A stencil is a stainless steel foil, typically 100 to 150 micrometres thick, with laser-cut apertures matching the pads, mounted in a frame for the screen printer. A squeegee pushes solder paste through the apertures onto the board, and aperture size and foil thickness set how much paste each joint gets. The area ratio limits how small an aperture can be and still release paste, and step stencils or reduced apertures handle mixed fine-pitch and large parts.
Cut the stencil at 4 mil, window-pane the QFN thermal pad at 60 percent and home-plate the 0402s.
Also heard as
- solder stencil
- paste stencil
- foil
Related terms
Solder paste
Mix of fine solder powder and flux printed onto pads to hold parts in place and form the joints in reflow.
Aperture
Shape and size used to draw or flash a feature in Gerber data, named for the opening in a photoplotter's aperture wheel.
Reflow
Soldering surface mount parts by melting printed solder paste in an oven that follows a set temperature profile.
Bridging
Solder defect in which solder joins two pads or leads that should be separate, creating a short.