Solder stencil apertures and paste volume: area ratio, aspect ratio, transfer efficiency, and a worked example
Every surface-mount joint starts as a brick of solder paste pushed through a hole in a stainless-steel foil. Whether that brick lands on the pad or stays stuck in the hole depends on one number, the aperture's area ratio, and how much solder the joint ends up with depends on the hole's volume, the share of it that releases and the half of the paste that is flux. This calculator takes the pad, an aperture reduction and the stencil thickness and gives the IPC-7525 area and aspect ratios with a verdict, the paste on the pad in nanolitres and milligrams, the solder after reflow, the thickest foil that still releases, the web between apertures at a pitch and the paste for a whole board, with the aperture drawn in cross-section while printing and after release, and the pads from above.
How to use the stencil calculator
- Type the pad's length and width (or switch to Round and type a BGA pad's diameter) and any aperture reduction as a percentage of each dimension; 0 for most pads, 10 % for fine pitch.
- Pick the stencil thickness and the paste in the menu row. The area ratio, the aspect ratio and the verdict appear, with the paste on the pad and the solder after reflow.
- Add the pitch for the web between apertures and the number of pads for the paste per board. The Thickness table tab shows the same aperture on every common foil, each with a Use button.
The area ratio
AR = L W / (2 (L + W) T) round: AR = d / (4 T)
When the stencil lifts off the board the paste in each aperture is pulled two ways: the pad holds it through the aperture's floor, the aperture holds it through its walls. The area ratio is the floor divided by the walls, and IPC-7525 sets 0.66 as the floor for reliable release with a laser-cut foil: above it the pad wins and the deposit is complete and repeatable; between 0.5 and 0.66 a growing and varying share stays in the hole; below 0.5 most of it does. The ratio falls with the foil's thickness and with the aperture's size, so the finest pad on a board decides the foil: a 0.25 mm BGA pad needs 0.25 / (4 × 0.66) = 94.7 µm or thinner, a 0402's pad is happy on 150 µm. Nano-coated or electropolished foils release a little below 0.66; electroformed nickel stencils lower still.
The aspect ratio and the web
The older rule for long, narrow slots is the aspect ratio, the aperture's narrow side over the foil thickness, at least 1.5; for most pads the area ratio is the stricter of the two, for a long thin QFP slot the aspect ratio can be. The other fine-pitch limit is the web, the strip of foil left between neighbouring apertures: at a 0.5 mm pitch with 0.3 mm apertures it is 0.2 mm, and much under 0.15 mm it flexes under the squeegee, paste creeps beneath it and the pads bridge. Reducing the apertures by 10 % of each dimension is the usual cure, and it also keeps paste off the solder mask between pads.
Paste, flux and solder
The aperture's volume, area times thickness, is the most the print can deposit: 1 mm³ is a microlitre, so most pads get tens of nanolitres. A transfer efficiency, typically about two thirds at an area ratio of 0.66 and over 90 % past 1.0, says how much of it reaches the pad; solder-paste inspection on a real line measures it to a few percent. Paste is 88–90 % metal by weight but only about half by volume, because flux is far lighter than solder, so at reflow the brick loses half its height and the joint is left with half the deposit's volume of metal. For a BGA that metal merges with the ball: the sphere figure in the results is the paste's contribution, for comparing with the ball's size.
Choosing the foil
A board gets one foil, so the finest pitch sets it: 120–130 µm suits boards down to 0.5 mm pitch and 0402 passives, 100 µm boards with 0.4 mm pitch QFNs and BGAs, 80 µm the finest; 150 µm and above is for boards of only large parts or where big connectors need the extra solder. When one fine part would starve everything else, a step stencil etched thinner over that part keeps the rest at full thickness. Large pads such as power tabs and QFN thermal pads get a windowpane of smaller apertures at 50–70 % coverage, since a full-size deposit floats the part and traps voids.
Your aperture, step by step
- Aperture: 1.35 mm × 0.27 mm in a 120 µm foil; area ratio 0.94, aspect 2.3: good release.
- Paste: 43.7 nL in the aperture, about 91% released → 39.8 nL, 0.167 mg; 100 pads take 16.7 mg.
- Solder: 50% of that stays as metal, 19.9 nL, a sphere of 0.336 mm.
- Foil: the thickest for AR 0.66 is 170 µm; the web at 0.5 mm pitch is 0.23 mm.
Worked example: a 0.5 mm pitch QFP on a 120 µm stencil
A QFP with 0.3 × 1.5 mm pads at 0.5 mm pitch, printed through a 120 µm laser-cut foil with the apertures reduced 10 % in each dimension to 0.27 × 1.35 mm. The aperture's area is 0.365 mm² and its walls 2 × (0.27 + 1.35) × 0.12 = 0.389 mm², so the area ratio is 0.94 and the aspect ratio 0.27 / 0.12 = 2.3: both pass, and the foil could be as thick as 170 µm before the ratio fell to 0.66. The aperture holds 43.7 nL; at a typical 91% transfer the pad gets 39.8 nL, 0.167 mg of SAC305 paste, which reflows to 19.9 nL of solder. The web between apertures is 0.5 − 0.27 = 0.23 mm, comfortably stiff; without the reduction it would be 0.2 mm and the deposit 50.3 nL. The 100 leads take 3.98 µL, 16.7 mg of paste.
Questions
Should apertures be the same size as the pads?
For most parts yes, 1:1 is the default. Reduce them 10 % for fine pitch (the web) and for pads whose mask opening is larger than the copper, and use a windowpane on large tabs. Enlarging them beyond the pad puts paste on the mask, where it balls up.
What transfer efficiency should I expect?
Around 80–90 % for apertures with an area ratio of 0.8 or more on a good laser-cut foil, two thirds near 0.66, and falling fast below. The spread matters as much as the average: measure it with solder-paste inspection rather than trust a curve.
Is 0.66 a hard limit?
It is IPC-7525's guideline for ordinary laser-cut stainless foils. Nano-coated and electropolished foils release reliably down to about 0.55, electroformed nickel to 0.5. Below that, thin the foil or step it.
How much paste does a board need?
Sum the deposits: the count field does it for one pad size, and a typical board of a few hundred pads uses a fraction of a gram. The jar lasts a long time; what runs out is its shelf life.