Paste Mask vs Solder Mask: Layer Purpose, Apertures and Data Checks

Follow the material through assembly

Solder resist stays on the board after fabrication and assembly. It protects selected surfaces and helps define where solderable copper remains exposed. Solder paste is a temporary mixture deposited through a stencil; after reflow, the joint and flux residues depend on the material and process. The stencil itself does not become part of the PCB.

These different lifetimes explain why the layers need separate ownership. The bare-board fabricator uses the mask data with the copper and construction. The stencil and assembly teams use paste apertures with the land pattern, component and printing process. An archive should identify both even when the same CAD export command produces them.

Copper, mask and paste comparison

Compare the three relevant shapes

Start with the copper land. Place the permanent-mask opening around or over that land according to the intended strategy. Then place the stencil aperture appropriate to the solder-paste requirement. All three may differ in size, shape and segmentation. A large exposed thermal pad often makes the distinction especially visible.

The worksheet uses separate dimensions for these rectangles. It reports the paste-to-copper area ratio and an ideal aperture volume from stencil thickness. A larger mask opening does not mean the aperture should grow by the same amount. The appropriate paste deposit also depends on release efficiency, separation, print settings, paste and package behavior.

Interpret percentage reductions carefully

A percentage can refer to area or to a linear dimension. Reducing width and length each by ten percent leaves eighty-one percent of the original rectangular area. Reducing area by ten percent while retaining aspect ratio requires a different scale factor. Check the actual rule definition in your CAD version rather than relying on the word reduction alone.

Altium's documented paste-mask rules distinguish absolute and percentage treatment. The output geometry is the important verification point, especially when migrating libraries or using object overrides. Preserve the intended aperture dimensions in the stencil review so a percentage setting cannot hide an unexpected physical result.

Treat thermal-pad segmentation as its own design

Several smaller apertures can distribute paste across a large thermal land. The segmentation pattern, web arrangement and total open area need review against the component recommendation and assembly process. A single area ratio does not tell you whether the deposit will collapse evenly, release well or produce acceptable voiding.

Keep nearby signal lands and vias visible in that review. Open vias can provide paths for solder loss; treatment of those vias belongs in the board construction. Do not attempt to solve an unreviewed via-in-pad condition merely by adding more paste. Ask the relevant teams to agree the combined land, via and stencil arrangement.

Use aperture volume as a comparison, not a promise

For a rectangular aperture, the ideal cavity volume is width multiplied by length multiplied by stencil thickness. It is a geometric starting point. Actual printed volume may differ because paste does not transfer perfectly and printing conditions vary. The tool deliberately does not convert its answer into a guaranteed joint height or a production yield.

When comparing alternatives, hold the units and aperture count constant. If one option uses segmented apertures, sum their individual areas rather than using the outer envelope that includes stencil webs. Record both the nominal geometry and any measured paste-deposit result so the design can be adjusted from evidence.

Package files so the layers cannot be swapped

Use descriptive layer filenames and include a readme or fabrication/assembly drawing that identifies top and bottom. Review the orientation and origin, particularly for bottom-side stencil data. A mirrored display can appear familiar while being wrong for the intended manufacturing operation.

The PCB stencil page owns stencil selection and quotation. Use solder-mask opening design for permanent coating geometry. Keeping those routes distinct helps an engineer reach the right review without reading two nearly identical generic layer descriptions.

Questions that prevent avoidable rework

Ask whether the stencil supplier is expected to optimize apertures and, if so, how proposed changes will be approved. Identify the source revision and request an aperture review for critical packages. Clarify whether the assembly provider requires paste files, copper files, package data or an existing stencil specification for the quotation.

If a board already has a printing problem, provide the measured deposit or defect location instead of only a screenshot of the mask setting. Include stencil thickness, aperture shape and the package reference. That information makes the next conversation about a real printing task rather than an ambiguous request to change the mask.

Review one critical footprint through the complete file chain

Choose a representative footprint whose geometry makes the mask decision important. Place its component recommendation, copper land, mask opening and paste aperture side by side. Name the role of each boundary before comparing dimensions. This prevents an aperture change intended for printing from being applied to the permanent coating by mistake.

Next, identify the source of every allowance. Nominal expansion, local opening growth and mask-to-copper registration describe different effects. A conservative tolerance budget can be appropriate, but only when it does not count the same variation twice. If the source of an allowance is unknown, preserve that as an open question instead of treating a familiar round number as verified process data.

Inspect the final exported files for that footprint and at least one important exception. This is where object overrides, library differences and rule scope become visible. The released output should match the documented design choice. A manufacturing partner's proposed change should be tied to the same feature and revision so both sides can assess its effect on assembly.

For a repeat order, retain the approved file set and any accepted exception. If a new supplier or mask grade is proposed, review the critical geometry against that route rather than assuming the old nominal dimensions are universally producible. The objective is a stable functional interface, not the smallest possible number in a capability table.

The enquiry can be brief: the marked region, exact component, relevant layers and the decision you need to make. That gives the reviewer enough context to discuss a real margin or opening strategy without forcing the customer to complete an unrelated cost calculator first.