Thin and thick film

 
Thin and thick film

What are thin and thick film resistors? Thin and thick film resistors are the most common types in the market. They are characterized by a resistive layer on a ceramic base. Although their appearance might be very similar, their properties and manufacturing process are very different. The naming originates from the different layer thicknesses. Thin film has a thickness in the order of 0.1 micrometer or smaller, while thick film is around thousands time thicker. However, the main difference is method the resistive film is applied onto the substrate. Thin film resistors have a metallic film that is vacuum deposited on an insulating substrate. Thick film resistors are produced by firing a special paste onto the substrate. The paste is a mixture of glass and metal oxides. Thin film is more accurate, has a better temperature coefficient and is more stable. It therefore competes with other technologies that feature high precision, such as wirewound or bulk metal foil. On the other hand, thick film is preferred for applications where these high requirements are not critical since prices are much lower. Thin Film Technology The resistive layer is sputtered (vacuum deposition) onto a ceramic base. This creates a uniform metallic film of around 0.1 micrometer thick. Often an alloy of Nickel and Chromium is used (Nichrome). They are produced with different layer thicknesses to accommodate a range of resistance values. The layer is dense and uniform, which makes is suitable to trim the resistance value by a subtractive process. With photo etching or by laser trimming patterns are created to increase the resistive path and to calibrate the resistance value. The base is often alumina ceramic, silicon or glass. Usually thin film is produced as a chip or smd resistor, but the film can also be applied onto a cylindrical base [… read more]

Carbon film resistor

 
Carbon film resistor

What is a carbon film resistor? Carbon film resistors are a fixed form type resistor. They are constructed out of a ceramic carrier with a thin pure carbon film around it, that functions as resistive material. Advantages and drawbacks Carbon film resistors are a significant improvement on carbon composition. However, in comparison to metal film and metal oxide film, the commercially available range steadily decreases. Metal and oxide film are not more expensive to produce, and have overall better properties. Carbon film resistor applications Typical use for carbon film resistors is in high voltage and temperature applications. Operating temperatures are up to 15kV with a nominal temperature of 350°C. Examples are high voltage power supplies, radar, x-rays and laser. Manufacturing Carbon film resistors are made with a deposition process. At high temperature and under a high pressure, a ceramic carrier is held in hydrocarbon gas. The gas (methane or benzene) is cracked at a temperature of 1000°C. The crystalline carbon is pyrolytically deposited on the ceramic substrate. Because of the precise distribution of the pure graphite without binding, these carbon resistors have a low noise. The desired resistance value can be obtained by choosing the right layer thickness, and by cutting a spiral shape in the carbon layer. The helical cut in the film increases the length of the current path. By decreasing the pitch of the helix, the length of the resistive path increases, and therewith the resistance value increases. Furthermore, by fine tuning the cutting of the spiral the resistor can have a higher accuracy of resistance value. Typical tolerance values for carbon film resistors are 2, 5, 10 and 20%. Because of the use of pure carbon, the carbon film resistor has a higher negative temperature coefficient than carbon composition. The resistive temperature coefficient lies between 2.5×10^-4 [… read more]