PCB Failure Analysis: Working From the Mechanism

Boards fail in the field for a small number of reasons, and most of them are present in the design long before the failure occurs. A failure analysis that begins with the mechanism rather than with the component reaches the cause faster.

Start With the Mechanism

A failure has a mechanism: a crack, a corrosion product, a void, a short, an open or a damaged device. Identifying the mechanism narrows the possible causes, and the identification is done by examination rather than by reasoning.

The examination begins with the unaided eye and continues under magnification. A surprising proportion of failures are visible as a discoloured joint, a lifted pad or a crack across a component body. Our board quality notes describe the appearance criteria.

Non-Destructive Examination

Optical inspection under magnification, X-ray and, where available, acoustic microscopy cover most of the non-destructive ground. Each has a resolution limit and each is blind to something.

X-ray sees inside a package but not a joint that has not merged; acoustic microscopy sees delamination but needs a coupling medium; optical inspection sees the surface but nothing beneath it. The choice follows from the suspected mechanism. Our BGA inspection notes compare the methods.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/pl149813980-oem_fast_turn_custom_pcb_rigid_enig_printed_circuit_board_quick_turn.webp" alt="Microsection of a failed solder joint under a microscope” />

Electrical Localisation

Where the failure is an open or a short, localising it electrically narrows the search to a small area before any destructive step is taken. A time domain reflectometer shows the distance to a discontinuity along a trace, which is often enough to identify the location.

For a short, a current injection with a sensitive detector finds the path by the field it produces. Both techniques preserve the board so that the evidence is not destroyed by the search.

Destructive Analysis and Sectioning

Sectioning exposes the internal structure of a joint or a hole and is the definitive method for plating and solder defects. The section has to be prepared carefully, because a poorly polished section introduces artefacts that look like defects.

The location of the section is the important decision. Sectioning through the failure preserves the evidence, while sectioning through a nearby good joint confirms what the process normally produces.

Chemistry and Contamination

Where corrosion or electrochemical migration is suspected, the analysis becomes chemical. Ionic contamination, halide residues and flux activators are each detectable, and each has a characteristic appearance.

The pattern of the corrosion indicates the mechanism. Dendrites growing between conductors indicate a metal migration, while a uniform tarnish indicates an environmental exposure. Our copper surface notes describe the related processes.

Thermal and Mechanical Evidence

A joint that has been cycled shows a crack that propagates from the interface, while one that has been overloaded shows a ductile tear. The two are distinguished under magnification, and the distinction tells you whether the cause is thermal or mechanical.

The location of the crack also carries information. A crack at the corner furthest from the neutral point of the package indicates thermal fatigue, while a crack at the same location on every board indicates a mechanical cause.

Process Records and Correlation

The analysis is only complete when the failure is correlated with the process that produced it. The panel identifier, the material batch, the plating parameters and the assembly profile are the records that make the correlation possible.

Without them, the analysis can identify the mechanism and not the cause, and the same failure will recur. Our traceability marking notes describe the records.

Corrective Action and Verification

A corrective action that is not verified is a hypothesis. The verification is a repeat of the test that produced the original failure, on a sample built with the correction applied.

Where the original failure took a long time to appear, the verification uses an accelerated test that reproduces the same mechanism rather than waiting for the same duration. Choosing that test is part of the corrective action.

Process Control and Verification

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.

Verification and Records

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.

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.

Points to Confirm at First Article

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

Documented Process Control

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.

Inspection and Traceability Notes

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it. Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

FAQ

Should the failed board be cleaned before examination? It should be photographed first, then cleaned only where the cleaning does not destroy the evidence. Contamination is itself evidence, and removing it before it is characterised loses the information.

How many samples are needed? One sample is enough to identify a mechanism and not enough to establish a rate. Where the failure is intermittent, several samples are needed before the cause can be distinguished from coincidence.

What does gopcb provide for a failure investigation? We provide the process records for the panel, the material certificates, the coupon data and, where the board is returned, a section through the failed area. We also compare the failure with the process records of the surrounding batches to see whether the cause is local.