Solder Mask Design Rules: Dams, Slivers and Openings

Solder mask is not decoration. It keeps solder where it belongs, protects the copper from the environment, and defines the resolution of a fine pitch layout. The rules that govern it are geometric and they collide with each other.

What the Mask Has to Do

The mask separates adjacent pads so that molten solder cannot bridge between them. It also keeps the surface insulation resistance high enough that a flux residue does not become a leakage path.

On a fine pitch part the two requirements pull in opposite directions. A wider dam between pads is better for bridging and worse for the space available, and the balance is the design decision.

Dams and Slivers

A dam is the strip of mask between two openings. Below a minimum width the dam will not survive the exposure and development, and it lifts or breaks away during assembly.

A sliver is the mask that remains between a pad and the copper beside it, and it is thinner than a dam. Slivers are the first feature to fail, so the layout should avoid relying on them. Our quality notes describe how the finished mask is judged.

Mask Expansion

The opening is usually larger than the pad, and the difference is the expansion. It compensates for the registration tolerance between the copper and the mask, and it is what keeps the mask off the pad surface.

Too little expansion leaves mask on the pad, and the joint does not form. Too much reduces the dam. The expansion must be stated per side and checked against the fabricator’s registration capability. Our fabrication notes notes list the attributes that should be stated.

Mask on Vias and Test Points

Tented vias save space and prevent solder wicking, but the tent can crack over a large hole during thermal cycling. Where the via carries a thermal load or sits under a pad, the tent is not enough.

Test points must be left open and their mask opening must be large enough for the probe to contact the pad without touching mask. A probe that lands on mask gives a false open and wastes a debug cycle.

Mask Under Components

Mask between fine pitch pads is often removed entirely to give the stencil a continuous surface, which trades bridging risk against paste release. The decision should follow the paste volume and the stencil design rather than a default.

Where a component has a large exposed thermal pad, the mask pattern under it controls the paste area and therefore the voiding. Our paste volume notes cover how the aperture relates to the joint.

Mask Type and Thickness

Liquid photoimageable mask gives the best resolution and is the choice for fine pitch. Dry film is used where the thickness needed exceeds what a liquid coat can hold in one pass.

The thickness matters as well as the material. A thick mask holds a dam better and stands further above the pad, which changes how the stencil seals and how the paste releases. Our stencil notes cover the printing consequence.

Colour and Appearance

The colour does not change the electrical performance, but it does change inspection. A dark mask under a bright finish gives good contrast for optical inspection; a finish of similar brightness to the mask makes the joints harder to see.

Appearance requirements should be stated if they matter, because they affect the process control that has to be applied and the cost of the panels that are rejected.

Verification

The mask is verified by inspection of the panel after development and by test of the assembled board. The panel check looks for dams, slivers and registration; the board check looks for mask on pads and for lifted mask after reflow.

The repair of a lifted dam is possible but local, and it reverses the resolution advantage the process was chosen for. Our inspection notes describe what the optical system can classify automatically.

Process Control and Verification

On a design of this kind, sliver is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

Process Control and Verification

On a design of this kind, sliver is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Process Control and Verification

On a design of this kind, sliver is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/AdobeStock_311524531-1.jpeg" alt="Mask opening around a probe test point” />

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Why does the mask keep lifting from a pad edge? Usually because the expansion is too small and the mask sits partly on the pad, where it has nothing to bond to. Increasing the expansion fixes it and reduces the dam.

Can the mask be removed between fine pitch pads? It can, and it is common on dense parts, but the stencil and the paste volume then carry the bridging risk and must be specified together.

What does gopcb provide for solder mask design? We provide dam and sliver rules matched to the fabricator capability, expansion values per side, via and test point treatment, mask patterns under thermal pads, mask type and thickness selection, and inspection of the finished mask before assembly.