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]

Metal film resistor

 
Metal film resistor

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. Metal film resistor definition Metal film resistors are axial resistors with a thin metal film as resistive element. The thin film is deposited on usually a ceramic body. Construction 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 [… 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]

Fixed resistor

 
Fixed resistor

What is a fixed resistor? Fixed value resistors have a defined ohmic resistance and are not adjustable. Fixed resistors are the most commonly used resistors and in general one of the most used electronic components. Fixed resistors are available in axial leaded and surface mount packages as well as more customized packages depending on their application. While axial leaded resistors used to be the most used resistors, nowadays the advantages of surface mount devices make the SMD resistors the most popular. Fixed resistor definition A resistor having a fixed, defined electrical resistance which is not adjustable. In an ideal world a perfect resistor would have a constant ohmic resistance under all circumstances. This resistance would be independent of for example frequency, voltage or temperature. In practice no resistor is perfect and all resistors have a certain stray capacitance and inductance, resulting in an impedance value different from the nominal resistance value. The resistor materials have a certain temperature coefficient, resulting in a temperature dependency of the resistor value. The different resistor types and materials determine the dependency of the resistance value on these external factors. Depending on e.g. the required accuracy, power dissipation and noise requirements, the type and material of resistor are selected. Some common types of fixed resistors are displayed below. For a complete overview of the basic properties and applications of standard resistors, take a look at the article ‘What is a resistor‘. Identifying fixed value resistors In the following table an overview of general purpose resistors is given. The types listed here are among the most used resistors in general. The carbon film is the most common axial leaded resistor which is used for applications where a very good tolerance and temperature coefficient are not necessary. The metal film is the general axial leaded resistor of [… read more]