Solder Mask vs. Solder Paste: Two PCB Layers You Should Never Confuse

Two exported layers may show nearly the same collection of pads while describing different manufacturing steps. Solder mask belongs to the finished bare board; solder-paste data helps define where solder paste is deposited during assembly. Checking the distinction is especially useful when ordering a stencil separately from the PCB.

What solder-mask data represents

Solder mask is the protective coating surrounding exposed features on a board. In a typical fabrication export, the mask artwork describes openings in that coating. A filled-looking pad in the viewer can therefore represent an area that will be left uncovered, rather than an extra patch of coating. Eurocircuits’ mask guidelines explain this negative-data convention; its numerical process limits are specific to its own service.

What the paste layer represents

The paste layer defines apertures used to deposit solder paste, commonly through an SMT stencil. KiCad treats solder-mask clearance and solder-paste clearance as separate settings. A paste aperture is not automatically identical to the copper pad or its mask opening. Use the component manufacturer’s land-pattern and assembly guidance when deciding how a particular package should be printed.

Review a few representative footprints

Instead of judging the package from a full-board thumbnail, inspect a large connector, a fine-pitch device and an exposed-pad component individually. Compare copper, mask and paste with only the relevant layers visible. Write down anything that cannot be explained by the intended assembly process.

• Does the footprint include the paste data required for the populated side?
• Are contacts that should remain free of paste clearly identified?
• Does a custom aperture arrangement agree with the assembly notes?
• If the board is panelized, does the stencil file represent the same arrangement?

Keep top and bottom unambiguous

Label the intended side and revision in the delivery package. For example, a two-sided board with components fitted only on the top may not need a bottom stencil. Confirm the actual assembly plan rather than ordering two stencils simply because the archive contains two paste files.

A useful handoff question

Ask: “Can the assembler identify exactly which paste pattern to use without guessing?” If the answer is unclear, add a short note and an assembly drawing. Avoid renaming a mask file as a paste file to satisfy an upload field; resolve the missing output in the design first.

Technical references: Eurocircuits: solder-mask data conventions; KiCad PCB Editor: mask and paste settings. Confirm manufacturing-specific limits with DIYIPCB for your order.