Resistors in printed circuit board: different types of resistors
Resistors are omnipresent. They are an integral part of the printed circuit board. Owing to this their performance is accepted for granted, given that they have functioned within their power, voltage as well as environmental ratings this is rational because after millions of gathered resistor-years knowledge there is small left for their fabricators to find out.
Nevertheless, there are still applications where identifying & applying resistors requires to be used with some caution.
Kinds of resistor – Surface mount chip
The most commonly used resistor is the surface mount chip kind which is thick film. Accessible in big quantities & very low costs, it’s the key type of resistor globe. The design is extremely easy and barely differs from fabricator to fabricator.
An alumina substrate having nickel-plated terminations contains a resistive ink film printed or else gathered on its top surface. The terminations are wrapped with a solder dip to make sure convenience of wetting when the component is etched into line, and the top of the component is wrapped with an epoxy glass layer to safeguard the resistive part.
Various fabricators make numerous claims for the toughness and efficiency of their parts nevertheless the fundamental features are similar. Power dissipation is hugely managed by the heat properties of the printed circuit board pads to which the chip is etched, and in case you are working near to the rated power of the component, it would be essential to verify that your pad design conforms with the fabricator’s recommendations.
You could also obtain metal film chip resistors for superior performance applications, nevertheless, these are costlier as compared to the general thick film. The resistive ink technology utilized for chips could also fabricate general axial lead resistors of miniature size, and could be utilized straightly onto a substrate to create printed resistors.
This technology is more often than not utilized in hybrid circuits & is extremely cost-efficient specifically when big nos. of similar values are need. It’s probable to print resistors straightly onto fiberglass PCB or printed circuit board, nevertheless, the outcome is of extremely inferior quality & can’t be utilized where a stable, expected value (compared with traditional kinds) is needed.
Metal film
The subsequent most general kind is the metal film, in its numerous guises. This is the general part for industrial as well as military objectives. The most common types of the leaded metal film are scarcely any more pricey as compared to carbon film and, provided their better characteristics, specifically temp coefficient, sound & power controlling capability, a lot of equipment manufacturers don’t locate it useful to trouble with carbon film. Variations of the common metal film serve for high or low resistance requirements.
The ‘metal’ in a metal film is a Ni-Cr alloy of differing compositions for various resistance ranges. An alloy film is plated onto an aluminum oxide ceramic substrate.
For leaded components, the end caps & leads are power-fitted to the tubular assembly & the resistive component is shortened to value by incising a helical groove of managed dimensions in it that facilitates the same film composition to be utilized on pretty a comprehensive range of nominal values. The entire part is then wrapped in epoxy & engraved.
The downside of the helical trimming procedure is that it intrinsically enhances the resistor’s stray inductance, & moreover restraints its pulse controlling ability.
Carbon
The most general leaded resistor for saleable applications is the carbon film. It’s pretty cost-effective – less than a coinage in quantity. It moreover has the least remarkable performance with regard to tolerance & temp coefficient, nevertheless, it is generally suitable for common purpose utilization.