Electronics Assembly
Nate 
Stenciling
Solder paste stenciling is a lot like silk screen stenciling a t-shirt. You have a mask that you lay over the thing you want to apply ink to. In our case we use a metal stencil to apply paste to very specific parts of a PCB (printed circuit board).
At SparkFun, we start with a PCB panel. Panels are a larger board with the design repeated several times over, to make handling easier. In this case we have 16 copies of the EL Escudo Dos. We make these larger panels so that we can stencil multiple boards at the same time. However, you can just as easily stencil one board at a time.
A small dab of solder paste is placed on each pad where a component will sit. To make this process faster, a stencil is laid over the board, and the paste is smeared over the stencil with a metal squeegee.
This is what solder paste looks like in the wild. It’s a mixture of metal alloys: 96.5% tin (Sn), 3% silver (Ag), and 0.5% copper (Cu). Most paste has an expiration date and should be stored in a cool place. This has to do with the flux that is added to the metals thus creating the paste consistency. Flux changes the surface tension of the metal when it is in liquid form helping it to flow into the joints.
We use lead-free solder paste but if you’re not planning on selling your widget in Europe then leaded paste is a little more forgiving and easier to use.
We design all our boards in Eagle PCB. The software outputs various layer files that the PCB manufacture uses to create the actual PCB. There are also two layers (top paste and bottom paste) that we send to our stencil fabricator. They use a high power CO2 laser to cut the stencil out of thin stainless steel sheets.
Stenciling can be done by hand. In the video above, Abe does a quick demonstration of how to line up a stencil and apply solder paste. Checkout this awesome presentation for some additional photos and information about stenciling.
There is no magic when globbing paste onto a stencil. We like to use off the shelf putty knives from the local hardware store.
Here's the stenciling machine at SparkFun. It's a lot of work to setup the machine, but for very large runs (greater than 500 pcs), it makes stenciling a bit easier.