Metal Film resistor

- the metal film resistor was the first major alternative to the carbon composition used up until the early 1960s.

As the name indicates, metal film resistors utilise a thin film of a metal allow to provide the resistive element.

These resistors provide an excellent level of performance and are now one of the preferred types for axial leaded resistors.

Although widely used for axial leaded components, surface mount resistor also use the same basic technology, but on a flat rather than a cylindrical body. However the term metal film resistor normally refers to the axial leaded component rather than the surface mount device as this is typically referred to as a surface mount resistor.

Surprisingly one of the first metal film resistors was manufactured by Dubilier in the 1930s. However the range never became popular.

Metal film resistors basics

Metal film resistors are manufactured as fixed leaded resistors, although as already indicated, the same basic technology is used for surface mount resistors.

They are use a ceramic rod as the base for the resistor. This rod is of high purity so that contaminants do not affect the metal and cause ageing and a shift of resistance later.

A thin layer of a metal or resistance material is deposited onto the rod using vacuum deposition. Materials such as Nickel chromium, or occasionally tin, antimony, tantalum nitride or even gold with antimony may be used.

The thickness of the deposited material is typically around 50 to 250nm. The thickness governs the resistance, the thicker layers giving lower resistance. The thickness also affects the long term stability - thicker layers give a better long term stability figure.

To create the required resistance a helical cut is made in the metal film, effectively lengthening the conductor and also making the conduction path narrower. In this way the resistance is increased. This is generally done using lasers to create very accurate cutting of the metal film.

Once the basic rod is complete, caps are pushed onto the ends to connect with the conductor and these enable the wire connections to be made.

With this complete, the resistor is covered with protective layers are added that are baked on. These provide protection against mechanical damage as well as moisture or contaminant ingress. They also have a high electrical strength, providing insulation from the exterior.

Metal film resistor characteristics

The metal film resistors provide some of the highest specification resistors. Typical specifications may include:

Typical Metal Oxide Resistor Performance Figures
Parameter Typical Figures
Tolerance ranges available 0.1, 0.25, 0.5, 1 and 2%.
Temperature coefficient of resistance 50 and 100 ppm/°C
Maximum operating temperature 250 - 300°C

By Ian Poole

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