Stencil Thickness: Matching Foil to Paste Volume
Stencil thickness is the one printing variable that sets both the height of every deposit and the smallest aperture that can be printed reliably. It is fixed when the stencil is ordered, so a decision made from a default value at the start of a project follows the product for its whole life. Choosing it deliberately is worth more than any amount of tuning on the printer.
The choice is a trade with two ends. A thicker foil delivers more paste, which is what a through-hole connector or a large thermal pad needs. A thinner foil releases paste from small apertures more cleanly, which is what a 0.4 mm pitch package needs. A board that contains both has to be resolved with geometry rather than with a single number.
What Stencil Thickness Controls
Deposit volume is aperture area multiplied by foil thickness, adjusted by how completely the aperture transfers its contents to the board. Everything else being equal, doubling the thickness doubles the volume. That makes thickness the coarse adjustment for paste volume and aperture size the fine adjustment.
Thickness also sets the wall height that the paste has to slide down during separation. A tall wall holds more paste by friction against the sides, and the deposit that remains in the aperture is the one that never reaches the pad. Print definition, which is how closely the deposit edge follows the aperture edge, degrades as the wall gets taller relative to the opening.
The Relationship Between Stencil Thickness and Area Ratio
Area ratio is the aperture opening area divided by the area of the aperture walls, and stencil thickness appears in the denominator. For a square aperture, the ratio is the side length divided by four times the thickness. A thicker foil therefore lowers the area ratio for every aperture on the board, including the ones that were already marginal.
The practical minimum for reliable release is usually quoted as 0.66, with a preference for higher on lead-free pastes and fine features. That figure translates directly into a maximum thickness for a given aperture. A stencil thickness chosen without that check is a guess, and working the calculation backwards from the smallest pad is how the number should be chosen, as the area ratio guide sets out.
Common Foil Thicknesses and Where They Fit
Foil for a given stencil thickness is stocked in a small number of values: roughly 0.10, 0.12, 0.15, 0.20 and 0.25 millimetres, with imperial equivalents that are still quoted in older documentation. The 0.12 millimetre foil is the usual starting point for a mixed-technology board, and 0.10 is the common choice for fine pitch.
The thicker foils belong to specific jobs rather than to general assembly. A 0.20 millimetre foil is used where a pin-in-paste connector needs a large volume, and a 0.25 millimetre foil is used for heavy through-hole filling. Using them across a whole board usually starves nothing but breaks the release and the print definition on the fine pitch apertures, so they are normally confined to a step region.
Paste Volume and the Joint It Has to Feed
The volume the stencil delivers has to become the volume the joint needs, allowing for the paste shrinking as the flux volatilises and for the solder that wicks into a plated hole. For a surface mount pad the calculation is straightforward; for a through-hole joint it is dominated by the barrel volume and the pin.
Working from the joint rather than from the print is the correct direction. Our notes on paste volume describe the arithmetic for both cases, and they show why a thin foil and a generous aperture can be a better answer than a thick foil with the same volume, because release improves as the aperture grows.
Fine Pitch: When Thinner Is the Only Option
At fine pitch, the aperture width is small and the wall height is set by the foil thickness, so the paste has very little area to grip and a long way to travel. The remedy is a thinner foil, and the consequence is that the deposit on every larger pad becomes shorter than the joint wants.
There is a second option, which is to improve release rather than to reduce thickness. A polished or coated aperture wall reduces friction, and an electropolished or nano-coated foil can print reliably at an area ratio that a plain laser-cut foil cannot. That buys back some thickness, and it is usually cheaper than a step stencil.
Step Stencils and Mixed Requirements
A step stencil changes stencil thickness across the same foil. A step down thins the region over a fine-pitch connector, and a step up thickens the region over a through-hole connector that needs more paste. The transitions are produced by etching, milling or welding a second foil into a recess.
Steps have their own rules. The squeegee has to travel over the transition without lifting, so a step up is normally placed so that the squeegee rides from the thin region to the thick one, and a step down is placed the other way round. The transition width, the depth tolerance and the direction of print all belong on the drawing. Our step stencil notes cover the geometry.
Thickness Tolerance, Frames and Foil Life
Stencil thickness is supplied with a tolerance, and the tolerance on a thin foil is a larger fraction of the nominal value than on a thick one. The stencil should be measured at several points on receipt, because a foil that is thick at one end and thin at the other will print a volume gradient across the panel that no printer setting can remove.
Foil life affects the same properties over time. Tension falls with use, aperture walls pick up scratches and the coating wears, so a stencil that printed perfectly when new may need a lower print speed after a few thousand cycles. Recording thickness and tension at receipt gives a baseline for the foil condition assessment later.
Verifying Thickness on the Shop Floor
Measurement is simple: a micrometer or a thickness gauge on a flat area away from the apertures, taken at several points and recorded against the stencil serial number. The number that matters for the process, though, is the deposit height measured by the paste inspection system, which shows how much of the aperture volume actually transferred.
A transfer efficiency that falls well below the expected value points at the aperture walls, the separation speed or the paste, rather than at the foil. When it falls on one region of the board only, the cause is usually a step, a damaged area or a support problem. Both checks belong in the same record, because a stencil thickness measurement alone does not describe print definition.
Specifying Stencil Thickness on the Drawing
The stencil drawing should state the foil thickness, the material, the aperture tolerance, the coating, the frame type and tension, and the location and direction of any step. Where a step is used, the drawing should show the boundary and say which side of the foil the step is on, because the two are not equivalent.
It should also state the smallest aperture and its area ratio, so that the fabricator can flag a design that will not print before the foil is cut. That single note prevents most of the arguments that follow a board which cannot be built, and it costs nothing at the time the stencil aperture data is released.
FAQ
Is 0.12 mm always the right stencil thickness? It is a sensible default for mixed-technology boards, not a universal answer. A board with 0.4 mm pitch parts needs thinner foil, and a board with a pin-in-paste connector needs a thicker region. Start from the smallest aperture and the largest volume the design requires.
Can I increase paste volume by raising squeegee pressure? Not usefully. Pressure influences how completely the aperture fills, but the volume is set by the aperture and the foil. Excess pressure forces paste under the stencil and creates a wider deposit rather than a taller one, which is a defect rather than extra volume.
Does a thicker stencil wear out faster? Wear is driven by cleaning, handling and squeegee contact rather than by thickness, but a thicker foil holds more tension at the same frame size and takes longer to settle. Tension measurement over the stencil life is the useful check, whichever thickness is chosen.
