Mesa Production 1958 Accelerated Life Testing Failure Analysis

# Accelerated Life Testing and Failure-Mode Analysis: Compressing a Year of Field Failure Into a Week of Oven Time

## 1. Why Waiting for a Device to Fail in the Field Is Not an Option

This step takes sample devices from the coated and uncoated populations alike and stresses them at elevated temperature and humidity for days at a time, then opens up every failure to determine exactly which mechanism killed it — because waiting for ordinary field use to reveal whether Step 7's coating actually improved reliability would take a year this line does not have before its next customer shipment. Accelerated testing works because the surface leakage mechanism Step 6 characterized is itself a thermally activated process, so running devices hot deliberately compresses time the same way a furnace compresses diffusion time — the relationship between stress temperature and the equivalent field life it represents follows the same Arrhenius form this project has used since the 1957 series' soft-bake and hard-bake steps:

$$AF = \exp\!\left[\frac{E_a}{k_B}\left(\frac{1}{T_{\text{use}}} - \frac{1}{T_{\text{stress}}}\right)\right], \qquad t_{\text{equivalent}} = AF \times t_{\text{test}}$$

where $AF$ is the acceleration factor, $E_a$ the activation energy of the dominant failure mechanism, and $T_{\text{use}}$ and $T_{\text{stress}}$ the field and test temperatures respectively. A test run for one week at an elevated temperature can represent months or years of ordinary field exposure, provided the mechanism being accelerated is the same mechanism that would eventually cause a field failure — which is precisely why failure-mode analysis has to accompany the stress test, not follow it as an afterthought: a test that accelerates the wrong mechanism tells this line nothing true about the field.

One Week Hot Stands in for a Year in the Field acceleration factor versus stress temperature, for a fixed activation energy ACCELERATION FACTOR VERSUS STRESS TEMPERATURE stress temperature, Tstress → AF field use temperature this test's stress point AF = exp[Ea/k·(1/Tuse − 1/Tstress)] — the test's value depends entirely on knowing Ea correctly getting the wrong mechanism's activation energy makes the whole acceleration factor meaningless

## 2. Real Diagram: Opening the Failure to See Which Mechanism Actually Won

A failed device by itself only says the device failed. Cross-sectioning it and examining the mesa edge under the microscope distinguishes between genuinely different mechanisms — surface inversion from mobile ionic contamination, corrosion of the exposed metal at the edge, and coating delamination from thermal cycling — each of which implicates a different earlier step and demands a different fix.

Three Failures, Three Different Culprits cross-sections of failed devices, each implicating a different earlier step SURFACE INVERSION mobile ions, step 6's measured leakage METAL CORROSION edge metallization attacked by moisture COATING DELAMINATION coating lifts, step 7's fix failing mechanically three mechanisms, three different fixes, one failure count if you only ask pass or fail

## 3. Why This Project's Earlier Reliability Steps Never Needed a Microscope After the Fact

The 1956 series' electrical test, at Step 19 of that series, measured cutoff frequency and current gain and reported a number — a device either met specification or it did not, and the article never needed to ask *why* a failing device failed, because the drift-field mechanism being verified was singular and well understood. This step cannot stop at a pass or fail count, because the exposed junction this series has been tracking since Step 5 can fail by more than one genuinely different mechanism, each implicating a different earlier step in this production line and each needing a different corrective action. Failure-mode analysis is the discipline this project adopts specifically because reliability, unlike the electrical parameters 1956 measured, turns out not to be a single number with a single cause.

Step 8 does not measure whether these devices are reliable; it measures *why* the ones that fail actually failed, which is the only information this line can use to decide what to fix next.

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