I use two main methods of PCB production; UV exposure and Toner Transfer. This page documents the Toner Transfer method, as this is the one I use most frequently - there's less mess, less chemicals, it's fairly quick and it's cheaper. The downside with the technique is that the results are not quite as nice as with photo methods; but 10/10 boards (and better) are quite achievable nonetheless.
In this example I'm creating a quick no-frills prototype board for a dsPIC30F2010 (with ICSP), which I intend to plug into a breadboard during the development of dsPIC30F2010-related projects. The bit of copper-clad board is just an old bit I've got lying around for just such purposes - as long as it's electrically sound it doesn't have to look brilliant, so bear that in mind when you're giggling at the wonky edges in my photos !
For schematic capture and PCB layout I use gEDA, which is an excellent suite of tools in the main. It has its nuances like every other bit of software, and there tends to be more work involved than with some other packages - particularly around symbols/symbol management - but being a vim person I like the key commands and shortcuts. And it's a native Linux app. Windows versions are available too.
Schematic capture tools are mainly personal preference or whatever is available, so I won't dwell on how to create the PCB layout on this page; rather, I will document how I produce the physical PCBs, assuming that a layout has been created and is ready to print.
As a side note, when I create the artwork for my PCBs I try to remove as little copper as possible as this increases the useful lifespan of the etchtant and also provides a good solid area for grounding. However, I tend not to leave floating areas of copper as this can cause unintended pick-up and other strange effects during circuit operation. I also try and avoid 90-degree angles in copper due to the increased probability of the traces lifting away from their adhesive, especially in places that are to be exposed to the heat of the soldering iron or likely to be scrubbed hard when cleaning the board - as far as I know, 90-degree angles do not increase EMI, so I do not avoid them for that reason. I have mentioned these two design choices since they impact directly on the process outlined on this page in a practical way, but there are many other facets to PCB design - too numerous to mention here. I suggest taking a look at David L Jones's fantastic PCB Design Tutorial as a starter-for-ten - it is also a good document as a refresher for things you tend to forget.
The idea behind the Toner Transfer method of PCB production is that toner from a laser printer can be used as an etch-resistant layer on a piece of copper-clad board; this means that it requires positive artwork (ie. what is printed is not to be etched). The main problem to be solved is getting the artwork from a piece of paper onto the board, unless of course your laser printer can directly print on copper-clad - if it can, there's no point in reading any further !
Some printers are better than others for this method, and the toner is critical too - it must be a copolymer-based type since the plastic needs to be able to melt and adhere to the board. My printer is a Brother HL-5270DN with an OEM toner drum and it seems to work quite well. There are plenty of other sites on the 'net detailing which printers and toners work best, so I won't attempt it here.
The three most critical parts of getting good results with Toner Transfer PCBs are the toner, the paper and the cleanliness of the board.
The type of paper is critical in getting a good result. I have tried many paper types, including Press-n-Peel. The Press-n-Peel actually produces good results, but it's a bit on the expensive side. I'll use it for final boards rather than the prototype boards like the example used in this page.
I found plain copier paper to adhere brilliantly, but its fibrous nature caused bleeding when touching up and it was also difficult to remove the layers. I also tried some glossy photo paper that I had on hand, but that was next to impossible to remove from the board; it basically melted, and the bits I could get off tended to lift the toner with it; although what toner was left on the board looked fantastic ! There are many different brands/types of photo paper, so you may have more luck with it than myself; maybe less heat would have been a good idea too. The key with Toner Transfer is really lots of experimentation and then settling on a combination that works well for your needs.
The paper I use is actually junkmail. I get glossy brochures posted through my door fairly regularly, or can pick them up easily enough from pretty much anywhere. Some magazines work well. The paper should be glossy and fairly thick, but thinner than photo paper. Try to get a piece that is mainly white - the inks won't generally affect the outcome since they're not resistant to the etchtant, but it makes spotting defects difficult if there's text or a picture covering the copper where there should be toner.
For this example, I've chosen to use a page from a beauty products catalogue. I know this works as I've etched a few other boards from it over the last few weeks. Besides which, all the bits of nice paper I'd put aside for this purpose mysteriously disappeared after a certain significant other 'tidied up' !
My printer has an MP Tray so that I can feed it non-A4 paper of a variety of thicknesses. I tend to cut the paper into a 4" x 6" piece and print onto that, which is a more than adequate size for a lot of my boards. If your printer does not have such a facility, it may be necessary to print onto an A4 (or whatever size paper your printer takes), and perhaps tape the (smaller) PCB paper to it. If you attempt this, be careful that the tape (and adhesive) doesn't melt/stick into your printer - and that the total thickness of paper is ameanable to your hardware too.
I print at a resolution of 1200x1200dpi; anything above 600dpi should produce decent results. I turn toner-saving mode off, set the maximum toner density available, and turn off duplexing - sometimes the paper will blister slightly due to the fixing temperature, so I don't want the paper to make more than one pass. I also turn off any page protection and set the paper type to transparencies; this lowers the fixing temperature (since transparencies may melt), which will allow the toner to be more easily transferred to the copper-clad board when it becomes time to iron. That's the theory anyway - though it also seems to work alright for me in practice !
The resulting artwork can be seen in the image on the right. The magazine print can still be seen but should not interfere with the etching. Note that this is artwork for the bottom-side of the board, and it is printed as seen from the PCB layout tool (ie. looking top-down). If printing artwork for the top-side of a board then it will need to be mirrored. It will be obvious when seen if the artwork is back-to-front anyway.
After the copper-clad board has been cut to size and sharp edges sanded off, it needs to be cleaned - it needs to be absolutely spotless. Any dirt on the board will prevent the toner from adhering. The materials used to clean the board are shown in the photo. These are:
- A pan scourer - nylon I believe. The abrasive side is the green side of the pad in the picture, which is the side used to clean the board.
- Washing-up liquid. I use Fairy to keep my hands fairy soft...
- Kitchen roll or paper towels. This is to dry the board and also to clean it; the material is not particularly important, the main thing is for it not to leave lint or other particles behind.
- Methylated Spirits (Denatured Alcohol), or a ketone-based solvent such as acetone will do.
The board is first scrubbed with the scourer and the washing-up liquid; this type of detergent is particularly good at removing grease, and is obviously commonly available. Scrubbing will remove the layer of oxidation that has built up as well as any grease from hands, etc. Scrub in all directions, not just up/down or left/right, and be careful not to touch the copper side after scrubbing - finger prints etch remarkably well. Rinse with warm water after scrubbing, then dry immediately with the kitchen roll to avoid water stains; wipe rather than dab, since textured paper may leave a slight imprint.
After drying, pour some methylated spirits onto a piece of kitchen roll and wipe the copper surface all over. The meths will evaporate, so thre is no need to dry it afterwards. Repeat this process until there is no more blackness (dirt) visible on the kitchen roll; notice this in the picture below (right).
The board is now ready for the toner; keep it completely clean from now until after the etching (ie. don't touch the copper !)
To the right is an image of the items that I use to perform the transfer. There is:
- A tray of cold water; no additives, just straight from the tap. Some people put soap in it to lift the paper more easily, but I found that not to be necessary.
- A toothbrush (an old one...don't put it back in the bathroom). This can be used to gently remove paper, in a much less abrasive manner than the scourer. A used one is preferable as it will be slightly softer than a new toothbrush, although you should make sure it's nice and clean.
- Masking tape; used to tape the artwork to the board.
- Scissors to cut the masking tape.
- An indelible marker pen with a fine tip is used to touch up any holes or broken traces after the transfer.
- A scalpel; used to remove small bits of paper after the transfer.
- An old iron; make sure this is not going to be needed (ever again) by your significant other - accidents do happen ! In an attempt to make PCB etching more exciting (aka 'Danger PCB-ing' !), I foolishly used the 'good iron' when she wasn't looking - then summarily proceeded to break it by leaning on it too hard. For Danger PCB-ing you need to be able to run fast and/or duck pretty well; in my case I had to buy her a brand-spanking-new sooper-dooper-mega-delux-does-everything iron - but at least I've now got my own iron now :)
- The paper with the artwork on it.
- The copper-clad board that needs to have the artwork on it.
- Two sheets of grease-proof paper; the stuff you use when baking. This is good for keeping the bottom of the iron clean and allows it to slide easily over the board without sticking.
- A firm bit of wood to rest on; the example in the picure isn't terribly firm or good for this purpose, but it's what I had to hand.
The first task is to turn the iron on. I have mine on the highest possible setting with any steam generation and any other fancy widgets turned off. Some people have reported that the iron shouldn't be at its hottest setting - I guess this depends on the iron and on what you're ironing. For example, I found that photo paper sticks to the board; a lower setting on the iron may prevent this. For the paper I use this isn't an issue. Experimentation is the key here. While the iron's warming up, move onto the next step.
The second task is to align the artwork on the copper-clad board. I cut the paper down to size but leave a long edge that I can use to fold around the board for taping in place (see pictures below). The adhesive on the tape will melt onto the board, and it's difficult to get off; make sure you don't get any on the copper side as it's a good etch-resist. For double-sided boards, I will have drilled two holes (before all the cleaning) - preferably in opposite corners of the board - that correspond to holes on the artwork. With two pins (from significant other's sewing basket) you can skewer both bits of artwork (with the board sandwiched between) and then tape them all together to hold everything in place; the pins can then be removed as they were only for alignment purposes. This hasn't been shown in the photographs as I only needed a single-sided PCB this time around.
When the iron is up to temperature place the board between the two pieces of grease-proof paper - artwork to the upside - and make sure that the artwork is flat on the board. Put the iron directly over the board and then lean on it hard for one minute without moving. If the board is bigger than the iron then this bit's not going to work too well ! You will have to repeat the process in multiple places.
The pictures illustrate the initial ironing - note the white knuckles from the hard leaning :) The idea behind not moving the iron is that the artwork needs to stick to the board so that it won't move around (and smudge) when ironing proper. The pressure is to lower the melting temperature of the toner slightly and to press it firmly into the board. For double-sided boards I will do this step for one side then flip the board over and repeat.
After the one minute has passed, switch to ironing the board with the edge of the iron - side-to-side, up-down. Do this for about one and a half to two minutes, making sure that you iron over everywhere - make sure you don't forget the corners. Flip the board over and do the reverse side if it is a double-sided board. Once the ironing has been done I drop the board into the tray of cold water by picking up the (somewhat cooler) greaseproof paper 'pouch' and tipping the board out; this way I don't have to burn my fingers by touching the rather hot bit of copper I've just ironed. Some people use a modified laminator instead of an iron for the toner transfer; this seems like a really good idea, and one which allows you to get an even pressure on the board. It's certainly an approach I'd like to try sometime - but for now I don't have a laminator.
I leave the board in the water for a minute or two, then try peeling away the top layer of paper. It will most likely come off in bits rather than one continuous strip. I remove the masking tape at the same time too. If you use the right type of paper, you can apparently just leave the board in the water for 15 minutes and it will peel itself; but being an impatient person, I tend to not want to wait that long.
I will gently rub the paper with my thumb and use the toothbrush to gently scrub the layers away. The toothbrush can be used fairly firmly, but don't brush too hard or agressively as the toner will lift. If the toner lifts with anything but the lightest of brushings then the board was most likely not clean enough, or the toner is not of the correct type. Usually, some areas will peel quicker than others when brushing and then the toner will start to flake away; or it may just be that a spot was missed with the iron. As long as the area in question is a small then it's nothing to worry about; that's why I have an indelible pen to hand (see the section on Touching Up for this).
I don't worry too much about removing the paper over the toner areas as it will not affect the etching. You will most likely not get all of the paper from between the areas of toner either, but this can be rectified later - again, see the section on Touching Up. I tend not to push my luck when brushing, and would rather leave little bits of paper bridging tracks than scrub off toner trying to remove the paper. Below are two pictures, the first from immediately after I've stopped brushing and the second after the board has dried in the air for a few minutes.
Notice that the dried paper has gone white; see the top-right of the board in the second photo for some white paper covering the copper between tracks. The second photo also shows some of the magazine writing, but this will have no effect on the etching as long as it covers toner. This board actually turned out to be quite tidy and had little lifted toner, which is not always the case; often there is a break in a track or the solid toner areas have pinholes (see top-left in the second photo). As long as these sort of defects are not too prevalent they can be touched up easily enough, which is the subject of the next section.
To touch up the transferred artwork I use a fine-tip indelible pen that is resistant to the etchtant and a scalpel; a craft knife or other similar sharp and narrow blade would work equally well. This is the time to look over the board and ensure that there are no broken, peeling or flaky tracks, and to ensure that there is no paper or toner bridging what should be bare copper.
The paper between tracks will be brittle and will simply scrape away from the board. The broken tracks can be carefully re-drawn with the marker pen; if there is bleeding when touching pen to paper then a different type of paper should be used - however, fibrous paper that causes bleeding should have been spotted long before this part in the process.
You can just about see the green marker pen in the image, although the contrast is not particularly good; this is more to do with the lighting in my photograph, as the marker is very visible to the naked eye. I will usually wait for the marker to dry, then go over it again to add another 'coat' as further protection against the etchtant. If I slip or wobble with the pen then the scalpel can be used to remove it from the copper areas when it is dry. If there are any small pinholes or areas that I think could do with re-inforcement, even the track is not broken, then I will touch-up. Pinholes can be a particular problem, especially for lower-resolution printers.
The board is now ready for etching. Ensure that the board is not left too long - say greater than a day or two - before etching, otherwise the copper will develop an oxide that will make it harder to be etched away, increasing the likelihood of the toner lifting before the metal does.
To etch the board I use a heated bubble-etch tank and a ferric chloride solution, which allows a decent resolution for the artwork details; ie. 10mil tracks/spacing are easily achievable. Below-left is a picture of the tank I use, and below-right is a picture of the rinsed board immediately after etching. This part of the process requires gloves and old clothes as the ferric chloride will stain anything it touches. Goggles are also a good idea because occasionally there are splashes. Just use common sense and follow the instructions that came with the chemicals/equipment. Spills are best avoided, but if necessary sodium bicarbonate (baking soda) can be used as a neutraliser.
The etching takes only a few minutes, depending on how old the etchtant is; the older the etchtant the more copper it has dissolved, thus the lower its effectiveness. I typically etch for a minute, check and rotate the board and then put it back in the solution - the rotation is intended to produce a more evenly etched PCB and avoid over-etching or under-etching parts, although this is less of a problem with a bubble-etch tank. I repeat this process several times with varying - often lessening - delays until I can see that the excess copper has been removed. I keep a plastic tub of water near the tank so that I can dip the board into it to quickly remove the etchtant and check the progress.
Sometimes during etching the marker pen from the touching-up, or perhaps some of the toner, will be removed to expose the copper underneath. If this happens then I dab the board with a paper towel and re-apply the marker pen before putting the board back into the tank - another good reason to take the board out regularly rather than leave it in the tank for minutes at a time.
On occasion, especially if dealing with fine traces or detail, I may take the board out of the etching tank while there are still copper bridges between tracks. As long as there aren't a great deal of them, and they are not too large, they can be removed later with a scalpel.
Rinse the board thoroughly under running water after etching to remove all traces of ferric chloride. The gloves can then be removed and the process of cleaning the board of toner can begin.
After etching, it's time to break out the trusty green pan-scrub and washing-up liquid again and clean the board just like in the Board Preparation stage. The toner and what's left of the paper will take a fair amount of elbow-grease to remove; a ketone-based solvent such as acetone can be used for much quicker results if you have any to hand. I use nail-varnish remover for this as it's very cheap and readily available. The picture below-left is of me just beginning to remove the toner - the white powder on my hands is from the gloves I used when etching.
After scrubbing, rinse the board in warm water and check it for any defects. Any broken tracks will mean either the board needs to be done again or has to be bridged with bits of wire when soldering; this looks messy and is fiddly, which is why I prefer a slightly under-etched board rather than compromising tracks by etching for longer. Any tracks still bridged with copper can be tidied up with the scalpel. A multimeter with an audible resistance test is handy for checking for shorted tracks without straining the eyes.
Once the board has been scrubbed and checked for defects/fixed, I use methylated spirits and kitchen roll to give it a final wipe-over. If acetone was used instead of washing-up liquid then this step can be skipped. After drilling you will need to give the board another wipe-over with meths to remove the dust too. Although not as imperative as during Board Preparation, take care not to touch the copper on the board until after soldering, just to reduce the possibility of creating dry joints. The picture above-right shows the final board ready for drilling, soldering and stuffing. Job done - at least in terms of board creation anyway !
