Solder Mask Defined vs Copper Defined Pads

A pad can be defined by the mask or defined as copper with the mask pulled back from it, and the two choices behave differently at the joint. The difference is small on a large pad and decisive on a small one.

What the Two Definitions Are

A mask defined pad has the mask opening smaller than the copper pad, so the mask overlaps the copper and the exposed area is set by the mask. A copper defined pad has the mask opening larger than the copper, so the exposed area is set by the copper.

The distinction matters because the mask has its own registration tolerance and the copper has its own. Whichever is larger controls the exposed area. Our board quality notes describe how the result is judged.

Process Control and Verification

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.

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.

Effect on Paste Volume

The stencil aperture is sized from the exposed pad, so the exposed area decides the deposit volume. A mask defined pad produces a smaller and more consistent exposed area, which makes the paste volume more predictable.

That predictability is the reason mask defined pads are used on fine pitch parts with small pads. Our paste volume notes describe the deposit targets.

Effect on the Joint

The mask overlapping the copper creates a step at the edge of the pad, and the joint has to form up that step. A copper defined pad has the mask away from the pad and gives the alloy more room to spread.

The larger exposed area of a copper defined pad produces a wider fillet and a lower stand off, which is an advantage where the joint has to carry current and a disadvantage where the parts are dense. Our X-ray notes describe how the resulting joint is inspected.

Registration Consequences

A mask defined pad is sensitive to mask registration, because an offset moves the exposed area and changes its shape. A copper defined pad is sensitive to copper registration, because an offset moves the pad relative to the aperture.

Where the mask process is well controlled and the copper is not, the mask defined pad is the better choice, and the reverse is also true. The decision is therefore a process question as much as a design one. Our SPI notes describe how the print result shows the consequence.

Where Each Is Used

Mask defined pads are used on a fine pitch device where the pad is small and the paste volume has to be tight. Copper defined pads are used on a general population where a larger pad gives a better joint and the density allows it.

Some designs mix the two, with the fine pitch part mask defined and everything else copper defined. That is a deliberate choice and it has to be stated rather than left to the shop.

What to Put on the Drawing

The drawing states which definition applies, and where a part is mask defined it states the required overlap. A general note that leaves the choice to the shop produces a board that is built correctly and does not match the stencil that was ordered from the artwork.

The stencil data and the drawing therefore have to be changed together. A stencil cut for one definition and a board built to the other is a mismatch that shows up as a volume problem rather than as a mask problem.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

FAQ

Is a mask defined pad always smaller? Yes, and the reduction is the overlap on each side, which is why the overlap is stated.

Does the definition change the paste aperture? It does, because the aperture is sized from the exposed area rather than from the copper.

What does gopcb provide for pad definition? We provide the definition stated on the drawing, the overlap controlled where the pad is mask defined, registration measured for whichever layer controls the area, stencil data changed with the drawing, and the result verified by paste volume and inspection.