01 — The Two Key Numbers
Area Ratio — IPC-7525 min: 0.66
AR = (L × W) / (2 × (L + W) × T)
Compares the aperture opening area to the stencil wall area. Above 0.66, paste adheres more strongly to the PCB pad than to the stencil walls and releases cleanly. Below 0.66, paste stays in the aperture.
< 0.66 Insufficient
≥ 0.66 Reliable release
Aspect Ratio — Rectangular apertures only, min: 1.5
AR = W / T
Checks whether the aperture is wide enough relative to the stencil thickness for a clean squeegee fill. A narrow, deep aperture does not fill completely, giving inconsistent paste volume. N/A for square or circular apertures.
< 1.5 Poor fill
≥ 1.5 Complete fill
02 — Stencil Thickness
0.10 mm
Ultra fine-pitch
0201 passives, fine-pitch BGAs (≤0.4 mm pitch). Area ratio is tight; apertures must be designed very carefully.
0.12 mm
Fine-pitch standard
Most common for mixed boards with 0402s, 0.5 mm pitch BGAs, and QFPs. Good balance of paste volume and release.
0.15 mm
General purpose
Standard mixed assemblies with 0603 and larger passives. Reliable paste volume for most connector pads.
0.20 mm
High volume / THT
Large connectors, high-current pads, pin-in-paste (PIP) applications. Too thick for small apertures.
⚠ Stepped stencil: When one thickness cannot satisfy both your smallest and largest apertures, use a locally milled thinner zone around fine-pitch areas. Calculate area ratio separately for each step.
03 — Solder Paste Powder Selection
Smaller apertures need finer powder. Particles that are too large bridge the aperture and prevent full fill. Use the largest powder type your aperture allows, finer powders cost more and oxidize faster.
| Type |
Particle range |
5-ball min (rect/sq) |
8-ball min (circular) |
Typical use |
| Type 3 | 25–45 μm | 225 μm | 360 μm | 0603, 0805, larger passives |
| Type 4 | 20–38 μm | 190 μm | 304 μm | 0402, 0201, 0.5 mm BGA ← most common |
| Type 5 | 15–25 μm | 125 μm | 200 μm | 01005, fine-pitch CSP, SiP |
| Type 6 | 5–15 μm | 75 μm | 120 μm | Ultra fine-pitch, microLED |
| Type 7 | 2–11 μm | 55 μm | 88 μm | Advanced packaging |
5-ball rule (rect/sq): aperture width ≥ 5 × largest particle diameter. | 8-ball rule (circular): aperture diameter ≥ 8 × largest particle diameter.
04 — How to Use This Calculator
1
Enter stencil thickness
Type the foil thickness in mm. Typical values: 0.10 to 0.20 mm. See section 02 above for guidance on which thickness suits your component mix.
2
Select shape and enter dimensions
Choose rectangular/square or circular. Enter dimensions from the PCB copper Gerber or CAD data, not from the stencil Gerber. BGA/CSP: use ball pad diameter. QFN: check perimeter pads, not the thermal pad.
3
Set the scale factor
Default is 90% (-10%), which is standard industry practice. For very small apertures with a marginal area ratio, increase toward 100% before reducing stencil thickness.
4
Read the results
The calculator gives area ratio with IPC-7525 pass/fail verdict, aspect ratio check, paste volume estimate, and powder type recommendation.