Printed Circuit Passive Components

Printed Circuit Passive Components

Resistors are ubiquitous. Because of this their performance is taken for granted, provided they are operated within their power, voltage and environmental ratings this is reasonable, since after millions of accumulated resistor-years experience there is little left for their manufacturers to discover. But there are still applications where specifying and applying resistors needs to be handled with some care.

Resistor types – Surface mount chip

The most common general purpose resistor is the thick film surface mount chip type. Available in huge quantities and very low prices, it is the workhorse of the resistor world. The construction is very simple and hardly varies from manufacturer to manufacturer. An alumina (aluminium oxide ceramic) substrate with nickel plated terminations has a resistive ink film printed or otherwise deposited on its top surface. The terminations are coated with a solder dip to ensure ease of wetting when the part is soldered into place, and the top of the part is coated with an epoxy or glass layer to protect the resistive element. Different manufacturers make various claims for the ruggedness and performance of their parts but the basic features are similar. Power dissipation is largely controlled by the thermal properties of the PCB pads to which the chip is soldered, and if you are running close to the rated power of the part it will be necessary to confirm that your pad design agrees with the manufacturer’s recommendations. You can also get metal film chip resistors for higher performance applications, but these are more expensive than the common thick film. The resistive ink technique used for chips can also produce standard axial-lead resistors (metal glaze) of small size, and can be used directly onto a substrate to generate printed resistors. This technique is frequently used in hybrid circuits and is very cost-effective especially when large numbers of similar values are required. It is possible to print resistors directly onto fibreglass printed circuit board, though the result is of very poor quality and cannot be used where a stable, predictable value (compared with conventional types) is required.

Metal film

The next most common type is the metal film, in its various guises. This is the standard part for industrial and military purposes. The most popular varieties of leaded metal film are hardly any more expensive than carbon film and, given their superior characteristics, particularly temperature coefficient, noise and power handling ability, many equipment manufacturers do not find it worthwhile to bother with carbon film. Variants of the standard metal film cater for high or low resistance needs. The “metal” in a metal film is a nickel-chromium alloy of varying composition for different resistance ranges. A film of this alloy is plated onto an alumina substrate. For leaded parts, the end caps and leads are force-fitted to the tubular assembly and the resistance element is trimmed to value by cutting a helical groove of controlled dimensions in it, which allows the same film composition to be used over quite a wide range of nominal values. The whole part is then coated in epoxy and marked. The disadvantage of the helical trimming process is that it inherently increases the resistor’s stray inductance, and also limits its pulse handling capability.

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