Metal Squeegee: 5 Rules for a Clean Wipe

A metal squeegee is the steel blade that pushes solder paste across the stencil. It is chosen because it keeps a sharp edge for a very long time, which makes the print repeatable from the first board of a campaign to the last, and because it does not absorb solvent from the paste.

The trade is that a metal blade is unforgiving. It transfers the full force of the print stroke to the foil, so the board support, the tension and the pressure setting all have to be correct, and any weakness in them shows up immediately as damage or as a print defect.

What a Metal Squeegee Has to Do

The blade has to wipe the foil clean, roll the paste in front of it and press that paste into the apertures. All three happen at the same time and at the same point of travel, which is why the angle, the pressure and the speed are set as a group rather than one at a time.

The clean wipe is the requirement that is most often compromised. Paste left on the foil is transferred to the underside of the stencil on the next stroke and then to the board, and the defect appears as stray deposits rather than as a missing one.

Blade Angle and the Paste Roll

The angle between the blade and the foil decides whether the paste rolls or slides. A steep angle pushes harder and fills apertures more aggressively, while a shallow angle rolls the paste and is gentler on both the foil and the blade edge.

Most settings sit between the two extremes, and the exact figure comes from the paste and the aperture size rather than from a machine default. Once the angle is set it should be recorded, because a blade fitted at a different angle behaves like a different tool.

Metal Versus Polyurethane Blades

A polyurethane blade conforms to an uneven surface and is tolerant of a board that is not perfectly flat, at the cost of a softer edge that wears into a rounded profile. A metal blade holds its edge and gives a more consistent volume, but it does not forgive a gap under the board.

The choice usually follows the product. Fine pitch stencil printing on a flat board suits metal, while a board with a large component on the underside or a rough surface may print better with a polymer blade. Our stencil tension notes explain how the foil behaves under each of them.

Edge Wear and the Wipe Quality

The blade wears at its edge, and the change is gradual. A new edge is sharp and wipes clean, while a worn edge is rounded and leaves a film of paste behind on every stroke. The film then accumulates, and the deposits grow heavier until the point where bridging appears.

Wear is faster at the ends of the blade where the pressure is concentrated and where the blade crosses the edge of the stencil frame. That is why a blade should be inspected across its whole length rather than at the middle, which is the part that wears least.

Pressure, Speed and Blade Deflection

Pressure has to be enough to keep the blade in contact with the foil across the whole board and no more. Excessive pressure bends the blade, which changes the effective angle, and it also loads the stencil and the support in a way that shows up as aperture damage over time.

Speed interacts with both. A fast stroke has less time to fill each aperture but produces a higher shear rate that thins the paste, while a slow stroke fills more completely and increases the chance of paste drying on the foil. The speed should be set with the paste, not with the takt time.

Blade Thickness and Stencil Thickness

A thin blade deflects more and is suited to a thin stencil and a delicate board. A thick blade holds its shape and is used where the print needs the volume that only a deep aperture can give, because the pressure needed to fill a deep aperture would distort a thin blade.

The pairing matters more than either figure alone. A thick blade on a thin stencil and a soft support is a recipe for foil damage, and our aperture area ratio notes explain why the depth of the opening is fixed by the paste volume requirement.

Coating, Plating and Blade Care

Blades are supplied plain, coated or plated, and the coating reduces friction and slows the wear. A coated blade costs more and lasts longer, and the decision is usually made on the total cost of ownership rather than on the purchase price.

Care is mostly about handling. A blade stored loose in a drawer will pick up nicks on its edge, and a nick produces a streak of paste that is transferred to every board. Blades should be stored in their holder or in a protective sleeve, and handled by the mounting block rather than the edge.

Setting Up a New Blade

A new blade should be fitted, aligned and then used to print a first article before production starts. The alignment check confirms that the blade sits parallel to the foil, because a blade that is out of parallel prints heavy on one side and light on the other regardless of pressure.

The first article should be inspected for the deposit pattern rather than only for the presence of paste. A pattern that is heavy at one end and light at the other is an alignment problem, and adjusting the pressure to compensate will only move the defect.

Inspecting the Edge for Wear

Inspection is a visual check along the full length of the edge under a bright light, looking for nicks, rounding and any build up of dried paste. A straight edge or a reference block makes the rounding visible, and a photograph at each inspection makes the trend obvious.

The interval should follow the number of print strokes rather than the calendar, because a line running three shifts wears a blade far faster than one running a single shift. Our skipped deposit notes describe the printing faults that a worn edge produces.

Records and Replacement Intervals

The record should show the blade type, its fitting date, the strokes it has run and the last inspection result. That record turns the replacement decision into a planned event instead of a reaction to a defect.

Where a blade is replaced, the first article should be repeated and the result recorded against the new blade. A change of edge changes the amount of paste in every deposit on the board, and the print that was acceptable with the old blade is not automatically acceptable with the new one. The SMTA library covers the print parameters that go with the blade.

FAQ

Can a metal blade be resharpened? Some can, and the result depends on the coating and on the supplier. A blade that has been ground back loses its coating and wears faster afterwards, so the economics should be checked before sending a set for rework.

Does a metal blade damage the stencil? It does not in normal use, because the blade rides on a film of paste and on the foil surface. Damage appears when the blade hits a dry stencil, when the pressure is excessive or when a worn edge has developed a burr.

Should both blades be replaced at once? They should be inspected together and replaced as a pair where the printer uses two directions. A worn blade on one side produces a difference between the forward and reverse prints that is otherwise hard to explain.