Characterizing Solder Paste Rheological Breakdown under Squeegee Forces
Excessive squeegee speed and downforce over-shear solder paste, reducing yield stress and causing severe deposit slump and bridging on fine-pitch components.
Paste Bead Rotation
Fluid Dynamics
The physical rolling of paste across a stencil surface ensures continuous aperture filling during printing. Adequate paste bead rotation occurs when the squeegee pushes the solder paste forward at a controlled angle. This movement mixes the paste and lowers its viscosity to prepare it for deposition.
The rolling action also prevents the paste from sticking to the squeegee blade.
Fill Performance
Insufficient roll leads to incomplete filling of fine-pitch apertures on the board. When the paste fails to rotate, it slides along the squeegee as a solid block instead of acting as a fluid. This slide restricts the downward pressure required to force the paste into the stencil openings.
The resulting deposits have inconsistent heights and missing volumes.
Process Optimization
Adjusting squeegee speed and angle stabilizes the fluid movement across the stencil. High printing speeds require a lower blade angle to maintain the correct roll. Squeegees made of metal or rigid polymers provide the necessary stiffness to prevent the blade from bending under pressure.
Maintaining a consistent roll throughout the production run ensures uniform paste transfer, which reduces the need for manual touch-ups or rework on the assembly line, keeping defects to a minimum even during high-volume production schedules.