HASS Testing Introduction
Customer designed HASS fixture
Crystal Instruments designed HASS fixture
HASS (Highly Accelerated Stress Screen) is a subsequent method of reliability testing after HALT. Once initial design weaknesses have been resolved in HALT, the product can be evaluated for production level manufacturing. HASS is intended to identify manufacturing anomalies to ensure the product will reflect the same level of reliability as prescribed.
HASS uses the Operational Limits identified within HALT Testing to define a profile. Unlike HALT, HASS is performed with an autonomous profile and functionality is most often only determined pre- or post-testing.
HASS is a pass/fail evolution and therefore it must be executed in an extremely sterile manner. Consistent, uniform test response levels must be established and periodically verified to ensure secondary variables are not introduced. All devices under test are expected to observe uniform thermal and vibration distributions, within acceptable level of variances.
Causes for failure in HASS:
Deviation from approved assembly/manufacturing procedures
Change in BOM components:
Selecting a lesser quality/more economical equivalent
Inconsistency of components produced by the manufacturer
Variation in production lines or locations
Prerequisites:
Proof of screen:
Initial thermal and vibration characteristics need to be verified on all UUT’s. Any variation from unit to unit can lead to erroneous test results.
Failure rate versus profile severity at this point can be used to extrapolate time to failure and thus revisions should be considered. There are several formulas to assist in determining these factors.
Fixture design:
HASS Screening is an expensive and time-consuming process. The fixture(s) should be designed with an approach which can incorporate the largest sample size per run while being conducive for efficient time to exchange units and enhancing the stressors being applied.
Profile Definition:
The parameters of the profile have an exponential impact within the HASS environment. The levels selected are expected to induce stress of nominal impact, but sufficient enough to exacerbate and identify inconsistencies. If the profile is too aggressive, unintended fatigue may be imposed, degrading the overall life expectancy.
HASS profiles are typically composed of two formats:
Precipitation and Detection. The nomenclature of each format is relatively self-explanatory. I will discuss these in detail in an upcoming article.
Primary components of a HASS profile: Number of cycles, upper and lower extremes (thermal and vibration), excursion rate and dwell times. Each attribute can be independently configured depending on the profile requirements.
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