Fluid Immobilization
The formation of low-velocity or stationary zones of paste within the apertures of a printing plate during the squeegee stroke limits the transfer efficiency of the deposition process. This state of immobility, known as stencil line stagnation, prevents the continuous roll of the solder paste from filling the finest apertures completely. If the paste becomes immobile along the squeegee path, it dries out and causes subsequent printing cycles to fail.
Process Disruptions
Inconsistent paste release resulting from this stationary behavior leads to solder volume starvation on the circuit board pads. During high-speed production, stencil line stagnation typically occurs when the squeegee speed is misaligned with the rheological properties of the paste. The paste fails to transition into a fluid-like state, remaining locked as a solid block that slides across the stencil surface rather than rolling.
This lack of roll hinders the paste from entering the small cavities, which severely reduces the yield of fine-pitch assemblies. This risk increases during extended pauses in production when the paste sits exposed to the environment.
Mitigation Strategy
Alleviating the build-up of stationary paste requires a combination of regular stencil wiping and optimized squeegee pressures. Automatically wiping the underside and the top of the stencil plate clears away the dried material before it can cause blockage. This preventative action maintains the paste in a highly mobile state throughout the printing shift.
