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...
What is a Foil Resistor? The metal foil resistor has the best precision and stability properties of all resistor types. The foil is made of an alloy of usually Nichrome with additives. It is mounted on a ceramic carrier with high heat conductivity. The foil has a thickness of only several micrometers. The desired resistance value is achieved by a photoetched resistive pattern in the foil. The metal foil resistor has a low Temperature Coefficient of Resistance (TCR), good long term stability, low noise, low capacitance, fast thermal stabilization and no inductance. The low TCR is one of the most important parameters that influence the stability. This means that the resistance value will vary only a small amount as the ambient temperature and the resistor’s internal temperature changes. Over a range from 0 till 60 deg C, a typical value for TCR is around 1 ppm per deg C. This depends on the construction (thermo-mechanical effects) and the properties of the foil material. In the planar foil the pattern is made parallel to reduce inductance (max 0.08 microH). Foil Resistor Definition A foil resistor is a high precision component to limit electric current. The opposition to current flow is provided by a very thin piece of metal. Metal Foil Resistor Characteristics The excellent properties for precision and stability are due to a combination of several characteristics, which are following from the design principles of the metal foil resistor. Temperature Coefficient, Resistance (TCR) – Foil resistors achieve a low TCR by taking advantage of two characteristics of the foil. The resistance of the foil naturally increases as temperature increases. The resistor is manufactured so that rising temperature causes compression of the foil. This makes the resistance drop as temperature rises. The total effect is one of very little resistance change as...