DIY SMD metal stencils – the definitive tutorial

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
• UPDATE: I noticed there are different soda cans, some work very well, some give poor results. I noticed that the ones that are very shiny (chrome like almost) will give flawless toner transfer. Coke and Dr Pepper cans seem to work well. I have a batch of known good cans saved for stencils. You might have to experiment to find a good brand or type of aluminum alloy cans that give you good results.

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, andreally 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