Saturday, January 5, 2013

Home-made solder paste stencil

SMD solder paste stencils are used to accurately "print" a thin layer of solder paste onto a PCB on which the components are placed before reflow soldering. The "printing" is done in a method similar to screen printing, where the solder paste is squeegeed through a 4 mil thick, stainless steel stencil onto the PCB.

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.

Materials

  • Aluminium sheet, preferably 4/1000 inch (4 mil) thick.
    I've sourced mine from an aluminium BBQ tray from my favourite shop, the Reject Shop. It's not quite 4 mil thick, but it's much thicker than "foil". Cut a flat section from the bottom at least as big as your PCB, very carefully with scissors.
  • UV Photosensitive film.
    This is the same film as used for PCB production in my previous posts.
  • Ferric Chloride solution.
    Yep, the same stuff you might use to etch copper off a PCB. I've got a very old bottle of solution, from back when Dick Smith actually sold things like this.
  • A transparency printed with the solder paste pattern you want to achieve.
    This is the same as for PCB production, but you want a positive, not a negative. This is, ink where you want the paste to go.

Method

Bond the UV sensitive film to one side of the aluminium sheet - this is the same procedure as for PCBs
Cover the other side of the aluminium sheet in a waterproof adhesive tape. I used clear packing tape, which in retrospect was too sticky, as it was hard to remove and caused folds in the sheet when removing. An alternative here may be to apply spray paint. Make sure the entire surface is covered, go around the edges if possible.
Expose the film on the sheet to UV light through the transparency - same procedure as for PCBs again.
"Develop" the film - same process as PCBs again.
You should have something like this at this point:
0805's and SOT-23's and one wide SOIC 20. All aluminium covered except where the paste is to go 
Next, place the sheet in a dilute solution of ferric chloride. I used a mix of about 50/50 water and ferric chloride. Warning: Aluminium reacts very strongly with ferric chloride. Take care!
After 20 minutes of lightly brushing the reacting surfaces in the etchant, it should look like this:
Exposed aluminium etched right through the sheet.
Now remove the tape. This turned out to be hard to do without damaging the stencil. Eucalyptus oil helps to "unstick" the tape.
Final stencil, rear side.
That's it - the stencil is ready for use.

Use

There are two uses for this stencil:
  1. As a stencil for where you DON'T want to apply protective lacquer to your copper board.
    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)
  2. As a solder paste stencil.
    I found using a plastic squeegee, like a credit card to be ideal for spreading the paste. Here's the result:
Solder paste applied with stencil
And the re-flowed board:
Re-flowed using hot air

Conclusions

Overall, reasonably successful. I was surprised how well the aluminium sheet retained the small fillets between the openings.
Things to watch out for next time:
  • Protect the rear side of the aluminium sheet with something other than clear packing tape. The removal of the tape almost ruined the stencil
  • Find some slightly thicker sheet - the amount of paste left on the board was bordering on the "too little" side. I saw a real nice looking BBQ tray at Woolworths the other day that felt just a little thicker.
  • Don't reflow with hot air, or next time, turn down the flow. The air caused the 0806 resistors to blow around once the paste became liquid. They did capillary into place nice and straight at first.

5 comments:

Unknown said...

I am not sure if you are in the US, but you can get shim stock from McMaster Carr. You can get it in a variety of materials including Copper, Aluminum, and Stainless Steel. The are reasonably priced, and get you the materials quickly. I have ordered something at 2:30pm to have it there by 10am all while using standard shipping. The longest I have had to wait is 3 business days.

bonybrown said...

Thanks for the tip, unfortunately I'm in Australia.
I'm sure I could obtain the shim if I went looking for it, but the cost would probably defeat the aim of creating a low-cost stencil.

s2list12006 said...

Just a heads up, but you can get professionally made stencils for low $$$. Check on ebay... item number
290748969337. I have not tried them myself, but I certainly will.
Cheers,
Sam

bonybrown said...

Nice find, Sam. That certainly looks interesting.
Please do let me know how they go, if you do try them.

Tony.

s2list12006 said...

Good day Tony,
I sure will. I found them by accident, as I was looking for some used PCB assembly equipment (pick and place machine), etc. I suspect that I will be trying them out in about 3-4 weeks... it should be interesting and possibly an excellent resource for prototyping.
Cheers,
Sam