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Choose a representative coupon structure

The coupon must represent the interconnection features under evaluation: through holes, selected via structures or other agreed chains. Preserve layer connections, material construction and processing relevance. A generic coupon with a different stackup cannot automatically represent the delivered product.

Locate and identify coupons so they remain traceable to panels and lots. The PCB test-coupon guide explains this specimen relationship. The coupon is evidence from a controlled sample; it is not a replacement for every board-level electrical test.

Define the complete thermal history

Record any assembly simulation, conditioning and prior exposure before the cycling sequence. Changing preconditioning can change the question being asked. A report showing cycles alone omits information needed to compare two constructions.

The public IPC method is a reference for the method's scope. Use the applicable controlled revision and an agreed test plan for execution. Published study conditions are specific to that study and should not be copied as universal acceptance requirements.

Interpret resistance on a consistent basis

Relative change is the current resistance minus the baseline, divided by the baseline. Measurements must use the same defined electrical path and comparable reference conditions. Temperature itself changes resistance, so unmatched measurement conditions can mislead the interpretation.

The tool calculates this arithmetic from entered values and compares it with your entered review threshold. It does not control an IST system, compensate temperature or determine the physical location of a crack. A zero or negative baseline is rejected because the ratio would be undefined or unsuitable.

Keep initial failures in the story

Record coupons that are open, shorted or otherwise unsuitable before cycling. Do not remove them from the narrative and report only surviving specimens. Define how initial-condition failures, interrupted tests and completed tests appear in the final report.

When comparing lots, preserve sample counts and the selection rule. A higher average cycle count in a small or differently conditioned group does not independently demonstrate better field reliability.

Comparable resistance-change worksheet

Investigate the failure mechanism

An electrical change identifies a response in the monitored structure. Determining whether the mechanism is a barrel crack, interface separation or another defect may require targeted physical analysis. Preserve the coupon and identify the affected net before destructive preparation.

Use the failure-analysis page to plan sample preservation and method selection. The most useful report links the electrical history to the observed failure location rather than presenting an unlabeled microsection image beside a cycle count.

Example: two similar numbers with different meanings

If a baseline is 100 milliohms and a comparable measurement is 104 milliohms, the relative increase is four percent. This arithmetic says nothing by itself about the permitted limit, thermal protocol or statistical meaning. Those belong to the agreed program.

If another coupon was measured at a different temperature, the same numerical change cannot be assumed to represent the same damage. Correct the comparison basis before interpreting the construction.

Prepare an IST review package

Send the stackup, via structures, coupon drawing, lot traceability, conditioning proposal, test purpose and required reporting. Identify whether the task is qualification, process-change evaluation or investigation of an observed defect.

Connect the request to the PCB manufacturing scope. Availability, subcontracting, sampling and acceptance requirements should be confirmed for the project rather than inferred from the presence of an educational page.

Questions to resolve before the next build

Can IST results predict years of field service? A cycle result from a defined coupon and protocol does not automatically translate into a service-life estimate. Such a claim needs an appropriate mechanism model and supporting validation for the product environment. Use the result to answer the stated qualification or comparison question and keep the sample, conditions and statistical limitations visible.

What if a coupon fails before cycling begins? Record its initial condition and the relevant preparation history. Do not simply omit it from the report and present the remaining coupons as the original sample. Agree how initial failures affect the program and whether further analysis is needed. The reason for excluding a specimen matters to the engineering interpretation and to any comparison between constructions or lots.

Design an evidence program around the engineering question

Write the engineering question in a form that can be answered by evidence. Comparing two constructions after a defined exposure is different from predicting product life, screening production units or investigating a returned failure. The question determines the specimens, stress, monitoring and interpretation. A test name alone does not define the study.

Identify what the specimen represents. Preserve the material, stackup, interconnection structure, process history and location information relevant to the mechanism. If the specimen is a coupon, explain which product features it reflects and which it does not. If it is an assembly, record fitted options, mounting and any prior rework or environmental exposure.

Plan the sequence before samples are consumed. Initial characterization, conditioning, stress, intermediate measurements and final analysis may each change the sample. Keep enough representative material for follow-up where practical. An interrupted test or an initially unsuitable specimen should remain in the report instead of disappearing from the population used to describe the result.

Use a consistent measurement basis. Temperature, connection path, instrument method and timing can affect a value independently of damage. Record those conditions and retain raw observations when needed to interpret a trend. A difference between two numbers is not automatically a physical degradation mechanism, especially when the measurement setup changed.

Define criteria through the responsible program. This includes what constitutes a failure, how uncertainty is handled and what happens when the test cannot continue as planned. Do not borrow a threshold from a study of a different construction without a justified technical basis. The same warning applies to turning an accelerated exposure into a field-life claim without an appropriate model and validation.

Review the distribution and the specimen history, not only the average. Different failure locations or early failures can be important even when a mean result looks favorable. Separate exploratory findings from a qualification conclusion and state sample limitations. A small comparison can identify the next engineering step without proving that all future production will behave identically.

Close the work with an actionable decision. Record whether the evidence supports the proposed construction, identifies a change or leaves a question unresolved. Update the relevant drawing, procurement requirement or process instruction so the result influences the next build. Keep the remaining uncertainty visible to the people who own product performance and release.