PCB@home -- using the toner heat transfer & muriatic acid + hydrogen peroxide etching procedures
There is an available pletora of how-to description regarding home made PCB boards. These are built around three different core trace transfer methods, the photosensitive method, the toner heat transfer method and the draw it yourself. The are a number of minor variations among them, from minor to greater tips on how to do this or that particular step. After doing some research and experimentation I decided for a procedure around a toner heat transfer method and present below the steps that led me to the best results.
The steps I've choose to work with were chosen based on:
- Ease of use
- Easily obtainable materials
- Use of non-specialized common tools wherever possible
So, when should I use this method? Well, that's up to you. Personally, I find fit using this method whenever I want to do a cheap, one time board, don't want to wait for a PCB house production/shipping time and for some reason a perf board wouldn't be enough.
Materials & Tools:
- Laser Printer
- Maganize paper
- Single-sided copper clad PCB board
- STAEDTLER Lumocolor® permanent universal
- Yellow tacky stuff
- Clothes electric iron
- Wooden cutting board
- Muriatic acid (HCl ~33%)
- Hydrogen Peroxide (~3%)
- Rubber gloves
- Protective Eyewear
- 2 Vats/tupperwear (around 1L in capacity each)
You should have a PCB layout that you either made, or gotten some other way. Given that I'm only considering a single-side board, such layout should have the Paths, Pads (and perhaps vias) of the bottom plane of a board. Given that the transfer will occur through the heat transfer method you SHOULD NOT mirror the layout.
Since I was fancying a ATmega Fusebit Doctor I got my test layout from here:
I've downloaded the latest project update and extracted the atmega_fusebit_doctor_V2h_bottom.pdf file.
I also designed a very simple second board using EagleCAD. This second board is just something to help me program my 28-pin ATmega chips and free up a breadboard and jumpers.
Above are the two board layouts I've used to test this procedure (my simple design on the right).
Step 1 -- Print the designs using a Laser printer
I've used magazine paper. Luckily, among my snailmail spambox at work I've found a book catalogue with pages just the right size (A4) and thickness (so as not to risk jamming the printer) and glossy enough that could be used to such effect.
Tips: If you don't have a laser printer at home and need to print your layout elsewhere, do several copies otherwise if something goes wrong with the first transfer...
Alternatives: Optimally is advised, by several people, the use of glossy photographic printer paper for this task, but I found the magazine paper to perform quite well. If you can't find paper the right size/thickness I've seen it recommend to just tape a piece of magazine paper to a normal sheet of paper (such piece should obviously be placed where the printing is going to happen and should be big enough to accomodate all of the effective print area). If you don't have access to a laser printer it is said that photocopying your layout will also work. However take in consideration that said photocopy should be also made using one of the aforementioned kind of papers.
Early test layouts printed on the aforementioned magazine paper.
Step 2 -- Toner Heat Transfer Method using a regular clothes iron
Turn on your iron (max temperature, NO steam). Set up a rigid heat-resistant board (I've used a wooden cutting board) on top of a table next to the iron. While the iron heats up put the PCB board with the copper facing up on top of the rigid setup. You should have pre-scrubbed the copper surface with something akin to Scotch Brite to remove copper oxides. Cut the printed layout to the desired/required size. Optimally there should be enough blank space left at the corners. In each corner place a tiny amount of yellow tacky stuff and then align the paper with the copper and press down on the "tacky zones" in order to firmly attach the paper into place. Place the PCB again on the rigid board now with affixed paper side facing up. Press down with the hot iron for a few seconds, until all the "tacky stuff" spreads and thins out (the paper should NEVER move out of place). The amount of "tacky stuff" used should be minimal and placed carefully in order not to spread over into the board layout paths. To cut the board into the right size just use a box cutter (x-acto knife) and a ruler to make a straight line from edge to edge. Notice the indentation in the photo below. Pass the cutter a few more times on the copper size, then turn the PCB around and from the other line can be seen clearly where the cut was made, align with a ruler an cut a few times on this size too. It should now be easy to cleanly break away the desired portion of the PBC by applying some pressure.
Among the technique variations that I've tried I've found that the more efficient one was doing 'micropressing short moves'. That is, using only the iron tip do short length presses in one direction while following a sweeping pattern that allows you to cover all the PCB transfer area. After the first full PCB sweep, rotate the board 90º full press it for a few seconds and repeat the previous process up until you have done a full 360º (3 turns, 4 full sweeps). See depicted procedure below:
After the iron pressing is all done carefully move the *scalding hot* PCB into a vat with cold water. Let the paper soak up until it is weak enough to be thumb-rubbed off. While, the PCB is still submerged carefully detach the "tacky" corners and then peel it the rest of the paper carefully. Rub off with your thumb all the paper fiber that you can from the board while it is still submerged. I found that some paper fibers adhere quite tightly to the board, mainly in-between some close paths and in the pad holes. These can be (and must be) easily removed with the help of a pin (tried the toothbrush way but found it damaging and unable to deal with tight spots). Just carefully scrap them off and they readily break away and can be washed off. It might require a bit of time and steady hand, but must be done.
Paper tends to get "stickier" in between some close paths.
If some of your paths/pads end up broken/gone/faded it is a good idea to do a little touch up using a permanent marker. I found the STAEDTLER Lumocolor® permanent universal marker to be particular useful to reconstruct elements that were lost during the transfer procedure. (The Sharpie brand is not commercially available around these parts). See below:
Step 3 -- Etching via HCl + H2O2
Now the fun, but also MOST DANGEROUS part. Extreme care should be taken while dealing with Muriatic acid. This acid is basically a solution of Hydrochloric acid which is sold in several concentrations and easily found on hardware stores, pool supply stores, etc. I believe the 33% HCl solution is one of the most common found (the one I use). You should work on a ventilated space, use eye protection, a mask wouldn't hurt (don't breath the fumes) and GLOVES. Hydrogen peroxide solutions are readily available at the supermarket, drug store, etc. It's easily found in 3% H2O2 solution and that is enough. (Have a pinch of salt at hand)
WARNING: If you are under age or over careless don't try this at home.
Recipe: Using a plastic cup measure 2 parts of peroxide into a plastic vat, and then carefuly add 1 part muriatic acid. Carefully stir the mix and then drop your board inside and gently keep on 'waddling' the vat (again, wear rubber GLOVES). After a few seconds you should see the mixture turning into a green colour and after a couple of minutes or so, you should start seeing the exposed copper disappearing and exposing the PCB (add a pinch of salt if you think the process is going slowly).
Once, all the copper is gone (leaving only the protected paths and pads) put the PCB into a vat with water. Wash it well, and just for paranoia's sake wash it again under plenty of running water.
Now, you just need to remove the toner from the copper with a bit acetone-soaked cotton.
And you're good to go... there's drilling to be done. But that's another story... or at very least some other chapters...
<INSERT REPORTS OF HANDDRILLING ~300 HOLES>
Step 4 - (Optional) Silkscreen, why not?
After drilling my boards I decided I could also transfer a silkscreen I had available for one of them. I've use basically the same method described above for heat transfer, just had to add a little extra alignment method.
Get your MIRRORED silkscreen print, your drilled board, some "yellow tack stuff" and a few pins.
Place a tiny bit of "tacky stuff" at each corner, use the pins and pierce the paper in a few strategic component placements. Put the respective pins through the PCB and align the print with the PCB using the pins as guides. Press over the "tacky" placement to set the paper into place.
Remove the pins, and apply the ironing technique described previously.
And 'voilá', after soaking and removing the paper I present you the silkscreen transfered.
Now it just needs to finish drying before it provides oportunity galore for soldering.
I would like to acknowledge the unlisted sources spread over the INTERNET and my fellow LMRians that helped me tune into this particular set of procedures, namely Hoff70, birdmun, Jerz, MetalMonkeeLad, TinHead & others (if you feel I forgot to mention you, then perhaps I did as much, so I'm truly sorry for that).
The outcome of my first two transfer & etching projects (ever) and using the above described techniques:
- The Fusebit Doctor project board:
Yes, it looks pretty. Alas, there are some wonky paths that did not turn out as good as they should, also my poor soldering skills have managed to create so solder bridging that I have yet to find. See my solder mess below:
Nevertheless, after correcting a few ragged traces (poor connections) and slightly banging against the board my atmega HV doctor now works and even has already "healed" the fuses of two atmega328p that I've gotten already "sick".
My second board was designed all by myself in EagleCAD, and while being a much simpler and less ambitious project turned out quite fine and is working like a charm.