FAQ 1: What is reliability?


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The simplest definition of reliability is quality over time. Since time is involved in reliability, it is often measured by a rate. Just as quality is usually measured in terms of rejects (we define the concept of 'quality' by observing the absence of it in devices that fail or perform in less than satisfactory ways), reliability is measured in terms of failures (or un-reliability).

Traditionally, the measurement of electronic failures has been straightforward. If a person assumed all failure rates were constant, as they might be in a large system or machine, then a MEAN time between failures (MTBF) would be expected. In contrast, most integrated circuits, including TriQuint GaAs devices follow the lognormal distribution, which rarely approximates a constant rate.

Although well understood since the 1970s, failure distributions were largely ignored in favor of the simpler constant failure rate and MTBF calculations. Even today, the simpler calculations such as: (the number of failures divided by the number of test hours) are preferred over the more complex relationships between failures and time as the probability density function over 1 minus the cumulative failure function.

Historically, failure rates were measured in percent failed per thousand hours of operation. The modern units commonly used today are in FIT. A FIT is a Failure unIT, which is equivalent to one failure per billion device hours. For comparison, one FIT is equivalent to 0.0001% per thousand hours, and 1% per thousand hours is equivalent to 10,000 FIT. However, as we've just discussed, a single rate is not sufficient to describe the reliability of semiconductors since their failure rates change over their lifetimes.

Generally, semiconductors have a very low wear out failure rate early in life, then have increasing failure rates as they wear out. At a point when about half of the devices fail in a group of circuits, the failure rate begins decreasing again. A very small part of an IC's population may fail early in life. These early failures have been associated with manufacturing or assembly defects. The early failures are sometimes called "infant" failures. As semiconductor reliability improves and more samples are stressed, the early failures become easier to detect and eliminate. TriQuint also utilizes feedback from high volume customers to identify causes of early failures so that defects can be continuously reduced. With long wear out lifetimes and very low rates of defects, TriQuint ensures high reliability.