Process Velocity
Constant drag forces applied during solder paste deposition define this mechanical movement across a stencil aperture. A continuous squeegee shear occurs when the blade maintains a stable forward velocity relative to the paste rheology and the board surface. High speeds introduce non-Newtonian flow behaviors where the solder paste viscosity drops abruptly under high stress.
Low speeds fail to maintain a rolling bead, forcing the paste to slide rather than roll into the stencil voids. Accurate control prevents aperture bridging and ensures consistent deposit volume during high density assembly.
Rheological Effect
Fluid dynamics dictate that solder paste exhibits thixotropic properties under this specific stress condition. Constant agitation from the blade keeps the metal spheres suspended in the flux medium. Excess speed leads to paste starvation if the rolling bead loses its mass, leaving insufficient material for the pad pattern.
Periodic cleaning of the stencil underside mitigates paste smearing caused by uneven pressure during this movement. Thermal stability of the paste remains a requirement for predictable performance under these operating conditions.
Fabrication Requirement
Assembly specifications mandate precise squeegee alignment to ensure that the shear force remains uniform across the entire stencil span. Variations in downward pressure distort the blade profile, leading to uneven paste heights that compromise joint formation on fine pitch components. Manufacturers establish optimal parameters by measuring the volume of deposited paste against the targeted stencil thickness.
Constant monitoring of these mechanical inputs detects wear on the metal or polyurethane edge that alters the resulting shear magnitude. Reliable solder joint integrity depends upon a stable shear environment throughout the printing phase of production.
