Where a "just send the Gerbers" package stalls
Zipping the full Gerber set with a note saying "please make a stencil from this" is the most common approach, and the most common reason files come back for more information.
The problem is not file format. SKPCB's stencil service accepts Gerber RS-274X, ODB++ and DXF, so format rarely blocks anything. What blocks review is information dimension. The paste mask is a flat set of shapes. It records every aperture position and outline precisely. But a stencil is a physical object, and several of its specifications have no corresponding data in a flat drawing at all.
Foil thickness is the clearest case. Whether paste releases cleanly is judged by area ratio: aperture area divided by aperture wall area, and the IPC rule of thumb keeps it at 0.66 or above. The denominator involves the aperture *wall*, and wall height is foil thickness. The paste layer has no thickness dimension, so a package containing only a paste layer cannot be checked for area ratio at all. Engineering cannot run that calculation and has to come back to you.
This article splits the package into three parts: layers you must supply, fields you must state in words, and the items review will otherwise ask about. The goal is not more paperwork. It is getting an aperture decision on the first review pass instead of burning days on back-and-forth.
1. Three required layers: paste, solder mask, outline
The stencil service page lists input files as Gerber RS-274X (paste + solder mask + outline layers). Each layer does a different job, and dropping one removes a cross-check.
Paste mask — the direct basis for cutting. Aperture positions and shapes come from here. This is the one layer nothing substitutes for.
Solder mask — used to cross-check against the paste layer. Apertures have to land on pads, and the solderable area of a pad is bounded by the mask opening. Overlaying the two layers is what exposes inconsistencies such as an aperture present on the paste layer with no mask opening underneath it. Engineering review cross-checks the paste layer against pads and solder-mask openings; without the mask layer that check cannot happen.
Outline — establishes the board boundary, which in turn establishes the usable stencil area and where the aperture array sits relative to the board edge. Without it, the effective stencil area is guesswork.
Top and bottom stencils each need their own paste layer. Top and bottom are two different aperture data sets. Do not send one side with an email note saying "the other side is the same" — unless it genuinely is symmetric, and that has to be shown in data, not asserted in a sentence.
2. Four fields the paste layer cannot hold
This is the core of the article. The four items below do not exist in any Gerber layer and must be written into the order notes or process requirements.
2.1 Foil thickness
SKPCB builds stencils day to day in 0.08 / 0.10 / 0.12 / 0.15 / 0.20 mm foil.
Thickness is not a parameter you can pick casually afterwards. It is coupled to aperture size, component pitch, paste and printing equipment. The service page states this plainly: stencil thickness cannot be selected separately from pad geometry, component pitch, paste and printing equipment, and thickness, apertures and area ratio are confirmed by engineering once paste, solder-mask and outline data are provided.
So there are two workable paths. Pick one and state it:
- You have already fixed the thickness — write the value. Engineering will check area ratio group by group at that thickness and report back any group falling under 0.66.
- You want engineering to recommend it — say so explicitly, and include the finest pitch on the board, the smallest pad dimension, and the paste type in use. Thickness is then confirmed during review against the data and equipment.
Leaving it vague is the slowest option. Data that reaches the review desk without a thickness can neither be checked nor advanced.
2.2 Frame type and frame size
Framed (glue-in) stencils and unframed foils are two different deliverables.
Framed stencils carry a permanent frame with a mesh border tensioning the foil, which suits high print frequency and long-term repeat production. Unframed stencils are foils without a permanent frame, fitted into reusable frames, which suits short runs, low volume and prototypes.
If you choose framed, the frame size has to match your printer. SKPCB's common framed sizes are 370×470, 420×520, 450×550, 550×650 and 584×584 mm. Only you hold this information — it depends on the printer on your floor. It is not in the Gerber and cannot be derived from board size.
2.3 Print side and whether to split stencils
The service page gives print side options as top, bottom, or top+bottom on one stencil / on separate stencils.
This is an explicit three-way decision, not a default:
- Top only (single-sided assembly)
- Top and bottom on separate stencils
- Top and bottom on one stencil
Combining both sides onto one stencil saves a stencil, but requires the usable area to hold both aperture sets and the print alignment method to suit. That decision is tied to how your line is organised, so it is not one the shop should guess.
2.4 Panel array, array count and fiducials
If the board ships panelised, state the panel arrangement and array count, and note any fiducial requirements. This is an explicit requirement on the stencil service page.
A single-image paste layer will not tell the shop that this is a 2×3 panel, nor will it convey panel spacing or rail width. Fiducials are the same story: stencil fiducials have to suit your printer's alignment method, and framed stencils with automatic printer alignment are the normal arrangement for volume runs. Blurred or missing fiducials show up directly as paste offset from pads.
If different zones of the board need different paste volumes — large connector pads taking more, fine-pitch areas taking less — raise it here. That points to a multilayer or step stencil; the design logic is in step stencil multilevel aperture design.
3. Three optional items worth stating anyway
These three have defaults and will not stall review. But if you have requirements on release consistency or service life, stating them saves a round trip.
Polishing — grinding polish is standard; electrolytic polish is an option that improves paste release. Worth asking for on fine-pitch designs.
Coating — optional nano-coating cuts wipe frequency and extends service life. Worth considering for lines running long uninterrupted shifts.
Fabrication method — laser cutting is the default and covers the large majority of products with faster turnaround and better value. Electroformed stencils are grown layer by layer in nickel, giving smoother trapezoidal aperture walls and more consistent paste release, which suits ultra-fine pitch work such as 0.3 mm pitch and 0201 parts, at the cost of a more complex process, higher price and longer lead time. At 0.4 mm pitch and above, a laser stencil with electrolytic polishing or nano-coating is usually enough. The trade-off is covered in laser-cut vs electroformed stencil fabrication.
4. Apertures: do not pre-shrink in the data
A frequent mistake is shrinking apertures by some ratio before exporting, then sending that.
The service page is direct about this: apertures must be reviewed against pad geometry, paste data, component pitch and equipment, and no fixed ratio or shrink value applies to every design. The common practice for standard parts is apertures 5%–10% smaller than the pads to suppress bridging, but that is a starting range, not a universal formula.
The more practical approach is to send pads as drawn and let the aperture plan be set in review. After files arrive, engineering reviews the aperture plan and flags items needing confirmation before quotation. The relationship between area ratio and aperture size is covered in stencil aperture area ratio and paste release.
One more alignment item: aperture design has to line up with what the board itself can hold. Check minimum trace/space, hole sizes and outline tolerances against manufacturing capabilities and tolerance standards. Aperture precision cannot exceed what the board can carry.
5. Data package self-check (run before you send)
- Paste layer exported, each side supplied separately (no substituting "same as the other side" for data).
- Solder mask and outline layers included, for cross-checking pad positions and usable stencil area.
- Format is one of Gerber RS-274X, ODB++ or DXF.
- Foil thickness stated, or "please recommend" stated explicitly with finest pitch, smallest pad and paste type attached.
- Framed or unframed chosen; if framed, frame size given and matched to your own printer.
- Print side declared: top only / top and bottom separate / top and bottom combined.
- Panel arrangement and array count stated; fiducial requirements noted.
- Any zone-to-zone paste volume differences stated (whether a step stencil is needed).
- Polishing, coating and fabrication preferences included if you have any.
- Apertures not pre-scaled by a self-chosen ratio, or the applied scaling stated so it is not shrunk twice.
Run the list, then send. Review can usually return an aperture conclusion on the first pass. Order quantity and quotation are themselves confirmed after review of the Gerber data, quantity and process requirements — complete data is what gives a quotation something to stand on.
6. After the data package: the stencil still gets handed off once
The data package only settles whether the cut stencil is correct. Once it reaches the floor there is another handoff: label against controlled data, orientation and fiducials, frame and foil tension condition, underside cleanliness, and first print. That is covered in SMT stencil incoming and line-handoff inspection.
To arrange the stencil alongside the boards, raise it on the SMT stencil service page or go straight to get a quote. Board types and materials are in products and the materials library; inspection flow is in quality assurance.
FAQ
Q: Can you build a stencil from the paste layer alone?
A: The paste layer is the one layer nothing substitutes for, but sending only that drops two cross-checks — the solder mask confirms apertures land inside the solderable pad area, and the outline establishes usable stencil area and aperture position relative to the board edge. SKPCB's stencil input requirement is Gerber RS-274X paste, solder mask and outline layers; with all three, review completes in one pass.
Q: Should I fix the foil thickness or let the shop choose?
A: Either works, but pick one explicitly. Stencil thickness cannot be selected separately from pad geometry, component pitch, paste and printing equipment. If you fix it, state the value (0.08 / 0.10 / 0.12 / 0.15 / 0.20 mm) and engineering will check area ratio per aperture group at that thickness. If you want a recommendation, say so and supply finest pitch, smallest pad and paste type. Saying nothing is the slow path.
Q: How is the 0.66 area ratio actually used?
A: Area ratio = aperture area ÷ aperture wall area, and the IPC rule of thumb keeps it at 0.66 or above. Below that, paste tends to stay in the aperture, producing insufficient deposits or open joints. Note that the wall height in the denominator is foil thickness — without a stated thickness the number cannot be computed, which is the direct reason thickness must be declared.
Q: Should I pre-shrink apertures by 5%–10%?
A: Not recommended. Apertures must be reviewed against pad geometry, paste data, component pitch and equipment, and no fixed ratio or shrink value applies to every design. Apertures 5%–10% smaller than the pads is common practice for standard parts, but it is a starting range. If you have already scaled them, state the scaling you applied so review does not shrink them a second time.
Q: What extra information does a panelised board need?
A: State the panel arrangement and array count, and note fiducial requirements. A single-image paste layer does not express the panel array, panel spacing or rails. Fiducials matter most: framed stencils with automatic printer alignment are the normal volume arrangement, and blurred or missing fiducials show up as paste offset from pads.
Q: Can top and bottom go on one stencil?
A: Yes. Print side options are top, bottom, or top+bottom on one stencil or on separate stencils. Combining saves a stencil but requires the usable area to hold both aperture sets and the alignment method to suit. You need to declare this in the package; there is no default.