Using a solder paste stencil is a quick way to get your production speed up when dealing with SMD parts (surface mount devices). At my house, its done because its fun and messy. But what exactly are they? A solder paste stencil is a metal or polymer plate with holes in it that represent the solder pads on your PCB.
All you have to do is to apply solder paste all over your stencil, remove it, and now a thin even layer of solder paste covers all your solder pads. Solder paste is liquid solder, often provided in a can or inside a syringe.
My preferred method is to place the target PCB in the middle of a wooden plate, align the stencil on top at the perfect spot, and later secure the target PCB with older PCS’s in all corners to keep it in place. Lastly, I have to secure the stencil in one corner perfectly aligned with the target PCB. If done correctly, I can now carefully replace the target as many times as I want, and flip the stencil over for a perfect fit with no hassle. I store it in a cardboard box for next time.