Performance and application of high precision resistors

The development and use of new technologies in the resistor industry have led to a sharp reduction in the size of resistors. New market demands have also driven electronic component manufacturers to develop products with better performance and stability. As the use of various types of precision resistors becomes more and more widespread, the performance of precision resistors has become more important. The main parameters of precision resistors include temperature coefficient (TCR), power coefficient (PCR), long-term stability, anti-static ability (ESD), thermal electromotive force (EMF), noise, etc.
The improvement of the overall performance of the system is achieved through each electronic component or subsystem. The overall performance is determined by the weakest link in the circuit system. Each component brings some performance limitations to the overall circuit system. In particular, short-term overload, long-term stability, frequency response, and noise. In the discrete resistor industry, there are four popular resistor technologies, wirewound resistors, thick film resistors, thin film resistors, and foil resistors, providing customers with solutions with different performance requirements and different budgets.
Table 1 briefly summarizes the advantages and disadvantages of various resistor technologies, thermal effects, mechanical stress, and electrical characteristics.
Stress, whether mechanical or thermal, causes changes in the electrical properties of resistors. If the resistor changes shape, length, configuration, or molecular structure due to stress, the electrical properties of the resistor will also change.
When current flows through the resistor element, it generates self-heating, and changes in temperature cause the material to expand or contract mechanically. The ideal resistor element has the ability to automatically balance these phenomena. The stability enhancement system can maintain the integrity of the physical properties of the resistor element through the manufacturing process and eliminate the need to compensate for self-heating and stress during the use of the resistor.
We will compare the characteristics of different resistor technologies and discuss the use of each type of resistor in precision circuits.
Foil Resistors
Special foil resistor alloys (nickel/chromium and additives) are bonded to a special ceramic matrix with controlled resistance to balance the stresses caused by heat and mechanical stress. The resistor path pattern is laser etched into the foil resistor alloy. This process uniquely combines the important characteristics of low temperature drift, long-term stability, non-inductive, insensitive to static electricity, low capacitance, fast thermal stability, and low noise in a single resistor technology.
These features bring high stability and reliability to the circuit system. No compromise between resistor accuracy, stability and response speed is considered. To obtain precise resistance values, Bulk Metal Foil metal foil is balanced and selectively eliminates internal shorting strips. When dealing with high resistance requirements, the area is selectively cut in known resistance increments to increase the resistance value in small steps.
Standard temperature coefficient ± 1 ppm/°C 0 °C to + 60 °C (0.05 ppm/°C Z-Foil), obtained from the foil resistor alloy and the thermal and mechanical stress balanced matrix it is combined with (Table 1).

The parallel resistor path design in the metal foil plane reduces inductance, with a maximum inductance of 0.08 μH. Maximum capacitance is 0.5 pF. The response time of a 1-kΩ resistor is less than 1 ns up to 100 MHz. Rise time is determined by the resistor value, but higher or lower values ​​are only slightly slower than intermediate values. No ringing is particularly important in high-speed switching circuits, such as signal converters.
The DC resistance of a 1-kΩ Bulk Metal Foil can be compared to its AC resistance at 100 MHz using the following formula:
AC resistance/DC resistance = 1.001
Foil Resistor Technologies produces resistors with special specifications that were previously unavailable and desirable. These include temperature drift of 0.2 ppm/°C, accuracy as low as ± 0.001 %, 1000-hour load life stability of ± 0.005 % (50 ppm), and static resistance of at least 25 kV.
How good does a good product really need to be?
Of course, not every circuit engineer designs requires overall high performance. In many applications, lower-performance resistors will suffice, so the answers fall into four categories.
1. Metal foil resistors can improve the overall performance of a device.
2. One or more metal foil resistors can achieve the best performance of a device.
3. Metal foil resistors can improve the development of circuit technology and improve many favorable properties of precision resistors.
4. Clearly plan to use precision resistors for long-term considerations (for example: metal foil resistors are more cost-effective than adding expensive devices to improve circuit performance slightly to maintain circuit accuracy).
In the second category, for example, a single parameter must be weighed against the overall design economy. Using a higher performance resistor can save costs because the compensation circuit can be reduced (and the cost of related components and their installation is saved). Improving the precision of the resistor is more cost-effective than improving the device, because improving the device to improve some performance is more expensive than precision resistors.
Precision wirewound resistors
Wirewound resistors are generally divided into power wirewound resistors and precision wirewound resistors. Power wirewound resistors will change greatly during use and cannot be used in applications that require precision performance of resistors. Therefore, we will not discuss power wirewound resistors.
During the manufacturing process of wirewound resistors, the inner surface of the resistor wire (the side close to the insulating skeleton) is squeezed, and its outer surface is subjected to tension. Permanent deformation occurs.
Deformation caused by this process and during subsequent annealing of the wire is artificial, as opposed to elastic or reversible deformation. Unpredictable permanent mechanical deformation causes random changes in the electrical properties and resistance of the wire. The result is a wide range of performance parameters.
Wirewound resistors are made of wire wound on an insulating skeleton. Wires of different diameters and lengths provide predictable resistance and characteristics. Precision wirewound resistors have better antistatic properties and lower noise than thin film and thick film resistors. Wirewound resistors also have lower temperature drift and better stability.
Wirewound resistors can have an initial accuracy of ± 0.005 %. Temperature drift can be as low as 3 ppm/°C typical; but for low resistance wirewound resistors it is generally 15 ppm/°C to 25 ppm/°C. Thermal noise is low, and tracking temperature drift outside the operating temperature range may also reach ± ​​2 ppm/°C. It should be clear that wirewound resistors do not have SMD packaging products (SMD is not molded, and some molded surface mount wirewound resistors are not SMD resistors), so wirewound resistors cannot be used in situations where the weight and volume of the resistor are required.
However, due to the winding structure of wirewound resistors, wirewound resistors are inductive, and similar windings produce capacitance. Special winding technology may be used to reduce inductance to improve the response time during the use of the resistor. Due to the inherent inductive and capacitive design, wirewound resistors do not have good high-frequency characteristics. Especially in applications above 50KHz. Due to inductance and price factors, precision wirewound resistors are currently on the verge of being eliminated.
Thin film resistors
Thin film resistors are made by evaporating a film material with a certain resistivity of 50 to 250 angstroms thick onto a ceramic substrate using a method similar to evaporation [1 angstrom (A) = 10 nanometers]. Thin film resistors can produce higher resistance values ​​in a given area than wirewound resistors and foil resistors and are cheaper. This makes thin film resistors a more economical choice in applications where medium-precision high-resistance resistors are required and space efficiency is considered.
Thin film resistors are sensitive to temperature. The appropriate thickness of the film can be selected to produce a variety of resistance values, but a specific film can only produce resistors with a certain resistance range. Therefore, films of various thicknesses are used to produce resistors with various resistance ranges.
The stability of the film is affected by the increase in temperature. The change process of the aging stability of the film depends on the thickness of the film to achieve the specified resistance value, so the resistance range is constantly changing. Chemical/mechanical aging also includes oxidation of the resistor alloy caused by temperature increase.
The temperature drift of the film of appropriate thickness can also have an adverse effect on the resistance value. High-resistance thin film resistors have a greater deterioration rate. Because the thinner the film, the more susceptible it is to oxidation. Because thin film resistors are very thin, they are easily affected by static electricity.
In addition, because the metal element part is very small, thin film resistors are also very susceptible to corrosion caused by moisture. When water vapor passes through the resistor package, it brings impurities and develops chemical corrosion, causing the resistor film to fail within a few hours of applying a DC low voltage load.
Through long-term multi-layer film deposition, high-precision resistance adjustment and post-screening, the best precision thin film resistors can also achieve a temperature drift of ±2ppm/°C and an accuracy of ±0.01%, as well as good long-term stability. However, the power is not high, the low resistance part has poor performance, it is not anti-static, the power coefficient is poor, it is difficult to meet the small batch supply, and the consistency of different batches is not good.
Thick Film Resistors
In the previous discussion, we explained that wirewound resistors cannot be used in chip applications due to size, packaging and weight. In less precise applications than wirewound resistors, thick film resistors are more commonly used because thick film resistors have denser resistance values ​​(high resistance/small size) and are cheaper.
Thick film resistors have faster frequency response, similar to thin film resistors and foil resistors, but among various resistor technologies, thick film resistors are the noisiest. When used in less precise applications, thick film resistors will be discussed and widely used in various types of circuits, including less precise parts of high-precision circuits.
Thick film relies on particle-to-particle contact on a glass matrix to form a resistance path. These contact points form the entire resistance. However, these particles are blocked by thermal stress during the use of the resistor. Because they have many parallel paths, the resistance path will not be broken. However, the resistance value will increase over time and as the temperature rises. Therefore, thick film resistors are not as stable as other resistor technologies.
The particle structure also means that thick film resistors have a high noise coefficient, because the aggregation and release movement of the resistor electrons through this structure generates relatively large noise. For a given size resistor, the higher the resistance value and the lower the metal content, the greater the noise and the less stable it is. The glass structure of thick film resistors forms a glass-like protective coat to protect the resistor during processing, which also makes thick film resistors more resistant to moisture than thin film resistors.
Through continuous improvement of thick film resistor pastes, the most precise thick film resistor technology has been able to achieve a temperature drift of ±5ppm/°C, and even by using multiple thick film resistor chips that can compensate for each other, it can eventually reach a temperature drift of ±2ppm/°C. Its highest accuracy can also reach ±0.01%. Precision thick film resistors are the mainstream technology in high-voltage, high-resistance, and high-precision applications. The disadvantage of thick film resistors is that it is difficult to achieve high precision and low temperature drift in the low resistance part, the noise index is not good, and the long-term stability is generally worse than other precision resistors.