Stencil Aperture Design For Paste Printing

The stencil is a thin metal sheet with a hole for every pad, and the aperture is that hole. The deposit that reaches the board is the volume of the aperture minus whatever paste stays stuck to its walls, so the size and shape of the aperture decide how much solder every joint receives.

This article covers what the aperture has to achieve, why the area ratio matters, and how the thickness and shape are chosen.

What The Aperture Has To Do

The aperture has to be filled with paste as the squeegee passes, and it has to release that paste onto the pad when the stencil lifts. Both steps depend on the same geometry. During printing, the squeegee rolls the paste across the surface and forces it into the openings, and the paste has to flow far enough to fill the aperture completely. During separation, the paste has to remain on the pad rather than following the stencil, which depends on the adhesion between the paste and the pad being greater than the adhesion between the paste and the aperture wall.

Those two requirements pull in the same direction for the aperture size and in opposite directions for the wall condition. A large aperture fills and releases easily; a small one does neither. A smooth wall releases better and holds less residue, and a rough or damaged wall holds paste that is then deposited in the wrong place. The way the deposit then behaves on the pad is described under placement order and pad positioning.

Area Ratio Explained

For a stencil aperture, the area ratio is the area of the opening divided by the area of the walls. A round aperture one unit in diameter in a stencil one unit thick has an area ratio of one to four. The larger the ratio, the more easily the paste releases, because the contact area with the pad is large relative to the contact area with the wall.

For a rectangular aperture the ratio is the opening area divided by the perimeter multiplied by the thickness, which for a narrow rectangle falls quickly as the width shrinks. The practical threshold is around zero point six: above it, release is reliable; below it, deposits become inconsistent and the process has to be changed. Since the thickness is fixed by the volume requirement, the ratio sets a minimum aperture width, which in turn sets a minimum pad size. The pad and aperture relationship is described under PCB pad design standards.

Thickness And Its Trade-offs

The thickness of the stencil sets the volume for a given aperture area, so a thicker stencil deposits more paste. For a fine pitch footprint the thickness has to be thin enough that the area ratio stays above the threshold, which means the volume per pad is limited. Where more volume is needed, the aperture is enlarged, which brings it closer to the neighbouring aperture and reduces the mask dam.

Where a board needs both a large volume for a heavy component and a small one for a fine pitch part, the two requirements conflict and the answer is usually a stepped stencil, with a thicker region where the volume is needed. A stepped stencil is more expensive and more difficult to clean, and it is used where the alternative is two printing operations. The trade is a matter of the assembly cost rather than of the paste.

Aperture Shape And Modifications

The aperture does not have to be the same shape as the pad. It is commonly reduced in one dimension to prevent bridging, or lengthened to increase the volume, or given rounded corners to improve release. Each modification changes the deposit, and the deposit is what the joint is built from, so the shape is chosen for the joint rather than for appearance.

A common modification is a home plate or a rounded shape at the toe of a fine pitch pad, which reduces the risk of bridging while keeping the volume at the heel where the fillet forms. Another is a reduction in width for an aperture between two fine pitch pads, which widens the process window. These changes are recorded in the stencil drawing rather than made at the printer, because a stencil that is modified by a supplier without agreement produces deposits that no longer match the process. The design rules that keep the stencil and the board consistent are under design guidelines for manufacturability.

<img src="https://www.gopcba.com/wp-content/uploads/2025/09/多层板.jpg" alt="Deposit volume measured on a paste inspection system” />

Alignment And Printing

The stencil has to be aligned with the board, and the alignment is normally done optically using fiducials on the panel and on the stencil frame. A misalignment of a fraction of the pad width shifts the deposit to one side, which changes the volume that ends up on the pad and the shape of the deposit. The measurement of deposit volume with a paste inspection system is the check that detects the shift.

The printing parameters interact with the aperture geometry. The squeegee pressure and speed set how the paste fills the aperture and how much is left behind; the separation speed sets how the stencil lifts away, which strongly affects release on a low area ratio aperture. A slow separation improves release at the cost of cycle time, and the optimum is normally found on a test board with the same geometry as the product.

Design Rules For The Stencil

The design rules should cover the minimum aperture width, the minimum gap between two apertures, the minimum area ratio for the stencil thickness chosen, and the modification allowed at the aperture for a fine pitch footprint. They should also cover the relationship between the aperture and the mask opening, since an aperture that is larger than the mask opening deposits paste on the mask.

Where the design is transferred to a new assembly house with a different printer, the stencil may have to be redesigned for the new equipment. A stencil that works on one printer with a particular separation behaviour may not work on another, and the difference appears as a change in the deposit volume rather than as a visible defect. Treating the stencil as part of the process rather than as a drawing is what keeps the two houses producing the same result.

The stencil drawing should also state the material and the manufacturing method, because a laser cut aperture and an electroformed one have different wall finishes and therefore different release. An electroformed stencil has smooth walls and releases better at low area ratios, which can make the difference on a fine pitch footprint that is marginal on a laser cut stencil. The choice belongs with the printing parameters rather than being left to the supplier.

FAQ

What is the minimum area ratio? Around zero point six is the usual practical threshold, above which release is reliable. Below it, the aperture has to be enlarged, the stencil thinned, or the process changed.

Can the aperture be smaller than the pad? It usually is, because a deposit that is slightly inside the pad boundary is less likely to spread onto the mask. The reduction has to be balanced against the volume needed for the joint.

Why does a stencil work on one printer and not another? Because release depends on the separation speed and the squeegee behaviour as well as on the geometry. The same stencil can produce different deposits on two machines.