Stencil area ratio and the 0.66 rule
Why some apertures release paste cleanly and others clog — and the arithmetic that predicts it before you cut anything.
Area ratio is the area of the aperture opening divided by the area of the aperture's walls. Process guides — IPC-7525 among them — put the working limit at 0.66: at or above it, paste normally releases cleanly from the aperture; below it, paste tends to stay in the hole and print poorly. The ratio is set by aperture size against foil thickness, so it is the check that ties the two together.
The formula, by aperture shape
The opening area sits over the wall area — the perimeter of the opening multiplied by the foil thickness:
- Rectangular aperture: area ratio = (W × L) ÷ (2 × (W + L) × T), where W and L are the opening sides and T is the foil thickness.
- Square aperture: area ratio = S ÷ (4 × T).
- Round aperture: area ratio = D ÷ (4 × T).
The related aspect ratio compares the smallest opening dimension to the thickness instead: for a rectangle it is W ÷ T, and the usual working threshold is 1.5. Both checks come from the same geometry — a bigger opening, or a thinner foil, raises them both.
A worked example
Take a rectangular aperture 0.20 × 0.30 mm in 0.12 mm foil. Opening area is 0.20 × 0.30 = 0.06 mm². Wall area is 2 × (0.20 + 0.30) × 0.12 = 0.12 mm². Area ratio = 0.06 ÷ 0.12 = 0.50 — below 0.66, so this aperture would be expected to print poorly and clog. Two ways to fix it: thin the foil to 0.10 mm (ratio ≈ 0.60, still marginal) or 0.08 mm (ratio ≈ 0.75, above the line) — or enlarge the aperture where the layout allows.
How thick can the foil be for a given opening?
Rearranged, the formula answers the question suppliers get asked most: for a given aperture, what is the thickest foil it can print from? The table below is computed for square openings at the 0.66 threshold — the minimum opening side for each stock thickness. Real designs usually add a margin rather than sitting exactly at 0.66.
| Foil thickness | Minimum square opening (ratio 0.66) | mils |
|---|---|---|
| 0.08 mm | 0.21 mm | 3.1 |
| 0.10 mm | 0.26 mm | 3.9 |
| 0.12 mm | 0.32 mm | 4.7 |
| 0.15 mm | 0.40 mm | 5.9 |
| 0.18 mm | 0.48 mm | 7.1 |
| 0.20 mm | 0.53 mm | 7.9 |
| 0.25 mm | 0.66 mm | 9.8 |
| 0.30 mm | 0.79 mm | 11.8 |
What to do when the ratio is low
When the maths says an aperture is too small for the paste demand, the options in order of preference are: use thinner foil (0.10 or 0.08 mm — fine for fine pitch); enlarge the aperture slightly where the pad allows; improve release with electropolishing or nano coating, which raise transfer in marginal cases; or, if only part of the board is the problem, keep the thin foil there and step the rest. A clean release is a print-process result — area ratio, finish and print parameters together.
Check a real aperture
Our stencil calculator takes your opening width, length and foil thickness and does this arithmetic — including the shape-specific formulas and the reversed check for foil thickness. For a whole board, send the Gerber: we run the ratio check across the layout, flag apertures below the limit, and quote the thickness that actually fits.
Common questions
Where does the 0.66 figure come from?
It is the industry working limit carried in process standards such as IPC-7525 and in stencil-supplier design guides. It is a threshold for reliable paste release from the aperture wall, not an exact physical constant — many shops treat 0.66 as the line, work happily above 0.70, and accept some risk between 0.50 and 0.66 with a good finish and tight print control.
Does nano coating change the limit?
Coatings improve paste transfer, especially in marginal apertures — that is largely what they are for. They do not rewrite the geometry: a ratio far below the limit remains a problem no coating fully rescues. Think of the finish as a way to widen the safe margin, not to ignore it.
My CAD tool shows a different aperture area — which is right?
The geometry is the layout's: the aperture drawn in the Gerber is what gets cut, and the foil thickness is what you order. If a calculator disagrees with your CAD, check whether it is using the pad size, the paste layer size, or an intentional enlargement — the paste layer in your file is the one that matters.
Do you check area ratios when you quote?
When you send a Gerber we look at the finest features against the requested thickness, and if the ratio is clearly below the limit we will say so and propose the thickness that clears it. That check is part of the free quote — better to adjust a thickness than to remake a stencil.
Related guides
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Which mounting suits your production.
Buying from China
Ordering direct from a Chinese factory, step by step.
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