Rheological Metric
Fluid dynamics during stencil printing determine how solder paste transitions from a solid-like block to a flowable cream. The squeegee roll shear rate represents the speed at which the solder paste is deformed between the moving squeegee blade and the stencil surface. This deformation reduces the viscosity of the solder paste, allowing it to roll smoothly and fill the tiny apertures.
Once the blade passes, the material must recover its viscosity quickly to prevent slumping.
Print Speed
Blade speed operates as the primary machine setting that controls how quickly the paste is sheared during the printing cycle. Increasing the squeegee speed raises the squeegee roll shear rate, which can lead to excessive thinning if the paste formulation is sensitive. When the shear rate is too high, the paste becomes too runny, leaking under the stencil and causing solder bridges on fine-pitch pads.
Conversely, a low speed fails to thin the paste sufficiently, resulting in incomplete aperture filling and dry solder joints. Operator training and automated inspection help maintain the correct print speed for each paste type.
Viscosity Recovery
Solder paste exhibits thixotropic behavior, meaning its viscosity decreases under shear and recovers when the shear is removed. High-recovery formulations ensure the printed bricks hold their shape before reflow. If recovery is too slow, the paste will slump, causing shorts after the stencil is lifted.
