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Hilaire Ananda Perera http://www.linkedin.com/in/hilairepereraLong Term Quality AssuranceHighly Accelerated Stress Screening (HASS)1.0 IntroductionAfter the design is ruggedized through Highly Accelerated Life Testing (HALT) and theDesign Verification Testing (DVT) completed, the End-Unit is ready for production. Theproduction process can introduce many failure modes that are not related to a faulty design,and the sustaining process can certainly introduce new design problems. HASS is intendedto catch these new failure modes more quickly and more effectively than traditional ESStesting done in production. The goal in HASS is to verify that no new "weak link" has creptinto the product since HALT that has shifted either the operational or destruct limits foundin HALT.An important first step to setting up HASS is the completion of HALT on the product. TheHASS limits have to be set based on the operational and destruct limits found in HALT.Prior to setting up HASS, it is important that corrective action has been implemented on allHALT failures and a verification HALT has been done.2.0 The HASS Process and EquipmentThe equipment used to do HASS is similar to that used in HALT, although often a largerchamber is needed to accommodate production quantities. The fixturing can be quite
Hilaire Ananda Perera http://www.linkedin.com/in/hilairepereraLong Term Quality Assurancedifferent in HASS, simply to accommodate the production flow. The speed with whichproduct can be fixtured in the chamber becomes important, as well as maximizing thenumber of products in the chamber. Quick release clamps are often used in lieu of nuts andbolts for securing the product.An important part of designing a fixture for HASS is the mapping of the fixture. The goal isto insure that the vibration and thermal stresses at each point in the fixture are roughly equal(although precise uniformity is not important). Mapping the fixture involves takingaccelerometer and thermocouple readings on a product in each of the fixture locations. It isimportant the fixture is completely loaded with product for the test, since the load will affectthe vibration characteristics. Thermal inconsistencies can be corrected by changing air flowthrough baffling or other air distribution changes. Vibration inconsistencies can be correctedthrough fixturing changes, with the introduction of dampening materials or changes inclamping mechanisms.During HASS, the stresses are applied simultaneously. Typically, the product is subjected tocontinuous vibration while the temperature is ramped between its limits, with short dwells atthe extremes.3.0 Defining the ScreenThe levels of the stresses to be applied during the screen are based on the limits foundduring HALT. There are two parts to the screen. The first part is the Precipitation Screen.This screen stresses the product beyond the Operational Limits and near the Destruct Limitsfound in HALT. It is intended to precipitate failures in the product due to latent defects.Because the product is being stressed beyond its operational limit, you do not expect it tofunction properly, so no testing is done on the product at this point. The product should bepowered, however, since applied power can be a significant stress for the product in itselfwhen combined with the other stresses of HASS. The second part of the screen is theDetection Screen. During the Detection screen the product is stressed to near the operationallimit found in HALT. Now, the product is being functionally tested. Any hard failuresinduced during the Precipitation Screen should be detected, as well as any soft failures thatmay be induced by the stresses.Figure 1 provides an overview of the purpose and limits of these screens. It shows themargin discovery curves, overlaid with the Precipitation and Detection Screens. The limitson the screens are set so that they are outside of the tails of the distribution of the failuremode(s) that define the operational and destruct limits for the product. Consequently,product which has no new latent failure modes should pass the screen undamaged. Any newfailure mode, however, will be exposed.There is one key problem with setting up the limits on the screens from this data - the smallsample size used in HALT means that we really have no idea what the distribution looks likeon these limits or where the tails may be. Consequently, a more empirical method is used. Abaseline for the stresses is derived by guard-banding the limits found in HALT. Typically,
Hilaire Ananda Perera http://www.linkedin.com/in/hilairepereraLong Term Quality Assurancevibration is reduced by 50% and thermal excursions are reduced by 20%. These limits canbe used as a starting point for the Proof of Screen (POS) Process.LowerDestructLimitLowerOperatingLimitProductSpecsESSDetectionScreenPrecipitation ScreenUpperOperatingLimitUpperDestructLimitStressFigure 1: Precipitation and Detection Screen LimitsReferenceHighly Accelerated Life Testing – Testing With a Different Purpose; by NeilDoertenback, QualMark CorporationHighly Accelerated Life Testing and Highly Accelerated Stress Screening(HALT/HASS) Guidelines by Honeywell Commercial Flight Systems,Minneapolis Operation