Printing Flow
A fluid flow behavior describes the movement of solder paste into stencil openings under the influence of a moving squeegee blade. Squeegee speed and angle generate a localized high-pressure zone that forces the paste downward, meaning aperture fill dynamics determine how completely the cavity is packed before the blade passes. Thixotropic materials must thin rapidly under this localized shear to fill fine-pitch cavities without leaving voids.
Paste with poor viscosity reduction under pressure will glide over the apertures instead of entering them. Hydrodynamic pressure generates a force that varies with the paste viscosity. When the viscosity remains too high, the paste rolls across the aperture rather than filling it.
Transfer Efficiency
Solder paste volume measurement determines the success of the stencil printing process before components are placed. Optimizing aperture fill dynamics ensures that the deposited volume matches the theoretical aperture volume, which prevents insufficient solder joints on dense board layouts. Stencils made from electroformed nickel promote smoother paste glide and improve the fill rate.
When the fill rate is low, the resulting solder deposit is thin or uneven.
Print Performance
High-speed 3D automated optical inspection identifies prints with insufficient volume or uneven profiles. Poor aperture fill dynamics lead to skipped pads and weak solder joints, which cause electrical open circuits after reflow. Adjusting squeegee pressure and slowing the stroke speed can compensate for poor paste flow.
Boards that fail inspection are wiped clean and re-routed to the printer.
