Review Soldermask Dams with Registration Allowance Included

A narrow strip of soldermask between pads may look correct in a CAD view and disappear after the openings are adjusted for manufacturing. The pad clearance, remaining web and mask registration allowance are linked. Review them together at the actual fine-pitch features rather than checking only a global minimum web rule.

Identify which geometry controls the joint

Determine whether the land pattern intends the copper or the soldermask opening to define the solderable area. Follow the component manufacturer’s package guidance and the assembly process. A generic mask expansion should not silently change a deliberately mask-defined pad into a different geometry.

Eurocircuits’ soldermask guidelines distinguish mask opening clearance, mask web and overlap over adjacent copper. Its published limits depend on its production process. Obtain the relevant limits from the actual fabricator rather than presenting another supplier’s minimum as a universal capability.

Calculate the remaining web

For two simple adjacent openings, the nominal web is their separation after both expansions are included. As an illustrative geometry, a 0.15 mm copper gap with 0.04 mm opening expansion from each pad leaves 0.07 mm of nominal mask between them. This arithmetic does not establish that the web is manufacturable or correctly registered.

Review tolerance and any proposed CAM adjustment. Enlarging openings may protect pad exposure while reducing the web. Reducing openings may preserve the web while risking encroachment on the land. The acceptable tradeoff depends on the package and process.

Inspect nearby copper and special features

Check traces between pads, vias, test contacts and exposed thermal lands. A mask opening can expose copper that was not intended to receive solder. Conversely, shrinking an opening can cover a test contact or alter a joint’s useful area.

Use the final mask artwork overlaid on the copper, including the fabricator’s proposed edits. Compare critical packages individually. A board-wide viewer at low magnification can conceal a few affected pads even when the rest of the mask looks consistent.

Agree on changes before fabrication

If the process cannot retain a dam, document the affected locations and review assembly implications with the assembler. Removing the dam is a specific design decision, not merely cosmetic cleanup. Consider whether a land-pattern change or a different process is required.

Inspect representative finished boards and the subsequent soldering result. Record the mask process, approved artwork and any accepted local exceptions. Recheck after changing mask color, package footprint or supplier because the same nominal expansion does not guarantee the same manufactured web and registration behavior.