In this third block our aim is first to help you understand how and why components fail, and then look at how we can minimise the effect that this has on the reliability of the final assembly.
We have made two main assumptions:
We are starting by asking the question ‘what is a failure?’, because this is not always a straightforward issue. Take the high mileage car which you traded in. Was this because it totally failed to function? More likely it was because it had started to exhibit increased fuel consumption, occasionally didn’t start too well, or didn’t look as smart as in its youth. All these represent failures, but at different levels.
Other kinds of equipment may still function as intended, but no longer be compatible with other requirements, standards or legislation – someone, somewhere must still have a working BetaMax VCR! Other kinds of failure in the consumer market relate to a product not being state-of-the-art, or reflecting the most modern styling, considerations which fuel the mobile communications and games industries.
In the same way, at the system level, electronic failures will vary very considerably from total failure to function, through to purely cosmetic issues. Whilst creating a product that looks the part, and is made to an acceptable cosmetic standard, is of major interest to the marketers and manufacturers of equipment, we will be looking primarily at failure of individual components, rather than systems, and only at those types of failure which result in system malfunction.
At the component level, electronic failures fall into four categories:
The effect that component failures will have on the system depends on the fault tolerance of the system, and the degree and type of failure. With high-reliability requirements, it is not uncommon for designs to be deliberately made to tolerate degrees of failure, particularly at a system level. An example of this might be a critical server application, where the operation of one system is mirrored by a second fully-functioning machine which takes over automatically if the main system fails.
In considering failure, we also have to bear in mind the possibility that there will be some knock on consequences, particularly of catastrophic failures. For example, failure in protection components can result in the failure of associated circuitry – service personnel will be familiar with cases where a cheap component fails and ‘takes out’ an expensive module elsewhere in the circuit. Sometimes this knock-on failure can be quite dramatic. The writer recalls a very expensive complex assembly becoming a total write-off because of the short-circuit failure of a ceramic capacitor placed across a high current supply, when it literally burnt a hole in the board!
A range of conditions cause electronic failures. Sometimes the mechanism is simple overload, but often misuse, misapplication, lack of full testing, or defects in manufacture can play a part. At the system level failures can be caused by:
Although our focus in this unit is on failure, it is important to keep in mind that the majority of components used in electronic assemblies do not have mechanisms that would cause the part to degrade sufficiently to fail during storage or normal use, provided that they:
These are important provisos, which we will consider further in Strategies for minimising failures, but remember that the quality of manufacture of modern electronic components is high, with typically fewer than 10 defective parts per million for complex components such as integrated circuits, and even less for simpler components.
The first three sections of this unit describe typical mechanisms for failure in common electronic components:
In the following three sections we shall be discussing some generic failure causes:
The final section of this unit relates to the need to understand the reasons for solder joint failure, starting with a review of stress and its effect on materials and why solder joints may crack. Whilst the text is targeted at solder failures, you will also get an insight into the potential for failure of other parts of the structures into which assemblies are placed.