Step Stencil Design for Mixed Paste Volumes on a PCB

A board rarely needs the same paste volume everywhere. A power component may want a thick deposit for a strong fillet, while a fine pitch package on the same panel needs a thin one to avoid bridging. A single foil thickness forces a compromise that satisfies neither. A step stencil solves that by varying the foil thickness across the board, and this guide explains how gopcb designs and prints one.

Why Mixed Paste Volumes Occur

Paste volume is the product of aperture area and foil thickness, adjusted by transfer efficiency. When one board carries both coarse pitch power devices and fine pitch packages, the optimum volume for each is different. Coarse pads generally want more paste to form a substantial fillet, while fine pitch pads want less to keep bridging risk low.

The conflict is strongest when a large thermal pad sits near a fine pitch connector. Increasing foil thickness to feed the thermal pad pushes the fine pitch deposits into a bridging regime, while thinning the foil to protect the connector starves the power joint. A step stencil removes that zero sum choice. The same problem appears in reverse when a board carries both small chip components and a large connector, where the connector benefits from volume that would bridge a zero four zero two pad.

How a Step Stencil Is Made

The stepped area is produced by selectively etching or milling the foil so that one region is thinner than another. The step is created from the same base material, with the thin area reduced to the required thickness while the rest of the stencil keeps its original gauge. The transition is machined rather than bonded.

Steps are usually quoted as a thickness and a direction, such as reducing from one hundred and twenty to one hundred micrometres over a defined region. The accuracy of the step depth and the quality of the transition zone decide how well the stencil prints, so the fabrication method deserves as much attention as the aperture design.

Step Up Versus Step Down

A step down thins the foil in a specific area, which reduces paste volume there. This is the most common approach, used to protect fine pitch devices from excess paste. A step up increases thickness locally, adding volume for power pads, connectors or press fit positions that need more material.

Sometimes both are used on one stencil, with the base thickness chosen to suit the majority of the board and both thin and thick regions added where needed. Each additional step increases cost and risk, so the design should use the minimum number of steps that solves the volume problem. Two steps are common, three are unusual, and a design that appears to need four is usually better served by changing the base thickness instead.

Stencil Thickness and Area Ratio

Area ratio is the relationship between aperture opening area and aperture wall surface, and it decides whether paste releases cleanly. Thin foil improves area ratio for small apertures, which is exactly why a step down helps fine pitch printing. Thick foil makes release harder, which is why fine pitch work avoids it.

The base stencil thickness should therefore be set for the most demanding fine pitch feature, not for the largest pad. Steps are then added where the remaining features need more volume. Choosing the base thickness the other way around forces a step on every fine pitch aperture, which is expensive and difficult to print.

Aperture Design on Stepped Areas

Apertures that sit inside a stepped region must be sized for the local thickness, not the nominal thickness. A designer who calculates the area ratio using the original gauge will specify an aperture that over delivers once the foil is thinned. The calculation has to be repeated for each step region.

Clearance around the step also matters. Apertures should not straddle the transition zone, because the varying thickness there produces an inconsistent deposit. Spacing the step boundary away from critical apertures, even by a small margin, keeps the printed volume predictable across the board.

Paste Release at the Step Boundary

The transition between thick and thin regions is the most difficult part of the stencil to print. Paste can collect against the step wall, and the squeegee sees a change in surface height that alters the local pressure. A poorly finished transition leaves a ridge of paste that eventually reaches an aperture.

A gradual, well machined transition reduces this effect. The boundary should also be aligned with a natural gap in the layout, such as a keep out area or a break between circuit blocks, so that even a small amount of paste disturbance occurs where no apertures are present.

Printing Parameters for Stepped Stencils

Stepped stencils need more care at setup. Squeegee pressure must be high enough to fill apertures at the thinner region without deforming the blade at the step. Print speed may need to be reduced to give the paste time to fill against the step wall, particularly on coarse apertures.

Separation also deserves attention, because the thin region releases differently from the thick one. Some printers allow a different separation profile for different areas, and where they do not, the parameter must be chosen to protect the most critical deposit. Verification with an inspection system closes the loop, as described in our paste inspection guide, and the printed result feeds the wider production process flow.

Inspection and Verification

Verification should measure volume in both the thick and thin regions, because a single average hides the difference that the step was created to control. Deposit height is especially informative, since it reflects the local foil thickness directly. Comparing measured height with nominal thickness confirms the step was manufactured correctly.

The first article should also check the step boundary for stray paste and any aperture that bridges. Once the process is stable, routine inspection can focus on the critical apertures, with the step region checked whenever a new stencil or a new paste lot is introduced. Any deposit found outside the window should also be confirmed at the end of the line, where optical inspection checks the joints themselves.

Cost, Lead Time and When Not to Use One

A step stencil costs more than a plain foil and takes longer to produce, because the step must be machined and measured. The cost is usually justified by a yield improvement on a difficult board, and the calculation should include the scrap that a single thickness would produce. Our notes on manufacturing cost reduction describe how to frame that comparison.

Step stencils are not always the answer. If the fine pitch features can be printed with a thin foil and the coarse features receive adequate volume through aperture enlargement, a plain stencil may be sufficient. Where only a few coarse pads need more paste, a dispensing step for those positions may be cheaper than a stepped foil.

FAQ

What is a step stencil used for? It varies foil thickness across a board so that different areas receive different paste volumes. It is used where coarse power pads and fine pitch packages share a panel and a single thickness would either starve one or cause bridging on the other.

How much thickness difference can a step stencil achieve? Steps of twenty to fifty micrometres are common, and larger differences can be produced depending on the base thickness. Each additional thickness change adds cost and printing difficulty, so the smallest step that solves the volume problem is usually the best choice.

Can a step stencil be used with nano coating? Yes, and the coating is often beneficial because the step boundary and the thin regions both benefit from improved release. The coating must be applied after the step is machined so that the finished surfaces are uniform.