Stencil aperture calculator

How the apertures are calculated

Solder paste is pushed through the apertures of a stencil and should stay on the pads when the stencil lifts. Whether it does depends mainly on how much of the paste touches the pad compared with how much touches the aperture walls. The area ratio and the aspect ratio express this as two numbers; the volume says how much paste a joint gets.

Formulas

• Opening area A and perimeter P: rectangle L × W and 2 (L + W); square s² and 4 s; circle π D² / 4 and π D; oblong (rectangle with rounded corners of radius r) L × W − (4 − π) r² and 2 (L + W) − (8 − 2π) r. An empty corner radius is W / 2, the full obround.
• Area ratio = A / (P × T), the opening over the wall area: rectangle L × W / (2 (L + W) T), circle D / (4 T), square s / (4 T).
• Aspect ratio = W / T, with W the smallest opening dimension (D for a circle).
• Aperture volume = A × T in mm³, the most paste the aperture holds; × the count for all apertures of that type; × the transfer efficiency (printed volume / aperture volume, measured by solder paste inspection) for the paste that reaches the pads.
• Guidelines: area ratio above 0.66 and aspect ratio above 1.5, the IPC-7525 values as widely cited (unverified: the standard was not read here). A ratio exactly on the guideline is not above it and is flagged. Both values can be changed.
• Thickness limit = the thickness at which a ratio equals its guideline: min(A / (P × 0.66), W / 1.5). A thinner stencil meets both.
• Minimum width at the given thickness: the smallest short side that meets both guidelines with the long side kept (an oblong keeps its corner radius in proportion to the width). For a circle or a square it is max(4 T × 0.66, 1.5 T), for a rectangle 2 × 0.66 × T × L / (L − 2 × 0.66 × T); the oblong is solved numerically. “Not possible” when even a width equal to the length is not enough.
• Lengths in µm, mm or mil (1 mil = 0.001 inch = 25.4 µm); areas and volumes are given in mm² and mm³.

Worked example (the tool's example)

1. A BGA at 0.5 mm pitch has round pads; the aperture is D = 0.28 mm in a 0.12 mm stencil.
2. Area ratio = D / (4 T) = 0.28 / 0.48 = 0.583: not above 0.66. Aspect ratio = 0.28 / 0.12 = 2.33: above 1.5.
3. Opening area π × 0.28² / 4 = 0.06158 mm², volume 0.06158 × 0.12 = 0.007389 mm³; the 144 balls take 1.064 mm³, of which 0.851 mm³ is printed at 80 % transfer efficiency.
4. To reach 0.66: a stencil thinner than 0.28 / (4 × 0.66) = 0.1061 mm, or an aperture larger than 4 × 0.12 × 0.66 = 0.3168 mm (often more than the pad allows). The example list shows the usual answer: the same aperture on a 0.10 mm step-down reaches 0.28 / 0.40 = 0.700.
5. In the list example two of the eight aperture types are below 0.66 (the 0201 pad, 0.603, and the 0.5 mm BGA on the 0.12 mm foil); all 652 apertures take 11.28 mm³ of paste.

Common mistakes

• Checking only the aspect ratio: long narrow apertures can pass it while the area ratio fails, and small square or round apertures fail the area ratio first (a square needs side / T above 2.64 for 0.66).
• Using the pad size instead of the aperture size: apertures are often reduced or reshaped against the pad.
• One thickness for the whole board when a few fine-pitch parts need less paste and a few large parts more: that is what a step stencil is for, with its own T on those apertures.
• Taking the aperture volume as the printed volume: the transfer efficiency falls as the area ratio falls.

Input and files

• Numbers may use a decimal comma or point; a comma gives commas in the results. The list columns are found by their header names (Name, Shape, L or D, W, T, Count, r), otherwise the numbers in the order L / D, W, T, Count.
• Files: CSV, TSV and TXT are read as if pasted (UTF-8, UTF-16, otherwise Windows-1257). XLSX: choose the sheet and the columns or a cell range; formula cells use the value Excel last saved, dates are kept as text. Files up to 20 MB are read in this browser only and never uploaded.
• XLSX files are read with SheetJS Community Edition 0.20.3, Apache License 2.0 (license text at the end of this file).

Questions

What is the stencil area ratio?

The area of the aperture opening divided by the area of its walls: for a rectangle L × W / (2 (L + W) × T), for a circle D / (4 T). It compares the force that pulls the paste onto the pad with the force that holds it to the aperture walls. Below about 0.66 the paste tends to stay in the aperture instead of printing.

Why are the 0.66 and 1.5 limits marked unverified?

They are the widely cited IPC-7525 design guidelines (area ratio above 0.66, aspect ratio above 1.5), but the tool quotes them from general practice: the standard itself was not read here. They are guidelines, not pass or fail limits; stencil technology (laser cut, electroformed, nano coatings) and the paste type move the practical limit.

Which one matters more, the area ratio or the aspect ratio?

For small square or round apertures the two say much the same, because the area ratio of a square of side W is W / (4 T) = aspect ratio / 4. For long narrow apertures the aspect ratio of the width can look fine while the area ratio is the one to watch. Check both; the tool flags each one.

How do I get the printed paste volume?

The aperture volume (opening area × thickness) is the most paste the aperture can hold. The paste that ends up on the pad is less: multiply by the transfer efficiency, which a solder paste inspection system measures and which falls with the area ratio. 80 % is only an example figure.