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This post was last edited by qwqwqw2088 on 2023-5-4 11:27
Almost sixty years have passed since physicist Dr. Felix Zandman invented the first foil resistor in 1962. Bulk Metal Foil resistor technology still far exceeds other resistor technologies in applications requiring high precision, high stability, and high reliability. Wistron Precision Measurement Group provides precision foil resistor products in a variety of specifications and packaging to meet various application requirements. U.S. Patent 4176794 is a patent for metal foil resistors applied by Angstrohm Corporation in the United States.
Israel's Vishay (Vishoy Precision Measurement Group, including AE acquired by Vishay) has great advantages in precision metal foil resistor technology. The Z-Foil metal foil resistor technology it developed has greatly improved various technical indicators. For example, within the temperature range of -55℃ to +125℃ and at a reference temperature of +25℃, the Z-Foil resistor has a typical TCR of ±0.2 ppm/°C.
The resistance value of a resistor will change due to various "stresses". High precision without stability is meaningless. For example, the precision of a resistor is ±0.01% when it leaves the factory. We pay a high price for this precision, but after a few months of storage or hundreds of hours of load, the resistance value may change by more than ±300ppm or even more. Another common situation is that the resistor is within the nominal accuracy range during incoming material inspection, but exceeds the nominal accuracy range after being soldered to the PCB. In addition, moisture, static electricity, etc. can cause irreversible changes in the resistance value of the resistor. What we want to emphasize is that stability should be considered first, rather than a one-sided pursuit of high precision.
Metal foil resistors are made by vacuum melting nickel-chromium alloy, rolling it into metal foil, bonding it to an alumina ceramic substrate, and then using photolithography to control the shape of the metal foil, thereby controlling the resistance. Metal foil resistors are currently the best resistors with controllable performance.
Metal foil resistors are made of special metal foil materials and are strictly controlled during the production process, making their performance far superior to other resistors. It is no exaggeration to say that high-precision metal foil resistors are ultra-precision resistors. So what are the advantages and characteristics of this resistor? A good precision resistor must have the characteristics of small aging, small temperature drift, and small deviation. At the same time, it is best to have high reliability, large power margin, small temperature rise, low noise, small series inductance and distributed capacitance, small voltage coefficient, and not easy to change due to welding, vibration, and stretching. Metal foil resistors have almost all of these advantages.
Of course, the most important parameter related to the benchmark is aging, followed by the temperature coefficient. Whether the resistor is marked with 1%, 0.1%, or 0.01%, this is just a deviation and does not directly represent the degree of "precision". Only when it remains highly stable under different temperature conditions and after a long period of use, can it represent true "precision".
What is "aging"? Aging is long-term stability, that is, the change in resistance after a long period of time (such as 1 year) when placed on a shelf at normal temperature and pressure. Aging is therefore often expressed in ppm per year. Aging is therefore an irreversible process, just like human aging, and can never return to its original state.
The temperature coefficient of resistance (TCR) indicates the relative change in resistance when the temperature changes by 1 degree, and the unit is ppm/℃ (i.e. 10E (-6)/℃). What is "temperature drift"? Temperature drift is the change in resistance of a resistor with temperature. Since the temperature drift of general resistors is not large, it is often expressed in ppm per degree, which is the temperature coefficient. If the temperature coefficient of a resistor is +100ppm/℃, it means that the resistance increases by 0.01% for every 1 degree increase in temperature. Similarly, a negative temperature coefficient means that the resistance of a resistor decreases with increasing temperature. When talking about the temperature coefficient, sometimes the /℃ at the end is omitted. For example, if the temperature coefficient of a resistor is 8ppm, it means 8ppm/℃.
Special metal foils with known and controllable properties are applied to special ceramic substrates to form thermomechanical balance forces, which are very important for resistor formation. Then, the resistor circuit is photolithographically etched using ultra-precision processes. This process combines important features such as low TCR, long-term stability, no inductive reactance, no ESD sensing, low capacitance, fast thermal stability and low noise in one resistor technology. These features help improve system stability and reliability without compromising accuracy, stability and speed. To obtain precise resistance values, large metal foil chip resistors can be trimmed by selectively eliminating inherent "short boards". When it is necessary to increase the resistance in known increments, the marked area can be cut to gradually increase the resistance by small amounts, as shown in the figure.
The internal structure of the patch sheet metal foil is shown in the figure.
Characteristics 1. Temperature coefficient (TCR)
“Why do you need resistors with very low temperature coefficients?” This is a question that may be asked when evaluating the performance and cost of a circuit system. The answer is due to a combination of circuit systems. The following pages discuss 10 different individual technical features of metal foil resistors that are important to precision analog circuits. While each feature is clearly discussed independently, many circuits require a specific combination of these features, and often all of them are required to be tested in the same resistor setup. For example, a certain feature is required to be tested using an operational amplifier.
In an op amp, the gain is determined by the ratio of the feedback resistor to the input resistor. The common mode rejection ratio of different amplifiers is based on the ratios of four resistors. In both cases, any change in the ratios of these resistors will directly affect the performance of the circuit. These ratios may change due to different temperature coefficients of the resistors, different heating effects (either internal or external), different tracking of ambient temperature changes, response time to different phase inputs or high frequency signals, differential Joule heating due to different power levels, different changes in resistance value over the design life, and so on. So it is easy to see that it is not uncommon for many circuits to rely on stable performance for many related applications - all at the same time, in the same setup. Bulk Metal Foil resistor technology is the only resistor technology that offers all of these tight characteristics in the same resistor device. Low noise is inherent in foil resistor technology and can be used in special applications where low noise is required. All of these characteristics are inherent in foil resistor technology. And all foil resistor products automatically have these characteristics. The
solution to these problems is to use a resistor with a low temperature coefficient to keep the effects of temperature changes to a minimum.
Initial Temperature Coefficient
Two predictable and opposing physical phenomena, the composite structure of the resistor's internal alloy and its matrix, are the key factors in achieving the low TCR of Bulk Metal Foil resistors. The
TCR of a foil resistor is achieved by matching two opposing effects - an increase in internal resistance due to an increase in temperature vs. compression - a decrease in resistance associated with the same temperature increase. Both effects occur simultaneously resulting in a generally low, predictable, repeatable, and controllable TCR result.
Due to the design of the Bulk Metal Foil resistors from Vespa Precision, this TCR is achieved automatically, without screening, without paying attention to resistance value or manufacturing date - even years later!
Improved Bulk Metal Z-Foil TCRs of ±0.2 ppm/°C are possible.
Foil resistor technology advances every few years, resulting in significant improvements in TCR.
Figure 1 shows the typical TCR characteristics of the various alloys that Vespa Precision uses to produce foil resistors.
The initial C alloy exhibits a chord with a positive slope and a negative response to temperature at the cold end and a chord with a positive slope to temperature at the hot end.
Next up is the K alloy, which has a negative sine slope with temperature on the cold side and a positive sine slope with temperature on the hot side. In fact, it provides a temperature coefficient curve that is approximately half that of the C alloy.
The latest development is the Z alloy and Z1 alloy foil resistor technology, which has improved upon the foil technology similar to the K alloy, providing a temperature coefficient curve that is many times better than the C alloy and five times better than the K alloy.
Using this technology, it is possible to create very low temperature drift resistors that have a near zero response to temperature.
As a result of this development, the resistor has greatly improved temperature drift performance relative to previous technologies, as well as other resistor technologies.
Typical Temperature Coefficient The typical temperature drift of a TCR
foil resistor is defined as the change in resistance versus the temperature (RT) curve, expressed in ppm/°C (parts per million per degree Celsius). The slopes are defined for 0 °C to + 25 °C and + 25 °C to + 60 °C (Instrument Temperature Range); - 55 °C to + 25 °C and + 25 °C to + 125 °C (Military Range).
These specified temperatures and the defined typical temperature drift chord slopes apply to all resistor values including low value resistors. Note, however, that with the exception of low value resistors with four-pin Kelvin connections, the lead resistance and associated temperature drift may have to be considered. All types of lead resistance and temperature drift measurements are made with the lead 1/2” as the reference point. For expected increases in temperature drift for low value resistors please contact our Applications Engineering Department.
Tracking Temperature Drift
"Tracking" is a comparison of the stability of two or more resistors. When more than one resistor is on the same substrate, (as in Figure 2), assuming two discrete resistors, drift tracking is a better description of how the resistance of resistors made from the same batch but with different technologies increases or decreases with temperature than drift. The resistance tracking ratio is affected by external heating (such as a rise in ambient temperature or by adjacent hotter components) as well as internal heating (self-heating due to power dissipation). Resistors may be screened to have good drift at the same temperature. But variations due to different internal temperatures (e.g., different power dissipation) or different locations (e.g., different heating from surrounding components) will track layer by layer and produce additional temperature-related errors. Therefore, low absolute drift is very important in precision applications. The
best analog design will be based on low absolute TCR resistors, because it minimizes the effects of ambient and self-heating on the resistor.
This is not possible for resistors with high temperature drift > 5 ppm/°C. Even resistors with good internal tracking will have drift less than 2 ppm/°C.
Characteristic 2: Power coefficient of resistance (PCR)
The TCR of resistors is usually given over a temperature range, which is obtained by measuring the resistance under two different ambient temperatures: room temperature and the temperature of a cold space or a hot space. The ratio of the change in resistance over the different temperatures produces a slope curve R/R = f (T). This slope is usually expressed in parts per million per degree Celsius T (ppm/°C). In this case, the temperature standard for measuring resistance is unified. In reality, however, the temperature rise of the resistor is also partly due to the power applied to the resistor, and part of the power is wasted in self-heating. According to the Joule effect, when current flows through the resistor, the resistor generates heat. Therefore, for precision resistors, the temperature TCR alone cannot represent the actual resistance change. Therefore, another parameter is used to describe the resistor's inherent characteristics - the power coefficient of resistance (PCR). The power coefficient (PCR) is expressed as one millionth per watt or one millionth of the rated power. For Z-foil metal foil power resistors, the power coefficient PCR at rated power is 5ppm typical, or 4ppm per watt typical. For example, for a metal foil power resistor with a TCR of 0.2 ppm/C and a PCR of 4 ppm/W, a temperature change of 50 C (from +25 C to +75 C) of 0.5 W will produce a R/R change of 50 x 0.2 + 0.5 x 4 = 12
ppm .
When voltage is applied to a resistor, it will self-heat. Foil resistors have low-temperature drift and power factor that minimize the effect of self-heating on the resistor. However, in order to achieve high precision, it is also necessary for the resistor to respond quickly to changes in environmental conditions or other stimuli. When the power changes, it is desirable for the resistor value to adjust quickly to a stable value. Fast thermal stability is important in some applications. The resistor must quickly reach a stable nominal value in response to changes in internal and external factors, with a deviation of the order of a millionth.
Most resistor technologies may take several minutes to reach their thermal stability, but foil resistors can reach stability immediately and within a few millionths of a second. The resistor responds accurately to changes in ambient temperature and power. When power is applied to the resistor, it will self-heat, causing mechanical stress on the resistor element and resulting in a temperature inversion phenomenon. Regardless, the performance of foil resistors far exceeds that of other resistor technologies. (Figure 3)
Feature 4: Resistance Accuracy
Why do people choose very precise resistors? A circuit system or a device or a particular circuit must function for a predetermined period of time. And at the end of its service life, it must still function properly. During this service life, it may have been subjected to adverse conditions, so the resistor may no longer maintain its original precision. One reason is to specify a resistor with a more stringent and traceable precision than expected at the end of the resistor's life, to allow for acceptable drift in precision during service. Another reason is to place more stringent precision requirements on resistors than on other resistor components.
Bulk Metal Foil resistors are made with precision of 0.001% by selectively etching various trim points on a photolithographic resistor foil (see Table 4). They provide predictable, step-by-step increases in resistance to the desired precision level. The trim pattern is made at these trim points, changing the current path through a longer path, so that the resistance is increased by a specific percentage. The trim process precisely increases the resistance at different locations. So the area within the etched grid remains reliable and noise-free. In the perfect resistance adjustment area, the resistance adjustment can achieve the final resistance value through slight changes with an accuracy of 0.001%. The final accuracy is 0.0005% (5ppm), which is the resistance adjustment resolution (as shown in Figure 5).
Feature 5: Load life stability
Why do designers care about stability under load? Load life stability is a typical indicator of the long-term reliability of a resistor. Military testing requires a limited amount of drift and a limited amount of failure rate within 10,000 hours. Precision foil resistors have the most stringent testing requirements. Whether or not they have been tested under military standards, the load life stability of foil resistors is unmatched and ensures long-term normal use.
Foil resistors have such stable performance due to their material construction. Bulk Metal Foil and a high purity alumina matrix. For example, the S102C and Z201 resistors tested at 0.3 W power at 125 °C for a 2000 hour load life test have a maximum resistance change of 150 ppm and a 10,000 hour load life test have a maximum resistance change of 500 ppm (see Tables 6 and 7). Conversely, a reduction in power results in a smaller resistance change, which reduces the temperature rise of the resistor element within the foil resistor. Table 6 shows the drift of foil resistors due to load life testing.
Figure 7 shows the drift of load life testing due to reduced power. Lowering the ambient temperature will have an effect on the load life test results. Figure 8 shows the drift of the load life test at different ambient temperatures at rated power. Figure 9 shows the load life test results of S102C foil resistors under low power and low temperature conditions.
Our engineering staff ensures the stability of metal foil resistors through several experiments and tests. Figure 10 shows the stability test results of metal foil resistors over 29 years. 50 S102C 10 kΩ resistor samples were tested continuously at 70 °C temperature and 0.1 W power. The average resistance change was only 60ppm.
Figure 11 shows the results of a customer-supplied VHP101 series foil resistor test over 8 years of shelf life. The average resistance change was less than 1ppm.
To evaluate load life stability, it is important to mention that there are two parameters, power and temperature, that can be combined into one for a given resistor. If the temperature rise of a resistor is determined at steady state, this temperature rise can be added to the ambient temperature and they appear as a combined temperature, (load introduction temperature + ambient temperature). For example, the S102C series VHP foil resistor will increase in temperature by 9°C for every 0.1W of power applied. This results in the following calculation:
If T = 75 °C, P = 0.2 W, t = 2000 hours
Self-heating = 9 °C x 2 = 18 °C
18 °C rise temperature + 75 °C ambient temperature = 93 °C R
R max = 80 ppm, as shown in the curve of Figure 12.
Figure 12 shows the resistance drift due to the combined temperature increase for a given continuous load life test. As explained above, the combined temperature includes the temperature rise due to power loading and the ambient temperature. The curve shows the maximum drift.
Feature 6: Fast response time
The resistor equivalent circuit, shown in Figure 13, combines resistance, inductance, and capacitance. Resistors can be viewed as R/C circuits, filters, or inductors, depending on their geometry. In wirewound resistors, the reactance is created by the coils and spiral voids formed by the windings. Figure 14 illustrates the increase in capacitance and inductance as the number of turns increases to increase the resistance. This assembly technique attempts to reduce the inductance of wirewound resistors, but the effect is limited. On the other hand, in planar resistors, such as Bulk Metal Foil resistors, the resistor path pattern is intentionally designed to be parallel geometric lines to cancel out the reactance.
Figure 15 illustrates a typical serpentine planar resistor resistance path pattern. The adjacent opposite currents minimize the mutual inductance and also reduce the capacitance.
Inductors and capacitors produce a reactance proportional to the operating frequency, which changes the effect of the resistor and the phase of the current and voltage in the circuit.
The reactance produced by inductors and capacitors can interfere with the input signal, especially in pulsed devices. Figure 16 Comparison of the response of the current to a voltage pulse. The metal foil resistor responds very quickly, while the wirewound resistor responds slowly. The
pulse width here is one billionth of a second, and the figure shows that the wirewound resistor will severely distort the signal, while the metal foil resistor segmented perfectly reproduces the signal.
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