Complete Circuit Board Lab & POV Business Card

Step 10 Screen the Solder and Populate the PCB

At this point we have the hard part done; we have a circuit board. We just need to solder on the parts. With a properly made stencil, it is a simple matter of wiping a thin layer of solder paste over board over the stencil and placing the components.

The Stencil
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Commercial PCB manufacturers use a metal solder stencil, etched out of stainless steel or aluminum for large runs of boards, but you can order them custom cut online out of thick mylar. If you have access to a laser cutter (laser cutter use number two) or vinyl cutter, you can cut your own out of acetate or transparency film. In a pinch, you can even print out your pattern onto a sheet of transparency  film and use an X-Acto knife to cut out the solder pads. Whatever works.

In my case, I had a friend  cut me a stencil out of thick acetate with his vinyl cutter. It took a bit of tweaking to get the settings right, as it is an unusually thick and brittle material to cut; thicker than most vinyls. Eventually we determined that the cutter needed to make at least four passes to cut all the way though.

Applying the Solder Paste
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For the actual screening, I made a jig out of a piece of thick silicone rubber. I cut out a piece the exact size of the PCB, then I stacked extra PCBs underneath the frame, thereby creating a cutout sunk exactly to the depth of the board. This gives a surface for the stencil to lay flay all the way across and also holds the board in place while the squeegee is dragged across.

Line up your stencil, then tape it down at the top with packing tape.Be sure to be wearing rubber gloves as the solder contains a lot of scary things, in my case, Lead. Scoop out a blob of solder paste and put it either on the squeegee, or at the top of the stencil, above the cutouts. The solder paste should be viscous, but not so viscous that it can't be spread. It's a non-newtonian material, meaning that it holds it flows more easily under pressure, and solidifies somewhat when the pressure is released.

Cheap decent-quality solderpaste and flux can be had from DealExtreme, though you should be prepared for slow (but free) shipping. If the solder paste is old, or simply too viscous, you may need to thin it with flux.

Hold the stencil tight at the bottom and run the squeegee across the stencil, taking care not to lift it off the surface. Repeat until all areas are filled and then carefully peel up the stencil.This part definitely takes practice and you will probably have to make several attempts before you get it right.

Now just place the board on a lazy susan and pick and place the appropriate parts with a tweezers or a vacuum pen. Then it's off to the reflow oven, where you hopefully don't bump the board and spill the parts you just meticulously placed all over the floor and have to pick hairs and dirt off the parts because it stuck into the paste still on them.

Tools & Supplies

• Solder paste stencil
• metal or rubber squeegee
• X-Acto Knife
• Vacuum pickup tool
• toothpick or other disposable mixing stick
• rubber gloves
• solder paste $3.37, DealExtreme
• Flux $3.61, Deal Extreme
• SIlicone rubber sheet
• packing tape

Comments

[dustinandrews]

I find a zip top bag with a needle hole in the corner works great for applying solder paste. If you do big runs, a stencil probably saves time. For a few boards at a time I think I spent less total time and effort with the "icing" method. A suitably sized needle on a syringe is also said to work well.