Components Reliability

The main difference between electrical and mechanical reliability is that generally speaking electronic systems do not wear out (with some exceptions). While there are debatably some wear out mechanisms such as electromigration and component parameter drift, electronic systems behave fundamentally different than mechanical ones.

Electromigration

Over time, high current densities in thin-film conductors on integrated circuits can cause voids or hillocks. This is called Electromigration.

Drift of component parameters

Over time analog components can drift from their specified values. This can be accelerated by factors such as temperature. Therefore, critical circuits need to be designed with a level of tolerance that can cope with parameter drift of components.

Transient electrical stresses

Modern electronic components are prone to damage from high currents due to their delicate nature and inability to sink heat. Thus transient stresses such as those due to electrostatic discharge (ESD), lightning, and power supply transients from switching or lighting can cause system failures. Some methods to protect against transient voltages include:

• Capacitors to absorb high frequency transients
• Opto-couplers to isolate sensitive portions of electrical system from damaging transients
• Resistors 1) between inputs and external connections to reduce transient voltage levels and 2) between outputs and external connections to prevent excessive currents in the case of a short to ground.

Excessive heat

Typically a problem in avionics and military equipment, excessive heat can wreck havoc in an electrical system. Component parameter values usually vary with temperature and it is important not to exceed the manufacture’s temperature range. Above such temperatures, parts are no longer guaranteed to be within specification. Thus thermal design can be an important aspect of a system’s over design. Components generate heat in operation and when combined with ambient temperature and solar radiation, excessive temperatures can be attained. Common methods to provide thermal protection include:

• Heat sinks for components that give off a considerable amount of heat.
• The use of a thermal conduction plane. Thermal conduction planes within printed circuits boards conduct heat away from generating components.
• Fans to improve airflow through enclosure.
• Liquid cooling for high power devices giving off large amounts of heat

Electromagnetic interference (EMI)

Electrical systems can emit electromagnetic radiation that can cause interference to itself or other systems. Particularly in digital systems, a conductor acting as an antenna can pick up electromagnetic signals and corrupt digital data. Thus to produce reliable electronic systems, emission of EMI must be limited as well as the system’s susceptibility to it. There are many different sources of EMI and one should consult a text on EMI for a full understanding of how to cope with it. Some ways to protect against EMI emission are:

• Suppression
• Screening

Some ways to limit EMI susceptibility are:

• Screening
• Filter our unwanted frequencies
• Isolation by opto-couplers
• Careful design, taking into account layout, packaging, etc.

Mechanical failures

Mechanical failures are quite commonly the cause for many system failures. Consider automobile wiring harnesses for instance. A wiring harness is a collection of wires that is routed throughout the automobile to keep all the wires together. Wiring harness damage is a common cause of electrical system failure is many automobiles. Damage to harnesses can occur from piercing by body screws, entrapment by an adjacent component, and chafing due to lost retaining clips to name a few.

Electronic systems must be designed to withstand mechanical shock, vibration, humidity and other environmental stresses. Since solder has rather poor fatigue properties, heavy components should be given extra support rather than simply relying on solder connections. Furthermore, cables need to be carefully supported and strapped down to avoid wear due to moving parts. Connector failure is often a common cause for electrical system failure and attention should be paid to their placement and mounting.

 

Reliability prediction of electronic equipment

There is a variety of reliability prediction modeling techniques. Instead of listing them here, they can be classified into five main categories:

Similar Equipment Techniques. In order to estimate the level of reliability, the equipment under consideration is compared with similar equipment of known reliability.

Similar Complexity Techniques. The reliability of a design is estimated by comparing its relative complexity with an item of similar complexity.

Prediction by Function Techniques. Correlations between function and reliability are considered in order to obtain reliability prediction of a new design.

Part Count Techniques. Reliability is estimated as a function of the number of parts involved.

Stress Analysis Techniques. Failure rate is a function of individual part failure rates and takes into consideration part type, operational stress level, and derating characteristics of each part

Useful in understanding these techniques is the exponential distribution. The exponential distribution is one of the most important distributions in reliability calculations. Specifically, it is used heavily for reliability prediction of electronic equipment. This is because of the general lack of a wearout mechanism. An exponential distribution has a constant failure rate, analogous to random system failures, not associated with wear, corrosion, etc.

An exponential distribution is good for modeling:

• Items whose failure rate changes negligibly with age
• Equipment whose infant moralities have been eliminated.

Not all electrical components follow an exponential failure rate. For instance, electrolytic capacitors can break down over time. Thus, it is not safe to say that electrical system can not wear out.