Solder Mask and Legend: Functional Not Cosmetic
The mask and the legend are applied at the end of fabrication and they are the layers most often revised for cosmetic reasons, which is why they are also the layers where a change can quietly invalidate a qualification. Treating them as functional rather than decorative avoids that.
What the Legend Has to Do
The legend identifies the components, indicates the polarity of the parts where it matters, shows the position of pin one and carries any marking the product requires. It is an assembly document rendered on the board.
Its legibility is therefore a functional requirement. A reference designator that is clipped by a pad opening or printed at a size that cannot be read under a microscope is a designator that will be checked against the drawing instead. Our assembly drawing notes describe the complementary document.
Legend and Pad Clearance
Legend printed on a pad will not survive, because the solder wets over it, and legend printed across a mask opening is removed with the mask. The clearance rules keep the text a defined distance from every opening.
Where the space is tight, the designator is moved rather than reduced, because a smaller font fails the legibility requirement before it fails the process. A leader line to a nearby position is sometimes used, and it introduces its own risk of confusion.
Polarity and Pin One
Polarity marks should be visible with the component fitted, which means they must not be placed under the body. On a small package the mark is usually outside the courtyard, and on a large one it may be at two corners so that it remains visible whichever way the board is viewed.
The convention should be consistent. A design that marks pin one with a dot on one package and a chamfer on the next invites an assembly error that the drawing will not catch.
Mask Openings and Expansion
The mask opening is larger than the pad by an expansion value, which exists to allow for registration tolerance. The expansion determines the dam width between adjacent pads, and the dam is what prevents bridging.
On a fine-pitch device the expansion may have to be reduced or set to zero, which is a deliberate decision rather than a default. It should be recorded, because it changes the appearance of the board and the risk of a solder defect. Our solder mask notes describe the rules.
Marking Content and Position
The marking that the product requires, whether a part number, a revision or a machine-readable code, has a content and a position. Both should be specified, and the position should be chosen so that the mark is readable on the assembled product.
Where the mark is a two-dimensional code, the quiet zone around it is part of the specification, and it is the item most often omitted. Our traceability marking notes describe the encoding options.
Colour and Its Consequences
The mask colour affects inspection as well as appearance. A dark mask makes a fine crack or a small solder ball easier to see, while a white mask makes the same defect harder to find because the contrast is lower.
Where an automated optical inspection is used, the colour and the finish affect the programming and the false call rate. The choice should be made with the inspection method in mind.
Revisions That Affect Only the Mask
A change to the legend or to the mask is often treated as cosmetic and released without the checks that a routing change receives. That is where an error enters, because the same data set now differs from the one that was qualified.
The safe practice is to treat any change to the mask or the legend data as a revision, with the same verification. The cost of the verification is minutes and the cost of the error is a batch of boards with an unusable marking.
Documenting the Requirements
The drawing should state the colour, the expansion, the minimum dam, the legend font and its minimum height, the marking content and the quiet zone. Those six items cover the queries that a mask and legend specification generates.
Leaving them out means the shop chooses, and the shop will choose for its own process rather than for the assembly. Our fabrication notes guidance lists the entries.
Process Control and Verification
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.
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.
Verification and Records
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.
Points to Confirm at First Article
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.
Documented Process Control
A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.
Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.
FAQ
Can legend be printed on the bottom side? It can, and it is often necessary when both sides carry components. The requirement is that the text is readable from the side it is on, which means it must be mirrored correctly in the data.
Why does legend sometimes disappear after assembly? Because it was printed over a mask opening or over a pad, and both are removed or covered during assembly. The clearance rules are what prevent it.
What does gopcb check on mask and legend data? We check the dam widths against our process, the legend clearance from every opening, the minimum font height, the marking content and the quiet zone around any code. Where a feature is below the limit we report it before the panel is released.
