Differences Between Thick and Thin Film Resistors
The resistance value of a linear resistor is relatively stable, and its volt-ampere characteristic curve shows a linear relationship. According to the manufacturing process, it can be divided into two types of resistors, which are thick and thin film resistors. The definitions of thin film and thick film are made to distinguish them according to the film thickness of the resistor body. Thick film resistors adopt the screen printing process, and the film thickness is more than 1 micron while thin film resistors adopt the sputtering process, and the film thickness is extremely thin, which can be made to be dozens of nanometers.
Understanding the thick film resistor
The main film components of a thick film resistor include a ceramic base, electrodes, a film resistive body, and a protective layer. The ceramic substrate material is generally made of aluminum oxide, which plays the roles of insulation and mechanical support. When manufacturing the resistor, the front electrodes and back electrodes are first printed on the insulating substrate. Then, the resistive element is printed to form an circuit connection with the electrodes. Next, each film layer of the protective layer is printed, and finally, the electrodes are formed through electroplating.
Understanding the thin film resistor
The earliest thin film resistors were carbon film resistors, which were prepared by the high-temperature decomposition of hydrocarbons in a film vacuum environment. Carbon film resistors are simple to manufacture and inexpensive, but their accuracy and temperature coefficient of resistance (TCR) are relatively poor. In the 1950s of the last century, metal film resistors and metal oxide film resistors were developed. Among them, metallic film resistors are resistors obtained by atomizing and spraying salt solutions such as tin and antimony onto a hot ceramic framework, followed by hydrolysis and film deposition process to form a metal oxide film. As it is an oxide by nature, its stability at elevated temperatures is rather high. It has good resistance to thermal shock and can handle a relatively large load. In addition, its accuracy and TCR - related performance are fairly good. However, it is prone to electrolysis under direct current, which causes the reduction of the oxide and makes its performance not very stable. Metal film resistors are film devices made by using vacuum coating technology to deposit metals or alloys on the surface of the substrate, and they exhibit good performance. With the development of the electronics industry, electronic components have gradually evolved towards being chip-shaped and miniaturized.
Differences between thick and thin film resistors
Compared with thick film resistors, thin film resistors have higher precision (up to 0.01% or even lower. Currently, the precision of high-precision thick film resistors can reach 0.1%, and the common precision is 1%). The temperature coefficient of resistance (TCR) of thin film resistors can reach 5 ppm/°C, while that of thick film resistors can reach 100 ppm/°C. Additionally, due to the differences in process and materials between the two, thin film resistors can have lower current noise. Therefore, thin film resistors have significant advantages in application fields such as signal sampling, signal processing, and precise measurement.
Why do thin film resistors have superior performance in terms of precision applications and TCR (temperature coefficient of resistance) compared to thick film resistors?
According to the manufacturing process of resistors, whether it is a thick film or a thin film resistor, the resistance value is detected in real-time during laser trimming process. As long as the measurement accuracy of the laser trimming equipment meets the performance requirements, the precision of thick film and thin film resistors should not be too different in theory. However, there is indeed a difference of several orders of magnitude between the two. So, where does the problem lie? Considering the factors that affect the precision of resistors, precision is closely related to the resistive material and the manufacturing process of the resistor.
When comparing the resistor materials of the two, the resistive layer of a thick film resistor is composed of a glass phase and a conductive phase. When the glass phase is affected by external stress, it may have an impact on the conductive surface network of the thick film resistor, resulting in a resistance value drift. In contrast, the resistive body of a thin film resistor is mainly composed of metals or metal alloys, and its environmental stability is much better. From the perspective of the processing film technology, the thin film formed by sputtering is very uniform. Thick film resistors, on the other hand, use screen printing. The film thickness of thick film resistors is dozens or even hundreds of times that of thin film resistors, and the uniformity of the film thickness in screen printing is poor, making it inherently difficult to achieve high-precision control. Under the current resistor manufacturing process, the diameter of the laser source for thin film resistors is relatively small. Therefore, in terms of processing precision, thin film resistors can also reach a higher level (this is a difference in equipment, and thick film resistors can also use this method to achieve higher precision).
In addition, during laser trimming, the laser will cause the resistive film to heat up. At this time, there will be a difference between the measured resistance value and the resistance value after the resistor cools down, which has a relatively large impact on the precision of thick film resistors and this is the effects of laser trimming.
TCR is a parameter that characterizes the temperature stability of a resistor. It is related to the inherent performance characteristics of the film material itself. Microscopically, the resistance of a metal is a characteristic resulting from the scattering caused by the interaction between free electrons and the thermal vibration of the crystal lattice. The higher the range of temperature, the more intense the vibration of the crystal lattice, so the resistance will also increase, that is, the TCR of a metal is positive. In thin film resistors, there are both continuous and discontinuous thin films. In the continuous power structure, its TCR is consistent with the temperature coefficients of resistance characteristics of the above-mentioned metal. However, the discontinuous thin film conducts electricity through the tunneling effect. As the range of temperature rises, electrons will gain more kinetic energy and are more likely to conduct electricity. Therefore, as the temperature rises, its resistance decreases instead, that is, the TCR is negative. Under the interaction of these two situations, the overall TCR of thin film resistors will perform relatively low.
Why does the thin-film resistor have lower current noise?
For thick-film resistors, the resistive material is made of metal and glass materials. Since glass materials are non-conductive, the direction of the current is changed by these glass particles, thus resulting in relatively high electrical noise. However, for thin-film resistors, from a microscopic structure perspective, the thin film is relatively uniform and continuous. When electrons move, the resistance they encounter is relatively small, and the current path is relatively smooth. Therefore, the noise performance is better.
Application Scenarios
Thin-film resistors are used in high-precision circuits, measuring equipment, amplifiers, sensors, signal processing, and other applications where high precision and stability are required. Thick-film resistors are used in general electronic devices, power supplies, household appliances, and other applications where the requirements for precision and stability are relatively low.
Cost Impact
The manufacturing process of thin-film resistors is complex and the material cost is high, so they are relatively expensive. The manufacturing process of thick-film resistors is simple and they can be mass-produced, resulting in lowest costs.
Electrical Properties
Since the film layer of thin-film resistors is relatively thin, they are easily damaged by mechanical stress and have poor impact resistance. The film layer of thick-film resistors is thicker, so they are more resistant to mechanical stress and are suitable for harsh environments.
