In the past, most acceleration tests were conducted using a single stress and a constant stress spectrum. Including periodic stresses with fixed cycles (such as temperature cycling between specified upper and lower limits, where the upper and lower limits of temperature and the rate of temperature change are constant). However, in accelerated testing, the stress spectrum does not have to be constant, and a combination of multiple stresses can also be used. Common non constant stress spectra and combined stresses include: step stress tests; Progressive stress test; High Acceleration Life Test (HALT) (equipment level); High Acceleration Stress Screening (HASS) (equipment level); High Acceleration Temperature and Humidity Stress Test (HAST) (Part Level).
The systematic use of environmental stimuli significantly exceeding the expected levels during product use in high acceleration testing requires a detailed understanding of the test results. The high acceleration test is used to confirm relevant faults and ensure that the product has sufficient margin for strength higher than the required level to withstand normal usage environments. The purpose of high acceleration testing is to significantly reduce the time required to expose defects. This method can be used for research and development experiments, as well as for screening.
HALT (High Acceleration Life Test) is a development tool, while HASS (High Acceleration Stress Screening) is a screening tool. They are often used in conjunction with each other. These are two relatively new methods that differ from traditional accelerated testing methods. The specific goal of HAL and THASS is to improve product design and minimize the impact of manufacturing deviations and environmental effects on product performance and reliability. Typically, quantitative life or reliability estimates are not associated with high acceleration testing.
Step by step stress spectrum test. Using a step stress spectrum, the test sample is first tested for a predetermined period of time at a given stress level, and then tested for another period of time at a higher stress level. Continuously increase the stress level and continue the above process until a test sample fails or the test is terminated at the maximum stress level. This method can make the product fail more quickly for analysis. However, it is difficult to establish an acceleration model correctly using this method, making it difficult to quantitatively predict the product's lifespan under normal usage conditions.
The amount of stress that should be added in each step is related to many variables. However, a universal rule that allows for such experiments in design is: assuming that the product has no defects, if it can ultimately exceed the expected stress in the usage environment with an appropriate margin, it can ensure that every individual in the population can withstand the usage environment and screening environment. Progressive stress spectrum testing to improve product lifespan or reliability. Progressive stress spectrum or "gradient test" is another common method, in which the stress level continuously increases over time. Its advantages and disadvantages are the same as step testing, but there is another difficulty, which is that it is difficult to accurately control the rate of stress increase.
HALT (High Acceleration Life Test). The term HALT was proposed by Gregg K. Hobbs in 1988. HALT sometimes refers to Stress Gain Life Test (STRIFE), which is a development test and a reinforcement form of stepwise stress testing. It is generally used to identify weak links in the design and problems in the manufacturing process, as well as to increase the margin of design strength, rather than to quantitatively predict product life or reliability.
HASS (High Acceleration Stress Screening) test. HASS is a form of accelerated environmental stress screening. It represents the harshest environment that the product has experienced, but usually lasts for a very limited period of time. HASS is designed to reach the fundamental limits of technology. At this point, a slight increase in stress will lead to a significant increase in the number of failures. An example of this fundamental limit is the softening point of plastic.
HAST (High Acceleration Temperature and Humidity Stress Test). With the rapid development of electronic technology in recent years, the acceleration tests that emerged just a few years ago may no longer be suitable for today's technology, especially those specifically designed for microelectronic products. For example, due to the development of plastic integrated circuit packages, it now takes thousands of hours to detect the failure of new integrated circuits using traditional and widely accepted temperature/humidity tests of 85 ℃/85% RH. In most cases, the test sample does not experience any failure throughout the entire experiment. A test that does not fail does not indicate any problem. And the product is bound to occasionally fail during use. Therefore, further improvement is needed for the acceleration test. HSAT is a method developed to replace the old temperature/humidity testing.

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