Step Stencil Control for Mixed Paste Volumes: 6 Rules

A step stencil carries two or more thicknesses in one sheet, so a heavy deposit can be printed on a power pad while a fine pitch area receives a thin one. It removes the compromise that a single thickness forces on a mixed board.

The step is a controlled reduction in the foil, produced by etching or by milling, and it introduces geometry that a flat stencil does not have. That geometry changes how the squeegee behaves and how the paste is released.

Why a Step Is Used

Fine pitch apertures need a thin foil to keep the area ratio in a range where paste releases cleanly. A power pad or a through hole pad often needs a heavier deposit to fill a large volume, which a thin foil cannot deliver.

A step lets both requirements be met on one board without a second print pass. The alternative is two stencils and two prints, which doubles the handling and the opportunity for a misprint, so the step is often the lower risk choice. A step also keeps the board on one machine, which matters when the print position has to stay registered to the placement program that follows it.

Step Depth and Area Ratio

The step depth is chosen from the two required volumes, and it is limited by how much the foil can be reduced without losing strength. A step that is too deep leaves a thin region that flexes under the squeegee and prints inconsistently.

The area ratio of every aperture is recalculated for the local thickness, because the same aperture behaves differently in a thick region than in a thin one. A design that works at one thickness may fall below the release limit at another. The calculation is done on the thinnest region, because that is where release fails first, and it therefore sets the lower limit on the step depth.

Relief and Clearance at the Step

The transition between thicknesses occupies space, and the transition is usually a ramp or a relief rather than a vertical wall. The ramp has to sit where it does not cover an aperture or interfere with a component on the board.

Clearance around the step also matters for the squeegee. A step that is close to a row of fine apertures changes the local paste pressure, and the deposit at the edge of that row is the first place a print defect appears. A ramp that lands between two rows of apertures is preferred, and that preference is stated on the stencil drawing rather than left to the supplier to decide. The supplier confirms the ramp position on the manufacturing drawing, and that confirmation is checked against the aperture layout before the stencil is made.

Squeegee Behaviour Across a Step

Squeegee pressure is set for the thinnest region, because pressure that suits the thick region will over-wipe the fine apertures. The blade then has to ride over the step without lifting, which is why a step with a long ramp behaves better than one with a sharp transition.

Blade hardness and angle are part of the same decision. A softer blade conforms to the step and keeps a consistent contact line, while a hard blade bridges over it and leaves a shadow of unfilled apertures. The paste itself is described in the flux solids notes. Pressure is verified on the first article by measuring deposit volume at both thicknesses rather than by watching the blade as it crosses the step.

Cleaning and Paste Trapping

The step creates a recess where paste can collect and dry, and dried paste there is released onto the board at random intervals. Cleaning has to reach the step face as well as the flat areas, which an under stencil wipe set for a flat foil may not do.

The solvent and the wipe frequency are chosen with the step in mind. Where the recess is deep, a manual clean at a defined interval is added to the automatic wipe, and the interval is set from inspection rather than from the calendar. Where the recess is deep, the automatic under stencil wipe is supplemented by a manual clean at an interval that the inspection result sets.

Print Verification

The print is verified at both thicknesses, with deposit volume measurements taken from the fine pitch area and from the heavy area. A single measurement on the heavy area says nothing about whether the fine apertures released.

Where a defect appears only at the boundary, the step position is the likely cause rather than the paste or the squeegee. The relationship between the printed deposit and the pad surface is described in the copper surface notes. Verification is repeated on the first article and after any change to the paste, the squeegee or the wipe settings, because each of them changes the release. Where the step has been reworked or repaired, the verification is repeated in full because the local thickness at that point has changed.

Wear, Damage and Life

A step stencil is more susceptible to damage than a flat one. The step edges catch a squeegee that is set too low, and the recess holds paste that hardens if the stencil is left dirty between shifts.

Welded or bonded step stencils can separate at the joint after many print cycles, and the separation shows as a gradual change in the deposit at the step. The stencil is inspected for step integrity at the same interval as the tension check. A step that has begun to separate shows as a gradual change in deposit volume rather than as a sudden failure, which is why the measurement is kept as a trend. A stencil that has been in service for a long period is re-measured rather than trusted, because the change is gradual enough to escape a visual check at the printer.

Records and Change Control

Records should include the step depth and position, the foil thickness at each region, the print parameters and the deposit volume at both areas. Together they describe the stencil as it was qualified.

A change of stencil supplier, step method or foil material changes the release behaviour, so it is qualified with a deposit volume measurement rather than with a visual check. The general checks around printing are listed in the fabrication notes. The stencil drawing, the print program and the verification result are kept together as one record for the product, so a later print problem can be traced to the stencil revision in use. A stencil that is replaced keeps its record, so the history of the product is not lost when the tool is scrapped.

FAQ

How deep can a step be? Deep enough to deliver the volume difference that the board needs, and shallow enough that the thin region keeps its stiffness. Beyond that limit the thin area flexes and the print becomes unstable.

Can a step be added to an existing stencil? It can be machined or etched into a foil that has enough thickness, but the apertures are affected by the process. A new stencil is usually the safer route.

Why is the defect always near the step? Because the squeegee pressure and the paste flow change at the transition. Moving the step away from critical apertures, or lengthening the ramp, is usually the fix.