A stencil defines the intended paste deposits
A solder-paste stencil transfers paste through openings onto the board lands. Its artwork comes from the assembly paste definition, not from a blind copy of every copper pad or solder-mask opening. The desired deposit depends on the component attachment and process, including exposed pads, fine-pitch terminals and mixed package sizes.
Stencil thickness and aperture geometry interact. The aperture area ratio compares opening area with aperture wall area and helps assess paste release. It is one input to a process review, not a universal pass/fail number independent of paste, stencil technology and printing conditions.
Choose the construction for the printing system
Laser-cut metal and electroformed stencil approaches offer different manufacturing routes. Framed and frameless formats must match the printer or fixture. Step stencils can provide different local thicknesses where a qualified process needs them; their transitions and usable spacing require review.
Compare options by the actual package mix, required transfer performance, board support and cleaning method. Material, coating, frame, step processing and inspection requirements can affect stencil cost. Avoid choosing solely by a nominal thickness or a claim that one manufacturing method is always best.
Release an unambiguous stencil package
- Identify board and assembly revision, side and fitted variant.
- Provide the approved paste layer and the matching outline or panel reference.
- Identify apertures that need specific reductions, divisions or exclusions.
- Confirm frame dimensions, fiducials and registration features.
- Record who may modify paste artwork and how approval is obtained.
Large exposed pads can require divided apertures or other device-specific treatment. TI’s QFN guidance discusses the relationship between layout and attachment requirements. Do not reuse its dimensions for a different package without checking the relevant component documentation and assembly process.
Inspect the stencil and the first printed board
Check the delivered stencil against the approved artwork and inspect for blocked, damaged or incorrect openings. Verify registration over the whole board or panel. A correct alignment near one corner does not establish correct scale or registration at every location.
Use first-print evidence to assess deposit position and the agreed paste characteristics before placing components. If a change is required, update the controlled stencil record and identify the affected revision. Keep cleaning, handling and storage practices compatible with the stencil and paste supplier guidance. A stencil reused after a design change should be treated as a reviewed tooling decision, not an automatic saving.