This is useful tip for anyone making or using surface mount components on PCBs.
It relies on access to a laser cutter, so if you don't have one, look away now.....
I cut a plastic stencil on the laser, align it with the PCB pads and then squeegee a thin layer of solder paste through the holes.
Peeling away the stencil will leave a perfect pattern of pads with the correct amount of solder on each one.
I have just successfully used this process on components with a 0.025" pitch (0.65mm) - that's VERY small,
The prototype in the pictures has been made by my good friends at Spirit Circuits in the UK, and is a full plated through, double sided board. with immersion silver coating.
Step 1: Prepare the Artwork
ALL PCB design programs these days will let you produce solder masks for surface mount boards. We take the solder mask output from the program, and direct it to plain, simple HPGL "plotter" files.
Here, I'm using Number One Systems "Easy-PC", which may look very familiar to anyone using DesignSpark's free offering.
Turn OFF all the other layers, turn ON the mask layer, change the output device to "plotter" and plot to file. I mirrored the mask, but I am not sure its important. Technically, the holes benefit the process if they are smaller on the TOP of the stencil than the BOTTOM. This ensures a clean peel at the later stages
Step 2: Take to Laser
Import the artwork on the laser cutter. I originally expected to CUT the stencil. THIS DOES NOT WORK.
The secret is to ETCH MULTIPLE times, to get very clean edges.
All I used for the stencil was OHP transparency. I suspect even paper might work, but the VOLUME of solder in the joint depends on the thickness you employ, and transparency is thicker and paper - and gives good results.
Step 3: TESTING
I don't show the "ouch" picture, but after cutting, my first plot was 90mm wide, which is a shame, because the pads were 88 mm apart.
You, like me, may need to re-run, and modify the output scale factor to suit,...Measure the test piece, and rescale to testpiece width/master width. Then re-run the job....
Step 4: USING
Since the process is very cheap to use (if you have a laser - sorry) you can make multiple stencils, or make free with a knife to cut the stencil to fit.
Here, I wanted to pretest the card's power supply lines, and had to lay down some components by hand to allow me to test it. I cut the stencil to fit around the components and taped it to the board.
Next, I applied a thin line of solder paste from the syringe provided.
Then I scrape the paste through the holes.......
Step 5: Final Step
...peel away the stencil
Step 6: Add Components and Heat
Add the components, and apply heat. I use my trusty Weller hot air gun, which is over 20 years old now.
Step 7: Finished Board.
Anyone who has tried to apply paste using a needle and the syringe can attest at the difficulties. This process save time and materials, and works even if you are making your own PCBs.