Beginners Guide to Precision Resistors

09 March

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Precision resistors refer to resistors whose resistance value tolerance, thermal stability (temperature coefficient), and distributed parameters(distributed capacitance and distributed inductance) all meet certain standards.

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I What is a Precision Resistor?

Precision resistors refer to resistors that meet the higher standards of resistance accuracy, temperature coefficient, load life, and long-term stability under the same resistance technology and installation method. The long-term stability of precision resistors is a very important indicator, and high precision without stability is meaningless.

Precision resistors are often associated with high-precision resistors. Here, &#;accuracy&#; represents the accuracy of resistance values. In fact, accuracy is affected by many factors, which are collectively referred to as "stress". Stress comes from many aspects, such as changes in ambient temperature, self-heating generated by the resistor itself after being powered, pressure or tension from the PCB, moisture from the external environment, and even corrosive gases, as well as welding, pulse, overload, static electricity, radiation, etc. All these "stresses" will change the resistance value of the resistor, which will also affect the resistance accuracy of the resistor.

For example, the accuracy of the resistor when it leaves the factory is ±0.01%, which may cost a lot of money. However, after several months of storage or hundreds of hours of load, the resistance may change by more than ±300ppm or more. Besides, though the resistance is within the nominal accuracy range during incoming inspection, it will exceed the range after the resistor is soldered to the PCB. Also, such as humidity, static electricity, etc. will cause irreversibly changes to the resistance value.

Figure 1. Failure and Stress Distributions in Power Electronic Systems

It should be emphasized that stability should be considered in the first place, and unilaterally pursuit of high precision is undesirable.

So what is a precision resistor? The answer is a resistor with both stability and accuracy.

II Difference between Precision Resistors and Ordinary Resistors

1.

D

ifference in Material Performance

Within a certain temperature range, the tolerance of precision resistors is much smaller than that of ordinary resistors.

2. Substantial Difference

The temperature stability of the materials used to make precision resistors is higher than that of ordinary resistors. For example, the temperature tolerance of ordinary resistors is 1% - 5%, while the tolerance of precision resistors is only 0.1% or less.

Explanation of tolerance: For example, if a resistor is of 0.1% accuracy, it actually indicates that at normal temperature (such as 10&#; -35&#;), all tolerances of the resistor should be guaranteed to be within 0.1% within 1 year), which is a composition tolerance in essence.

3. Difference in Cost

Because precision resistors use materials with higher temperature stability, they are much more expensive than ordinary resistors.

4

. Difference in Digital Models

Ordinary resistors are labeled with 3 digits, the first two are valid numbers, the latter is the power. For example: 152 means 15 * 102= ohm. Precision resistors are labeled with 4 digits, the first three are significant figures, the latter is the power, for example, means 150 *102 = ohm.

The Three and Four Digit System

5

. Difference in Color Wheel Reading

The color wheel reading of ordinary resistors and precision resistors are almost the same. The first two digits of ordinary resistors are numbers. For precision resistors, the first three digits are numbers, the fourth ring is multiplying power and the fifth wheel is the tolerance, so the precision resistor is also called the five-color ring resistor.

Figure 2. A 5-Band Resistor

6

. Difference in Application

Compared to ordinary resistors, precision resistors are more costly, so they’re mainly used in high-precision circuits of signal conversion, bridge circuits, sampling circuits, precision voltage division circuits, and so on.

In a circuit, resistors are generally used for current limiting and voltage dividing. The precision of the resistance need not be considered in the current limiting part, while for the voltage dividing part, there are two situations:

(1) If the accuracy of the voltage does not affect the subsequent circuit too much, an ordinary voltage dividing resistor is chosen;

(2) If the accuracy of the voltage has a great impact on the subsequent circuit. Here, the voltage dividing resistor should be of high precision.

Figure 3. Simple Resistive Voltage Divider

In short, precision resistors can replace ordinary resistors, but ordinary resistors cannot replace precision resistors. If you replace precision resistors with ordinary resistors, it will cause differences in the results of experiments or tests.

III Classification of Precision Resistors

We often consider the tolerance value when using the precision resistors, but the premise is that we have selected the precision resistor of a certain technical material. Therefore, there are three types of precision resistors: metal film precision resistors, wire wound precision resistors, and metal foil precision resistors.

1. Metal Film Precision Resistors

This kind of precision resistor has high precision, but the resistance temperature coefficient and the distribution parameter index are slightly lower. It is usually cylindrical and mainly used in communication equipment, medical electronic equipment, test and measuring equipment, automotive electronics, industrial products, etc.

Figure 4. Construction of A Metal Film Resistor

2. Wire-wound Precision Resistors

This type of precision resistor is made of constantan, manganin, or nichrome alloy wire wound on a ceramic skeleton. The surface is often coated with protective paint or glass glaze, and there are mainly two types: fixed type and adjustable type.

The packaging materials of such resistors include insulating varnish, silicone resin, coating, ceramics, aluminum case, etc. Most precision wire-wound resistors are packaged in silicone resin because this package does not have a high-temperature sintering process, so the electrical properties of the winding material will not be affected.

Because it is wound on the ceramic skeleton, this resistor has high-temperature resistance, good thermal stability, small temperature coefficient, high power, high resistance accuracy, high-temperature coefficient index, and the working temperature is up to 300°C. The wire-wound precision resistor is generally cylindrical, flat cylindrical, and rectangular, which is mainly used in precision instruments, electronic equipment, and other AC and DC circuits.

Figure 5. Basic Construction of a Wire Wound Resistor

3. Metal Foil Precision Resistors

A metal foil with known and controllable characteristics is laid on a special ceramic substrate, and then the ultra-precise photoetching process is used for the resistor circuit.

This process combines the important characteristics of low TCR, long-term stability, no inductive reactance, no ESD induction, low capacitance, fast thermal stability, and low noise in one resistor technology. The precision, temperature coefficient and distribution parameters of the resistance value of this kind of precision resistor are all very high.

Besides, this kind of precision resistor is often in the shape of a square or a sheet, which is widely used in aerospace, marine inertial navigation, power distribution control, precision measurement, and other systems. They're also used as current and voltage standard of comparison, constant current and constant voltage circuit components, and various shunt and voltage dividing proportional networks.

Figure 6. Cutaway View of A Metal Foil Resistor

It is worth noting that the precision resistor mainly uses thin film and foil technology. Among them, the electrical performance of the foil resistor is more excellent, which can achieve almost zero temperature drift and 0.001% accuracy. Therefore, compared to other chip resistors its advantages are very obvious, and its considered to be the closest to the ideal resistance. on account of this, it is only used in higher-end products or in places where the requirement for resistance is very harsh.

IV Main Parameters of Precision Resistors

1. Accuracy

Accuracy is an important indicator for incoming inspection. Do all precision resistor manufacturers do a 100% accuracy test before the products leave the factory? The answer is negative. Although there are many different processes and materials for precision resistors, almost all precision resistors need to be adjusted to achieve "high precision". For example, laser trimming is performed on the surface of the precision thin-film resistors, and precision foil resistors are adjusted by the cutting-off method.

The process of adjusting resistance is actually a measurement process, but the adjusted product is not a finished product, and it must go through some subsequent processes such as packaging. This process may affect the resistance of the resistor. Also, the accuracy of the measuring instrument and the correct measurement method is also very important, especially for precision resistors with more than one ten-thousandth accuracy, as well as milliohm resistors.

Figure 7. Commonly Used Laser Trim Kerf Shapes

2. Temperature Excursion

In the specification of the resistor, we often see only one temperature drift index, such as ± 5ppm/°C. But in the actual situations, it&#;s very likely that this index does not cover the working temperature range required by the product, which means that the temperature excursion of the same resistor is different in different temperature intervals.

The specifications of most precision resistor manufacturers do not clearly define the temperature drift index in different temperature ranges. It&#;s a common phenomenon that some manufacturers only select the best &#;curve&#; in the entire operating temperature range as the temperature drift index in the specification.

Another fact is that the temperature drift index is difficult to be measured 100% before the products leave the factory. The measurement is of high cost, and accurate measurement may be destructive to the product itself. For example, for chip resistors, it’s generally recommended to solder it on the PCB before the temperature drift measurement. In addition, the stress caused by the temperature drift measurement will change the resistance value.

The control of temperature excursion is mainly based on the resistance material itself and the manufacturing process. For example, temperature drift of precision foil resistors can achieve near zero by using special low-temperature drift alloys and stress compensation methods.

3. Load Life

Load life is related to three factors, power, temperature, and time. Derating can help slow down the change in resistance.

Figure 8. Temperature Derating Curve

The active period of the change in resistance value is often the first several hundred hours in the using process, and the resistance tends to stabilize as the use time increases. This is because the resistance element itself stabilizes, or the stress between the resistance element and the substrate is gradually released over time.

The index of load life can only be monitored by sampling test because this kind of test requires at least 1,000 hours and even 10,000 hours for testing aerospace applications.

Beyond that, the test is destructive. Power training of precision resistors before use can effectively accelerate the aging of the resistor and stabilize it, but it will change the resistance of the resistor.

4. Shelf Life

Shelf life is used to examine the stability of the resistance value under standard or specified storage conditions. As with the life of the load, the longer the resistor is stored, the more stable the resistance will be. This is why some precision instrument manufacturers do not use precision resistors immediately but store them for months or even years before using them.

Of course, storage will change the resistance value, but this change is getting smaller and smaller. So for the old batch of resistors, as long as the accuracy is within the nominal value and the solder pins are not oxidized, its stability is better than the new batch of resistors.

Figure 9. Basic Number of the Shelf-life for Different Technologies

We should pay special attention to humidity control in the storage of resistors. Humidity will have a great impact on the resistance of any resistor. For example, an electrolyte will be formed in the resistance layer of all kinds of film resistors once moisture enters, which will seriously affect the resistance.

In addition to moisture, there are various corrosive substances in the air, such as sulfur and salt spray. The standard resistor used as a metering application will be completely sealed after being filled with oil, thereby isolating the impact of external environmental stress on the resistance value and reducing the annual change rate of the resistance.

V Pros and Cons of Mainstream Precision Resistor Technologies

1. Precision Thick Film Resistor

Through continuous improvement of thick-film resistor pastes, the most precise thick film resistor technology can already achieve a temperature drift of ±5ppm/°C, and even by using multiple thick film resistor chips that can compensate each other, it can finally reach a temperature drift of ±2ppm/°C. Its highest accuracy can also reach ±0.01%.

In the application of high voltage, high resistance, and high precision, precision thick film resistor is the mainstream technology. The disadvantages of thick film resistors are that it is difficult to achieve high precision and low-temperature drift in low resistance parts, and the noise label is not good. The long-term stability is generally worse than other precision resistors.

Figure 10. A Precision Thick Film Resistor

(1) Main Performance Parameter Range

Size Specifications: , , , , , etc.

Resistance Range: 10Ω- 1MΩ

Resistance Accuracy: + 0.1%, + 0.25%, + 0.5%

Rated Power: 1/20, 1/16, 1/10, 1/8, 1/4 (W)

Temperature Coefficient of Resistance (TCR): + 50ppm/&#;

ESD(Electro-Static discharge) Withstands: ESD2KV

(2) Advantages

Low manufacturing cost and high-cost performance;

In the shape of chips, suitable for SMT placement;

The outer glass protects the structure and has good moisture resistance.

(3) Disadvantages

Large temperature coefficient of resistance value;

Poor electric shock performance and low power;

Poor resistance value stability and loud noise.

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In the glass phase of the thick-film resistor, the particles are connected to each other. Part of the resistance path will be blocked by thermal stress during the use of the resistor, the resistance will increase, and its stability will become poor.

The particle structure will result in a high noise factor. The higher the resistance value, the lower the metal ratio; the greater the noise is and the more unstable the resistance will be. Among various precision resistors, thick film resistors are the noisiest.

Figure 11. Experimental results for resistance noise spectrum

(&#;&#;before pulse stressing, &#;&#;after pulse stressing) for catastrophically failed thick&#;film resistor with the initial resistance

R = 111.92 kΩ

(4) Application

Widely used in general precision circuits, or local circuit parts requiring less precise in high precision circuits.

2. Precision Thin Film Resistors

The technical development of precision thin-film resistors represents the precision resistor technology that can be used in a large number of commercial applications and is also the most popular precision resistor technology at present. Through long-term multilayer film deposition, high-precision resistance adjustment, and subsequent screening, the optimal precision thin film resistor can reach a temperature drift of ± 2ppm/°C and accuracy of ±0.01%, as well as good long-term stability. However, the power is not large, the indicators of low resistance values are not good. Besides, it&#;s not antistatic, the power coefficient is poor, which is difficult to meet the supply of small batches, and the consistency of different batches is not good.

Figure 12. Thin Film Precision Chip Resistor

(1) Main Performance Parameter Range

Size Specifications: , , , , ,

Resistance Range: 1Ω- 2MΩ

Resistance Accuracy: + 0.05%, + 0.1%, + 0.25%, + 0.5%

Rated Power: 1/16 1/10 1/8 1/4 1/2 3/4 1 (W)

Temperature coefficient of Resistance(TCR): +5 +10 +15 +25 +50 (ppm/&#;)

Antistatic Ability: ESD 2KV

(2) Advantages

High resistance accuracy and low-temperature coefficient;

Less loss and low noise;

Low parasitic effect, excellent high-frequency performance;

In the shape of chips, suitable for SMT placement;

(3) Disadvantages

The thin film will oxidize in high temperatures, chemical corrosion, and other occasions, which will continuously change the resistance value. The film of the high resistance film resistor is very thin and is more susceptible to oxidation and is easily affected by ESD.

Figure 13. ESD Safety

(4) Application

Thin-film resistors have the largest application and use in the precision resistor market. It has a wider range of resistance values and is cheaper than wire wound resistors and foil resistors. Thin-film resistors are a more economical choice for applications with medium-precision, high-resistance resistors, and space efficiency considerations.

3. Precision Metal Film Resistor

Precision metal film resistors are similar to precision thin-film resistors. Wafer metal film precision resistors are gradually being replaced by chip thin film precision resistors, but pin metal film precision resistors are still mainstream low-cost precision resistor technology. Like precision thin-film resistors, the resistance adjustment will cause a hot spot effect, which will affect the stability and reliability of the resistor.

Figure 14. A Precision Metal Film Resistor

4. 

Precision Wire-wound Resistor

As the earliest precision resistor technology, the temperature drift of high-precision wire-wound resistors can achieve ± 1ppm/°C and its accuracy can reach ±0.001%, which is impossible for thin film and thick film resistors. The best precision wire-wound resistors have a resistance value close to 50M, which is suitable for ultra-precision high-resistance applications. Due to the development of other resistance technologies, precision wire-wound resistors tend to be eliminated, because they are expensive and having an inductance.

Figure 15. Precision Wire-wound Resistors

(1) Main

P

erformance

P

arameters

Size Specifications: lead type and patch type

Resistance Range: 1mΩ- 275KΩ

Resistance Accuracy: ±0.001%, ±0.005%, ±0.1%, ±0.5%

Rated Power: 0.5, 1, 5, 7, 10 (W)

Temperature coefficient of Resistance(TCR): ±3, ±10, ±20, ±30 ppm/&#;

Antistatic Ability: ESD 25KV

(2) Advantages

High resistance accuracy and small temperature coefficient;

High power, high voltage surges resistance, good short-term overload performance;

High temperature withstand, fast heat dissipation, can still work normally at an ambient temperature of 170&#;;

Low working noise, which is lower than thin-film resistors and thick film resistors;

Stable and reliable, good static stability, no change in resistance value over the years.

(3) Disadvantages

Large volume, not suitable for small electronic equipment;

Low resistance value, mostly below 100KΩ;

The wire-wound structure has large parasitic capacitance and inductance, and cannot be used in high-frequency circuits.

(4) Application

Occasions that require high precision and high stability like resistance boxes, measuring instruments (multimeters), telecommunications instruments, etc..

Figure 16. A Resistance Box

5. 

Precision

F

oil

R

esistors

Although the precision foil resistor was invented as early as , it is still the most precise resistor technology so far. By attaching a nickel-chromium alloy to a ceramic substrate for balancing stresses, a temperature drift close to zero is obtained. The resistor pattern is etched by laser and trimming, an accuracy as high as ± 0.001% can be obtained. The best foil resistors have a resistance drift of only ±2ppm after stored for 6 years, which is antistatic with no inductance and capacitance, no hot spot, low noise, and low voltage coefficient. The disadvantage of the foil resistor is that the resistance cannot be very high. The maximum size of the chip resistors can only be 150K, and the maximum size of the pin resistors can only be 2M.

Figure 17. Nickel-Chromium Alloy

(1) Main Performance Parameter Range

Size Specifications: , , , , ,

Resistance Range: 1mΩ- 150KΩ

Resistance Accuracy: ± 0.005%, ± 0.01%, ± 0.05%, ± 0.1%, ± 0.25%, ± 0.5%

Rated Power: 0.1 ~ 8 (W)

Temperature coefficient of Resistance(TCR): ± 0.2, ± 2, ± 5, ± 10 (ppm / &#;)

Antistatic Ability: ESD 25KV

(2) Advantages

Metal foil precision resistors are considered to be ideal resistors due to their low-temperature drift, long-term stability, no inductance, insensitivity to static electricity, low capacitance, fast thermal stability, and low noise.

(3) Disadvantages

The resistance value cannot be increased. At present, it can only be KΩ, MΩ, or even TΩ.

The price is expensive.

VI What Kind of Precision Resistor Should We Choose?

1. For precision sampling resistors with resistance values from 1 milliohm to 1 ohm, generally, only precision foil resistors can be used. Because only the resistance of the alloy material can be stable under low resistance and large current. The temperature drift can be as low as ±5ppm /°C, and the accuracy can be ±0.1% or even higher.

The temperature drift is the main factor that determines the price of this type of resistor. Because its main application is current detection, the four-pin structure facilitates precision sampling. Use a larger size or reserve more power space, and increase the auxiliary radiator to help reduce the temperature of the resistor surface and improve the stability of the sampling resistor. Precise current-sampling resistors can replace Hall current sensors and have cost advantages.

Figure 18. Structure of an Open Loop Hall Effect Current Sensor

2. Resistance values from 1 ohm to 10 ohms are a challenge for any resistance technology. Because this resistance value range belongs to the low resistance value range, only a thick resistance material, and a short current path can achieve a low resistance value. Thick resistive materials are not good for combining with the substrate to balance temperature drift, and short current paths are not good for precise resistance adjustment.

It is difficult for precision thin-film resistors and precision metal film resistors to provide excellent temperature drift in this resistance range, ± 20ppm/°C is the best drift they can achieve. The temperature drift of precision wire-wound resistors is completely based on the alloy wire itself, which can achieve about ±10ppm / °C, but wire-wound resistors generally only have pins and inductance. Within this resistance range, the temperature drift of the precision foil resistor can be controlled within ±5ppm / °C, the accuracy can be ±0.1% or better, and both chips and pins can be provided.

3. 10 ohms to 150K ohms is a conventional resistance value range. Many products can be selected based on different precision requirements within this resistance value range.

If the requirement for temperature drift is below ±2ppm / °C, or there is a requirement for long-term stability, only precision foil resistors can be used, and the pins of the patch can be selected. If the temperature drift is required to be about ± 5ppm / °C, and the long-term stability requirements are not so high, thin-film resistors or metal film resistors can both be considered.

The highest accuracy of the foil resistor can be ±0.001%, and that of the film resistor and metal film resistor can be ±0.01%. It should be noted that different sizes of chip foil resistors have different resistance value ranges. For example, the maximum resistance value of chip foil resistors is 10K. In addition, there are many manufacturers of precision chip thin-film resistors. The parameters are marked the same, but the actual performance gap is very large. There are many cases of temperature drift and accuracy exceeding standards, and resistors of different brands have different indicators of long-term stability and noise.

Figure 19. A Foil Resistor

4. The resistance range from 150K to 1M belongs to the middle resistance range. In this section, if chip precision resistors are needed, only precision film resistors can be used.

For temperature drift below ± 2ppm /°C, generally, only pin precision foil resistors can be used. However, the higher the resistance value, the higher the price of this resistor, because multiple resistor chips are needed to achieve the required resistance value. Some pin film resistors can provide temperature drift as low as ± 5ppm / °C, but the long-term stability is worse than precision foil resistors. The pin precision wire-wound resistor can also meet the high precision and temperature drift is as low as ± 2ppm / °C, but there is no price advantage.

5. 1M to 50M is a high-resistance section for precision resistors. The highest resistance that precision thin-film resistors can support is generally 10M, the best temperature drift is ± 5ppm /°C, and the highest accuracy is ±0.01%. Precision thick film resistors can provide accuracy and temperature drift similar to thin-film resistors, but they can support the entire range of resistance values. A single-precision foil resistor currently supports a maximum resistance of 2M and is very expensive. Another expensive precision resistor is wire-wound resistors. A single wire-wound resistor can provide resistance values up to 50M, and the temperature drifts are as low as ± 2ppm /°C, and accuracy is as high as ±0.001%, which is significantly superior to thin-film and thick-film resistor technologies especially for long-term stability.

6. The demand for 1M-10T high-voltage precision, we can only choose precision thick-film high-voltage resistors, because only thick-film technology can work at high voltages and provide high resistance values. Important technical indicators of such high-voltage resistors are voltage coefficient, voltage stability, long-term stability, noise, temperature drift, and accuracy, among which the indicators of voltage coefficient and voltage stability are very important.

The resistance of most thick film resistors tends to drift to a small value under a high-voltage environment. This is because part of the insulating medium is activated to form a parallel resistance, and the quality of the resistance paste and the thick film process determines the quality of the voltage coefficient.

In other applications, low-noise high-voltage resistors are required. The noise is in the defected part of the resistance film layer. The defect may already exist or may be added after the resistance adjustment. The more such defects, the larger the noise. Luckily, the thick film process helps to improve the noise index.

The selection of precision resistors is briefly introduced above according to different segmented resistance values. The actual situation is much more complicated. It is important to choose the right precision resistor regarding the actual application, but the electrical parameters that all applications focus on are nothing more than long-term stability, temperature coefficient, absolute accuracy, power coefficient, voltage coefficient, and noise.

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