Stencil Design Rules for SMT Printing

A stencil is a thin metal foil with apertures cut to match the pads, and it is the tool that decides how much paste reaches each pad. The printing process has little room to correct a stencil that is wrong, so the stencil design rules that govern the foil thickness, the aperture size and the special features are worth more attention than the printer settings that are adjusted to compensate for them.

Foil Thickness

The foil thickness sets the volume of paste that a given aperture will hold. The deposit volume is approximately the aperture area multiplied by the foil thickness, so a thicker foil delivers more paste for the same opening.

The thickness is chosen from the paste requirement of the finest pitch feature on the board. A fine pitch device needs a thin foil so that the aperture can be small, while a power device with a large thermal pad needs a thick deposit. A single board with both requirements is the reason that stepped stencils exist.

The usual thicknesses run from about 0.08 mm for the finest work to 0.20 mm for a board with large pads and no fine pitch parts. The choice should be recorded with the stencil, because a replacement ordered from the same artwork with a different thickness will not produce the same deposit.

Area Ratio and Release

The area ratio is the area of the aperture opening divided by the area of the aperture walls. It describes how much of the paste is in contact with the wall relative to the opening, and it predicts whether the paste will release.

A high area ratio releases well because the opening is large relative to the wall area. As apertures become smaller and the foil stays the same thickness, the ratio falls, and below a certain value the paste is held by the walls and does not transfer cleanly to the pad.

The accepted working figure is that the area ratio should stay above roughly 0.66. When a design falls below it, the response is to reduce the foil thickness, to reduce the aperture by a small amount in the length direction or to accept a lower paste volume and a narrower process window.

Aperture Reduction and Shape

Aperture size is usually slightly smaller than the pad, which reduces the chance of paste bridging to a neighbour and compensates for the tendency of the deposit to spread. The reduction is applied in different ways depending on the feature.

For a fine pitch row, the length is often left at the pad length while the width is reduced, so that the deposit keeps its area for the joint while the gap between apertures is increased. For a large thermal pad, the aperture is often divided into a grid or a set of stripes.

The division of a large aperture is worth explaining. A single large opening releases unevenly, can leave voids and can cause the component to float on a bed of paste during reflow. Splitting it into a grid with narrow webs keeps the volume while improving the release and reducing the void content.

Aperture Home and Registration

The aperture has to be positioned relative to the pad, and the position is set by the pad data and by the fabricator registration. A systematic offset produces a deposit that is off centre on every pad, while a random variation points at the stencil tension or the printer.

The stencil frame and its tension matter, because a foil that is not taut will stretch during the print stroke and shift the apertures. The tension should be checked when the stencil is received and after a period of use, since a foil that has lost tension will put the deposit in the wrong place even though the artwork is correct.

The fiducial arrangement on the stencil should match the board. A stencil with two global fiducials aligns to the board as a whole, and a stencil with additional local fiducials allows the print to be aligned to a fine pitch area at the expense of the alignment at the board edge.

Steps, Cavities and Special Features

A step is a region where the foil thickness is reduced, and it allows a thinner deposit in one area of the board while the rest keeps a thicker one. Steps are made by etching or by laminating a second foil, and both leave a transition region where the paste release differs.

A cavity is a pocket formed in a thicker foil so that a component or a feature can sit inside it while the rest of the board is printed with a thicker deposit. Cavities are used where a tall feature would otherwise hold the stencil away from the board.

Both features add cost and lead time and both should be justified. Where a board has a single fine pitch device, it is often cheaper to accept a compromise thickness than to order a stepped stencil, and the decision should be made with the paste volume data rather than in advance of it.

Printing Parameters That Follow

The squeegee material and the print speed follow from the stencil. A metal squeegee wears the foil less and holds its edge, while a polyurethane blade conforms better to an uneven surface. Speed has to suit the aperture size, since a fast stroke over a small aperture does not fill it completely.

The separation speed is the speed at which the board drops away from the foil after the stroke, and it controls the release. A slow separation helps the paste detach cleanly, and a fast one can pull the deposit apart, especially at a low area ratio.

The wipe frequency follows the aperture density. A dense stencil needs a wipe more often because paste accumulates on the underside, and the accumulation is transferred to the mask and becomes the source of stray solder later in the process.

Stencil Care and Records

A stencil should be cleaned at the end of a shift and stored flat. A foil that is stored leaning against a wall will deform, and the deformation is not visible until the deposits no longer line up with the pads.

The stencil should be labelled with the part number, the revision and the foil thickness, and the thickness should also be recorded against the work order. When a printing problem appears, the thickness is one of the first items to check, and a stencil that has been replaced without recording the change makes the diagnosis much slower.

The condition of the apertures should be checked at intervals. A laser cut aperture has a slightly tapered wall that helps the release, and repeated cleaning with an abrasive will remove the taper and change the printing behaviour without any change to the printer settings.

Practical Rules

Fix the foil thickness from the finest pitch, keep the area ratio above the working limit, and reduce the aperture width rather than the length on a fine pitch row. Divide a large thermal aperture into a grid.

Record the thickness and the revision with the build records and the defect history, and review the printing defects and the paste volume data whenever the release changes.

FAQ

What decides the stencil thickness? The paste requirement of the finest pitch feature on the board. A thin foil lets a small aperture release; a thick foil is needed for a large thermal pad.

What is a good area ratio? Above roughly 0.66. Below that value the paste is held by the aperture walls and does not transfer cleanly to the pad.

Why split a large thermal aperture? A single large opening releases unevenly and can leave voids or float the component. A grid keeps the volume while improving the release.