Reliability Testing

Introduction

Reliability testing is the key to the success of a specific product design in an age when even the smallest failure in a product may lead to a massive recall, brand damage, and even safety risks. Reliability testing is a short-term test; unlike quality control, which looks at the short-term manufacturing flaws in a product, reliability testing looks at how a product degrades and fails over the long term. It provides the answers to such basic questions as ‘How long will this product last?’ In what circumstances will it be unsuccessful? What are its least strong elements? Through accelerated life testing (ALT) and highly instrumented failure analysis, engineers can squeeze years of normal operation into weeks of laboratory tests, finding ways of failure before mass production.

Fundamental Principles and Core Concepts

The concept behind reliability testing is probability theory and the physics of failure. The fundamental assumption is that the failures of products have predictable patterns, which are usually represented by such statistical distributions as the Weibull or exponential distribution. The testing is concerned with three stages of the bathtub curve: early life failures (infant mortality because of manufacturing defects), constant failure rate, and wear-out failures (degradation at the end of useful life). The important measures are MTBF (Mean Time Between Failures) in the case of repairable devices, MTTF (Mean Time To Failure) in the case of non-repairable objects, and Failure Rate.

Testing Equipment and Instrumentation

Controlled, repeatable stresses need specialized, typically capital-intensive equipment. Environmental chambers have high accuracy in temperature (-70°C to +180°C), and humidity (5 to 98 RH), and the transition rates to thermal cycling are high. Vibration shakers (electrodynamic or hydraulic) use controlled profiles of vibration (sinusoidal or random) or shock vibration across many axes. 

Combined Environment Chambers: Temperature, humidity, and vibration are combined. Power cycling and burn-in racks are automatic power cycle tools that automatically power cycle electronic assemblies during performance monitoring.

Industry-Specific Applications and Standards

The programmes of reliability testing are very industrialized in terms of risks and regulations. Other standards like MIL-STD-810 on environmental conditions and MIL-HDBK-217 on prediction of reliability are also applied in aerospace & defence; the costly results of failure are devastating, and therefore testing is extreme. The automobile industry, particularly electric cars and ADAS, conforms to ISO 16750 (environmental testing); large-scale component-level tests (ALT) are applied to products, like ECUs, sensors, and battery packs, with a service lifetime of 15 years on average.

Challenges and Common Pitfalls

Even though it is significant, reliability testing has some significant challenges. Time-Cost Trade-off is invariable: full testing is both expensive and time-consuming, imposing a burden on corner-cutting. The physics of failure can lead to invalid extrapolations; an incorrect assumption about the physics of failure may be made (e.g., an Arrhenius model where the failure is not thermal). Limitations to sample size may invalidate results; it might be necessary to test hundreds of units during thousands of hours to achieve statistically significant results with high levels of reliability (e.g., 99.99% over 10 years); this is often impossible.