A solder mask dam, also called a mask web, is the narrow strip of cured solder resist left between neighboring pad openings. It helps keep adjacent solderable areas separate. It is not the copper gap, and its presence does not guarantee a bridge-free assembly.
For fine-pitch parts, decide whether the web is manufacturable before releasing the PCB. The useful question is not simply “Is this a 0.4 mm pitch package?” It is “How much mask remains after the pad widths and openings are accounted for, and can the selected fabrication process reproduce it?”
What the dam separates, and why copper clearance is different
The dam lies between openings in the solder mask layer. The copper pads below those openings have their own dimensions. Enlarging an opening consumes some of the available mask web even when the copper pads do not change.
| Feature | Where to measure | Why it matters |
|---|---|---|
| Copper gap | Between facing copper edges | Defines conductor separation, not remaining mask width |
| Mask expansion | From a copper edge to the corresponding opening edge | Provides exposed area and registration allowance for an NSMD land |
| Mask dam/web | Between facing mask-opening edges | Must remain a reproducible, adherent strip after processing |
For a non-solder-mask-defined land, or NSMD land, the opening exposes the copper pad and normally extends beyond it. For a solder-mask-defined land, the mask intentionally overlaps copper and defines the exposed area. Do not shrink an NSMD opening until it covers the pad simply to manufacture a wider web; that changes the land definition and needs a package-level review.
A dam is also not a solder-paste stencil feature. Mask belongs to the bare board, while the stencil controls paste deposition during assembly. A sound mask layout cannot compensate for an unsuitable paste aperture or printing process.
Calculate the nominal web before changing the footprint
For two parallel rectangular pads, subtract the facing mask expansions from the copper gap. Use dimensions measured in the direction between the pads, not the pad length along a lead.
Nominal web = copper edge-to-edge gap − left opening expansion − right opening expansion.
For equal pad widths and equal expansion on each side, this becomes web = pitch − pad width − 2 × expansion. Pitch means center-to-center spacing. The expression is a two-dimensional geometry check, not a manufacturing tolerance model.
Take a hypothetical footprint with 0.50 mm pitch, 0.25 mm pad width and 0.05 mm expansion per facing edge. The copper gap is 0.25 mm, while the nominal mask web is 0.50 − 0.25 − 0.10 = 0.15 mm. If the same pad and expansion are used at 0.40 mm pitch, only 0.05 mm remains. The pads are still separate in copper, but the mask situation has changed substantially.
These numbers are examples, not PCBtry capability limits. A positive answer only means the drawn openings do not touch. Compare it with the fabricator’s requirement for the selected mask process and the intended finished condition.
A zero result means the nominal opening edges meet. A negative result means they overlap; there is no nominal web between them. Do not label that as a tiny positive dam. Inspect the actual exported shape, especially where rounded corners, unequal pads or nearby via openings change the narrowest location.
Here is a useful review note to copy into a drawing query: “At U3 pins 12–13, pitch is 0.50 mm, facing pad width is 0.25 mm, and expansion is 0.05 mm per side. Nominal web is 0.15 mm. Please confirm the finished-web requirement and any CAM adjustment.” It names the location, inputs and unanswered question instead of merely asking for “better mask.”
When to keep individual openings or use a shared opening
Keep separate openings when the package guidance and fabrication process support them. If they do not, review the available choices with both the fabricator and assembler before removing dams or altering the land pattern.
Analog Devices AN-772 gives a specific LFCSP example in which the available space at 0.4 mm pitch does not support the stated web and opening requirements, so a trench opening is recommended. That is guidance for the illustrated geometry, not a rule that every 0.4 mm footprint must use a trench.
Conversely, Worthington Assembly’s fine-pitch guidance emphasizes retaining dams and coordinating pad and opening widths. The apparent disagreement is a reason to compare the actual dimensions and manufacturing assumptions, not to choose whichever advice is easier.
| Route | When to consider it | Approval needed |
|---|---|---|
| Retain individual openings | Nominal geometry and the agreed process support reliable webs | Fabricator confirms width, alignment and coverage requirements |
| Revise expansion or select a finer process | A web is valuable but current opening/process settings are unsuitable | Pad exposure, registration and process capability reviewed together |
| Use an approved shared/trench opening | Package and assembly guidance permit a shared opening | Assembler reviews exposed copper, paste deposition and bridging controls |
Changing copper pad width is a separate decision. It can affect the component land pattern and joint geometry. Do not make it a hidden CAM workaround for a mask-width problem. Keep the package drawing and recommended footprint available during that discussion.
Why a web visible in CAD may not survive fabrication
CAD shows nominal geometry; fabrication must form and retain the material. Imaging, development, surface preparation and the selected mask system affect the result. The relevant capability is not just whether a line can appear on a screen.
Separate alignment from opening size. If an entire mask image shifts rigidly to one side, the distance between two openings in that image does not change, but their alignment to the copper does. If the openings become larger, the intervening web becomes narrower. Local distortion can produce a more complicated combination. Therefore, do not automatically subtract twice a quoted registration tolerance from the web and call that a validated worst-case calculation.
Ask what the supplier’s minimum actually describes: the design-file web, the finished web, the minimum opening, or mask-to-copper alignment. Also identify mask type, color, copper topography and any process restrictions. A capability quoted for one build does not automatically apply to another.
The broader solder mask manufacturing process explains how the coating is patterned and cured. For a local fine-pitch problem, however, the immediate task is to locate the weak feature and agree its acceptance basis, not to rewrite every process setting.
Review the release files and agree the assembly checks
Approve the exported mask geometry and its assembly consequences before production. A fabrication preview and a copper-only DRC report answer different questions; retain both the design intent and the released data.
- Identify the tightest location. Record the component, pin pair, board side and revision. Include corners and pad-to-via gaps, not only a straight row of equal pads.
- Measure the nominal geometry. Record copper gap, facing expansions and remaining web using consistent units. Check whether dimensions refer to each side or the total opening increase.
- Open the exported layers together. Inspect copper and solder mask in a CAM viewer. Confirm polarity, side assignment and whether small webs have already been merged or removed.
- Resolve the factory query. Ask for the required web basis and a marked proposal. Do not authorize an unspecified “optimization” of every footprint.
- Coordinate assembly. If openings merge or pad exposure changes, have the assembler review stencil apertures, paste amount, placement and the applicable reflow process. Do not assume the previous build remains equivalent.
- Verify the first build. Compare critical areas with the approved revision using suitable magnification and calibrated dimensional inspection where specified. Record missing or damaged webs and pad encroachment separately from actual solder bridges.
| Release item | What the reviewer should be able to confirm |
|---|---|
| Copper and top/bottom mask files | Actual openings, narrow locations and exposed conductors |
| Package drawing and footprint revision | Whether a proposed change preserves the intended land definition |
| Mask type/color and critical-location notes | Which process capability and inspection requirements apply |
| Marked CAM proposal | Exactly which webs or openings would change |
| Assembly review and first-build criteria | How changed geometry will be evaluated before repeat production |
When comparing PCB fabrication options, compare these agreed requirements rather than one advertised minimum. A missing dam may be an approved feature or a defect, depending on the released design. An electrical test can check specified net connections but cannot, by itself, prove that every mask web is present and adherent.
Send PCBtry the fabrication package with the marked fine-pitch location, mask requirements and package reference. Ask for confirmation of the proposed opening geometry and any exceptions before approving production.
Solder mask dam questions
Are solder mask dam, web and bridge the same term?
They commonly refer to the mask strip between openings. State “mask web” when a “bridge” might be confused with an unwanted metallic solder connection.
Is 0.10 mm always an acceptable dam width?
No. Confirm whether that figure is a nominal artwork value or a finished requirement, and whether it applies to the chosen process and board.
Does a narrower mask opening always solve the problem?
No. It can increase the nominal web while reducing pad-exposure margin. Review alignment and land definition before changing it.
Can two separate copper pads share a mask opening?
Yes, when the package and assembly plan permit it. The copper remains separate, but the intervening mask barrier is absent and assembly needs review.
Does changing solder mask color preserve the same minimum web?
Do not assume so. Obtain confirmation for the actual material, color and process rather than applying a value from another order.
Can silkscreen provide a missing solder mask dam?
No. Legend ink is not a substitute for the specified solder resist geometry. Correct or approve the mask design itself.
Should every absent dam be rejected?
Compare it with the approved files and acceptance criteria. An intentional shared opening is different from an unapproved removed or damaged web.
What should a supplier send before removing a narrow web?
A marked location, original and proposed mask geometry, the process reason and the required customer approval. Preserve the agreed revision for assembly.
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