HASS (Highly Accelerated Stress Screen) received its name in 1988 after being invented by Gregg K. Hobbs, Ph.D., P.E. in 1979.
The test levels selected should impose the greatest possible stress to expedite time to completion. The stress factors utilized provide damage accumulation which further minimizes the need for ancillary equipment and technical oversight. HASS development utilizes (vibration, thermal, and electrical, etc.) limits found in HALT, so the values derived therein are a prerequisite for profile development.
The presence of numerous stressors may be applied simultaneously during HASS, so a simplified equation such as the Arrhenius method doesn’t always apply. Miners Law is better suited for HASS since it can incorporate multiple variables.
Profile Development: The thermal aspects are typically reduced by 20% of the operational limits found in HALT. Vibration is usually at 50% of the operational limit. All screening/testing methods remove some level of life expectancy from the devices tested. This fact should be considered when developing a HASS screen.
The failure modes encountered during HALT testing and extent of mitigation should also be considered before defining a HASS profile. If the parameters of the profile HASS should exceed those of operational limitations, these anomalies should be acknowledged and expected. Refer to Figure 1 for statistics of failures related to test type during HALT.
Precipitation Screen: is intended to evoke latent defects rapidly and reliably. These screens are typically of a higher stress level than detections screens for this reason. The values of this screen may exceed the operational limits if design constraints inhibit functionality. A few common methods are stated below. These can be used discreetly or in varying combinations.
High vibration (gRMS): accumulates stress quickly.
Significant, rapid thermal excursions: creates low cycle fatigue in previously stressed components. Elusive failure modes may become patent.
Cycling/varying input power: further low cycle fatigue.
Detection Screen: intended to further scour for latent failures not detected or induced by Precipitation Screen levels. The setpoints utilized in this segment are typically less aggressive than those of the Precipitation Screen.
Modulated excitation or intermittent/decreased “tickle” vibration: varying gRMS in a linear manner or cycling from high/low levels of vibration. Typically from Upper Vibration Level to chamber minimum and back to Upper limit (5 gRms for Qualmark chambers).
Intermittent failure(s) can occur and remain undetected during high vibration dwells. When the vibration is decreased, these may become apparent. If the vibration is removed completely, they may never be detected.
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