A wide range of capacitors and inductors are used within electronic circuits, and all of these can potentially fail. Film capacitors, for example, can be damaged by excessive temperature. The materials used being thermoplastics, the problem is less of dielectric failure, leading to short-circuit conditions, as of failure between the external connection and the internal metallising, leading to intermittent or permanent open circuit. Most polymer materials will also absorb moisture, and increased losses can be expected when film capacitors are stored or used in damp environments. Depending on the surface finish, and on the voltage applied, surface tracking and breakdown may also occur.
For electrolytic capacitors, the major problems are the application of over-voltage and reverse voltage, about which more will be said later. These are an inevitable consequence of the way in which the dielectric is grown and the counter-electrode made. With time, many larger aluminium electrolytics will also show some adverse effects due to the drying out of the gel dielectric, increasing both ESR and losses.
All types of capacitor will exhibit changes in time and temperature in both insulation resistance and values. Given their initially high values, the former is not usually a problem, but capacitors may well drift out of tolerance, and this can impact on certain types of circuit.
Only NP0 ceramic capacitors should be regarded as sufficiently stable for critical applications, other types exhibiting a continued reduction in value with time. This is related to a change of state which takes plate at the Curie temperature: taken above this temperature, which is typically under the melting point of solder, capacitors with a high dielectric constant reach a maximum and reduce thereafter at a logarithmic rate of 1–2.5% per decade hour. That is, the component will lose 1% of its initial value in the first hour, a further 1% in the next 10 hours and so on. This is in addition to the changes in capacitance that take place over the temperature range.
Film capacitors are generally more stable with time, but may drift if mechanical pressure is applied.
Inductors are relatively stable and reliable, with the exception of parts where the leads are subject to cyclic stresses. However, there may be significant value changes with ferrite-cored components if the ferrite is subjected to shock or other cause of fracture. This is because the fracture creates a break in the magnetic circuit and consequent changes to inductance and mutual inductance. Transformers built with silicon-iron laminations, rather than ferrite cores, also deteriorate with time, although failure is usually gradual, being caused by changes in the insulation between laminates. It is not unusual for both laminated cores and ferrites to become loose and noisy, even if the electrical performance does not deteriorate.
All potted and coated components have potential for failure caused by the absorption of moisture or by temperature cycling (due to the difference in CTE between the materials). The latter problem is particularly severe in constructions such as tantalum capacitors, where some polymer layers are comparatively thick, whilst others are thin. The lack of inherent ‘balance’ in the structure leads to stresses, which may cause failure.
Other mechanical stresses leading to failure can be the result of the poor mounting of larger components. Vibration in particular can cause cyclic component movement which fatigues joints.
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