In previous posts (here and here) I described my efforts and research to develop a DIY method to make good quality SMD metal stencils at home. I have since experimented some more and I believe I found the best method (so far) to do this with very cheap materials, yielding very good results.
Since transferring the toner to the soda can aluminum alloy was the most challenging part, I kept experimenting with different transfer mediums and other ways of possibly facilitating and perfecting the transfer.
Finally the solution is here: consistent, repeatable, “perfect” toner transfer, every time.
Step 0 – Be safe
Be safe and wear a respirator mask, goggles, gloves and use ventilation! Your lungs were not designed for breathing acetone and acid vapors.
Step 1 – Prepare soda can aluminum
- Cut and unbend the aluminum on a flat surface
- Heat the aluminum with your iron for a minute. This will make the next step much easier
- The trick is to then chemically remove the interior can coating *without* sanding it off. Even grit 2000 sandpaper/steelwool will scratch the interior enough that toner will not transfer very well or could be inconsistent between attempts. A friend told me the interior coating is a resin epoxy layer. To avoid sanding, it can be removed with acetone from HomeDepot (the nail polish remover is not strong enough). Rub the interior coating with a paper towel soaked in acetone, it will wipe off fairly easily if you preheat the metal, exposing shiny metal. Wipe off the exterior paint as well.
Step 2 – Print the toner mask
See my first step through post or the youtube video for details how to prepare the cream layer and shrink the pads before printing – this is important because the etched pads should be ~30% smaller than the PCB pads to avoid dumping too much paste and cause solder bridging. How much you shrink pads is up for experimentation. In my case 6-8mil smaller on all sides works well.
For toner transfer I tried many things and initially I recommended transparency film. It works quite well. But there’s something even better: vinyl ! I use self-adhesive shelf vinyl from the dollar store. I got a roll of several feet for 1 dollar. Can’t beat that. It’s perfect because it’s very thin/flexible and does not wrinkle, also very easy to spread evenly. The adhesive backing sticks to printer paper, will never jam the printer and the print is high quality. Adjust your toner density if you can to print as much toner as possible.
Step 3 – Transfer the toner
I then use a stack of sticky notes to transfer the toner. I peel the topmost sticky note a little and sandwich the metal and vinyl in the opening then push the sticky note back flat. The sticky note’s adhesive keeps the vinyl from moving. I’m sure there’s many ways to do this, I just found sticky notes work well and it’s quick.
Then set your clothes iron to maximum heat, drain any water to avoid steaming. Let it sit on the sticky note stack for 30 seconds. Then start applying pressure back and forth for another 30 seconds or so. Then lift the metal and dip it in cold water for rapid cooling. Remove the sticky note. Then remove the vinyl gently, it peels off easily, no soaking, no rubbing. And voila, perfect transfer! Feels good doesn’t it?
Step 4 – Touch up and mask rest of metal with clear tape
The laser printer like everything ephemeral will sometimes have less than perfect prints with tiny tonerless spots. You can reprint or touch those up with a sharpie pen after the transfer. Then I tape everything in clear tape avoiding air bubbles on the back where pads etch through.
Step 5 – Etch and cleanup
I etch the stencil in 1:3 muriatic acid to hydrogen peroxide solution. In a few minutes the pads come through the other side. Watch for over-etching. Then rinse the acid in water, remove the tape and wipe off the toner with the HD acetone. Done!
Costs
- Soda can: free from recycle bin
- Acetone 1qt – $7 – HomeDepot (lasts a long time)
- Muriatic acid 1gal (31.5%)- $7 – HomeDepot (lasts “forever”)
- Hydrogen Peroxide (3%) – $1 – pharmacy
- Self adhesive shelving vinyl roll – $1 - dollar store
- Clear tape, sticky notes: $1
- Sharp tip sharpie pen: $1-2
- Your time: priceless
- Tools: laser printer, clothes iron, etc. (you already have these right?)
Conclusion
Metal stencils are a magnitude better than plastic stencils. Laser cut stencils are higher resolution than etched stencils but significantly more $$$. But when you spread the paste it won’t be a huge difference if you get the pads shrinking and toner transfer right. Just play around, you might mess up a few times, but this is really simple, really cheap, and really quick … takes about 30 minutes at most from start to finish, and it gets better as you practice.
Some eye candy to prove this works
 A stencil … |
 … and the final result (yes … 0.5mm pitch!) |
Felix,
The tutorial is awesome, but it left me with a few gaps to fill in… I did my best as was nearly 100% successful with a VERY detailed stencil on my first try… I got the scaling of the images used for the stencil slightly wrong since I had to shuffle images back and forth from Windows VM’s to the real world of Ubuntu… but on the second attempt… Utterly successful… and I learned a few tips that I’d liek to pass along that worked perfectly for me.
1. Rather than risk my Cheap (but still rather expensive color laser printer with funky papers I bought the cheapest black and white laser printer I could find… a Samsung ML-2545 for $69 from Frys Electronics. A littel fiddleing got me a very niuce device driver for Ubuntu from Samsung that installed easily.
2. Use Glossy Phopto Paper to print on (I bought a little packet of 50 sheets of 4″x6″ Canon “Photo Paper Glossy”, cost something like $12. I bought the small sheets becuase I will end up wasting a lot of the paper as I trim it to size for later.
3. Use Felixs’ recommendations for Viewmate (Windows.. ugh!) to finesse the gerbers prodiuced by Eagle, I ended up shrinking my D codes by only 0.005 since some of the pades for the TQFP 48 I’m using are too small to shrink any further, I think the resulting etched slots are around 2-3 thou is width… and ONE MY FIRST attempt I only had one gap between pads break through… ONE…
4. In order to clean the epoxy/varnish from the surface of the aluminium in the cans, Acetone works fine, but using paper towels to rub the stuff away is a pain and takes for ever… I used a conventional Scotch Brite Heavy Duty dish cleaning pad, the harsh green scrubbing side with only gentle pressure, almost immediately removes the epoxy with acetone and leaves a very gentle swirled pattern on the aluminium which I think may help adhere the toner in the following steps. Yes, I was surprised that the Acetone didn’t just dissolve the pad or the sponge… it was totally unaffected.
5. now comes the lucky part… transferring the image to the paper… I set the Samsung printer up to print to card stock (the only 4″x6″ paper size was listed as a postcard… whatever), this was a manual feed option for the paper, there is a slot on the front of the printer to feed manual paper supplies into.
I told the printer to print to the card, made sure that the image was exactly the right size… you can scale the image in the printer setting dialogue, i didn’t have to but it may be useful to move it in that dialogue to position it better on the card..
AND THE PRINTER JAMMED… it took the card in, deposited toner on the card in the exact position I wanted and jammed.. or at least couldnt; feed the card properly, opening up the printer and removing the toner cartridge I can see the card sitting there in the printer, with toner on it… I reached in and took it out to unjamm the printer… it wasn;t stuck there at all I guess it just missed the feed through to the heated rollers that bind the toner to the paper.. thats right the toner was perfectly layed out on the paper but had not been melted on a slight touch and its woudl all be messed up. I thought this might be worth try to I held on to that card with teh unfixed toner while I prepared the next steps.
6. Rather than try the Hot Iron method for transferring the toner to the aliminium, I hav an old cheap laminating machine (probably $40 ten years ago, about the same now). I very carfully put the aliminium and the unfixed toner printed card into the laminating pocket and fed it though the laminator a few time (I think 4-5 times).
7. The photo card sticks to the Aluminium with the toner so dousing it in large amounts of cold water helps break down the paper and then another miracle… you can scrub away at the paper with an old toothbrush to scrub the soggy paper off the aluminium, as the paper is pretty embedded in the toner but the toner is VERY firmly attached to the aluminium… almost looks like its galvanised into the metal. you can SCRUB away to get the last traces of paper off the metal, the slightly rough feel of the toner goes away completely when the last traces of paper are gone and the metal looks smooth and black with toner. Too much scrubbing will eventually cause a few thin spots to wear through the toner but I’m amazed at how attached this stuff is to the metal.
8. Etch with the recommended 1:3 (Muriatic acid from homedepot:Hydrogen peroxide 3% from pharmacy), and in less than ten minute I have a very nice, very very detailed stencil that I can clean up with a little more scrubbing with the acetone and the pan scrubber.
Perfect.
And although I probably spent $150 on bits and pieces if you count everything including the 10 year old laminator… if i make 3 stencils, I’ve broken even and every one after that is profit.
I tried using acetone (the strong stuff from home depot) to remove the paint from a soda can (cherry coke) and had a rough time getting it to come off. I tried heating then rubbing, soaking in acetone, rubbing with a scouring pad, but it is very slow going. Any suggestions? Have you found certain types of can work better? I see Dr Pepper in your video, maybe I’ll buy a 12 pack and try that.
I wouldn’t worry about the paint so much. It’s more important to remove the epoxy coating inside. I leave the hot iron on the can aluminum for 1 minute and that helps removing the epoxy. I just use a paper towel soaked in acetone and rub it hard without scratching the metal underneath, comes off pretty quick.
Use a Scotch Brite type of dish scrubber, the harsh side soaked with acetone as a fluid works really well. It leaves a very light surface swirl effect on the aluminium when its worked its way through the epoxy and is much faster than using a paper towel. I thin the slightly roughened aluminium surface that is left is great for helping attach the laser toner when it is heat transferred on. I leave the painted side of the can for later as the appearance of the pain color through the aluminium is a great indicator that you are getting close to done with the etching process (use a flashlight from below to make it very clear when the etch is getting close to done, I use a clear Tupperware tub as my etch tank) with the Muriatic acid+hydrogen peroxide etch solution, it takes around 10 mins to complete.
Felixs’ suggestion of using normal printer paper with a thin layer of craft shop transparent adhesive film is PERFECT for toner transfer, its easy to print, its very cheap, and gets perfect transfer with no need to clean up the aluminium afterwards, no paper residue etc.
Thanks for the fast reply. I did some experimenting this weekend and was able to produce a usable stencil after a while. I was never able to get the coating off with just a paper towel, even after soaking over night in aceton. I ended up using a dish scrubber, and that worked great. Transferring the toner was a bit tricky at first, but eventually after leaving the iron on for a few minutes I was able to get a usable transfer with shelving vinyl. My printer has a “reduce paper curl” option that I found helped as well, perhaps it lowers the fusing temperature? Or maybe I was just getting the hang of it. Either way thanks for the write-up, it was very helpful. To think I almost spent $25 on a mylar stencil…
Hey David – great to hear you’re getting results, and yeah it gets better and really pays off. I messed up many times (imperfect transfer, over etching etc) until I got the hang of it but I would never pay $25 for a plastic stencil and wait days until it gets to me .. I just can’t afford that time delay. Plus – these stencils will outlast your PCB revision. I’ve been using one same one to assemble hundreds of PCBs, still as good as new.