Reliability and Accelerated Life Testing

A company cannot become a business if its device is not made into a timely and cost-effective product. To meet the time-to-market and cost objectives, the product must be qualified to serve in the given capacity and reliable enough to successfully operate in this capacity for the required period of time under the given conditions. Typically, the acceptable level of reliability is proven based on the acceptance criteria of qualification tests (QTs). However, it is the accelerated life tests (ALTs) that reveal the physics of failure. By accumulating reliability statistics, the ALT can provide information about the product's reliability before it is shipped to the customer. Adequately designed, carefully conducted and properly interpreted ALTs provide a consistent basis for obtaining the most convincing information of the reliability of a product — the probability of failure.

Although the body of knowledge in ALTs has come quite a long way in a rather short time, design, implementation and interpretation of ALTs are still some of the major challenges associated with the development of new products. The Table illustrates how ALTs occupy a very special place among other types of accelerated tests, in terms of their objectives, end points, follow-up activities and ideal test conditions.

Profits decrease as failure rates increase. Too low a reliability can lead to a total loss of business. At the same time, there is a permanent struggle between the recognition that adequate product reliability is a must and a strong business pressure that tends to compromise reliability in order to shorten the time-to-market and reduce manufacturing costs. The cost of improving or maintaining a certain level of product reliability must always be weighed against the benefits obtained. From the cost and business point of view, there is always an adequate, though less than perfect, level of reliability appropriate for the given product or system.

Qualification standards and requirements are only good for what they are intended — to confirm that the given product (provided it passes the tests) is indeed "qualified" to serve in a particular capacity. In some cases, especially for new products and new technologies, when no experience has yet been accumulated, the general qualification standards based on the previous generations of the device or on other "similar" devices and components might be too stringent. Unreasonable "torture" QTs that do not reflect the actual use conditions might result in a rejection of a good product (vendor's risk), while QTs that are not stringent enough will be unable to detect a faulty product before it is shipped out to the customer (consumer's risk).

If a product passes the standardized QTs, it is not always clear why this product was "good." And it certainly does not mean that there will be no failures in the field, nor does it indicate how likely or unlikely these failures might be. If the product fails, it is equally unclear what can be done to improve its reliability. Since QTs are not supposed to be destructive, they are unable to provide the most important information about the reliability of the product — the information about the probability of its failure.

ALTs, on the other hand, deal with the two major areas of reliability engineering — physics and statistics of failure. ALTs provide a consistent basis for the prediction of the probability of failure. ALT data can be extremely helpful in understanding what should be changed in order to design a viable and reliable product. Indeed, any structural, materials and/or technological improvement can be "translated," using the ALT data, into a reduced probability of failure. This is, in effect, the substance of a probabilistic approach to physical (structural) and functional (electrical or optical) design of a component or device.

As the time to develop and produce products is permanently and rapidly decreasing, application of ALTs can provide valuable quantitative insight and help strike the right balance between the demands for the product's reliability on one hand, and time-to-market, cost and schedule on the other.

Carrying out of ALTs should not be delayed until the device is made. Rather, based on my experience, ALTs should be conducted prior to, in addition to, and, in many cases, instead of QTs (if QT specs exist in the given area of engineering). If no sufficient experience has been accumulated yet in a particular field, then the necessity and urgency of conducting ALTs becomes especially significant. If, for one reason or another, ALTs are delayed and QTs are conducted in the hopes that the product will pass all the qualification specifications (which typically does not happen), then ALTs should be designed and conducted for the type of tests the device was unable to pass. After the appropriate improvements are made, based on the analysis of the ALT data, then the corresponding QTs should be revisited.

Unfortunately, different manufacturers often have to run the same ALTs and learn reliability lessons from their own mistakes. This is because ALT methodologies, studies and, especially, test data are generally considered highly proprietary information. ALTs can provide significant help to a manufacturer making its device into a product. They should play an important role in the evaluation, prediction and assurance of device reliability, as well as in business decisions.