Service life

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A product's service life is its expected lifetime. It is the time for which MTBF applies and is usually 2 to 5 years for most commercial and consumer products (for example computer peripherals and components). Most items to which this applies follow a bathtub curve of reliability and the service life is the width of the well at the bottom of the curve.

A product's end of life is determined by wear out mechanisms in certain components, other than random failures due to variations in component stress withstand as a result of small manufacturing defects and other than variations in the actual stress on the components as a result of unintended overloads.

For mechanical parts like fans or automobile motors the end of life is determined by loss of material from these parts to such an extent that the intended operation is not possible anymore or with greatly reduced performance. For small fans in electronic equipment have a typical life of 10,000 to 60,000 operating hours.

For electric lamps there is a degradation mechanism, the incandescent lamp has a life expectancy of 1000 operating hours and the halogen lamp 3000 operating hours. Fluorescent lamps can reach operating service life of 18,000 hours, and special quality fluorescent lamps 30,000 hours.

In these devices material changes occur, as a result of processes like chemical reactions, sublimation of materials, sputtering and diffusion of poisoning materials, leading to degradation of performance and finally to total inability to operate (broken filament in an incandescent lamp).

Electrodeless fluorescent lamps can reach 100,000 hours service life and LEDs intended for lighting purposes, if sufficiently derated 50,000 hours. It is then important to define the end of life condition, for instance 70% of the original light output.

In much consumer products mechanical parts like switches, potentiometers or the other mechanical parts, for instance in videocassette recorders, can be the life defining factor. It is difficult to express the life in number of operating hours or number of actuations, since it depends on a lot of other factors. If Printed Circuit Boards experience large mechanical stress, the life can be limited by mechanical fatigue (rupturing of Board or solder joints).

In electronic circuits there are parts that have a relatively well defined service life limit.

Electrolytic capacitors are devices with a well defined wear out mechanism, the evaporation of the electrolytic fluid used in these capacitors. High quality electrolytic capacitors have a service life of 10,000 hours at 105 °C, the life roughly doubling for every 10 °C lower temperature. Very cheap small electrolytic capacitors have a service life of 1000 hours at 85 °C.
Other components with limited service life are rechargeable batteries. Their service life can be influenced by method of charging the batteries.
Magnetic components like transformers and inductors can also have a limited service life, since the isolation lacquer of the enameled wire or other isolation materials degrade at high temperature. This will only come into play if (part of) the components are continuously operating at temperatures close to the temperature class of the wire or isolation material, for instance if internal hot spots are present.
In memory devices like EEPROM and flash memory charge leakage can be a limiting factor for the life. EEPROM memory also has a limited number of write cycles (in the order of 100 000 cycles).
Optocouplerss LEDs gradually degrade since the light output decreases. Service life depends on the End Of Life definition. If 50% light reduction is permitted operational life is in the order of 150,000 hours.
Electromechanical relays can have an operational life of 100,000 switching cycles at rated switch current. The life depends very much on load type. The life is limited by arcing, [eroding] the contact surfaces. Reed relays can reach an operational life of 100 million switching cycles.
In high voltage transformers internal corona effects can limit the components life. This is mainly dependent on the operating voltage level. Internal corona can occur in small gas cells in the isolation material and degrades the isolation.
In theory also other electronic components like film resistors have a limited service life. With film resistors material sublimation will occur at very high temperatures. This will change the resistance value. In well designed electronic circuits this is not very often a life limiting factor.
In most semiconductors also small material changes can occur at high operating temperatures like diffusion, oxidation or electromigration. This is not very often a life limiting factor.
In the early days of electronics vacuum tubes were also parts with limited service life due to the poisoning of the cathodes (diffusion of performance degrading materials into the cathode surface). The related Cathode Ray Tubes, as used in computer monitors and televisions and magnetrons used in microwave ovens have similar life limiting processes.

Another life limiting mechanism is thermal fatigue as a result of differences in Coefficients of Thermal Expansion (CTE) or temperature differences in materials. This is an important factor for solder joints on Printed Circuit Boards and for [power semiconductors]. For power semiconductors, apart from the temperature difference also the average temperature plays an important role, the lower the average temperature, the higher the number of thermal cycles that will be survived. Large power modules will survive 20,000 cycles at 100 °C temperature difference and 80 °C average temperature, increasing to 5 million cycles for a temperature difference of 30 °C. (1). This translates to a factor 2.5 more cycles for every 10 °C lower temperature swing at the same average temperature. Every 10 °C lower average temperature increases the number of cycles by a factor 1.8.

Thermal fatigue can play a role in electronic equipment where frequently high temperature changes are present, for instance in automotive electronics.

References

1. U.Scheuermann, U.Hecht: Power Cycling Lifetime of Advanced Power Modules for Different Temperature Swings, Proc PCIM Nürnberg, May 2002, pp 59-64

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