all means, use that ironing board, but put a piece of wood under your PCB (at least as long and wide as the iron is long). That’ll stop the PCB getting an uneven heat and will improve your transfer immeasurably.

The time that you take with the iron is another bone of contention. Some folk say to iron that board for 4 minutes, well, I’m here to tell you that I iron for about 2 minutes, 2 1/2 minutes tops. I’ve had the paper char and start to smoke if I go even close to 4 minutes.

Soaking is good … but soak in hot water. I will boil the jug before I set the iron to the board. When I’ve ironed the transfer enough, I’ll set the iron aside and pour about 3 cm of boiling water into an old ice-cream container and then drop the board straight in. When the water is cool (that is, almost cold), then I’ll take the board out of the water and rub the paper away.If you need to, boil the kettle again and re-soak to get any persnickety bits of paper away from the board.

When you’ve removed the paper, you may notice that you’ve got some gaps or holes in your transfer. Well, that happens. The thing to do to recover from this situation is to fill in the gaps with a pen that uses an etch resistant ink. If you go online and you search for etch resistant pens … expect to pay a lot. If you are not as rich as Croesus, then go to your stationers or art shop and buy a Staedtler Lumocolor Permanent marker. They come in a range of thicknesses from (what I’ve seen, this isn’t a definitive list) M medium to F fine … there’s a S small? in there too … but I haven’t really looked at it’s nib. I have a Medium tip, and it does me pretty well and only cost me $4.05 AUD from the art supply store (Artery, in Hobart). I tried using a Chinagraph pencil as well, for comparison, and found that, while it does lay down an etch resistant material, the line is sketchy and leaves me with a pitted and scratched looking trace. I guess that is because of the way that the pencil works on a hard surface. I also tried some free-hand squiggles with the marker, and it worked out just dandy. Of course, Staedtler isn’t the only manufacturer that makes a permanent ink marker with etch resistant properties, but that’s what I’ve tried and succeeded with.

The art supply shop also showed me some other etch resistant materials. One, that comes in a 120ml bottle at $44 is used in lithography and so I guess that would work well. The other a snip at $22 for a 200ml bottle was a very thin latex that is used in a variety of other artistic pursuits, such as watercolour and silk screen printing. The down side to these other materials is that they are applied with a paint brush. Now, I have plenty of fine paint brushes from my war gaming days and I concede that on a per millilitre basis, even the $44 bottle is much cheaper than the marker, but I baulk at having to paint it on with a brush … my eyesight ain’t that good any more. At this stage, you should also clean off the sticky residue from the sticky tape with some Isopropyl alcohol (also from the Chemist). The sticky crap is also acid resistant and will leave messy copper splotches on your PCB.

When it comes to acid baths, there are really only two choices. Ferric Chloride or Cupric Acid. Ferric Chloride is too much of a pain for me and is much more expensive. Cupric Acid, on the other hand, is much cheaper and easily obtained. Simply mix 2 parts Hydrogen Peroxide with 1 part Hydrochloric Acid and you get the basis of your etchant. The Hydrogen Peroxide comes from the chemists (yes, that’s right, the stuff people use to make their hair blonde). The Hydrochloric Acid comes from the hardware store (in the brick cleaning section). The main downside to this acid is that the peroxide evaporates fairly quickly, so only make enough for what you are doing today, don’t expect it to be useful tomorrow. Also, make sure you add the acid to the peroxide, not the other way around. Exploding baths of acid in your face can be a bummer. Dispose of your waste material by taking it to a chemical waste site … NOT by tipping it down your sink. Cupric acid will take a while to etch your board, so don’t plan on either populating your board immediately, OR, leaving it unattended. It’s really not a good idea to sit your PCB in the acid and then go shopping, too many things could happen that could really ruin your day.

When your board is properly etched it will be a bit transparent. You can test the etchedness of your board by taking it out of the bath and giving it a thorough rinse and dry and then hold it up to the light. You should see your copper traces clearly through the board. If this is OK, then get out your multimeter and switch to continuity test. Test the board by placing the anode (or cathode, it doesn’t matter which) on a trace and the other one (cathode or anode) onto the field of the PCB … where there isn’t supposed to be any copper. If you get a beep, then it’s back to the bath. You might also want to do a regular continuity test from one end of each trace to the other to make sure that your traces are, well, continuous.

So now your board is thoroughly etched, rinsed and tested. Now you want to clean off the etch resistant toner/marker. For this, you’ll need to raid your wife/daughters vanity for some acetone. Alternatively, you can buy your own when you go to the chemists to buy your hydrogen peroxide. Apply some acetone to a cotton swab, cotton bud, tissue, thumb, and rub the offending marks away “Out! Out! Damned spot!” ( I thought that Lady Macbeth was trying to get rid of her dog). You may need to discard the blackened cloth/swab/bud and get another to complete the job.

If you didn’t remove all of the sticky taper earlier, you will probably need to scrape the copper splotches away from the PCB where they were.

At this stage, you should have a usable PCB that is just waiting to be drilled and populated. Compare it to your on-screen circuit design, it is not uncommon to have your PCB back to front because you mixed up the positive and negative images. I know I’ve done it enough. You can cut your dud PCB into little pendants and earrings for your Wife/Girlfriend/Daughter, honestly … they’ll love them (sure).

If you are feeling a little adventurous, you can print out the silkscreen side of the PCB (also mirror) and transfer toner print the front of the board too! If you do, drill your holes first so that you can better match up the silk screen print to the underside of your PCB.

Anyway, that’s it for now. Have fun!

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