Choosing stencil thickness when fine-pitch parts and large pads share a PCB
Illustrative image: Unsplash contributor on Pexels
For a 0.20 × 0.20mm square aperture, reducing stencil thickness from 0.10mm to 0.075mm raises the area ratio from 0.50 to about 0.67. It also cuts the aperture volume by 25%. That trade-off explains why a thickness that improves fine-pitch printing can leave a large connector termination short of paste.
The starting point is the aperture geometry, rather than the component's pitch alone. Area ratio compares the aperture opening with the area of its sidewalls. As the opening gets smaller or the stencil gets thicker, there is more wall area holding the paste relative to the opening through which it must release.
For a rectangular aperture with sides a and b and stencil thickness t:
Area ratio = ab / [2t(a + b)].
For a square aperture with side length a, this simplifies to a / 4t. All dimensions must use the same units.
The opening example is hypothetical, not a recommendation for a particular package. Changing thickness changes both the release geometry and the amount of paste available for the joint.
Indium's stencil-design guidance uses 0.66 as a conventional minimum area-ratio guideline. It is a screening value rather than proof that a particular paste and stencil will produce a capable process. Surface finish, coating, powder size and printing conditions also affect release.
Resolve the competing deposits
Start with the assembly's difficult features: the smallest apertures, large connector terminations and thermal pads with their required paste patterns. Record both opening geometry and intended deposit volume. A single thickness should be assessed against all of them, not selected from the finest component alone.
Where one thickness cannot meet the requirements, a step stencil can provide different thicknesses in different regions. Its transition must be planned with the stencil supplier so that the squeegee and neighbouring apertures can work with the resulting geometry. Changes to aperture shape or subdivision may provide another route, particularly on large pads.
Powder choice belongs in that discussion. Finer particles can help with small apertures, but changing powder size also changes paste behaviour. Indium notes that finer powder increases the surface area exposed to oxidation, which matters during reflow and can contribute to graping.
Measure what reaches the board
Transfer efficiency is the deposited paste volume divided by aperture volume. It separates the amount available in the stencil from the amount actually printed. Solder-paste inspection can show whether the smallest features release consistently and whether larger pads receive the intended volume.
A repeatable deposit is not the final acceptance criterion. Reflowed joints must also meet the assembly requirements. A print trial can distinguish a small aperture that releases poorly from one that releases consistently but holds too little paste; those faults call for different changes to the stencil.
