Surface Mount Stencil Aperture Sizing Calculations for Fine Pitch Components
Maintain stencil aperture area ratios above 0.66 and aspect ratios above 1.5 to achieve reliable solder paste transfer efficiency on fine pitch components.
Squeegee Pressure Setpoint
Machine Parameter
Machine parameters determining the downward force applied by the blade during the solder paste printing process ensure a clean wipe of the stencil surface. A squeegee pressure setpoint is chosen to balance the need for complete aperture filling with the risk of scooping paste out of larger openings. This value is usually measured in kilograms per centimeter of blade length.
Printing Consistency
Excessive force can cause the metal blade to flex and dig into the apertures which leaves insufficient solder on the pads. When the squeegee pressure setpoint is too low, a film of paste remains on the stencil and leads to smearing or shorts on the next board. Finding the ideal setting requires testing the paste viscosity and the hardness of the squeegee material.
Mechanical Wear
Higher pressures accelerate the erosion of the stencil foil and the degradation of the blade edge. Technicians monitor the squeegee pressure setpoint throughout a production run to compensate for changes in temperature that might alter the flow of the paste. If the pressure is not adjusted correctly, the resulting variations in solder volume will cause defects like head on pillow or insufficient joints.
This setting is one of the most critical variables for achieving a high first pass yield in the SMT process.