A board surviving a hot or humid test does not automatically demonstrate a stated number of years in service. Translating stress exposure into use life requires a justified model and an understanding of the failure mechanism. Report what the test actually establishes before making a projection.
NIST’s acceleration-model overview relates time-to-failure to stress for a particular mechanism. A factor derived for one mechanism should not be applied indiscriminately to every component and interconnect on an assembly.
State the purpose of the test
Decide whether the work is intended to reveal weak points, compare design alternatives, screen production, or estimate reliability at use conditions. These purposes can require different samples, measurements, and analysis. A failure-discovery experiment may be useful without supporting a quantitative service-life claim.
Define the expected use environment and the mechanism being evaluated. Record electrical bias, duty cycle, temperature, humidity, mechanical loading, or other relevant conditions. Nominal chamber settings alone may not describe the stress at the critical component or joint.
Review the projection evidence
| Evidence | Question |
|---|---|
| Failure mechanism identification | Does the observed failure represent the mechanism of interest? |
| Acceleration relationship | What supports the mapping between stress levels? |
| Life-distribution model | Is the statistical model appropriate for the data? |
| Use-condition definition | Which field environment does the projection represent? |
Review whether the applied stress introduces behavior that would not occur in normal use. A damaged material system or a changed operating mode can invalidate the intended relationship even when the test produces convenient failures.
Retain the evidence needed for reanalysis
- Track each unit and its actual exposure history.
- Record failure time or the interval in which failure was detected.
- Preserve surviving and withdrawn-unit records.
- Investigate failure modes before combining data.
- Document model assumptions and uncertainty in the projection.
Do not pool unrelated failure modes into one simple acceleration factor without justification. Involve the responsible reliability specialist when selecting models and interpreting censored data. The conclusion should distinguish measured observations from extrapolated behavior. Keep the assumed model parameters and their sources available so reviewers can evaluate sensitivity to uncertain inputs.
If the evidence supports only survival under the tested conditions, state that result plainly. It can still inform design decisions and qualification planning. Avoid converting hours into years using an unexplained multiplier borrowed from another product.
The zero-failure guide addresses the statistical limits of a test with no observed failures, while the censored-record guide covers units whose complete failure time is not observed.
Move from reading to a build-ready project
Use the fabrication and assembly checklists to prepare drawings, production files and test requirements.
Open the DFM checklist