Creating SMD Stencils for Solder Paste

[Rochey]

Hello folks,

thought I'd post some more details here on making SMD stencils.
I'd let you guys who aren't sure what I'm talking about do a google for them. (for the sake of keeping this post shorter...)

Yes, I could get them laser cut and wait a few days for one to arrive in the post. But I'm too impatient.

Here's the background story

- I have a design for a SMD tool controller that I've recently had PCB's made for.
- I want to make (and sell) these as an assembled board for folks on groupdiy and other forums to use, when assembling future expat audio boards
- I have the design already done in Eagle - and have the ability to export the Solder Paste locations (known as Top Cream in Eagle) as a gerber file, or print to a lazer printer.


Experiments already done.
- I purchased some laminating pouches for ID cards, like these:
- I printed out the image on my lazer printer on to regular paper.
- I removed one side of the pouch (the thinner side that has the adhesive... the other side is clean, thin plastic.
- I taped the paper beneath the plastic, and cut through the plastic to make the apertures with a sharp blade.

This worked, but the holes aren't clean, and solder paste tends to collect and stick to the uneven edges. - annoying.
Pictures to be uploaded soon.

Next Experiment
- Use my CNC machine to drill small holes for each SMD pad.

Questions
- Will a circular deposit of paste be just as effective as a deposit of paste that fits perfectly over the pad?


Cheers
/R

[Chae Ham]

Fun stuff!

If you get frustrated with x-acto knifing stencils, I'd be happy to do it for you.  Precision laser cutting mylar film stencils is inexpensive and in my opinion easier to work with than steel stencils.  It's $1/sq. inch ($25 minimum).  Add a squeegee and some flux to lube the mylar as you swipe it and you're in business! 

Will a circular deposit of paste be just as effective as a deposit of paste that fits perfectly over the pad?

Generally speaking yes, but you may have to experiment with the hole size/stencil thickness to get the correct amount of paste on there. 

[Rochey]

Chae,

Thanks for the offer - I'm too impatient though! (and I have a CNC machine here... would prefer to try and get that running first!)

Here's what I'm planning to do for my experiment...

- Generate only the SMD pads:
- Convert to Gerber:
- Load into Cambam, my friendly CNC GCode generator:
- Select all the pads, then press "Drill":
- Update the tool diameter for my 1mm pcb drill:
- Set up the CNC machine, and PRAY!

[Chae Ham]

If you're cutting the stencil out of steel, add holes that line up to the PCB's mounting holes.  This way you can use those mounting holes to fasten it to the big PCB array and swipe until your arm gives out without worrying about the stencil moving.

[Rochey]

I don't say this very often, but everything worked as planned!

I'll post pictures soon, but in the meantime, my feedback is:

It took 2 minutes to set up and drill the stencil.
2 minutes to align the pcb with the stencil
1 minute to squeegee the paste through the stencil!

The result is a very clean pcb with a perfect dot of solder paste on each pad.

Pictures to come!

/R

[Rochey]

Pictures with proof!

The stencil, after cnc drilling it with my zen toolworks CNC machine.
Notes: The drill will naturally want to pull the plastic up. I had to tape it down on all edges, and hold the plastic down near each cutting point.
Cutting time was less than 2 minutes.



Results: Clean amount of solder paste on each pad.
Notes: I deliberately didn't put paste on every single pad on the TSSOP microcontroller. It would have been a mighty mess as the hole would have been larger than the footprint. Instead, I put two blobs of solder - one at each corner to tack down the device.





Sorry for the poor quality.

[Chae Ham]

Nicely done!