Evaluating Step Stencil Keep out Zones for Mixed Pitch Assemblies
Step stencil keep out zones require a 10 to 1 distance ratio per mil of step height difference to prevent squeegee levitation and solder paste bridging.
Squeegee Wiper Blade
Paste Distribution
Mechanical sweeping across a stencil foil transfers solder paste into apertures and clears the surface for the next print cycle. A squeegee wiper blade consists of a metal or polyurethane strip held in a rigid mount. This tool must maintain a sharp and straight edge to ensure an even deposit of paste across the entire board.
Its material properties determine the amount of pressure required to achieve a clean sweep.
Mechanical Operation
Movement of the tool across the stencil creates a rolling bead of paste that is forced into the openings. The angle and speed of the squeegee wiper blade influence the shear rate of the paste and its ability to fill fine pitch features. Metal blades are preferred for high precision because they do not scoop paste from large apertures.
Polyurethane versions offer better compliance on boards that are not perfectly flat.
Wear Management
Maintenance involves regular cleaning and inspection for nicks or rounding of the contact edge. A damaged squeegee wiper blade produces streaks or uneven paste heights that compromise the quality of the solder joints. Replacement should occur according to a scheduled cycle or when automated inspection shows a decline in print repeatability.
Proper alignment of the blade to the stencil prevents premature wear of the foil.