Visual Criteria for Solder Joint Acceptance

A visual acceptance criterion is a sentence and a photograph, and the two have to agree. Where they do not, the criterion is interpreted differently by different inspectors, and the difference shows up as an inconsistent yield rather than as a defect.

What the Criteria Describe

The criteria describe a fillet that wets both surfaces, an angle that shows wetting rather than a lump of alloy, a surface that is not disturbed, and the absence of a bridge or a void that reaches a surface.

Each of those is a shape rather than a measurement, which is why the illustration matters as much as the text. Our joint criteria notes set out the standard the sentences come from.

Wetting and Its Appearance

Wetting is a low contact angle between the alloy and the surface, and it looks like a thin feather rather than a ball. A joint that sits on the pad without spreading has not wet it, whatever else is true of it.

The distinction between a joint that has wet and one that has not is the first thing an inspector learns and the last thing that is reliably applied, because an incomplete fillet can look like a small one. Our board quality notes describe how the judgement is calibrated.

Disturbed Joints and Their Cause

A disturbed joint has a grainy, frosted surface, and it was moved while the alloy was solidifying. The cause is mechanical: a conveyor vibration, a board that was handled too early, or a component that shifted.

The appearance is unmistakable once it has been seen, and it is often mistaken for contamination or for a poor alloy. The correction is at the machine rather than at the material.

Fillet Height and the Acceptable Range

The fillet height is described as a proportion, usually as a fraction of the joint, and the proportion has a lower limit and sometimes an upper one. A fillet that is too low suggests insufficient alloy and one that is too high suggests excess.

The band is what makes the criterion usable. A sentence that says a fillet is required without a proportion leaves the reading to the inspector.

Two inspectors who agree on a written criterion can still disagree on a board. The way to remove that is a set of samples that includes a marginal joint, and a periodic session where the inspectors read the same boards.

The sample set is worth keeping, because it also serves as a reference when a new product is introduced. A criterion that has never been calibrated is a criterion that has not been applied consistently. Our inspection notes describe how the machine is programmed from the same standard.

An inspection programme is built from the same criteria, and the thresholds it uses are the proportions in the standard. Where the programme is tuned for a low call rate, it is no longer applying the standard, and the two have to be reviewed together.

That is the reason a change to the visual standard is a change to the programme. A standard that is updated and a programme that is not is a line that inspects to the previous revision.

The criterion needs a disposition for each class: accept, rework, or refer. A class that is referred without a named person to refer it to becomes an accepted class in practice.

Writing the disposition into the standard is what makes the yield figure meaningful. A yield that counts a referred board as accepted is a yield that describes the recording rather than the process. Our first pass yield notes describe how the classes are counted.

Checks Before Release

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

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. Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

Verification and Records

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

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

Points to Confirm at First Article

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Is a photograph part of the standard? It is, because it is what makes the sentence reproducible between inspectors.

Can a marginal joint be reworked? It can, and the standard should say whether it is to be reworked or referred, because the two have different costs.

What does gopcba provide for visual criteria? We provide criteria written with proportions and illustrated, a calibration sample set that includes marginal joints, periodic reading sessions for inspectors, an autoinspection programme built from the same standard, and a named disposition for each class.