Failures occur at every stage of the product lifecycle, and for a variety of reasons. Understanding the root causes of failure offers the chance to improve product quality by making changes to product design or manufacturing process.
At the prototype and design phase, component failure can be frequent and the reasons usually obvious - poor handling, hand soldering, use of test probes and 'mod wires' all have an impact on relibility of prototypes.
Designers will often develop a knowledge of which components fail, and which never do. While some devices certainly require more careful handling, it may also be a pointer to other issues such as lack of in-circuit esd protection, power supply transients or sequencing, thermal bonding or simply an incorrect component value.
Production is often carried out by an external supplier, usually including some level of module test before delivery. Problems which require rework by the supplier during test may not become apparent until bad enough that the supplier decides to flag the issue as a problem. Issues found here may result from some problem with the design, the manufacturing or test processes, or component supply.
Final Assembly may expose issues with the overall design or the manufacturing process.
Failures in-service can be the most troublesome to track down. Problems can be due to operator error, tampering (malicious or mischevious), installation, site-specific environmental, or simply use-case.