Homemade, doublesided SMD circuitboards using the toner transfer method.
There are several ways to make your own PCB's at home. Back in the old days, when most projects used mostly through-hole components, it was relatively easy to make your own PCB's at home. Depending on the technique you used, you could get good results without too much of a fuss. The most surefire way of getting consistent results was, and still is, using photosensitive PCB's.
For quite some time now, another option has been available: the so-called toner transfer method. Essentially, what how it works, is by printing your layout to a piece of glossy paper and then use heat to transfer the toner, which is a form of plastic, to a naked copper clad board. The toner then forms the etch-resist material. Using the toner transfer method, you can get really good and consistent results, providing you use a laminator and proper paper. It can be a tricky a job if you do this for the first time, or have bad luck finding proper paper.
I thought it would be fun to try this method out for myself. In this article I present the results of my toner transfer experimentation.
NOTE: The solvent I use to clean the board (and dissolve toner) is called 'wasbenzine' in dutch and in the Netherlands it it readily available in supermarkets. It is a slightly less dangerous solvent compared to thinner and works well as a degreaser. Do not use terpentine to clean the board as it is not usable as a degreaser. I was not able to find the english translation for 'wasbenzine'. Petroleum Ether and Lingoine were two terms I found, but am not sure if those substances are actually the same as 'wasbenzine'. I found that acetone did not work for me. Thinner will surely work, but I do not use it because it is even more of a nasty substance than 'wasbenzine'.
Some useful facts
- This method will only succeed if proper paper is used
- Scrubbing and cleaning the copper is a very important step in the process
- A laminator will save you a lot of hassle, especially for double sided designs
- Using this technique I have easily created boards with traces and pads up to 8mil (0.2 mm)
- Aligning double sided designs is tricky and requires practice
- Single sided, through-hole-only designs are a lot easier to do than dense, double-sided SMD designs
Before you get started, you need to make sure you have all the stuff you need.
- Hi resolution bitmaps of your pcb layout (use 1200 dpi)
- Glossy paper
- copper clad boards
- A laserprinter capable of 600 dpi or more
- A laminator (I got mine at a DIY store called "GAMMA" for 15 euros)
- A sheet of plexiglass or normal glass. Use this to align two sided layouts
- A very bright lightsource (at least 60 Watts when using an incandescent bulb)
- Scotch tape
- A scotchbrite sponge
- Pritt glue or some other glue-on-in-a-stick
- A pan with clean, warm soapwater
- Toilet paper
- A solvent (thinner or 'Wasbenzine')
I have tried at least fifteen different types of paper; laser printer photo paper, inkjet photo paper, thin glossy magazine paper, thick glossy magazine paper, regular white printing paper and more. Eventually I found a medium thickness, glossy paper which works perfectly. It is the paper used for the magazine for Dutch supermarket called "Nettorama" and the magazine is called "Boodschappen". I read that lots of people have good luck using photo paper, but I much prefer using magazine paper. It is both cheaper (and even free in some cases) and has better qualities for toner transfer. Just try all the glossy papers you can get your hands on and you will find a good paper type in no-time.
I found out that the paper has some very specific requirements.
- It must accept being printed on by a laser printer. Some papers I tried showed signs that the printer was not able to properly print on them because the toner would not stick to the paper, thus creating holes in the layouts.
- It must not deform when being printed on. Quit a few types of magazine paper I tried ended up getting warped by the transportation mechanism and / or the heat of the fuser, making it impossible to properly align my double sided layouts.
- It must release the toner with ease. After you have ironed or laminated the paper to the board, you need to soak the paper off of the board. Some papers, like photo paper for laser- and inkjet printers, are very thick and somewhat plasticized and do not easily absorb water. This means that you need to soak for a very long time, which can be annoying, and even then the paper does not always release the toner, causing holes in your layout. The best types of paper only needs soaking for no more than 10 to 15 minutes and then can be pulled off the board very easily. This type of paper is usually a bit thinner than regular white printing paper and feels very smooth and looks glossy.
- The paper must not remain embedded in the toner. Some types of paper are never completely removed from toner, making the traces and pads 'hairy'. These hairs are paper remnants that are molten into the toner and they are bad because they can easily create shorts, especially on dense layouts. Glossy paper hardly ever has this problem because the toner isn't able to penetrate it very far. The gloss prevents the toner from entering the paper. That is why you want to use glossy paper.
- For double sided designs: The paper must be as contains as little dark print as possible, because you need light to shine through the paper to be able to align the top and bottom layers. Just select the pages that contain the lightest or no images and you'll be fine.
Create 1200 dpi bitmaps of the top side and the bottom side of your layout. I use Cadsoft Eagle (link) for my designs. In my 'eaglerc.usr' (found at C:\Documents and Settings\username\Application Data\CadSoft\EAGLE) I have the following shortcuts to easily create the top and bottom layouts:
Brd.Key.C+F11 = "dis none 16 17 18 19 20 116"
Brd.Key.C+F12 = "dis none 1 17 18 19 20 116"
Brd.Key.SC+F11 = "EXPORT IMAGE bot MONOCHROME 1200;"
Brd.Key.SC+F12 = "EXPORT IMAGE top MONOCHROME 1200;"
Control+F11 displays all of the top layer items and shift+control+f11 exports that image to the file "bot.png" at 1200 dpi. F12 does the same for the bottom layer. See the page on Eagle tips for more info (link).
Prepare the bitmaps for printing
The Gimp is very useful for this. Most important thing to do is:
- invert the images so that all copper is black;
- flip the top layer horizontally;
Also , see my page on Eagle tips for more info (link)
Two layers on one page
For double sided designs, a useful thing is to print both layers to a single page; this saves time and paper. After generating the top and bottom images of you layout, load them in the Gimp. For the top layer do the following:
- Make all copper black : colors/value invert
- Mirror the to layer : image/transform/flip horizontally
- Resize the canvas : image/canvas size
- set units to millimeters
- make both x and y size 150 (or something else that fits your needs)
- select 'all layers'
- now click resize
- Fit the image view to the window : view/zoom/fit image in window
Now open the bottom layer and do the following:
- Make all copper black : colors/value invert
- Select all : select/all
- Copy all : edit/copy
Now activate the top layer and place the copied bottom layer into it. Drag the pasted layer to the bottom right corner and save the image as a png file. Answer yes to all the defaults when saving the image (flatten, compression etc.). You now have both layers in a single image and ready for printing.
Fortify the paper
I have found that the best paper for toner transfers is too thin to be properly handled by a laser printer. The thin paper is too fragile for the laser printing beasty and will usually jam up somewhere in the fuser. To prevent this from happening, you have to fortify the glossy paper by glueing it to a sheet of thick paper. Pritt glue is very good for this (link).
Apply a line of glue close to the top edge of a sheet of glossy paper. Then turn it around and stick it to a sheet of thick paper, close to the top edge of the thick paper. The glue does not need to dry; you can use the paper for printing.
Warning: If you do not apply the glue close enough to the top edge of the glossy paper, it will still fold up inside the printer. This will cause a jam in the printer and if you are unlucky, it will smear glue on your printer's drum, causing it to print black smears on all your subsequent printouts. This is bad. To fix a dirty drum, you have to remove it from the printer and clean it with a very soft cloth. Toilet paper also works. If the glue is dry and doesn't come off, then use a little spit. Pritt glue dissolves in water. But be careful: If you scratch the drum, all your printouts will contain smudges or white holes and you will probably have to buy a new drum. So be careful. If have gotten Pritt on the drums of both my HP Laserjet 5L and Brother HL2035 a few times and have succesfully cleaned both drums. Oh, and watch out for the toner: Most, if not all drums, are an integral part of the toner dispenser. There is a chance that toner will leak out. And let me tell you that you do not want toner on your clothes or in your lungs. Be careful.
Print the bitmaps to the glossy paper and inspect the printouts for any imperfections. I always place the glossy paper in the manual feed tray. If on inspection there are imperfections on the print, then throw it away and make a new print. For printing I always use the Windows application 'Irfanview'; it's a lot smaller and faster than the Gimp. The printer settings are very important. Things to watch out for:
- Use the setting "Original size (from image DPI)". This means that the image will be print to scale.
- Set your printer to use at least 600 or 1200 dpi. 600 dpi is usually enough.
- Do NOT use toner save mode. You want to have a thick layer of toner, so do not use toner save mode.
The result after printing:
Now cut the printouts to a manageable size. Slightly larger than your board is a good idea for single sided designs. Double sided designs require different treatment, as described next.
Cut out the two layers as shown in the illustration. One long and one short side are cut close to the edge of the board. The other two side have about 5 centimeters of spare paper. This extra paper will serve these purposes:
- to stick the paper to the glass sheet during alignment; the extra paper is needed for the sticky tape
- to form a sort of an 'envelope' during the laminator process; after algning the two layers, they will be glued together on this extra paper
If creating a double sided PCB, then align the top and bottom layouts using the sheet of glass and a bright light source. I have a special technique for this which I got from this fellow (link).
Grab a (plexi)glass sheet and place a bright light source under it. Place one of the two layers on the sheet, with the printed side up. Use some sticky tape to fix the paper to the sheet, as shown in the illustration. You now see one of the reasons why we left some extra paper on the cutout: we now have some extra paper to put the sticky tape on.
Now place the other layer on top of the one you just placed on the sheet. This layer should have the printed side towards the bottom, so both printed sides will be facing each other.
Now align the two sheets as good as you can. This can be tricky if you've not done this before. Also, sometimes it is impossible to align the pieces because some types of paper deform when they are fed through a laserprinter. In that case, you need to find a better type of paper; one that does not deform.
When the two layers are aligned, glue them together using a glue stick like Pritt.
Now, we are done aligning. Do no forget to cut away the sticky tape from the paper. If the tape somehow gets into your printer, it will deposit glue on your drum and also melt in the fuser section. You do really want to avoid that from happening.
Clean the board
Scrub and rinse the copper clad board. Do this just before you intend to use it. If you clean it and let it lay around for a while, the copper will oxidize, which is bad for the transfer process. So, a very important part of the process is cleaning the copper clad board. It must be really, really, REALLY clean. No fingerprints, no dust, no big scratches. First, place the board on a cloth, to prevent scratching the bottom side. Then remove the oxides by scrubbing with a scotchbrite-like sponge. First scrub the board horizontally, until it is nice and shiny, then scrub it vertically. Double sided boards naturally get both sides scrubbed. Try not to get any deep scratches on the board.
Next remove the remaining dirt and copper dust from the board. After scrubbing with the sponge, use a piece of cloth, or toilet paper, and place some solvent (thinner) on it, and then rinse each side until no more dirt comes off the board. When done, carefully store the board in a piece of cloth and prevent it from becoming dirty. Be especially careful not to get any fingerprints on the board. The sweat, salt, fat and acids from your skin will not do your board any good.
The actual transfer process
In this step, the toner will be transferred to the board by heating the paper and the board and simultaneously applying pressure to both. The toner will melt and stick to the copper. Two factors are very important here: Heat and pressure. The heat needs to be just right. Too much heat will cause the toner to smear out and too little heat will prevent the toner from properly sticking to the copper. As for pressure: It needs to be big enough to force the toner onto the copper and evenly distributed so that all parts get evenly compressed. Too much pressure or unevenly distributed pressure will cause the toner to smear and too little pressure will prevents the toner from sticking to the copper.
There are basically two methods for the heat/pressure step: One involves ironing the paper onto the board and the other involves using a laminator. Use a laminator. I have tried using an iron to transfer the toner, but the results were terrible. I got smeared-out traces in places where it got too hot and on other spots the toner didn't adhere to the copper because it didn't receive enough heat. And that was just on a single sided board. Double sided boards would have been even harder to get right. So I bought a cheap laminator and immediately got good results. Others have reported good results with an iron, so you might want to give it a go and try your luck. For double sided boards I would not even try using any iron. Go straight for a laminator. They're cheap and save you a lot of hassle.
Some people modify their laminator to deliver more heat. The only thing I changed was to cut out a large part of the plastic cover to allow me to better guide the boards through the laminator.
First pre-heat the board by placing it in a piece of clean scrap paper and feed it through the laminator about six times. The toner will stick much faster to a hot board than to a cold board.
Next, place the board under or between the layout(s), depending on if you have a single- or double sided layout, and the feed it through the laminator for about 30 times at several different angles. The first two or three times, you need to guide the board and the paper with your hands, because the toner is not yes stuck to the board. After two or three times, the toner will be stuck to the board and you can let it go freely through the laminator because the paper will not shift any more. Here you can see my laminator in some spectacular action shots.
After the heat-transfer bit is over, place the PCB in a pan with warm soap water and leave it to soak for a long as needed. The duration depends heavily on the type of paper used. The paper I currently use only needs 15 minutes to come loose completely and easily.
Carefully remove the paper. If the paper gets pulled off too roughly, then it might take along some toner and ruin the transfer. Sometimes not all the paper comes off on in one go. If the stuck paper is still a bit dry, then soak it some more. Otherwise you can gently rinse of the paper with your finger, under a running tap to wash away the debris. Usually you can be quite rough while rinsing the board. I was surprised to see how well the toner sticks to the board. But don't try your luck; if you accidentally rub off a track or a pad, then you can start anew.
Also, usually there will be a very thin layer of ink, or glue, from the glossy paper, still covering your board. This layer will sometimes clog up very small holes, like vias and drill-aid-holes. So gently try to rub away any goo that is clogging any holes.
Check the board for any imperfections. If there are any damaged pieces of the layout which are not easily fixed, then you will want to start over. Also, check double sided layouts to see if they are aligned properly. This is not very easy to determine by eye. But very badly aligned layouts will be obvious, so you can at least check for that. If a transfer has failed, then proceed to prepare the board for re-use. Place the board on a piece of cloth, to prevent deep scratches on the bottom side. Then take a couple sheets of toilet paper, fold them a few times, and place some solvent on it. Then proceed to rub off all the toner on the top side. Do the same for the bottom side. Make sure you properly take off all the toner. Now the board is ready for re-use. Do not forget to scrub and clean the board again before using it, as described above. The board must be absolutely clean.
If the toner transfer is complete, then the board is ready for the next step: Etch! Here is a snapshot of my homemade bubble etcher. Notice the nice blue colour of the etchant! Fresh etchant is translucent, but as the copper dissolves and oxidizes, it turns the liquid into a very beautiful blue. Did I mention that this stuff is lethal when ingested and a single drop in your eye can blind you for life?
That's it. Feel free to comment on this article.