Highly Accelerated Life Testing
Original article date: June 1998
GEORGE COLEMAN of Sharetree Systems explains which criteria are important when evaluating highly accelerated life testing (HALT) techniques.
A highly accelerated life test (HALT) evaluation exercise should be applied to a new production before production release. The exercise will increase (or prove) the robustness of the product and establish the basis for a production highly accelerated stress screening (HASS) process. It should initially be carried out on pre-production samples of all versions of the product.
It is important that key company personnel understand the reasons for undertaking a HALT evaluation exercise and the advantages of any subsequent HASS process. This need is best met through an initial tutorial. The tutorial presentation should cover the theoretical basis for various of stress testing and its practical application during the development and production phases of a product. The most effective stress methods are usually rapid temperature cycling and vibration through power cycling and power margining may also be useful.
After the briefing stage the approach should consist of the following stages:
- Development of the evaluation program
- Collection and consideration of existing information
- Selection of vibration method and vibration survey
- Vibration HALT evaluation
- Thermal product survey
- Thermal HALT evaluation
- Summary of results and desirable design changes
- Recommended production HASS process
It is possible to perform HALT evaluation using a combined vibration system and thermal stress chamber. In order to understand the failure mechanisms however it is usual to apply the two screening methods independently. After completing an HALT exercise on pre-production units it is entirely feasible to establish a production screening process which will combine the thermal and vibration screens at appropriate levels.
A small in-house design team should be formed to guide the HALT evaluation exercise which should include representatives form all departments – engineering production quality assurance and so on. When starting the investigation a number of decisions will need to be made before the HALT evaluation exercise can be implemented – for example the number of samples to be tested test conditions number of stress steps. These will depend on factors including product complexity method of construction and known or suspected weaknesses.
The product build standard should be as close as possible to the production version. Where a lot of build variants are possible the exercise should be planned to minimise the amount of testing by evaluating the most representative versions.
Methods of electrically powering and monitoring the product will need to be agreed in order to be able to detect intermittent operation during vibration or thermal stress. Electrical loading of power supply outputs is recommended during thermal evaluation – to maximise the stress in certain power components especially during power cycling.
Full use should be made of existing information on failure modes where available. Examples are results from previous testing in-house inspection and rework test results and field data on related products.
This information is a valuable guide to the most appropriate stress conditions and a useful check on the relevance of failure modes to those found under actual use conditions. Special attention should be given to design features differing significantly from previous products.
Similarly at the end of the HALT evaluation it is important to summarise the results and to discuss them (if not already done) with the in-house team. Desirable design changes may be impractical for a number of reasons such as economic or production constraints and alternative solutions may need to be agreed.
Once the design changes have been implemented the product should be tested under similar stress conditions to prove the effectiveness of the changes. A long-term life test may also be carried out to ensure that the proposed stress screen does not significantly reduce the useful life of good units.
Careful consideration should be given to the failure modes detected and to potential process problems when deciding the best production stress screening (HASS) method. In general stress levels for production will be somewhat below the maximum levels achieved without failures during the evaluation exercise to allow for process variables. Many other factors like procurement and operating costs will also need to be examined before choosing the optimum HASS facility.
| Vibration method survey |
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| Vibration HALT evaluation |
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| Thermal survey/HALT evaluation |
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Sharetree Systems has launched a range of standard test chambers designed for temperature cycling and climatic test conditions with a temperature of +150deg to -40degC or -70degC The Cyclone chambers are available in four sizes from 500 to 1000mm.
Cyclone chambers are suitable for both constant temperature and temperature cycling applications. Evenly distributed airflow ensures consistent conditions throughout the chamber volume and therefore more meaningful results.
Close control of temperature/humidity conditions is achieved by means of a microprocessor-controlled dual loop programmer. An electronic capacitive humidity probe is fitted for high reliability and minimum maintenance.
Humidity generation requires a supply of de-ionised water. This is supplied from the built-in reservoir which may be filled automatically from a mains supply. Chamber condensate is collected by the drainage system and normally run to waste to avoid contamination build-up. However a recirculation pump can be fitted if required.
Options and accessories include boost heating and liquid nitrogen cooling. Computer control may be used to set up and monitor chamber conditions which is particularly useful for simplifying the programming of complex cycles.
- Sharetree Systems
- George Coleman
June 1998