T
HE CONSUMMATE ENGINEER
by George Novacek (Canada)
Accelerated Testing Testing is essential to projecting your product’s reliability in the field. This article examines different types of testing, including accelerated life testing, stimulation as accelerated stress testing, and highly accelerated life testing.
E
very manufacturer worth his salt is concerned with his product’s reliability in the field. This is often expressed as mean time between failures (MTBF) and can also be used to calculate budgetary provisions for warranty, realistic warranty length to be offered to the customer, spare parts allocation, and so forth. Field product reliability should be given to design engineers as a goal and its status should be monitored concurrently with the design process. Unfortunately, with the shortage of resources, reliability prediction is often considered to be something of a useless exercise and, consequently, is calculated after the fact just to satisfy the specification.
STATISTICS & LIES
It frequently becomes an issue when the MTBF calculated after the fact doesn’t meet the specification goal. Worse, at times the calculated MTBF is just a few hours less than what is called for in the specification. Panic ensues with engineers tweaking the numbers merely to hit the goal and satisfy some bureaucrat in program management. This only proves the sad truth that, more often than not, no one understands what the reliability prediction is about and that the Time calculated MTBF is nothFigure 1—Reliability growth with time. The blue ing more than a statistical trace indicates the reliability growth based on probability based on a com- field data. The red trace indicates the reliability ponent’s historical data, its achieved by accelerated testing. Reliability
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stress levels, and so forth. The former British Prime Minister Disraeli once stated that there are lies, damned lies, and statistics. The calculated MTBF is just a statistic. It indicates the product’s reliability ballpark. It is a good start, but not much else. To arrive at the actual product reliability, the rubber must literally hit the road. As the product enters the service, the manufacturer tracks its performance through field returns. Every failure is analyzed and a corrective action is taken. Discovered “weak” components are replaced with “stronger” ones, or some operating parameters may be adjusted. This process grows the reliability until it settles at a value that reflects balanced design with no weak spots. This is shown in Figure 1 by the blue trace. Unfortunately, this takes a long time, often years, based on the product type and volume.
TESTING TO FAIL Because this process is so slow and costly, as manufacturers must satisfy warranty claims, it is desirable to achieve the final reliability in the shortest period of time so that the product can be released into the field with its reliability already established. This is where accelerated stress testing (AST) comes in. The principle of AST is that applying a high amount of stress to the product for a short period
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