Applying solder paste
Next is the messy bit. Of course, you’ll need some solder paste. I used this solder paste, which is a lead-free variant.It basically means that you won’t kill yourself slowly with lead poisoning, which is always a good thing. The other good thing about this solder paste is that it can stay on the boards for up to 8 hours without drying up. That’s plenty of time to get these boards loaded and soldered.
The process is pretty simple. Apply solder paste to the edge of a card so that you have a nice even line.Credit cards work well as they are ridged with a bit of flexibility, but you really should use a card that is bigger than the PCB and only make one pass over the board.The reason for this is that the stencil doesn’t really lie completely flat.When you apply solder paste the stencil can move up slightly as you pass over. As it moves up, it can produce a small vacuum that sucks solder back on to the board.When you pass over it again, the stencil pushes that bit down onto the PCB. The problem gets worse, the passes you make. And once you have solder paste underneath the stencil, things go from bad to worse and you’ll have to just stop, clean both the PCB and stencil, and start again.Next you’ll need to remove the stencil very carefully. Any slight movement and you’ll mess up the solder.
For example; the second batch of boards I moved the stencil very slightly. It completely stuffed everything up. If you do this, you’ll need to thoroughly clean both the PCB and stencil, and start again.
After you’ve removed the stencil, visually inspect to make sure everything is OK.Then cleanup. Pack away any spare solder back into the container. Set aside the PCBs you’ve pasted and cleanup the stencil. You don’t want any dry solder paste stuffing things up later.