Inspection Lighting and Magnification Choices

An inspector finds what the light shows and the magnification resolves. Both are equipment decisions, and both are usually made once and then left as they were installed, which is why a defect that is visible under one light becomes invisible under another.

What the Lighting Has to Reveal

A fillet is a shape and it is revealed by a light that comes from an angle rather than from directly above. A bridge between two pads is a surface feature and it is best seen by a light that grazes across it.

The lighting therefore follows the defect rather than the bench. A station set for one class of defect is a station that is weak on another. Our board quality notes describe the standard the inspection is applying.

A diffuse light shows colour and a directional light shows shape. Most inspection benefits from the two together, which is the reason a ring light and an oblique light are often both fitted.

Colour matters less than the contrast it produces. A light whose colour matches the board reduces the contrast, and a complementary colour increases it.

Magnification is quoted as a factor and what matters is the resolution it gives at the object, which depends on the lens and the working distance as well as the factor. Two systems with the same factor can resolve different features.

The limit is set by the defect that has to be found. A magnification chosen for a fine pitch part is uncomfortable for a general inspection, which is why the two are often separated. Our inspection notes describe the machine side of the same question.

A binocular view gives depth, which is what allows an inspector to judge a fillet rather than a silhouette. A monocular view at high magnification shows more detail and less depth, so the two are used for different classes.

That is the reason a microscope with a shallow depth of field is poor at judging a fillet and good at reading a marking.

Inspection is a vigilance task and detection falls over a shift. The countermeasures are rotation, breaks and a limit on the time spent at the station, and they are as much a part of the process as the lighting.

The rate at which a defect is missed rises before the inspector notices the change. Our first pass yield notes describe how the escape appears in the data.

The station is verified with a sample that contains the defect classes it is expected to find. A station that is verified with a good board only has been verified for nothing.

The sample set is kept and re-used, and the result is recorded. A station that is verified once and never again is a station whose lighting has been changed without anyone noticing. Our placement capability notes describe the same verification discipline for a machine.

Acceptance and Its Evidence

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.

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 environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. 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.

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it. A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process.

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 an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

Checks Before Release

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 process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Verification and Records

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 measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Is more magnification always better? No. A higher magnification reduces the field and the depth, and it slows the inspection for no gain on a large feature.

Can a ring light find every defect? It shows a fillet poorly, because a ring light removes the shadow that gives the fillet its shape.

What does gopcba provide for inspection stations? We provide lighting chosen for the defect class, a combination of diffuse and oblique light, magnification set from the feature rather than from the factor, rotation and break limits to manage fatigue, and a station verified with a sample that contains the expected defects.

Points to Confirm at First Article

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.