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 with axial leads. In this case, more often the term metal film resistor is used.
Thin film is usually used for precision applications. They feature relatively high tolerances, low temperature coefficients and low noise. Also for high frequency applications thin film performs better than thick film. Inductance and capacitance are generally lower. The parasitic inductance of thin film can be higher if it is executed as a cylindrical helix (metal film resistor). This higher performance comes with a cost, which can be factors higher than the price of thick film resistors. Typical examples where thin film is used are medical equipment, audio installations, precision controls and measurement devices.
Thick Film Technology
In the 1970s thick film started to gain popularity. Today, these are by far the most used resistors in electrical and electronic devices. They come usually as chip resistor (SMD), and have the lowest cost compared to any other technology.
The resistive material is a special paste with a mixture of a binder, a carrier, and the metal oxides to be deposited. The binder is a glassy frit and the carrier exists of organic solvent systems and plasticizers. Modern resistor pastes are based on oxides of ruthenium, iridium and rhenium. This is also referred to as a cermet (Ceramic – Metallic). The resistive layer is printed onto a substrate at 850°C. The substrate is often 95% alumina ceramic. After the firing of the paste on the carrier, the film becomes glasslike, which makes it well protected against moisture. The complete firing process is schematically depicted in the graph below. The thickness is in the order of 100 micrometer. This is approximately 1000 times more than thin film. Unlike thin film, this process is additive. This means that the resistive layers are added sequentially to the substrate to create the conducting patterns and resistance values.
The temperature coefficient typically ranges from 50 ppm to 200 ppm/K. Tolerances are between 1% and 5%. Because costs are low, thick film is generally preferred in case no high tolerances, low TCR or high stability is required. Therefore, these resistors can be found in almost any device with an AC plug or a battery. Advantages of thick over thin technology are not only lower cost, but also the ability to handle more power, provide a wider range of resistance values and withstand high surge conditions.
Thin versus thick film; what are the differences?
In the table below the main differences between the two technologies are listed. The components may look the same, but the way of producing and the electrical properties are definitely different.
|Film thickness (µm)||±0.1||±100|
|Manufacturing process||Sputtering (Vacuum Deposition)||Screen and stencil printing|
|Trimming||Abrasive or Laser, for complex patterns photo etching||Abrasive or Laser|
|Resistive Material||Uniform metallic film, usually Nichrome||Paste of Ruthenium Oxide or other alloy.|
|Resistance Values (Ω)||0.2 – 20M||1 – 100M|
|Tolerance (%)||±0.1 – ±2||±1 – ±5|
|Temperature Coefficient (ppm/K)||±5 – ±50||±50 – ±200|
|Maximum Operating Temperature (°C)||155||155|
|Maximum Operating Voltage Umax (V)||50 – 500||50 – 200|
|Current Noise (µV/V)||<0.1||<10|
|Power Rating P70 (W)||1/16 – 1||1/16 – 1/4|
|Stability at P70 (1000h) ∆R/R %||±0.15 – ±0.5||±1 – ±3|
|Moisture resistance||Thick film is more resistant to moisture, since they are glass like.|
|High frequency behavior||Thin film features lower parasitic inductance and capacitance. However, inductance may be high if thin film is executed with a cylindrical shape that is spiral cut.|
|Typical areas of use||High precision: Measuring or monitoring equipment, medical or audio applications, precision controls.||Very wide, almost any electrical device with battery or AC connection. The average PC contains well over 1000 thick film chip resistors.|
|Cost||More expensive than thick film.||Lowest cost resistor type on the market. Preferred solution if performance requirements are low.|
in analogue circuits