A mask web is an intentional feature
A solder-mask web, sometimes called a dam or bridge, is the strip of coating between adjacent openings. It is not the same as an unwanted bridge of solder between electrical connections. An opening deliberately exposes a pad or other copper feature; defects arise when the manufactured geometry fails the agreed design and acceptance requirements.
Positive mask expansion normally makes an opening larger than its copper pad, leaving registration allowance. In a solder-mask-defined land pattern the coating overlaps the copper and helps define the exposed land. Use the exact package recommendation and assembly review to choose the approach; do not apply one convention to every package.
Review a tolerance problem, not one nominal number
Consider the smallest copper-to-copper gap, opening size, available mask web and registration allowance together. Increasing expansion can expose an adjacent trace or remove the web entirely. Reducing expansion can leave coating on a soldering land if the layers are misregistered.
Request the fabricator’s achievable web and registration limits for the selected coating, color and process. If individual openings cannot be maintained, a shared opening may be proposed. The assembler and design owner should approve that change because solder control then depends more heavily on the land pattern, stencil and process.
Check special features separately
For BGA and QFN regions, review openings alongside pad dimensions, via treatment and paste apertures. Check test points for probe access and edge contacts for the intended contact area. Avoid letting a global mask rule silently cover a test pad or expose copper next to a mounting hole.
Via tenting instructions should identify the required side and construction. If the assembly needs a flat solderable land over a via, discuss filling and capping rather than assuming a mask tent provides the same result.
Verify the released artwork
Overlay copper and both mask layers in a manufacturing viewer at the dense areas, then review the complete board for missing openings. Keep layer names, units and origin consistent with the drill and outline files. A paste preview is not a substitute for the mask preview.
Request approval of CAM changes where the mask geometry is functional. Save the approved artwork with the fabrication revision, and use agreed optical acceptance to check registration and exposed areas on the first article. Electrical testing does not by itself prove that the mask pattern is correct.