Stencil Aperture Design And Paste Release
The stencil is the only part of the printing process that decides how much solder paste lands on each pad, and the aperture design is what the stencil maker actually cuts. Every joint on the board, and therefore every solder fillet, void, bridge and open, begins as a deposit whose volume was fixed by the shape and the size of an opening in a thin sheet of metal.
This article explains what the aperture controls, how the area ratio rule decides whether paste will release, how apertures are adjusted for difficult pads, and how the result is verified in production.
What The Aperture Controls
The volume of paste deposited on a pad is the aperture area multiplied by the stencil thickness, reduced by the fraction that stays on the wall of the aperture instead of transferring to the pad. Since the volume that a joint needs is set by the joint itself, the aperture design is a calculation of area that satisfies a required volume at the stencil thickness that the process uses.
The stencil thickness is chosen first, usually from the finest pitch on the board, and the apertures follow from it. A board with 0.4 millimetre pitch parts and a 0.15 millimetre stencil cannot also carry a connector with a large thermal pad on the same thickness without some form of local adjustment, which is where stepped stencils and area reductions come in.
The Area Ratio Rule
Two ratios describe how easily paste leaves an aperture. The aspect ratio is the width of the opening divided by the stencil thickness, and the area ratio is the area of the opening divided by the area of its walls. The area ratio is the more useful figure on a small opening, and the accepted rule is that it should be at least 0.66, with 0.7 or more preferred for a robust process.
The rule is a statement about adhesion. Paste sticks to the wall of the aperture and to the pad, and the transfer is complete when the force pulling it onto the pad exceeds the force holding it in the aperture. As the opening gets smaller relative to its depth, the wall area grows faster than the pad area and the paste stays behind, which produces a deposit that is short of volume and a joint that is weak or open.
Designing For Paste Release
Where the area ratio falls below the limit, the aperture is enlarged so that the deposit is not the full pad. The opening is usually reduced in one direction only, leaving the dimension along the pad the same, because solder needs to flow along the joint rather than sideways onto the mask. An opening of this kind is described as area reduced, and the reduction is applied to the axis where the extra paste does least good.
A second technique is to split one large aperture into a grid of smaller ones. The total area is kept close to the original while each individual opening has an area ratio that releases cleanly, and the paste deposits merge during reflow. The approach is standard on thermal pads under power devices, where a single large opening would never release and a grid prints consistently.
Thickness, Steps And Special Cases
Stencil thickness is normally between 0.1 and 0.15 millimetres for fine pitch work, and up to 0.2 millimetres where the finest feature allows it. A thicker stencil delivers more paste for a given area, which is what a large through hole pad or a power device needs, and it is also what makes release harder on the smallest apertures on the same board.
A stepped stencil resolves the conflict by thinning the metal in the fine pitch area and leaving it thick elsewhere. The step is created by etching or by laser ablation of the sheet, and it introduces a transition region that has to be placed where no aperture sits, since an aperture that straddles the step will print an uncontrolled volume. Where the board has both extremes, the alternative to a step is to print twice with two stencils, which is slower but far easier to control.
Home Plate, Rounding And Wall Finish
The shape of the aperture affects release as much as its area. A rounded corner releases better than a sharp one, because paste tends to remain in a corner, and an aperture with a radius at each corner of about a quarter of the width is a common compromise between the printed shape and the release. Apertures are also sometimes drawn with a home plate or a bullet shape for the same reason.
The wall finish matters as well. A laser cut stencil has a slightly tapered wall, which helps release, while an electroformed stencil has smoother walls and holds a smaller paste volume per unit area. The taper produced by laser cutting is roughly five degrees, and it is one of the reasons a laser cut stencil performs differently from a chemically etched one even when the aperture dimensions are identical.
Where The Design Meets The Assembly Line
The aperture design cannot be separated from the paste, the printer and the board finish. A paste that is formulated for good paste release tolerates a lower area ratio than one that is not, and a printer with a well aligned squeegee and a controlled separation speed will release where a worn machine will not. The properties of the paste that govern this are described under solder paste rheology and viscosity.
The pad itself also has to be right, because the aperture is placed relative to the pad and the mask. The dimensions that the joint needs are covered under PCB pad design standards, and the whole set of rules that makes a design printable is set out under manufacturable design guidelines.
Verification And Process Control
The deposit is verified by measuring the solder paste volume on the pads, either with an inline inspection system that reports the volume of every deposit or with a sample weighed before and after printing. The measurement is compared with a window around the target volume, and the window has to be tight enough to catch a trend before it produces a defect.
The stencil itself is verified on receipt. The aperture dimensions are measured against the drawing, the thickness is checked at several points and the tension of the frame is confirmed, because a stencil that has lost tension will not sit flat on the board and the gap between stencil and pad will vary across the panel. That variation is one of the most common causes of a printing problem that cannot be found anywhere else in the process. The wider sequence is described under PCBA development process.
Openings Near The Board Edge And Fiducials
Apertures are not only drawn over pads. Fiducial marks need openings as well, and a fiducial that is printed with paste will not be recognised by the placement machine, so its aperture is either omitted or reduced to a size that leaves no deposit. The same applies to tooling holes and to test points that must stay clean for a probe, both of which are protected by removing the opening rather than by adjusting the print.
Near the board edge, the squeegee has to bridge the gap between the stencil and the panel without the stencil sagging into the aperture. Apertures close to the edge of a small board are therefore more difficult than the same apertures in the middle of a panel, and the usual remedies are to add support under the panel, to increase the stencil tension or to move the feature inwards when the layout allows it.
FAQ
What area ratio should be used as a limit? A minimum of 0.66 is widely quoted, and 0.7 or better is preferred for a stable process. The figure should be confirmed with the printer and the paste that the line actually uses, since release depends on both.
Should the aperture be the same size as the pad? Usually it is 1:1 in one axis, and reduced in the other where the area ratio is marginal. Enlarging an aperture beyond the pad is only done deliberately, because solder that lands on the mask will not form part of the joint.
How much does the stencil thickness affect the joint? Directly and proportionally, since the paste volume is the aperture area multiplied by thickness. A change from 0.12 to 0.15 millimetres increases the deposit by a quarter, which is enough to turn a marginal joint into a bridging defect.
