Board Level Reliability Test Plan and Sample Size

A reliability test answers a specific question, and the question has to be written before the test is chosen. A plan that lists tests without stating what each one decides produces data that cannot be used to make a decision.

Starting From the Question

The questions are usually about a mechanism: whether a joint survives power cycling, whether a coating keeps moisture out, whether a connector survives mating. Each maps to a different test.

A test that does not map to a question is an expense. Our thermal cycling notes describe one of the mappings.

Choosing the Stress

The stress should reproduce the mechanism rather than the environment. A damp heat test reproduces humidity damage, and a thermal shock test reproduces the mechanical strain of a fast change, and they are not interchangeable.

Using the wrong stress produces a pass that means nothing. Our failure investigation notes describe how the mechanism is identified.

Sample Size and Confidence

The sample size follows from the failure rate that has to be detected and the confidence required. A test with three samples can show that something works and it cannot show that the failure rate is low.

The sample size should be stated with the result. Our yield analysis notes describe the statistics in the production context.

Failure Criteria

The criterion should be measurable: a resistance rise, a leak rate, a change in a functional parameter. A criterion based on a visible crack gives a different answer from one based on resistance.

The criterion has to be fixed before the test starts. Our joint criteria notes describe the measurement that supports it.

Acceleration and Its Assumptions

Acceleration is a model, and the model assumes a mechanism. Where the stress is increased far enough, the mechanism changes and the model stops applying.

The assumption should be written into the plan. Our fatigue notes describe the extrapolation in this case.

Correlating with the Field

A test result becomes useful when it is compared with returns from the field. The comparison requires the field conditions to be known, which means knowing the duty cycle of the product.

Without that, the acceleration factor has no anchor. Our field failure notes describe the data that is needed.

Verification

The verification is a written question for each test, a stress chosen to reproduce the mechanism, a sample size stated with the result, a failure criterion fixed in advance and a field comparison that states the assumed duty cycle.

Our quality notes describe how the records are kept.

Additional Considerations for This Build

Practical attention to test plan pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating test plan explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Deliberate attention to accelerated test pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating accelerated test explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Careful attention to field correlation pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating field correlation explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, test plan is the item that decides how the rest of the board is arranged. 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.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

Process Control and Verification

On a design of this kind, test plan is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, test plan is the item that decides how the rest of the board is arranged. 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.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

How many samples are enough? It depends on the failure rate to be demonstrated and the confidence required. The number should be calculated rather than chosen for convenience.

Can one test cover several mechanisms? Rarely well. A combined test is a compromise, and a failure in it is harder to attribute.

What does gopcb provide in a reliability plan? We provide a written question for each test, a stress chosen to reproduce the mechanism, a sample size stated with the result, a failure criterion fixed before the test starts, and a field comparison that states the duty cycle it assumes.