Solder Mask Dam and Sliver Design Rules

A solder mask dam is the strip of mask between two adjacent pads, and it exists to stop solder from bridging between them during reflow. When the dam is absent, the two pads behave as one wettable surface and the bridge is only a matter of how much paste is present.

The width of the dam is constrained by the pad pitch, the registration tolerance of the mask and the resolution of the mask process. Where those three cannot produce a dam, the design should accept that the part will need an alternative approach rather than assuming the process will cope.

What the Dam Does

The dam provides a physical barrier between pads and a surface that solder does not wet. It also sets the solderable area of each pad, which is the basis of the footprint dimensions used for paste aperture calculations.

Removing the dam, which is sometimes done to gain pad width, increases the risk of a bridge on that footprint. The gain in solderable area rarely compensates for the loss of process margin.

Dam Width and Pitch

The achievable dam width falls as the pitch falls, and it is limited by the mask resolution, the registration of the mask to the copper and the thickness of the mask. A typical fine pitch footprint has a dam of about the same order as the space between the pads.

The registration is the part that is usually underestimated, because it depends on the fabricator’s process and on how the mask was exposed. A design that assumes perfect alignment will produce slivers and open dams on a fraction of the panels.

Slivers and Their Consequences

A sliver is a fragment of mask that is too narrow to adhere, so it lifts during processing or in service. A lifted sliver becomes a loose particle that can lodge between pads and cause the very bridge the mask was meant to prevent.

The minimum sliver width is a fabrication rule that varies with the mask thickness and the process, and it should be respected even when the geometry would allow a narrower feature. A design that produces slivers is worse than one that omits the dam deliberately.

Mask Webs Between Pads in a Row

On a connector or a fine pitch package the mask often forms a continuous web between all the pads in the row. The web has to be supported at both ends, or it will lift at the end of the row where it has no anchor.

Where the pad row ends at a larger pad or at a via, the mask can be anchored there. Where it ends in open space, the design should terminate the web with a wider strip rather than leaving a free end, which is the same anchoring principle applied to mask openings in pad design standards.

Mask Thickness and Aspect

A thicker mask is more resistant to solder but harder to resolve, so a design that needs a narrow dam also needs a thin mask. Mask thickness is chosen with the finish and the surface requirements rather than in isolation.

The aspect of the dam is the ratio of its width to its thickness, and a dam that is narrow relative to the mask thickness is unstable. Where the ratio cannot be met, the footprint should be modified rather than the mask specification pushed.

Paste Aperture and the Dam

The paste aperture is normally the pad opening in the mask, so the dam defines the aperture. A narrow dam produces a narrower aperture, which reduces the paste volume and may make the deposit insufficient for a heavy joint.

When volume and dam width conflict, the usual resolution is to reduce the pad dimensions and accept a smaller joint, or to move to a different package. Adjusting the stencil alone cannot create paste volume where the mask leaves no room.

Alternative Approaches

Where a dam is impossible, the options are a mask defined pad with a reduced opening, a different pitch, or a process change such as a stepped stencil or a jet. Each has a cost, and the choice should be made deliberately during layout.

A mask defined pad, where the mask opening is smaller than the copper, is common on fine pitch parts because it removes the risk of a bridge at the expense of pad width. The trade is accepted widely for small packages and should be documented so that the fabrication and assembly teams work from the same assumption.

Inspection and Defects

Inspection of the dam is a check of width, continuity and adhesion, done at magnification and usually against a sample. A dam that is present but cracked will fail in service as the crack opens during thermal cycling.

The defects to record are open dams, lifted slivers and mask on a pad. All three are visible before assembly, and all three are cheaper to correct at that point than after reflow.

Documentation

The drawing should state the mask type, the side it is applied to, the minimum dam width and the treatment of the pads that are mask defined. Where a fabricator needs to thin the mask to hold the dam, that should be agreed rather than assumed.

These notes belong with the fabrication data described in fabrication notes, because the mask specification is one of the few items that is both a fabrication and an assembly requirement.

Process Control and Verification

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Can a bridge be prevented without a dam? Not reliably at fine pitch, because once the pads share a wettable surface the outcome depends on paste volume and placement accuracy.

Is a wider dam always better? Up to the process limit it is, and beyond that the thickness and the sliver rules take over.

Why does a sliver matter if it stays in place? It usually does not stay in place, and a loose fragment between pads is a bridge waiting to happen.

Should mask defined pads be used everywhere? No. They reduce the solderable area, so they are used where pitch makes a dam impractical rather than as a general practice.