Frequently Asked Questions
Reliability is related to weaknesses or faults in manufacturing processes (infant mortalities), weaknesses in process control (random failures) or natural wearout of electronics devices, components, boards or systems. It involves failure due to some physical phenomena such as insufficient joint area, corrosion, moisture ingress, stress, creep or fatigue. As a consequence of this, the failure may be described by a mathematical equation, leading to the prospect of reliability measurement and prediction. Equally, the failure can be accelerated, leading to the ability to reduce testing time but still demonstrate the service failure mechanism.
Infant mortalities can be detected by 'burn-in' (e.g. 125°C for 168 hours) and 'environmental stress screening' (e.g. 20 cycles from -10 to +70°C ramped at ±10°C/min). Random failures occur throughout the life of a system/product and require the monitoring and measurement of field failures and the generation of acceptable quality levels (AQL). Wearout is an inevitable consequence of design, choice of materials, technologies and service conditions.
To test for service reliability, it is essential to know the details of the product (design, materials, processing procedures, etc) and the service conditions to be seen by the product. An analysis of expected failure mechanisms leads the Reliability Engineer to design a suitable accelerated test, from which failure data can be derived, and a reliability formula developed from which to predict lifetime. It is essential to ensure that:
- the failure mechanism is representative of the true failure mechanism in service
- the test regime does not change the physical state of any of the materials, which could unduly influence the result (change failure mechanism), e.g. a thermal test above the glass transition temperature (T g) of a polymer.
- the reliability equation or formula is validated against known data or within a justified limit of extrapolation
A typical accelerated test for many components involves a combination of temperature and humidity at the level of 85°C, 85%RH. In certain circumstances, this can be highly accelerated (e.g. 108°C, 90%RH) to shorten the duration of testing significantly.
Further reliability support is found in MIL-HDBK-217 which was developed for the military market, but many experts believe this handbook is out-of-date for current materials, environmental conditions, and the change in reliability approaches for commercial products.
FAQ: What is burn-in testing related to electronics devices?
FAQ: Why do electronics devices fail?
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