Print Stroke
Controlled deposition of solder paste through a metal mask relies on the movement of a metal or polyurethane blade. Executing a squeegee sweep pushes the paste across the stencil surface and forces it into the open apertures. This linear movement is performed at a specific speed and pressure to ensure complete aperture filling.
The angle of the blade is typically held at sixty degrees to maintain a consistent roll of paste.
Process Mechanics
The mechanical action of the blade generates a localized shear force that temporarily lowers the viscosity of the thixotropic paste. During the squeegee sweep, the paste rolls ahead of the blade, which is critical for pushing the paste down into the finest apertures. Incorrect blade pressure can result in either leaving paste on the stencil surface or gouging paste out of the apertures.
The speed of the stroke must be balanced against the viscosity of the paste. A roll of paste that is too small cannot generate the hydraulic pressure needed to fill deep apertures, while an excessive amount can cause paste to spill over the sides of the squeegee.
Volume Control
Consistent deposit volume across the entire board surface requires a uniform blade speed from start to finish. If the squeegee sweep is erratic, the resulting print deposits will vary in height, causing defects during reflow. Automated inspection systems verify these deposition results before the board proceeds to component placement.
