DIY solderpaste stencil etching

in DIY by DP | 13 comments

IMG_2117_v1

Here’s how to etch a solderpaste stencil. The process is similar to the UV photo etching method used for DIY PCBs.

This is all well and good, but commercial stencils are expensive, and are massive overkill for prototype boards. Here is a method for producing a “good-enough” stencil using aluminium sheet via a method similar to PCB production via the UV photosensitive film method.

He also provides a guide on how to use the solderpaste stencil to apply a soldermask to your PCB.

Just use the stencil to put down a thin layer of petroleum jelly instead of solder paste, then apply spray circuit lacquer to the board. Once dry, wash the petroleum jelly off with methylated spirit. You will be left with the board protected by the lacquer, except where the solder paste is to go. This will help prevent the paste from flowing down the tracks of the copper board (effectively, a solder mask)

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Comments

  1. Bertho says:

    This I have got to try myself. There are so many times that a stencil is much easier, but the cost is 4..5 times that of the PCBs. For small series it looks like to be nearly ideal in cost/benefit.

    One thing; the pad-openings were not scaled down for the solder-paste. Usually only 60..80% of the pad is covered in paste. This is important for small pitch parts (0.4, 0.5 and 0.65mm pitch) or the paste will flow over onto adjacent pads and create short in reflow.

    • Tony says:

      Good point about the scaling down of the pads – that’s easily addressed when you create your transparency – but I didn’t recall seeing any such option in Eagle, where I exported the (top cream) layer from.

      As I said, the amount of paste applied was a bit low, which I put down to the sheet being too thin – so pad “over-coverage” is compensated somewhat by the “under-thickness” of the stencil. From the results so far, I would be reluctant to reduce the opening size, unless you’re using a thicker sheet material.

      Thanks for picking up the post, DP!
      Tony.

  2. Filip says:

    Insted of the tape, clear lacquer could be used to insulate the back…from acid… Also framing it might come in handy…

  3. Stan Pry says:

    What thickness of foil?

  4. James says:

    I seem to recall seeing somebody do this with flattened out aluminium drink cans recently, on youtube or a blog maybe.

  5. Bertho says:

    Just been looking for aluminum here, but that is a bitch to get. However, 0.1mm copper foil is cheap and readily available. That should also work.

  6. Winston says:

    When is someone going to offer cheap laser cut plastic film stencils via emailed Gerbers for low volume hobbyist boards and prototypes? Or have they already? I can imagine a very easy way to automate the entire production process.

    • Bertho says:

      Well, to cut through stainless steel or aluminium, you need at least a 1000W laser (CO2). That is a very expensive buy and you have to make very, very many stencils. For plastic/kapton you need 250+W laser to get clean cut edges. That is also an expensive laser. If the cutter can run 3 years and cost $50k, then you need (about) $50k/year revenue just to pay the bills. At an average price of $5 per stencil (if you are talking cheap) you are talking about 40 stencils per day.

      • Winston says:

        This outfit offers 2″ x 2″ mylar stencils for $25 with each additional square inch costing another dollar. Cheap, but not as cheap as I’d want (of course). I doubt that they use the totally automated process I was thinking about:

        http://www.pololu.com/catalog/product/446

        “Our stencils are made from 3-mil or 4-mil mylar with a low-power CO2 laser. The finest pitch we can handle is approximately 0.5 mm, and we strongly recommend that fine-pitch stencils be cut from the 3-mil mylar. The stencils are just a piece of thin plastic with a bunch of holes cut in it. There is no frame, squeegee, or solder paste included. The maximum material size we can handle is approximately 24″ x 12″, so the stencil area will need to be smaller than that.”

    • Sleepwalker3 says:

      They already do http://ohararp.com/Stencils.html
      or for homemade plastic ones, you can read here, though it’s a lot of mucking about. http://www.sparkfun.com/tutorials/383

      • Winston says:

        Thanks for those links!!! The first one is very reasonable at $25 for an generous 8.5″ x 11″ size stencil. The Sparkfun DIY one requires a $1000+ vinyl cutter that isn’t sufficiently accurate for smaller pitches.

  7. Sleepwalker3 says:

    Lol, No Probs. You can get decently priced Stainless ones too, at least down-under, but I imagine somebody in US, etc. would be doing them.

    Tony, I just use Brass shim stock available in a pack from the local engineering supplies. I use 4 or 5 thou, but they have various. In Aus I got it for about $8 for a pack of 6 sheets about 150mm x 150mm (if you want details contact me, but plenty of places in Newy should have it) and you can get much bigger. It’s stiff, springy and worked. You can of course get other shim materials, haven’t tried for Aluminium. Another place quoted me about 3 times the price for the same thing, so just shop around.
    And if you add citric acid (which you premix with H2O first) in the mix, like I’ve pointed out in other places, it will cut faster, better and last longer. I just grab it from the supermarket for $2 a pop and that does about 1 bottle of the Tricky Dicky stuff (not that they sell it anymore).
    Mine weren’t as well defined, but I was very rough with it and the material was thicker, so more etch-out on the side walls, but better results could be had with care. The Aluminium might perhaps cut more defined, but I think it’s mostly the thickness.

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