Highly Accelerated Life Test
A Highly Accelerated Life Test (HALT), is a
stress testing methodologydeveloped by Gregg K. Hobbs. It is commonly associated with electronicsand is performed to obtain information about a product's reliability. Individual components, printed circuit boards both populated and unpopulated, and whole electronic systems can be subjected to different HALT tests. Testing is typically performed on a statistically significant sampling of the product. The size of the sample is governed by many factors including the number of samples available, cost, type of stresses applied, and physical size. For example, component manufacturers can typically test thousands of individual components at one time. Operating at increased temperature is the most common form of failure acceleration for electronic devices, but testing may include other stress mechanisms such as mechanical vibration, humidity, or ionizing radiation.HALT is not a reliability qualification test. It is just an empirical method used across industry to point out limiting failure modes of a product and the stresses at which these failures occur.
A significant advantage of accelerated life testing is that it can be conducted during the development phase of a product to weed out design problems and marginal components. Thus a consumer products company can achieve better customer satisfaction because fewer products have to be returned for repair, and can also save money on warranty returns, or an aerospace manufacturer can avoid catastrophic failures in aircraft or space vehicles. Another major advantage is that the design team can be moved on to designing new products rather than becoming occupied with problems in older products.
Usually three types of stress are applied in an HALT test:
#Combined Environment stress
Typically, cold step testing is performed in order to discover the failure modes at lower temperatures,objective would be to find out the lower operating limit of the equipment under test as well as lower destructive limit. Hot step testing is performed to discover the failure modes at higher temperatures and it also be used to find out the upper operating limit for the equipment as well as upper destruction limit for the equipment under test. Combined environment testing is performed to find out the failure modes at both vibration as well as temperatures variations.
Ideally, when a product or device's reliability is investigated, it is tested under the end-application stress levels and tested until the product has failed. However, under normal operating conditions, reliability failures often do not occur for weeks or months, which precludes producing failure data in a timely manner. Accelerated life testing allows manufacturers to increase the failure rate for a batch of devices or products and then use the obtained data to project the reliability of that type of device far into the future. For electronic devices, the failure rate is most commonly accelerated by operating the devices in a powered-up condition, under temperatures much higher than the normal operating temperature of the devices. Then, the first few failures are plotted, and fitted to a curve determined by the
Arrhenius equation, which has been found to apply not just to chemical reactions but to thermally driven failures in electronic devices. The single curve for a given "burn-in" temperature is then extrapolated for other temperatures, producing a set of reliability failure curves. Using this method, in theory only asingle device failure data point is needed to extrapolate the failure rate for that type of device at different operating temperatures and for years into the future.
When HALT test is promptly applied,it can produce a very robust product without undue cost, because improvements are targeted only where they are needed. As failure modes are discovered,understood the product life can increase significantly. This makes the product more robust and risk of failure reduces drastically.
Individual components such as
resistors, capacitors, and diodes, printed circuit boards, and whole electronic products such as cell phones, PDAs, and TVs, eventually fail at different rates under different end-user stress levels. For example, a typical consumer-owned television will not likely be operated at temperatures outside the range of normal living accommodations, or subjected to mechanical stress by being dropped. A cell phone, on the other hand, may be dropped from 3 or 4 feet off the ground fairly often, and subjected to a varying range of vibrations. A commercial telephone switch may be required to operate in remote installations ranging from Barrow, Alaska to Phoenix, Arizona, at an ambient temperature range of from minus 50 to plus 120 degrees Fahrenheit. Components used in military and aerospace applications may be subjected to even more severe operating temperature requirements as well as high G-forces and ionizing radiation, sometimes simultaneously, to meet specific MIL-SPECstandards.
Therefore, failure rate data used to select any device in a product must correlate with the stress levels in the product or application. To accomplish this, the required lifetime and operating conditions for the product into which the components are designed must first be determined. For instance, in the above examples, the television set may only be required to operate through its warranty period, whereas the telephone switch may be required to operate without being serviced for ten years or more. Components used in a missile may only be required to operate for a few hours of testing and a few minutes of actual use, but their failure rate will be expected to be zero during that period. Devices used in satellites or space vehicles where replacement is not possible are expected to have a zero failure rate for the lifetime of the vehicle. Knowing the required failure rate as determined by the application, components can be selected based on the failure rate data supplied by the manufacturer as described above.
Typically, components used in consumer devices are chosen by finding the least expensive component which will meet the requirement for the warranty period. At the other end of the scale, components used in aerospace applications are more likely to be chosen for maximum reliability independent of cost. Moreover, due to cost, warranty failures are usually not expected to be zero during the warranty period, but rather to not exceed a level which might subject the manufacturer to unwanted publicity or legal action. Additionally, the failure rate for critical components may be required to be lower than for other components in a system. For instance, components in an automobile which may cause so-called "walk-home failures" are usually subject to higher reliability requirements than are components in the automobile's entertainment or security systems.
Once the product or device is deployed or sold into the marketplace, proper quality control procedure requires that the "quality loop" be closed by retrieving all components which fail in the field during the predicted lifetime, analyzing them to determine why they failed before they were predicted to fail, and determining where the reliability failure prediction was in error. Information from these analyses should then be used for appropriate corrective action in the reliability failure prediction methods.
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