Solder Mask Colour and Its Process Effect

Solder mask is supplied in a range of colours, and the colour is a pigment rather than a property. The pigment changes the cure, the resolution and the inspection contrast, which is why a colour change is a process change.

What the Colour Is

The mask is a resin with a pigment and a filler, and the pigment is what gives the colour. A darker pigment absorbs more of the light that cures the mask, which is the reason the cure differs between colours.

The effect is largest on a thick mask and on a fine feature, where the light has to penetrate further into the opening. Our board quality notes describe how the cured surface is judged.

A dark mask needs more exposure energy than a light one for the same degree of cure, and the figure is set per colour rather than per product. A program that is not changed for a colour change produces an undercured mask.

The undercure shows as a soft surface that blocks up at the fine dam, and it appears after assembly rather than at the mask step. Our coating notes describe the same cure reasoning in a coating process.

The pigment scatters light inside the mask, so a fine opening is formed less cleanly in a dark mask than in a light one. The practical minimum dam width is therefore different for each colour.

A design that is transferred from a green board to a black one can lose its dam, and the loss appears as bridging during reflow. That is the reason the colour is stated on the drawing rather than chosen at the shop.

Inspection contrast depends on the difference between the mask and what is on it. White silkscreen on a green mask has a high contrast, and white on a white mask has none.

An automatic optical inspection system is calibrated for a contrast, so a colour change is a program change. Our inspection notes describe how the program is built.

The colour is used for identification between products and it is also used for marketing. Neither of those is a technical reason and both are legitimate where the process is adapted.

The adaptation is the point: a colour is a process setting rather than a cosmetic choice. A product that changes colour without a process review is a product that is running a mask it has not qualified.

The colour is recorded with the mask type and the manufacturer, and a change to either is a change to the product. The exposure energy and the development are recorded with the colour so that the two travel together.

That record is what allows a colour change to be made deliberately rather than discovered. Our first pass yield notes describe how a colour related change appears in the data.

Acceptance and Its Evidence

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. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. 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 first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected. The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected.

Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

Checks Before Release

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. The sequence of operations is part of the specification, because a different order produces a different result from the same steps.

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. 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 result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

Verification and Records

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. 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.

Does the colour affect the electrical properties? Not appreciably. It affects the cure, the resolution and the inspection contrast.

Is black mask harder to process? It is harder to cure and to resolve, so the fine dam limit is larger than it is for a lighter colour.

What does gopcba provide for mask colour? We provide exposure energy set per colour and thickness, the practical dam width stated for the colour in use, the colour recorded with the mask type and manufacturer, the inspection program built for the contrast, and change control when the colour changes.

Points to Confirm at First Article

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. Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.