HALT (Highly Accelerated Life Tests) is a method aimed at discovering and then improving weak links in the product in the design phase. HASS (Highly Accelerated Stress Screens) is a means of finding and fixing process flaws during production. Both techniques use stresses far beyond the normal stress levels. These methods are discovery testing in which problems are found by testing to failure using accelerated stress conditions. HALT is a discovery test as opposed to a compliance test, that is, we want to find problems and we do everything necessary in order to do so and then to remove the weaknesses found. The old paradigm of qualification or design verification testing was one of trying to pass. In a compliance test, every attempt is made not to discover problems, so that the design phase can end and the production phase can begin. Any failure that occurred was usually declared to be unusual or due to overstress and therefore not relevant. This latter approach is called success or compliance testing by the author and is still in use today. As you can see, the two paradigms are diametrically opposed. The HALT and HASS techniques represent a paradigm shift of major proportions. Companies using these evolving techniques correctly have obtained outstanding reliability, yet most of them do not publish their results given the significant competitive advantage that these techniques provide to them.
HALT is an acronym for Highly Accelerated Life Tests that was coined by me in 1988 after having used the term “Design Ruggedization” for 18 years. In these tests, every stimulus of potential value is used to find the weak links in the design and fabrication processes of a product during the design phase. These stimuli may include vibration, thermal cycling, burn-in, voltage, humidity, and whatever else will expose relevant weaknesses (including stresses that will not occur in the real world1 if they generate real world failure modes). The stresses are not meant to simulate the field environments at all but to find the weak links in the design and processes using only a few units and in a very short period of time. Hence, these techniques are called Time CompressionTM. The stresses are stepped up to well beyond the expected field environment in order to obtain time compression in finding design weaknesses. HALT has, on many occasions, provided substantial (5 to 1000 times) MTBF gains. Even when used without production screening it has reduced the time to market substantially and also reduced the total development costs.
HASS is an acronym for Highly Accelerated Stress Screens that was also coined by me in 1988 after using the term “Enhanced ESS” for some years. These screens use the highest possible stresses (frequently well beyond the “QUAL” level) in order to attain time compression in the screens. Note that many stimuli exhibit an exponential acceleration of fatigue damage accumulation with stress level1, and so a drastic reduction in screening equipment and manpower is obtained by the use of higher stress levels. The screens must be, and are proven to be, of acceptable fatigue damage accumulation or lifetime degradation using Safety of HASS techniques1. HASS is generally not possible unless a comprehensive HALT has been performed as, without HALT, fundamental design limitations will restrict the acceptable stress levels to a great degree and will prevent the large accelerations that are possible with a very robust product. It has been proven that HASS generates extremely large savings in screening costs because much less equipment (shakers, chambers, monitoring systems, power and liquid nitrogen) is necessary due to time compression in the screens. HASS, too, is discovery testing as compared to success testing. For the complete paper, click on this link