Squeegee Pressure And Solder Paste Printing

The squeegee is the moving blade that rolls the solder paste across the stencil and presses it into the apertures. Three settings describe it: the pressure that holds it against the stencil, the speed at which it travels and the angle at which the blade meets the surface. Together they determine whether the paste fills an aperture completely, whether it is scraped off the surface cleanly and how quickly the blade wears.

This article explains what each setting does, how they interact, and how a printing problem is traced back to the squeegee rather than to the stencil or the paste.

What The Squeegee Does

The blade does two jobs at once. It rolls the paste ahead of itself, which mixes it and gives it the shear history that the rheology depends on, and it presses the paste into the apertures so that they fill completely. The rolling action means the paste is always moving, which is why a print that is interrupted and restarted behaves differently from one that runs continuously.

The blade also has to clear the surface. Any paste left on the stencil after the blade has passed will be pressed into the next aperture or smeared onto the board underneath, which is the mechanism behind the defect known as paste on the underside of the stencil. Both jobs have to be performed by the same blade at the same travel.

Pressure, Speed And Angle

The pressure is the force per unit length of the blade, and it has to be enough to keep the blade in contact with the stencil across its whole width without bowing. Too little pressure leaves the blade lifting over a feature and skipping paste, while too much compresses the blade, wears it faster and pushes paste under the stencil at the edges. The correct figure is the lowest that produces a clean wipe, and it is found by increasing the pressure until the skipping stops.

The speed sets the shear rate applied to the paste. A higher speed lowers the viscosity, which helps the aperture to fill, and it also gives the paste less time to recover before the stencil is separated. A lower speed gives a more complete fill but allows the paste to dry at the edges of the print. The angle then governs the rolling: a blade held at about 60 degrees from the stencil rolls the paste well, while a vertical blade scrapes it.

Blade Material And Wear

The blade is usually metal or polyurethane. A metal blade holds its edge and is used where the print has to be sharp and the life is long, while a polyurethane blade is softer, conforms better to a stencil that is not perfectly flat and is gentler on the stencil. The choice affects the pressure that is needed and the wear that the stencil sees.

Wear is the variable that is easiest to lose track of. A metal blade develops a nick or a rounded edge, and a rounded edge stops wiping cleanly while a nick leaves a streak of paste. A polyurethane blade absorbs solvent and swells, which changes its hardness and its angle. Both are inspected on a schedule, and the blade is replaced on condition rather than on a count that has no relation to the work it has done.

Snap Off, Separation And Support

The stencil has to separate from the board after the print, and the way it separates affects the deposit. The board is held up against the stencil by the tooling pins and by the vacuum, and the separation is either a snap off, in which the board drops away, or a controlled separation in which the board is lowered slowly. A slow separation gives the paste more time to release from the aperture wall and produces a more complete deposit.

The support under the board matters as much as the separation. A board that is not held flat against the stencil has a gap under part of it, and the paste is pressed into that gap rather than into the aperture. The support pins are positioned to hold the board where the apertures are, and their placement is part of the setup rather than an afterthought. The paste and stencil properties that the settings interact with are described under solder mask ink thixotropy in the analogous case of mask printing.

Diagnosing Print Defects

An incomplete deposit has several possible causes, and the pattern of the defect points to which one. A deposit that is short on one side of a pad suggests a stencil that is not flat against the board, while one that is short in the direction of travel suggests a speed that is too high or a pressure that is too low. A deposit that is short everywhere on a fine pitch area is more likely to be an aperture that cannot release.

Paste on the underside of the stencil is the clearest indication of excessive pressure or of a blade that has lost its edge. Bridging between adjacent pads after printing indicates a paste that has slumped, which is a paste property rather than a printing one. The distinction matters because the remedies are on different equipment, and the assembly sequence that the print belongs to is described under PCBA development process.

Setup And Documentation

The print parameters are recorded as a recipe: pressure, speed, angle, separation speed and the support arrangement. The recipe is specific to the product and to the stencil, and a change of either requires a new setup rather than a transfer of the numbers. The deposit measurement is the check that the recipe is working, and the target volume with its window belongs alongside the settings.

Where the print is critical, the volume is measured on every board by an inline inspection system and the results are trended. A drift in the mean volume over a shift indicates a change in the paste, in the blade or in the room, and it is the earliest warning of a defect that would otherwise appear on the board. The rules that make the pad geometry printable are described under manufacturable design guidelines, and the placement that follows the print under placement order and pad positioning.

Additional Considerations for This Build

Practical attention to paste release pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating paste release explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, squeegee pressure is the item that decides how the rest of the board is arranged. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

FAQ

How much squeegee pressure is correct? The lowest pressure that wipes the stencil clean across the full width of the print. Higher pressure accelerates blade and stencil wear without improving the deposit.

Does a higher print speed increase throughput safely? It increases throughput and lowers the viscosity, and beyond a point it leaves the aperture partly filled. The limit is found by measuring the deposit rather than by observing the board.

When should the blade be replaced? When it shows a nick, a rounded edge or, in the case of polyurethane, a change in hardness from solvent absorption. The condition is checked on a schedule rather than judged by the age of the blade.