Joint Appearance and Its Relation to Reliability

A joint that looks acceptable is not necessarily a joint that will last, and the two questions are answered by different methods. Appearance is a proxy for the process, and reliability is a property of the structure.

What Appearance Predicts

Appearance predicts whether the process did what it was set to do. A fillet that wets both surfaces and has a smooth surface is evidence that the flux worked, the profile was in range and the surfaces were clean.

That is a strong indication and it is not a measurement of strength. Our joint criteria notes set out the appearance that is accepted.

Where the Two Diverge

The divergence appears where a defect is hidden or where the structure differs without a visible sign. A void inside a joint, a thin intermetallic and a grain structure that was cooled too slowly are all invisible from the outside.

The second divergence is the joint that looks marginal and is sound. An inspector who rejects it costs a board and an inspector who accepts a voided joint costs a return. Our board quality notes describe how the judgement is calibrated.

The Hidden Defects and Their Methods

A void is found by X-ray, an intermetallic thickness by a section, and a grain structure by an etched section. None of the three is visible optically and all three affect the life.

The methods are destructive or slow, which is why they are applied to a sample rather than to the product. The sample is what connects the appearance to the reliability. Our inspection notes describe how the optical check is programmed.

The link between appearance and reliability is established by a qualification: a set of joints that meet the criteria are cycled and the failures are counted. Where the failures are acceptable, the criteria are a sufficient proxy.

That is the assumption behind every visual standard, and it is worth stating. A standard that is applied outside the range it was qualified for, such as a much thicker board or a different alloy, is an extrapolation.

The modes that appear later are fatigue at the intermetallic, fatigue in the bulk alloy and a crack that starts at a void. Each has a driving condition, and each is accelerated by thermal cycling rather than by any single excursion.

That is why the qualification is a cycling test rather than a shock test where the dominant mode is fatigue. Our thermal cycling notes describe how the test is constructed.

The practical position is that appearance is the control and reliability is the qualification. The appearance check is applied to every board because it is fast, and the qualification is applied once to establish that the check is meaningful.

Where a change is made to the design or the process, the qualification is revisited. A standard that has been qualified for one construction is not automatically valid for another.

Acceptance and Its Evidence

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. 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.

Checks Before Release

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

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

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

Verification and Records

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

Can a joint look good and fail in the field? It can, and the causes are the hidden ones that the optical check cannot see.

Does X-ray on every board replace the appearance check? No. X-ray sees the voids and not the wetting, and the two defects are different.

What does gopcba provide for joint reliability? We provide appearance criteria applied to every board, X-ray and sections on samples for the hidden defects, a qualification that links the criteria to a cycling result, the qualification revisited after a change, and records that tie the result to the construction.

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. 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.