Confusing paste mask with solder mask
They serve different purposes. Confirm the manufacturing file used for the stencil and exclude nonprinting mechanical details.
Services / PROJECT FIELD GUIDE
PCB Stencils
Match stencil design to the paste printing needs of the assembly.
TAKE THIS PAGE INTO YOUR NEXT REVIEW
Less guesswork. A clearer release.
- Understand the relevant choices
- Compare requirements and next steps
- Download useful notes for your review
What to know before you decide
A PCB stencil controls where solder paste is deposited and how much is available before reflow. Stencil selection should consider aperture size, foil thickness, package recommendations and the printing process together. A thicker foil supplies more nominal paste but can make small apertures harder to print consistently. Use a package-by-package review for mixed assemblies, and assess actual deposits before treating a stencil drawing as a proven process.
01 / DECISION GUIDE
Make the trade-offs explicit
01
Begin with the difficult apertures
Identify the smallest openings and the features requiring the most solder. Ask whether a single foil thickness can satisfy both after aperture optimization. Do not resize every aperture by the same percentage without checking package guidance. Thermal pads, fine leads and small passives can need different treatments for reasons that are not visible from pad area alone.
02
Use area ratio as a screening aid
For a rectangular opening, area ratio is opening area divided by its wall area: ab divided by 2t(a+b), with all dimensions in the same units. It helps compare candidate apertures but does not include paste behavior, surface treatment or machine settings. Treat marginal geometry as a prompt for process review and measured transfer, not an automatic pass or fail.
03
Specify the physical interface to the printer
Confirm framed or frameless format, foil dimensions, image position, fiducials and identification. Align the stencil revision with the panel and PCB revision. A correctly designed aperture pattern can still be unusable if its frame or image location does not match the production equipment. Include the intended top or bottom side prominently in the release.
04
Keep optimization traceable
Record aperture changes approved after the first print or assembly review. Preserve the original design and the manufacturing revision so a replacement stencil can reproduce the accepted process. When paste or package mix changes, review the earlier settings rather than assuming the old stencil is still appropriate. This is particularly valuable for recurring low-volume builds.
Compare the choices
| Choice | Potential advantage | Review before release |
|---|---|---|
| Single thickness | Simpler tooling | Small and large deposit compatibility |
| Stepped stencil | Local solder-volume adjustment | Step clearance and printing feasibility |
| Framed stencil | Defined equipment interface | Frame size and printer fit |
| Frameless foil | Compatible reusable tensioning system | Mounting format and image position |
| Aperture optimization | Package-specific deposits | Measured print and assembly results |
Questions that prevent rework
Universal aperture reduction
A blanket reduction can create insufficient paste on one package while helping another. Review the actual mix.
Relying only on nominal volume
Nominal aperture volume is not the deposited volume. Check transfer consistency at the difficult sites.
Get a clear answer
How is stencil area ratio calculated?
Divide the opening area by aperture-wall area. For a rectangular opening with sides a and b and foil thickness t, use ab/[2t(a+b)]. Keep units consistent and use the result as a process-screening metric.
Does a higher area ratio guarantee good printing?
No. Paste, aperture finish, support, alignment and printer settings also influence the deposit. Actual print inspection is needed to assess repeatability.
What files should accompany a stencil order?
Provide the approved paste layers, board or panel outline, side identification and stencil format requirements. Include any package-specific aperture changes and the shared PCB revision.
Technical references
Check each source’s scope and revision against your project requirements. Confirm the proposed construction with your manufacturing partner.
YOUR PROJECT / YOUR NEXT STEP
Build a useful project brief
Identify stencil review priorities; this tool does not select a production foil thickness.
Use this planning aid to prepare a review. It does not approve a design or estimate a price. Selections are processed locally and are not submitted by this tool.