Fluidic Force
Solder paste deposition during stencil printing requires specific mechanical energy to ensure the material enters the board features completely. Aperture filling hydrodynamic pressure represents the calculated stress exerted by the squeegee on the paste volume as it traverses the stencil opening. This force drives the metal spheres and flux carrier into the copper-defined geometry before the stencil liftoff phase occurs.
Insufficient magnitude in this action leaves voids or incomplete deposits, while excessive intensity forces paste beneath the stencil foil. The variable depends upon squeegee speed, the hardness of the blade material, and the viscosity of the printing medium.
System Velocity
Constant squeegee travel speed prevents irregular pressure spikes across the pattern area. Rapid motion reduces the time available for the paste to flow into narrow apertures, which mandates higher downward force to maintain consistent filling. Slower speeds allow the thixotropic material to shear properly and occupy the entire volume defined by the stencil thickness.
Precise control over the head movement produces uniform deposit heights across dense component arrays and large pads.
Material Shear
Metal particle size and flux chemistry dictate the response of the paste to the applied hydrodynamic energy. Smaller solder spheres require higher shear rates to initiate flow compared to coarse particles. High viscosity formulations hold shape better after printing but necessitate greater pressure for full volume displacement during the stroke.
Proper calibration of the squeegee pressure ensures that the paste rheology supports high-density assembly requirements without causing bridging or starved joints.
