Solder Mask Colour and Its Effect on Optical Inspection

Solder mask colour is usually chosen for appearance, and it has a measurable effect on how well an inspection system can see the board. The effect is not about quality and it is about contrast.

How Colour Changes Contrast

An inspection system detects edges by the difference in reflected light between two areas. A mask that reflects strongly against a bright component produces a clear edge, and one that matches the component produces a weak one.

The relevant figure is the reflectance under the lighting that is used. Our AOI notes describe the programming that depends on it.

Effect on AOI Programming

A colour change invalidates the learned reference images, so the program has to be re taught. Where the change also lowers contrast, the thresholds have to be relaxed and the false call rate rises.

Both effects should be expected at the change. Our AOI programming notes describe the re teaching.

Effect on Visual Inspection

A mask that is too dark for the illumination at the station makes the operator increase the magnification or the light, which reduces the field of view and slows the inspection.

The lighting should be chosen for the mask rather than the reverse. Our quality notes describe the station.

Component Contrast

A black mask with black components and black silkscreen is the worst case for a camera. A light silkscreen on a dark mask or the reverse gives the edge that the system needs.

Our silkscreen notes describe the contrast that is worth designing for.

Thickness and Gloss

The mask thickness and its gloss also change reflectance, and both vary with the process. A gloss that is not controlled produces an inconsistent image across a panel.

The two are specified with the colour rather than separately. Our mask control notes describe the specification.

Choosing a Colour

Where there is no other constraint, the colour that maximises contrast with the components and the silkscreen is the one that makes inspection easiest. That is usually a mid tone rather than the extreme.

The choice should be made with the inspection method in mind. Our mask colour notes describe the options.

Verification

The verification is a reference image re taught after a colour change, a false call rate compared before and after, and a lighting setting at the visual station that suits the mask rather than one inherited from a previous product.

Our first pass yield notes describe how the comparison is recorded.

Process Control and Verification

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.

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. 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. 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. 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.

FAQ

Does mask colour affect solderability? No. It affects inspection and appearance, and the solderability is set by the surface finish.

Are dark masks harder to inspect? They are harder at low magnification and with poor lighting, and they are entirely workable with the right illumination.

What does gopcb provide regarding mask colour? We provide a colour chosen with the inspection method in mind, silkscreen contrast that keeps the edges visible, thickness and gloss specified alongside the colour, programmed reference images re taught after a change, and a false call rate compared before and after.