Metal Film Resistors
Metal film resistors have a thin metal layer as resistive element on a non-conducting body. They are amongst the most common types of axial resistors. Other film type resistors are carbon film and thick and thin film resistors. In most literature referrals to metal film, usually it is a cylindrical axial resistor. However, thin film chip resistors use the same manufacturing principle for the metal layer. The appearance of metal film resistors is similar to carbon film resistors, but their properties for stability, accuracy and reliability are considerably better.
The resistive element is a thin metal layer that is usually sputtered (vacuum deposition) on a cylindrical high purity ceramic core. Sometimes other techniques than sputtering are used. The deposited metal is artificially aged by keeping it for a long period at a low temperature. This results in a better accuracy of the resistor. The resistance material is often nickel chromium (NiCr), but for special applications also other alloys are used such as tin and antimony, gold with platinum and tantalum nitride. The stability and resistance are strongly dependent on the thickness of the metal film (50-250 nm). A larger thickness of the layer results in a better stability and a lower resistance value. On both ends a metal cover is pressed with the connection leads. After this, the desired resistance is achieved by cutting a spiral shaped slot in the thin metal layer. This is usually done by lasers, while in the past sandblasting and grinding techniques were used. Carbon film resistors use the same technique to trim the resistance. The resistor is covered with several coating layers that are baked individually. The coating protects against moisture and mechanical stresses and preferably has a high dielectric strength. The resistor value is marked by color code bands or with text. The metal film resistors are available with tolerances of 0.1, 0.25, 0.5, 1 and 2%. The Temperature Coefficient of Resistance is usually between 50 and 100 ppm/K.
Metal film resistors have good characteristics for tolerance, stability and TCR. Furthermore, the resistors feature low noise properties and a high linearity because of a low voltage coefficient. Therefore, in circuits where tight tolerance, low temperature coefficient and low noise properties are important, often metal film resistors are used. Examples of applications are active filters or bridge circuits.
Reliability of metal film resistors
For a good reliability, metal film resistors are normally operated between 20 and 80 percent of their specified power rating. Reliability is generally increased by derating 50%. However, in very specific situations, at lower than 20% of the power rating in a humid environment the reliability decreases. Compared to wirewound or carbon composition resistors, these resistors are easier damaged by voltage surges and power overloads.