Stencils for SMD solder pasting are becoming less expensive. Companies like ohararp.com makes your custom Kapton-polymide film stencils for $25 per 8×11″ sheet. Not bad, but I’m not sure if they allow you to fill that sheet with more than one design. It’s plastic and it will wear out fast, fold easily, etc. My DIY adventures involve mistakes sometimes and I appreciate mistakes because they teach me a lot, but not at a rate of $25+shipping and a week of waiting per mistake. I want better.
Professionally made metal (steel) stencils are still in the very expensive realm for any DIY/prorotype purposes. They are laser cut with special high power lasers and are very high quality and very durable. But for my small SMD projects I don’t have a budget for laser cut steel stencils. Instead I like to tinker with an idea until something good comes out of it. I’ve been thinking of how I can make metal stencils at home for a few weeks, so I speed up the assembly of Moteino and other upcoming SMD projects. Applying solder paste with a syringe on every pad is very slow and tedious. I want much better.
First I experimented with finding the right metal for the job. It had to be 3-5mil thick, flexible, and etchable. Copper or aluminum sounded like good candidates so I bought a 3mil 9×12″ copper sheet online for ~$10. Expensive but worth a shot. In the meantime I noticed that the aluminum food trays from food orders (which we trash or recycle) are the right thickness and have flat areas that can be cut out and used for stencils. Perfect. I had 2 metals to try. Next up is figuring out how to transfer toner to these.
An alternative to toner transfer is photo chemical resist etching. That works well with pre-sensitized PCBs, I’ve done it several times before and I like the method – it produces very good results. But how do I get photo resist on my metal sheets? I thought about trying spray photo resist then continue the process in similar fashion to PCB etching. Essentially here’s how photo chemical etching works:
However here’s many potential issues with the spray-resist method:
- you have to spray it evenly in a controlled low dust dark environment
- from what I’ve researched people say spray photo resist is not as good as the presensitized version (mainly because it’s hard to match the resist application quality that manufacturers have), also hard to find, and expensive
- there’s relatively cheap film photo resist on ebay which you can laminate to your PCB/metal and get good results. I actually ordered some from China but that will take forever to get here and I’m too impatient. I want better and faster. I want NOW.
I tried various approaches to get an etch mask on the metal, none of which produced even decent results. Copper was less friendly than aluminum. I tried the glossy/magazine-paper toner transfer and that went nowhere:
- the paper would often jam inside my printer (maybe my printer doesn’t like unconventional sized thin paper). Thicker magazine paper worked fine but failed in later steps.
- when transfering with an iron, if the temperature wasn’t exactly right, the paper would either burn and smear the toner (even a tiny amount would mess up small pads) or it wouldn’t transfer well enough. If by chance I would get a good toner-metal bond, removing the paper by soaking it in water would remove enough toner with it to make it unusable.
- bottom line: paper toner transfer really didn’t work for me
Next I tried taping the metal to the paper to try and print the toner directly to the metal. I first printed the board outline to the paper for guidance, then taped the metal over it, then printed the cream layer on the same spot. This didn’t work at all, apparently the toner won’t stick to aluminum or copper, and only produced a very smeared useless print. On a second thought I think it actually makes a lot of sense why laser printing on metal sheets doesn’t work. Laser printers dump toner dust on the media while magnetizing the media to make the toner stick to it, then it gets passed through a heat element which melts the toner and bonds it to the media.
My next idea was the one that actually worked quite well. Why not print on transparencies, then transfer that to the metal? Transparency film is very cheap, sustains a lot of heat without melting, and is transparent 🙂 a very welcome helping feature.
Yesterday I was drinking a can of Coke and it dawned on me that it was also made of aluminum. A much harder/flexible version (probably anodized in some way). Which is mostly a good thing. The food tray aluminum would bend/dent easily and keep the shape. This beverage can aluminum alloy had shape-memory and while could be easily bent/creased, it seemed to be a better candidate than anything I’ve considered before. (I’ve actually had very decent success with food tray aluminum as well, but you have to be very careful not to crease/dent/poke it with sharp objects).
This story is getting long so in my next post I’ll describe the actual steps in making the stencil, from Eagle to properly modifying and printing the cream layer and then etching the stencil and applying solder paste.