What is a carbon film resistor?

Answer 1

It's exactly what is says. A resistor made of a film of carbon deposited on a substrate. Cuts are made in the film to allow generation of different resistance values.

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A spiral is used to increase the length and decrease the width of the film, which increases the resistance. Varying shapes, coupled with the resistivity of carbon, (ranging from 9 to 40 µΩm) can make for a variety of resistances. Carbon film resistors feature a power rating range of 1/6 W to 5 W at 70 degree celcius. The resistance ranges from 1 ohm to 10M ohm. The carbon film resistor features an operating temperature of -55 to degree celcius to 155 degree celcius. It has 200 to 600 volts maximum working voltage range.

Thick film resistors became popular during the 1970s, and most SMD resistors today are of this type. The principal difference between "thin film" and "thick film resistors" isn't necessarily the "thickness" of the film, but rather, how the film is applied to the cylinder (axial resistors) or the surface (SMD resistors). In thick film resistors the "film" is applied using traditional screen-printing technology.

Thin film resistors are made by sputtering the resistive material onto the surface of the resistor. Sputtering is a method used in vacuum deposition. The thin film is then etched in a similar manner to the old (subtractive) process for making printed circuit boards: ie the surface is coated with a photo-sensitive material, then covered by a film, irradiated with ultraviolet light, and then the exposed photo-sensitive coating, and underlying thin film, are etched away.

Thin film resistors, like their thick film counterparts, are then usually trimmed to an accurate value by abrasive or laser trimming.

Because the time during which the sputtering is performed can be controlled, the thickness of the film of a thin-film resistor can be accurately controlled. The type of the material is also usually different consisting of one or more ceramic (cermet) conductors such as tantalum nitride (TaN), ruthenium dioxide (RuO2), lead oxide (PbO), bismuth ruthenate (Bi2Ru2O7), nickel chromium (NiCr), and/or bismuth iridate (Bi2Ir2O7).

By contrast, thick film resistors may use the same conductive ceramics, but they are mixed with sintered (powdered) glass and some kind of liquid so that the composite can be screen-printed. This composite of glass and conductive ceramic (cermet) material is then fused (baked) in an oven at about 850 °C.

Traditionally thick film resistors had tolerances of 5%, but in the last few decades, standard tolerances have improved to 2% and 1%. But beware, temperature coefficients of thick film resistors are typically ±200 or ±250 ppm/K, depending on the resistance. Thus a 40 kelvin (70° F) temperature change can add another 1% variation to a 1% resistor.

Thin film resistors are usually specified with tolerances of 0.1, 0.2, 0.5, and 1%, and with temperature coefficients of 5 to 25 ppm/K. They are usually far more expensive than their thick film cousins. Note, though, that SMD thin film resistors, with 0.5% tolerances, and with 25 ppm/K temperature coefficients, when bought in full size reel quantities, are about twice the cost of a 1%, 250 ppm/K thick film resistors.