This is just an Excerpt from a larger document, click here to view the entire document.Advanced Concepts in Accelerated Testing
Historically, most accelerated testing is done using a single stress and a constant stress profile. This includes cycled stress (e.g., temperature cycling between specified limits) where the cycle (upper and lower temperature limits and rate of change of temperature), rather than the temperature, is fixed. In accelerated testing, however, the stress profile need not be constant and a combination of stresses may also be used. Some common non-constant stress profiles and combined stress profiles include:
Step Stress Profile Test
Progressive Stress Profile Test
Highly Accelerated Life Test (HALT) (Equipment-level)
Highly Accelerated Temperature and Humidity Stress Test (HAST) (Part-level)
Highly accelerated testing is the systematic application of environmental stimuli at levels well beyond those anticipated during product use. Thus, the results need to be carefully interpreted. It is used to identify relevant faults and to ensure that products have a sufficient margin of strength above that required to survive the normal operating environments. Highly accelerated testing attempts to greatly reduce the time needed to precipitate these defects. The approach may be used either for development testing or for screening.
HALT is a development tool and HASS is a screening tool. They are frequently employed in conjunction with one another. They are relatively new, and differ from the classical approach to accelerated testing. Their specific goal is to improve the product design to a point where manufacturing variations and environment effects minimally affect performance and reliability. Quantitative life or reliability prediction is not usually associated with highly accelerated testing.
Step Stress Profile Testing. Using a step stress profile, test specimens are first subjected to a given level of stress for a preset period of time, and then they are subjected to a higher level of stress for a subsequent period of time. The process continues at ever increasing levels of stress, until either all the specimens fail, or the time period at the maximum stress level ends. This approach precipitates failures more rapidly for analysis. However, with this technique it is very difficult to properly model the acceleration and, hence, to quantitatively predict the item life under normal usage.
The amount by which the stress should be increased in any single step is a function of many variables and is beyond the scope of this discussion. However, the general rule to follow in the design of such a test is to eventually exceed the expected environments by a comfortable margin so that all members of the population can be expected to survive both the field environment and the screen environments, assuming of course that they are defect free.
Progressive Stress Profile Testing. A progressive stress profile or "ramp test" is another frequently used approach, where the stress level is continuously increased with time. The advantages and disadvantages are the same as those for step stress testing, but with the additional difficulty of accurately controlling the rate at which the stress is increased.
HALT Testing. The term HALT was coined in 1988 by Gregg K. Hobbs. HALT, sometimes referred to as stress plus life testing (STRIFE), is a development test, an enhanced form of step stress testing. It is typically used to identify design weaknesses and manufacturing process problems, and to increase the margin of strength of the design rather than to predict quantitative life or reliability of the product.
HASS Testing. HASS is a form of accelerated environmental stress screening. It presents the most intense environment of any seen by the product, but it is typically of a very limited duration. HASS is designed to go to "the fundamental limits of the technology." This is defined as the stress level at which a small increase in stress causes a large increase in the number of failures. An example of such a fundamental limit might be the softening point of plastics.
HAST (Highly Accelerated Temperature and Humidity Stress Test). With the recent improvements in electronics technology and the speed with which these technology improvements are occurring, accelerated tests which were designed just a few years ago may no longer be adequate and efficient for today's technology. This is especially true for those accelerated tests intended specifically for microelectronics. For example, due to the improvements in plastic IC packages, the traditional, universally accepted 85°C / 85% RH Temperature/Humidity test now typically takes thousands of hours to detect any failures in new integrated circuits. In most cases, the test samples finish the entire test without any failures. A test without any failures tells us very little. Yet we know that products will still fail occasionally in the field. Thus, we need to further improve our accelerated tests. HAST is intended to replace the older temperature/humidity test.
