There are few alternatives. For example, standard polymer film capacitors are made of polyester and cannot be used at temperatures above 150°C because their mechanical integrity and insulation resistance begin to break down.
Polymer films, such as PTFE, are mechanically and electrically stable at higher operating temperatures—showing minimal changes in dielectric constant and insulation resistance even after 1000 hours of exposure to 250°C. However, these films also have the lowest dielectric constant and are the most difficult to manufacture in very thin layers, which severely reduces the energy density of capacitors.
The best option is to use stacks of temperature-compensated ceramic capacitors. New ceramic dielectric materials continue to offer improved high-temperature stability via tailoring of the microstructure or the composition of barium-titanate-based mixtures. One particularly promising composition is X8R, which exhibits the energy density of X7R but has a minimal change in capacitance to 150°C.
Since flow of electrons through any medium develops heat, all resistors are subject to temperature changes. The effect of these temperature variations depends upon the type of construction and is known as temperature coefficient.
Temperature coefficient indicates how temperature changes affect the resistor’s value and may be either positive or negative. In general, composition resistors have a negative temperature coefficient, and metallic or wire-wound resistors have a positive coefficient. This means that composition resistors decrease in resistance with an increase in temperature, while metallic types increase in resistance with an increase in temperature.
A low temperature coefficient indicates slight change in resistance per degree of temperature change. High-quality resistors have a low or even zero temperature coefficient. These, of course, are the most desirable, especially in precision work.
Composition resistors are commonly found in electronic circuits. The element’s resistive material is moulded into a small rod or deposited upon an insulating core. Wire leads are coaxially attached to each end of the element and an outside covering of natural bakelite is applied for insulation. The resistance value is marked on the body using EIA colour code.
Resistance values range from a fraction of an ohm to several million ohms. Exact values are difficult to manufacture and usually are not required. Therefore tolerance limits of 5 and 10 per cent are often used. In each tolerance group only certain preferred values are made. So no overlapping of values is possible due to normal manufacturing variation.
Exact-value precision resistors are available for applications that require extremely high accuracy. Common power ratings range from 1/4 watt to 2 watts. Physical size increases with required wattage. For good stability, actual power dissipation in service should not exceed 50 per cent of the rating. Since power is dissipated in the form of heat, this is a good rule to follow, because excessive heat will decrease resistance.
Overheating may cause permanent damage to resistors. So when soldering these units in place, care must be taken to prevent these from over-heating. Excessive heating will cause discolouration of the resistor body and colour code stripes. For precision applications, a low-wattage, 1 per cent composition type made of pure carbon deposited in a spiral groove on a ceramic rod is also made.
Resistance values are generally marked on the body in English. The physical size of precision resistors may vary between manufacturers and may often be misleading as they are somewhat larger for a given rating than the common composition type. They cost several times more than ordinary-composition resistors.
As mentioned earlier, wire-wound resistors’ resistance increases as these heat up. This change in resistance is quite small, but care should be taken to keep the resistor as cool as possible for resistance stability. Resistors should be mounted in a well ventilated position and should be capable of at least twice the power dissipation required. In other words, if calculations show that 5 watts will be dissipated, the resistor used should be rated at no less than 10 watts. This rule should be followed invariably, although overheating is more likely to cause permanent damage to composition resistors than wire-wound units.
PCBs and substrates
PCBs and substrates provide mechanical support for components, dissipate heat and electrically interconnect components. Above their glass transition temperature (Tg), however, organic boards have trouble performing these functions. They begin to lose mechanical strength due to resin softening and often exhibit large discontinuous changes in their out-of-plane coefficients of thermal expansion. These changes can cause de-lamination between the resin and glass fibres in the board or, more commonly, between copper traces and the resin. Furthermore, the insulation resistance of organic boards decreases significantly above Tg.