A process for uncovering design defects and weaknesses in electronic and mechanical assemblies using a vibration system combined with rapid high and low temperature changes. The purpose of HALT is to optimize product reliability by identifying the functional and destructive limits of a product at an early stage in product development.
Highly accelerated life test (HALT), is a stress testing methodology for accelerating product reliability during the engineering development process. It is commonly applied to electronic equipment and is performed to identify and thus help resolve design weaknesses in newly-developed equipment. Thus it greatly reduces the probability of in-service failures (i.e., it increases the product's reliability). Progressively more severe environmental stresses are applied building to a level significantly beyond what the equipment will see in-service. By this method weaknesses can be identified using a small number of samples (sometimes one or two but preferably at least five) in the shortest possible time and at least expense.
A second function of HALT testing is that it characterises the equipment under test, and identifies the equipment's safe operating limits and design margins. Data from a HALT test is therefore used as a basis for the design of an optimal "HASS" or "ESS" test, which is used to screen every piece of production equipment for latent manufacturing defects and defective components. HASS or "highly accelerated stress screening" is an extension of HALT, but is applied during production.
Individual components, populated printed circuit boards, and whole electronic systems can be subjected to HALT testing. The size of the test 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 whereas often it is not economically feasible to write off more than a few items of very expensive equipment because production quantities or the application does not justify the cost. A general principal is that while HALT test can and should be conducted at unit level, it is very desirable to conduct it at sub-assembly and piece-part level as well.
Temperature cycling and random vibration, power margining and power cycling are the most common form of failure acceleration for electronic equipment. HALT does not measure or determine equipment reliability but it does serve to improve the reliability of a product. It is an empirical method used across industry to identify the 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.
Several standards and test methods are available for a HALT test. Different stresses are applied with different failures occurring during each. The types of stress typically employed are:
1. cold step
2. hot step
3. rapid temperature cycling
4. stepped vibration (random)
5. combined environment stress (temperature cycling and random vibration plus power switching and power margining)
In HALT these stresses are applied in a controlled, incremental fashion while the unit under test is continuously monitored for failures. Once the weaknesses of the product are uncovered and corrective actions taken, the limits of the product are clearly understood and the operating margins have been extended as far as possible. The result is that a much more mature product can be introduced much more quickly with a higher degree of reliability.
When HALT testing is applied during the design process, it can produce a very robust product without undue cost, because improvements are targeted only where they are needed. As failure modes are discovered and understood the product life can increase significantly. This makes the product more robust and risk of failure reduces drastically.