A bare PCB and a tray of components become a working circuit through a sequence that mirrors industrial SMT lines, just shrunk to a kitchen bench. It starts with the stencil. A laser-cut stainless steel or polyimide foil sits over the board, apertures aligned to every solder pad. Geometry is governed by the area ratio: the aperture opening’s area divided by its wall area. Below ~0.66, paste clings to the walls instead of releasing onto the pad, which is why fine-pitch parts demand thinner foils and electropolished apertures. Solder paste is a suspension of alloy microspheres in flux: - Sn63/Pb37: eutectic, melts sharply at 183C - SAC305 lead-free: liquidus near 217C - Type 3 or 4 powder, 25-45um or finer for dense layouts A squeegee drags paste across the stencil at a controlled angle and pressure, shearing it into each aperture, then the board snaps off cleanly to leave crisp deposits. The flux is thixotropic, stiff at rest and fluid under the blade. Pick and place sets each part...