Solder Mask Design: Openings, Dams and Tenting

Soldermask is often treated as a cosmetic layer, and it is anything but. It defines where solder may go, protects the conductors from the environment and from handling, and its openings constrain the stencil and the component footprint.

What the Mask Has to Do

The mask has to cover everything that must not be soldered, adhere to the copper and the laminate beneath it, survive the thermal excursion of assembly and remain electrically insulating over the life of the product.

Each of those requirements has a process consequence. Adhesion requires a clean and properly roughened copper surface; insulation requires a minimum thickness; and surviving assembly requires a chemistry that does not degrade at reflow temperature. Our solder mask process notes describe how the layer is formed.

Openings and the Mask Dam

Each opening in the mask exposes a pad, and the space between two adjacent openings is a dam. A dam that is too narrow will not survive the imaging and development steps, and a dam that fails produces a bridge between two pads before any solder is applied.

The minimum dam width is a process parameter, and it is usually larger than the minimum space between conductors. A design that places pads on the minimum conductor spacing may find that the mask between them cannot be produced. Our paste inspection notes describe how the print is affected.

Tenting and Via Coverage

A via may be tented, meaning the mask covers it completely, or left open at one or both ends. Tenting protects the via from contamination and prevents solder from wicking into it, while an open via can be probed and can be used as a test point.

The choice affects the assembly process as well. A tented via that is not completely sealed can trap flux beneath the mask, and an open via beside a pad can wick solder away from the joint.

Mask Thickness and Its Measurement

The mask is applied as a liquid or as a dry film and is typically between fifteen and thirty micrometres thick over the copper. The thickness over a conductor is less than the thickness over the laminate, because the mask follows the profile rather than filling it.

The measurement is normally made on a cured coupon rather than on the board, and the acceptance criterion refers to the thickness over the copper, since that is where the electrical insulation matters.

Colour, Finish and Appearance

Green remains the default because it is the cheapest and because the imaging process was developed around it. Other colours are available and some of them behave differently: white and yellow are thicker to achieve opacity, and black is difficult to inspect optically.

A matte finish hides defects better than a glossy one, which is one reason it is popular for products that are visible to the user. The choice should be made with the inspection method in mind rather than only with the appearance.

Plugs, Dams and Via Fill

Where a via lies inside a pad, the via has to be filled and the surface planarised before the mask is applied, or the mask will not sit flat and the solder will not spread. The fill is a separate process step and it should be specified explicitly.

The alternative is to move the via out of the pad and connect it with a short trace, which removes the need for the fill at the cost of a small amount of routing area. The decision belongs at the layout stage.

Compatibility With the Assembly Process

The mask has to be compatible with the paste and the flux it will meet, with the cleaning process if one is used, and with the conformal coating if one is applied. An incompatibility usually appears as poor adhesion rather than as an electrical fault.

Where a coating is to be applied, the mask surface energy matters, because the coating has to wet it. That is one reason the coating and the mask are usually specified together rather than independently.

What to Put on the Drawing

The drawing should state the colour, the finish, the minimum dam width and whether vias are tented. It should also state the requirement for any via that has to be filled, and whether the mask is required between fine-pitch pads.

Those five items cover most of the queries that a mask specification generates. Leaving them out means the shop chooses, and the choice will be made for its own process rather than for the assembly. Our fabrication notes guidance lists the items to state.

Process Control and Verification

On a design of this kind, solder mask is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

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.

Process Control and Verification

On a design of this kind, solder mask is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

Process Control and Verification

On a design of this kind, solder mask is the item that decides how the rest of the board is arranged. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

FAQ

Can the mask be used to insulate a high voltage gap? It contributes to the insulation but should not be relied upon alone, because the mask can be damaged and because its dielectric strength is lower than the laminate’s. The spacing should be adequate without the mask, and the mask then adds margin.

Why do pads sometimes come out larger than drawn? Because the mask opening is normally larger than the pad, to allow for registration tolerance. That expansion is a design parameter and should be stated rather than left to the shop, since it affects the dam width between fine-pitch pads.

What does gopcb check on a mask specification? We check the minimum dam width against our process, the mask expansion, the tenting requirement, the via fill where needed and the compatibility with the specified finish and any coating. We report any item that cannot be produced before the panel is released.