To prepare the boards for surface mount (SMT) soldering; a PCB stencil, solder paste and a paste applicator is needed. The stencil is a stainless-steel sheet in a metal frame that consists of small laser-cut holes where the SMT pads are located on the PCB. The size of the openings relative to the pads is important and specific to the type of solder paste used. If too much paste is applied then the solder may overflow and cause solder bridges to form. Too little and the connection between the component terminals and the pads will be weak.
Stencils are included in all Seeed Fusion PCBA orders and will be shipped to you with the populated PCBs. We will carefully control the size of these openings when they produce the stencil to match the lead-free solder paste used in the assembly facilities, ensuring the best wetting on the solder joints.
During application, the stencil is locked in place in the automated paste printer and the plain printed circuit boards are fed in, one by one, and aligned using cameras. Lead-free solder paste is then applied to the surface and the squeegee blade is dragged across to evenly deposit the paste onto the desired areas. The resulting boards come out of the other end of the machine and are inspected for paste uniformity.
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