Nov. 28, 2011 — Vishay Precision Group, Inc. (NYSE: VPG) today announced that its Vishay Foil Resistors division (VFR) has released a new series of ultra-high-precision Bulk Metal® Foil chip resistors offering flexible terminations, low TCR of ±2.0 ppm/°C typical from –55°C to +125°C, +25°C ref.,and load-life stability to ±0.01% at +70°C for 2,000 hours at rated power.

Offered in 1206, 2010, and 2512 chip sizes, the VSMF series features a wide resistance range from 10Ω to 125 kΩ, with tolerances to ±0.01%. Any resistance value within this range is available at any tolerance with no additional cost or lead time effect. The devices provide a power rating to 400 mW at +70°C and withstand electrostatic discharge (ESD) to at least 25 kV. The resistors feature a rise time of 1.0 ns, with effectively no ringing, a thermal stabilization time of < 1 s (nominal value achieved within 10 ppm of steady state value), current noise of 0.010 µVrms per volt of applied voltage (< -40 dB), a voltage coefficient of < 0.1 ppm/V, and a thermal EMF of 0.05 μV/°C. The RoHS-compliant devices offer a non-inductive (< 0.08 μH), non-capacitive design.

Typical chips in sizes of 1206 and larger occasionally fall off the printed circuit board (PCB) or develop substrate cracks, mainly due to bending during assembly and other stresses introduced through handling of the PCB, excess thermal gradients, or thermal shock. Thermal stresses can occur due to changes in ambient temperature or internal heat generation in the resistor itself. Whatever the cause, stress cycling due to thermal phenomena can cause material fatigue and cracks.

The VSMF series eliminates the problems of cracked substrates and board delamination by providing flexible terminations for strain relief, thereby increasing reliability. In addition, the Bulk Metal Foil resistor has documented more precise load-life stability than any MIL-qualified resistor. This is because the resistor’s element is a bulk metal alloy that has been applied to a substrate with a unique process that results in a stress-free adhesion. The adhesion is maintained throughout the manufacturing process, including termination welding, encapsulation, marking, and curing. It then maintains adhesion integrity through all environmental and application stresses.

“Allowances must be made for the stresses – especially mounting and thermal cycling – that will change a resistor’s end-of-life tolerance,” said Yuval Hernik, senior director of application engineering for Vishay Foil Resistors. “These allowances depend on the specific resistor application and the service conditions of the equipment in which it is installed. The less stable a resistor is, the more allowance for instability is required. These allowances are considered cumulative and when subtracted from the end-of-life error budget, impose a tighter tolerance at the point of purchase.”

“Foil resistors with flexible terminations are the most stable resistors, and thus require the least error allowance. The lower in-service error allowance means that more error allowance can be transferred to active devices – resulting in lower costs – or applied to the Foil resistors themselves, allowing for looser initial tolerances than would be required for other resistor technologies.”

Samples and production quantities of the VSMF series are available now, with lead times of five days for samples, and eight weeks for standard orders.

Nov. 21, 2011 — Vishay Precision Group today announced that its Vishay Foil Resistors division (VFR) has introduced a new series of ultra-high-precision Bulk Metal® Z1-Foil hybrid chip resistors designed for demanding high-temperature applications to +240°C. The devices are designed for hybrid circuits with aluminum wire bonding technology.

The HTHA series is based on VFR’s next-generation Z1-Foil technology, which provides an order of magnitude reduction in the Bulk Metal Foil element’s sensitivity to temperature changes – both external and internal – while providing long-term stability in high-temperature environments. HTHA devices offer TCR of ±1 ppm/°C from -55°C to +125°C and ±2.5 ppm/°C from -55°C to +220°C, +25°C ref. The resistors feature exceptional load-life stability to ±0.05% at +220°C for 2,000 hours at working power, long-term stability to ±0.05% at +240°C for 2,000 hours (no power), and tight tolerances to ±0.02%. The devices are capable of withstanding electrostatic discharges at least to 25 kV without degradation.

The HTHA series is offered in six case sizes ranging from 0603 to 2512 and features working power to 150 mW at +220°C. The devices provide a wide resistance range from 5Ω to 125 kΩ, with any resistance value within this range available at any tolerance with no additional cost or lead time effect. The resistors feature a rise time of 1.0 ns, with effectively no ringing, a thermal stabilization time of < 1 s (nominal value achieved within 10 ppm of steady state value), current noise of 0.010 µVrms per volt of applied voltage (< -40 dB), and a voltage coefficient of < 0.1 ppm/V.

The HTHA series is optimized for a wide variety of high-temperature applications, including geothermal measuring equipment, turbine engine control, and environmental test chambers in military, industrial, automotive, and down-hole drilling systems. Many analog circuits in these applications require passive components such as resistors to have a minimal drift from their initial values when operating above +175°C and in humid environments. In high-temperature applications, the most important factor is the end-of-life tolerance, and to a lesser extent, the initial tolerance. HTHA resistors provide stabilities well under the maximum allowable drift required by customers’ specifications through thousands of hours of operation under harsh conditions.

The Z1-Foil technology allows VFR to produce customer-oriented products designed to satisfy unique and specific technical requirements. In addition to the special chip stabilization under extreme environment conditions in the production line, additional specially oriented post manufacturing operations (PMO) are offered for high-temperature applications that require an even higher degree of reliability and stability.

“Resistors are the passive building blocks of an electrical circuit. They may be used for dropping the voltage, buffering the surge when the circuit is turned on, providing feedback in a monitoring loop, sensing current flow, and more,” said Yuval Hernik, senior director of application engineering for Vishay Foil Resistors. “When their application requires stability over time and load, initial accuracy, minimal change with temperatures higher than +200°C, resistance to moisture, and a number of other characteristics, only the new generation of VFR resistors has the attributes required.

“Compared to Bulk Metal Foil, thick and thin film resistor elements are produced with a non-controllable material,” Hernik added. “Heat or mechanical stresses on the resistive element cause the particles forming the film to expand. However, after these stresses are alleviated, the particles in the film don’t return to their exact original position, which reduces their overall stability. For high-temperature applications where stability is a main concern, Vishay Foil resistors offer the best resilience against time at elevated temperature.”

Samples and production quantities of the HTHA series are available now, with lead times of five weeks for samples, and eight weeks for standard orders.

Sept. 28, 2011 — Vishay Precision Group, today announced that its Vishay Foil Resistors division (VFR) has introduced a new series of ultra-high-precision Bulk Metal® Z1-Foil hybrid, surface-mount chip resistors connected using gold wire bonding. The HTH series is designed for high-temperature applications to +240®C and offers improved heat dissipation, providing nearly a 100°C extension in operating temperature range over precision thin film chip resistors.

The HTH series is based on VFR’s next-generation Z1-Foil technology, which provides an order of magnitude reduction in the Bulk Metal Foil element’s sensitivity to temperature changes – both external and internal – while providing long-term stability in high-temperature environments. HTH devices offer TCR of ±1 ppm/°C from -55°C to +125°C and ±2.5 ppm/°C from -55°C to +220°C, +25°C ref. The resistors feature exceptional load-life stability to ±0.05% at +220°C for 2,000 hours at working power, long-term stability to ±0.05% at +240°C for 2,000 hours (no power), and tight tolerances to ±0.02%. The devices are capable of withstanding electrostatic discharges at least to 25 kV without degradation.

The HTH series is offered in 5x5, 15x5, and 15x10 chip sizes, in addition to six case sizes ranging from 0603 to 2512. The devices feature two different chip design layouts according to their size. The HTH series provide a wide resistance range from 10Ω to 125 kΩ, with any resistance value within this range available at any tolerance with no additional cost or lead time effect. The resistors feature a rise time of 1.0 ns, with effectively no ringing, a thermal stabilization time of <1 s (nominal value achieved within 10 ppm of steady state value), current noise of 0.010 μVrms per volt of applied voltage (<-40 dB), and a voltage coefficient of <0.1 ppm/V.

Relatively few of today’s passive components are rated for such high-temperature operation, and even when they are, their useful operating life may only be a matter of hours or days. The HTH series is optimized for a wide variety of demanding high-temperature applications, including geothermal measuring equipment, turbine engine control, and environmental test chambers in military, industrial, automotive, and down-hole drilling systems. The Z1-Foil technology allows VFR to produce customer-oriented products designed to satisfy unique and specific technical requirements. In addition to the special chip stabilization under extreme environment conditions in the production line, additional specially oriented post manufacturing operations (PMO) are offered for high-temperature applications that require an even higher degree of reliability and stability.

“In high-temperature applications, the most important factor is the end-of-life tolerance, which is part of the stability and, to a lesser extent, the initial tolerance,” said Yuval Hernik, senior director of application engineering for Vishay Foil Resistors. “Changes in the ambient temperature, heat from adjacent components, destabilizing thermal shocks from suddenly applied power, long-term exposure to applied power, and the repetitive stresses from being switched on and off in environments with temperatures above +175°C all can affect stability. For high-temperature applications where stability and the total error budget are the main concerns, this new generation of Vishay Foil resistors addresses all of these parameters and offers the best resilience over time and temperature.”

“The key to stability and reliability is to keep circuits cool, and unfortunately, in extreme-temperature applications such as down-hole electronics, that is not always an option. Vishay Foil resistors are known to exhibit very low aging, and thus can achieve reasonable end-of-life tolerance (or total error budget) even at high temperatures. In applications that require stability under extreme environments, the short-term cost savings of using a thick or thin film resistor may easily be lost due to field failures and time spent on calibration. That’s why the marginally higher cost of Vishay Foil resistors often represents the best investment a designer can make.”

Samples and production quantities of the HTH series are available now, with lead times of five weeks for samples, and eight weeks for standard orders.

VPG Releases New FSR Secondary Standard Resistor Based on Bulk Metal® Foil Technology for Laboratory Calibration Standards and DMMsC

Breakthrough Device Offers High Accuracy to 10 ppm, Long-Term Stability of ±0.0005%, and TCR to ±0.3 ppm/°C from +15°C to +45°C Aug. 17, 2011 — Vishay Precision Group, today announced that its Vishay Foil Resistors division (VFR) has introduced a breakthrough secondary standard resistor for laboratory calibration standards and digital multimeters (DMM). For test equipment designers, the new FSR offers exceptionally high accuracy of 10 ppm at the terminals, long term stability of ±0.0005 % (5 ppm) at +25 °C for one year, temperature coefficient of resistance (TCR) to ±0.3 ppm/°C from +15°C to +45°C (Max TCR: 10 ppm to 30 ppm total deviation over temperature range), and resistance tolerance to ±0.005 % (50 ppm).

Offering a direct plug-in device for most DMMs, the FSR is based on a new design utilizing Bulk Metal® Foil technology, resulting in a combination of standard features with new benchmark capabilities for accuracy, stability, and speed. By using VFR precision resistors instead of the inductive resistors featured in most secondary standard boxes, the FSR allows for controllable accuracy at the box terminals through calibration, while extending the frequency range up to 10 times.

The device released today is intended for adjustable, direct reading resistance and serves as a substitution component for RTD, bridges, attenuators, voltage dividers, multipliers, adjustable feedback resistors for use with operational amplifiers, and network ladder elements. In addition, the FSR greatly extends the range of usefulness for these instruments due to its high-frequency performance, making them ideal for use in inspection stations, in the standards laboratory, and for component evaluation.

“Virtually all available conventional and specialized secondary standard resistors present a number of problems, including instrument accuracy; resistance shifts caused by load, temperature, or environmental changes; difficulty in measurement of the last digits; limitations on usable frequency; size; and ease of setting,” said Yuval Hernik, senior director of application engineering for Vishay Foil Resistors. “After an intensive analysis of these issues, we have developed a completely new design. With the unequalled combination of the Bulk Metal Foil resistors’ performance parameters, the FSR offers a level of accuracy, stability, and versatility never before combined in one secondary standard resistor, eliminating or reducing these problems to insignificance.”

The FSR offers virtually infinite resolution in resistance setting and readability. The device is available with up to six significant digits (e.g. 9.99962), and a wide resistance range from 1Ω to 150 kΩ, with any resistance value within this range available at any tolerance with no additional cost or lead time effect. For increased reliability, the resistor standard features an RF shielded case, while its hermetically sealed resistive element eliminates the effects of humidity. The FSR offers a small size and robust, four-terminal construction with an additional banana socket for external ground, and is supplied with a certificate of accuracy traceable to NIST.

The resistor standard features a rise time of 1.0 ns, with effectively no ringing, a thermal stabilization time of <1 s (nominal value achieved within 10 ppm of steady state value), current noise of 0.010 μVrms per volt of applied voltage (<-40 dB), and a voltage coefficient of <0.1 ppm/V. The device offers a non-inductive (<0.08 μH), non-capacitive design, and is capable of withstanding electrostatic discharges at least to 25 kV without degradation.

Production quantities of the FSR are available now, with lead times of 3 weeks.

August 31, 2011 — Vishay Precision Group today announced that its Vishay Foil Resistors division (VFR) has released a new series of ultra-high-precision Bulk Metal ® Z1-Foil wraparound, surface mount chip resistors with extended pads for high-temperature, high-power applications. The new extended pad designs are optimized for applications at temperatures as high as +225°C, and offer improved heat dissipation for high working power to 1,200 mW at +70°C and 330 mW at +200°C

The FRSH series is based on VFR’s next-generation Z1-Foil technology, which provides an order of magnitude reduction in the Bulk Metal Foil element’s sensitivity to temperature compared to the classical Foil – both external and internal – while providing long-term stability in high-temperature environments. FRSH devices offer TCR of ±1 ppm/°C from -55°C to +125°C and ± 2.5 ppm/°C from -55°C to +200°C, +25°C ref.

The resistors feature exceptional load-life stability to ±0.05% at +200°C for 2,000 hours at working power, long-term stability to ±0.05% at +225°C for 2,000 hours, and tight tolerances to ±0.02%. The devices’ full wraparound terminations ensure safe handling during the manufacturing process, as well as providing stability during multiple thermal cycling. The devices are also capable of withstanding electrostatic discharges at least to 25 kV without degradation.

Offered in six chip sizes from 0603 to 2512, the FRSH series features a wide resistance range from 10Ω to 125 kΩ, with any resistance value within this range available at any tolerance with no additional cost or lead time effect. The resistors feature a rise time of 1.0 ns, with effectively no ringing, a thermal stabilization time of <1 s (nominal value achieved within 10 ppm of steady state value), current noise of 0.01 μVrms per volt of applied voltage (<-40 dB), and a voltage coefficient of <0.1 ppm/V. The RoHS-compliant devices offer a non-inductive (<0.08 μH), non-capacitive design, and are available in matched sets upon request.

The TCR of the FRSH resistor is achieved through a relatively thick cold-rolled foil that maintains the same molecular structure as the raw alloy in high-temperature conditions. This represents the basis of the foil resistor, because the foil must act as a monolithic structure with a fixed and known linear coefficient of expansion over any temperature range the resistor might experience throughout its design life. The next most important element in the resistor’s construction is the adhesive that holds the foil to a flat substrate. It must withstand high- and low-temperature exposure, pulsing power, moisture incursions, shock and vibration, and more, while still securely holding the foil element to the substrate. With these characteristics, the basic technology of foil resistors combines the essential stress compensation that defines the foil technology.

“There has been considerable growth in the demand for precise, stable, and reliable resistors that can operate in harsh environments and especially at high temperatures above +200°C and +220°C,” said Yuval Hernik, senior director of application engineering for Vishay Foil Resistors. “Many analog circuits in these applications require passive components such as resistors to have a minimal drift from their initial values when operating above +175°C and in humid environments. But the most important factor is the end-of-life tolerance, and to a lesser extent, the initial tolerance. The new Vishay Foil resistors provide stabilities well under the maximum allowable drift required by customers’ specifications through thousands of hours of operation under harsh conditions.”

The FRSH series is optimized for a wide variety of demanding high-power, high-temperature applications, including power supplies, geothermal measuring equipment, turbine engine control, and environmental test chambers in military, industrial, automotive, and down-hole drilling systems. The Z1-Foil technology allows VFR to produce customer-oriented products designed to satisfy unique and specific technical requirements. In addition to the special chip stabilization under extreme environment conditions in the production line, additional specially oriented post manufacturing operations (PMO) are offered for high-temperature applications that require an even higher degree of reliability and stability.

Samples and production quantities of the FRSH series are available now, with lead times of five days for samples, and six weeks for standard orders.

VPG Releases New VCS1625P Ultra-High-Precision Foil Surface-Mount Current Sensing Chip Resistor With High Power Rating of 1 W, Low TCR to ±2 ppm/°C, Load Life Stability of 0.015% (2,000 hours, +70 °C), and Tolerances to ±0.1%

July 20, 2011 — Vishay Precision Group, today announced that its Vishay Foil Resistors division (VFR) has released a new ultra-high-precision Foil surface-mount current sensing chip resistor with low TCR of ±2 ppm/°C from -55°C to +125°C, +25°C ref., and tolerances to ±0.1% (0.01% and 0.05% are available). For high-power applications, the VCS1625P offers a power rating of 1 W at +70°C and maximum 5 A current rating, and a four-terminal Kelvin configuration for increased accuracy.

The VCS1625P features a wide resistance range from 0.01Ω to 10Ω, with specific “as required” values within this range (e.g. 1.234Ω vs. 1Ω) available at no additional cost or delivery time. The resistor features a rise time of 1.0 ns, with effectively no ringing, short time overload of <0.005% (50 ppm), current noise of 0.010 ΜVrms/V of applied voltage (<-40 dB), and a voltage coefficient of <0.1 ppm/V. Offering the utmost in electrostatic discharge (ESD) immunity, the device withstands ESD to at least 25 kV, for increased reliability, and offers a non-inductive (<0.08 μH), non-capacitive design. The VCS1625P is offered with tin/lead or lead (Pb)-free gold or tin termination options, and with additional temperature treatments (PMO) to extend the operating temperature from +150°C to well above +200°C. The design of the VCS1625P allows for designers to use only a single device to measure larger currents than previously possible by dissipating up to 1 W in the same 1625 size package.

The Bulk Metal® Foil technology of the VCS1625P provides a significant reduction of the resistive component’s sensitivity to ambient temperature variation (TCR) and to the “self-heating” effect caused by changing loads. These capabilities allow designers to guarantee a high degree of stability and accuracy in fixed-resistor applications. The resistor’s load-life stability at +70°C at rated power is an order of magnitude better than typical current sensing resistors (0.015% after 2,000 hours versus 0.05% after 1,000 hours). The device’s improved stability makes it ideal for tightened-stability reference voltage and precision current sensing applications in forced-balance electronic scales, measurement instrumentation, bridge networks, motor controllers and medical and test equipment. In addition, the VCS1625P can be tested in accordance with EEE-INST-002 (MIL-PRF 55342) for military and space applications.

The resistor’s design results in a very low thermal EMF of 0.05 ΜV/°C typical, which is critical in precision applications. The VCS1625P’s all-welded construction is composed of a Bulk Metal Foil resistive element with plated copper terminations. The flat terminations make intimate contact with the resistive layer along the entire side of the resistive element, thereby minimizing temperature variations. In addition to the low thermal EMF compatibility of the device’s metals, the uniformity and thermal efficiency of the design minimize the temperature differential across the resistor, thereby assuring low thermal EMF generation at the terminations. This further reduces the thermal EMF voltage, or “battery effect,” exhibited by most current sensing or voltage reference resistors.

The device released today is characterized by extremely low excess noise when compared to other resistor technologies. Additionally, the current in adjacent current-carrying paths runs in opposing directions, cancelling the parasitic inductance of these paths. Also, path-to-path capacitances are connected in series, which has the effect of minimizing the parasitic capacitance of the resistor. The low-inductance/capacitance VCS1625P resistor is characterized by non-measurable peak-to-peak signal distortions.

Samples and production quantities of the VCS1625P are available now, with lead times of five working days for samples, and six weeks for standard orders.

VPG Releases New Ultra-High-Precision CSM3637Z

Febryary 9, 2011 -- Vishay Foil Resistors (VFR) division has released a new ultra-high-precision Bulk Metal® Foil surface-mount Power Metal Strip® current-sensing resistor with a load-life stability of ±0.2% at +70°C for 2,000 hours at rated power—a significant improvement over the load-life stability of ≥1% offered by typical current sensing products. The new CSM3637Z also features an absolute TCR of ±5 ppm/°C maximum from -55°C to +125°C, +25°C ref., and a tolerance of ±0.1%. The CSM3637Z utilizes proprietary processing techniques that result in extremely low resistance values of 3 mΩ to 50 mΩ.

The improved resistance stability of the CSM3637Z makes it an excellent choice for tightened-stability voltage division, precision current sensing, and pulse applications in switching and linear power supplies, power amplifiers, feedback circuits, measurement instrumentation, bridge networks, satellites and aerospace systems, and medical and test equipment. The device’s stability can be further enhanced by post-manufacturing operations (PMO) which are uniquely applicable to Bulk Metal Foil Technology, such as temperature cycling, short-term overload and accelerated load-life.

Traditional passive current sensors and shunts generate heat under power, which changes their resistance, and thus their voltage output. The CSM3637Z’s low absolute TCR reduces errors due to temperature changes, thus reducing a major source of uncertainty in current measurement. Consider a typical current sense resistor with a TCR of 20 ppm/°C. While dissipating a power level that raises its temperature by 100°C, it exhibits a 2000 ppm (0.2%) resistance change. That’s a 0.2% error in the measured current flowing through the resistor. Moreover, the temperature rise is not linear with power dissipation, and the measured current has an additional non-linearity relative to the current flow through it. The only way to reduce this error is with a very low absolute TCR as in the CSM3637Z. Further, the CSM3637Z can withstand unconventional environmental conditions, including the extremely high temperatures and radiation-rich environments of down-hole oil exploration and well logging, or the deep-sea underwater repeaters in cross-ocean communications.

The CSM3637Z features a low thermal electromotive force (EMF) that is critical in many precision DC applications. The device’s all-welded construction is composed of a Bulk Metal resistive element with welded copper terminations, plated for soldering. The terminations make true ohmic contact with the resistive layer along the entire side of the resistive element, thereby minimizing temperature variations. The resistor element is designed to uniformly dissipate power without creating hot spots, thereby minimizing parasitic contact resistance and contact resistance variation for a low thermal EMF of <3 μV/°C.

Featuring a four-terminal (Kelvin) design for precise and accurate current measurement, the CSM3637Z offers a power rating of 3 W for a resistance range of 3 mΩ to 10 mΩ and 2 W for 10 mΩ up to 50 mΩ. VFR foil resistors are not restricted to standard resistance values, and can be supplied with “as required” values (e.g. 10.2345 mΩ vs. 10 mΩ) at no extra cost or delivery time. The device features a maximum current up to 31 A, very low inductance of 0.5 nH, excellent frequency response to 50 MHz, and short time overload of ±0.1% typical. The CSM3637Z’s specifications are based on tests performed in accordance with methods prescribed by appropriate MIL-PRF standards (MIL-PRF-55342 and MIL-PRF-49465). Available in waffle-pack or tape-and-reel packaging, the resistor is available with lead (Pb)-free and tin/lead alloy terminations.

Designers often unnecessarily pay for tighter tolerances than required simply to accommodate the resistance stability shifts they know to be imminent in an application due to the large application-related changes in the components they selected. Selection of a high-stability component like the CSM3637Z in these applications eliminates the need for shift allowance due to “planned instability,” and allows initial tolerances to be looser than would be necessary with current-sensing resistors based on other technologies. Additionally, the overall system cost is often reduced by eliminating the necessity of additional compensating circuitry or temperature-controlling systems.

Samples and production quantities of the CSM3637Z are available now, with lead times of five days for samples, and eight weeks for standard orders.

The hidden variable: circuit stability as a function of resistor stability

Even close-tolerance instrument components can drift from their specified values. Because of their method of construction, bulk metal foil resistors are stable over a wide temperature range.

Yuval Hernik, Vishay Precision Group -- EDN, August 26, 2010

Designers of instruments or control systems often find that component performance limits overall equipment performance in areas such as stability, frequency response, noise, and ESD. This is particularly true of resistors, which must also be able to meet stringent size, bandwidth, accuracy, and tracking requirements.

Selecting the right resistor type from wirewound, thick- or thin-film, or foil components involves tradeoffs due to the interlinked effects of thermal and mechanical forces on resistor electrical characteristics.

Stresses, whether mechanical or thermal, cause a resistor to change its electrical parameters. If such aspects as shape, length, or diameter are changed by mechanical or other means, the electrical parameters also change.

When current passes through a resistor it generates heat, and the thermal reaction causes mechanical changes by differential expansion in the different materials. Ambient conditions have the same effect. The ideal resistor would undergo no mechanical change during manufacturing, eliminating the need to compensate for the effects of heat or stress during use.

Wirewound resistors can be as good as ±0.005% with their initial tolerance, with a TCR (temperature coefficient of resistance) as low as 5 ppm/°C typical but usually about 15 to 25 ppm/°C. Thermal noise is low and tracking to ±2 ppm/°C over a limited temperature range is possible, though it adds considerable cost.

There are certain important manufacturing factors that affect wirewound resistor properties. For example, the wire is wound under tension around a core, which can elongate the wire and alter its diameter. In addition, during formation of the coil, each turn of wire has the inside surface under compression and the outside surface under tension. These deformations are permanent and irreversible. They affect stability and unpredictably shift the TCR from its original value. Over long periods, the wound element tends to change physically as the wire attempts to regain its original shape.

Permanent mechanical changes, which occur randomly and in unpredictable ways, cause equally random and unpredictable changes in the wire’s electrical parameters. Therefore, after winding, resistance elements can have highly variable electrical performance characteristics ranging from excellent to poor.

Wirewound resistors have relatively high inductance values due to their coiled-wire construction. There is also an intercoil capacitance. These units thus exhibit poor high-frequency characteristics, especially above 50 kHz.

It is difficult to make two wirewound resistors of equal value that can accurately track each other over a specified temperature range. Such tracking is of great importance in high-precision circuitry.

Traditional wirewound manufacturing methods do not isolate the resistive element from the various stresses arising out of handling, packaging, insertion pressures, and lead forming, for example. Here, one must consider tension applied to axial leads on mounting and pressure on the package exerted by mechanically induced forces.

Thick-film resistors possess good frequency response but have other characteristics that prevent their use in precision applications. Accuracy is generally in the range of ±5 to 20%, although tolerances of 1% can be attained. The temperature coefficient of thick film can be held through selection to 50 ppm/°C and can be selected to provide ±20-ppm/°C tracking. Shelf-life stabilities are about 2 to 5% of resistance value (20,000 to 50,000 ppm/year), and noise is appreciable.

Thin-metal-film resistors, evenly deposited on various substrates, offer accuracies of ±0.1% at best. The nature of the manufacturing process is such that a TCR better than ±10 ppm/°C is extremely difficult to produce. Moreover, the coefficient change of any two resistors falls randomly into the 20-ppm/°C span of the product.

As with most other resistor types, environmental effects and load-life changes cumulatively add up to, and exceed, their initial tolerances. Resistance changes can reach ±1 to 2% regardless of whether the initial resistor tolerance was ±0.05% or ±1%. When changes amounting to ±2% occur, a total design tolerance span of 6% would have to be considered in a circuit employing ±1% units, or 4.2% for ±0.1% units. This degree of instability prohibits the use of evaporated metal-film resistors as direct replacements for precision wirewound types.

Various and complex factors contribute to the instability of thin-film resistors. These include lattice distortion, discontinuous aggregate formations, occlusion of gas at crystal interfaces, oxidation of the film to form oxide semiconductors, and mechanical strains. The principal virtue of thin-film resistors is that they can be used in high-speed applications where precision and stability are not major factors, and where price is a consideration. In addition, because thin-film devices can be deposited on a single substrate in large resistor arrays, they can require fewer external connections. Thin-film resistors are very sensitive to ESD and can drift above 2500V or can open-circuit.

The comparative advantages of bulk metal foil resistors begin with how the devices are manufactured (Figure 1). Bulk metal foil resistors are produced by cementing a cold rolled alloy film on a large-area substrate. The film is then photoetched with very fine lines and precise spacing to obtain resistances that are easily adjustable down to 0.001% (10 ppm) with matching of pairs to 0.002% (20 ppm). The standard ±0.05-ppm/°C TCR (between 0 and 60°C) with the new Z-Foil technology is derived from the properties of the alloy and its match with the substrate.

The planar design with a parallel patterned element compensates for inductance effects; maximum total inductance of the resistor is 0.08 μH. Capacitive effects are 0.5 pF (max). A 1-kΩ resistor, for example, has a rise time of less than 1 nsec without ringing. Rise time depends on resistance value, but higher and lower values are only slightly slower than midrange values. The absence of ringing is especially important in high-speed switching: for example, in computer conversion ladder networks.

Foil resistors and networks have been used extensively in precision measurement and control instrumentation, in precision current applications, and in defense and space electronics. They are particularly advantageous in systems exposed to extreme environmental conditions and ambient temperature where only small changes in resistance can be tolerated. They have also proven valuable for use in devices such as analog-to-digital and digital-to-analog converters, where high speed and stability are important factors, along with close tolerances and precise ratios.

For example, one firm designed an energy-absorbing winch that uses a small computer. This controller automatically regulates the braking force during high-stress aerial recovery operations to provide a constant line tension or payload acceleration.

The energy-absorbing controller must reach a level that will not damage the package being retrieved or place undue loads on the aircraft executing the retrieval, and maintain that level throughout the runout.

Because the computer functions in an aircraft, it must operate over a temperature range from –65 to 120°F and in a high-vibration environment. Early models used wirewound resistors and precision thin film, but temperature stability was a problem. The wirewound resistors and precision thin film caused drifting, and at high temperatures, the power rating fell off considerably. Constant adjustments were necessary because the resistors just weren’t stable over the operating temperature range. Acceptable operation could be achieved only by utilizing much larger but derated wirewound resistors. A switch to bulk metal foil resistors based on the Z-Foil technology solved the drift problems, and provided increased board density and lower mass for vibration resistance.

Many air data parameters needed for navigation by high-performance aircraft are a function of static and total air pressures around aircraft during flight. To generate such information requires sophisticated equipment. Before these parameters can be calculated, the total and static pressures must be precisely transduced from pneumatic to electronic digital signals. Since digital computation is inherently accurate, overall stability of the air data computer depends mainly on the degree of stability of the pneumatic to electrical transducer and related circuits such as A/D and D/A converters, sample and hold amplifiers, and precision power supplies. Stability is of particular concern, with ±0.01 psi per year (±0.07% of full-scale) essential.

By using bulk metal foil resistors in place of military metal film devices, a 30 to 50% increase in the long-term performance of the analog circuits was achieved over a period of 650,000 flight hours. The transducers have an initial accuracy of ±0.04% full-scale maintained over a period of one year.

Author Information

Yuval Hernik holds a B.Sc in electrical engineering from the Technion (Israel Institute of Technology). He has been a director of application engineering at Vishay Precision Group—Bulk Metal Foil resistors—since 2008.

The hidden variable: circuit stability as a function of resistor stability

Introduction

The increasingly small form factors of electronic systems and components have brought many benefits to users, but they also complicate the electronic design process. The increasingly small form factors of electronic systems and components have brought many benefits to users, but they also complicate the electronic design process.One such problem is thermal electromotive force (Thermal EMF, or TEMF), which introduces an error-voltage signal in DC circuits. This article shows how to minimize thermal EMF by paying close attention to material and component selection, thermal management of critical areas, and proper circuit layout.

Basics of noise sources in low-level DC circuits.

When designing a low-level DC circuit, it is necessary to keep the influence of noise sources to minimum. The externally induced sources are EMI and RFI. There are also several internally generated noises. Some of these are Johnson noise, current noise, shot noise, and popcorn noise. There is also vibration-induced EMF, triboelectric, and electrostatic noise. In 1821, Thomas Seebeck discovered that when two wires made of dissimilar metals are joined at both ends and the opposing junctions thus created are maintained at different temperatures, a current will flow through the loop (Figure 1).

Figure 1: Seebeck effect generates current flow in the dissimilar wire loop

The magnitude of this current is proportional to three factors:

1. The temperature difference between the junctions at the two ends
2. Seebeck coefficients (proportionality measures based on different metals) of the two wire materials
3. The loop resistance

If one end of the loop is opened and a voltmeter is attached to the two wires (Figure 2), with the newly created meter-to-wire junctions kept at the same temperature as before, a voltage can be read on the voltmeter.


Figure 2: Opening the loop results in a voltage is proportional to the temperature difference at the two ends of the wires and to the difference between the Seebeck coefficients of the two wire materials.

This voltage is proportional to the temperature difference at the two ends of the wires and to the difference between the Seebeck coefficients of the two wire materials. Because the meter has a practically infinite input resistance, no current flows in the circuit and therefore produces no voltage drop in the wires, and the loop resistance is no longer a factor.

This is the basic principle of operation for thermocouple thermometers. The meter-side junction is kept at a fixed reference temperature (usually 0°C) by an ice bath or an electronic circuit simulating zero-degree conditions and the other end is at the heat source.

The Seebeck coefficient is also called “thermal electromotive force coefficient” or simply thermal EMF coefficient. Because it is very difficult to predict the thermal EMF coefficient of an alloy from its element composition, all the values for metals and alloys commonly used are obtained by measurement. Measurements are made by comparing the selected metal or alloy to a common reference metal such as platinum or copper. Thus, the thermal EMF coefficient for each metal is stated as referred to the same reference metal.

All thermal EMF coefficients are temperature dependent. The numbers given are averages between 0°C and 100°C. Except for very demanding applications, the small error generated by using these averages is negligible.

The unit of thermal EMF referring to copper or platinum is µV/°C. When the value of thermal EMF for a material referred to platinum is known, the value referred to copper can be calculated. This is done by subtracting the thermal EMF value for copper vs. platinum (7.6 µV/°C) from that same material’s thermal EMF value vs platinum. Similarly, the thermal EMF between any two metals can be found just by subtracting their individual thermal EMFs referenced against copper.

Design Considerations

A host of metals, alloys, and metal composite conductors are used in electronic components. Some of these components, such as resistors and semiconductors, generate internal heat (which is known as self-heating or the Joule effect). Therefore it is very difficult to avoid thermal EMF generation in electronic circuits. However, good results can be achieved by paying close attention to material and component selection, thermal management of the critical areas, and proper circuit layout.

Another way of looking at thermal EMF is to imagine small batteries in series with every component. Each battery voltage would be equal to the temperature across each metal junction, times the thermal EMF coefficient defined for that junction’s materials.

There are several points to keep in mind when you’re designing low-level DC signal-processing electronic circuits. First, nearly 30 times more heat will be transmitted by the thin copper foil of a printed circuit board than by the glass-epoxy board itself. In low thermal-EMF circuit designs it is crucial to keep all junctions at the same temperature. This means designing the low-level and sensitive areas of the circuit in an isothermal island, where the temperature is kept even over the entire area.

It is as important to reduce the heat flowing into a junction as it is to reduce heat flowing out. Heat can flow into or out of a junction in three ways:

• First, by conduction through the copper foil or the component leads or terminations
• Second, by radiation from a hot component nearby
• Third, from forced or natural air convection within the housing

As much as possible, low thermal EMF-generating materials and components should be used. Because of the close thermal-expansion match of Kovar alloy to glass, many electronic components are sealed using glass seams with Kovar terminations or eyelets leading to the outside. All hermetic-package leads are made from Kovar or from something very similar. Unfortunately, the thermal EMF coefficient of Kovar against copper is one of the highest among the alloys used in electronics (−35 μV/°C).

Analog sensor circuits have sensor outputs in the microvolt range; typically, this is about 5 μV. That voltage would normally be amplified up to a more-usable voltage level, typically about 5 volts, through a series of amplifiers headed by a differential amplifier.

However, if a component in the sensor circuit has just a 0.02°C temperature differential with a 35 μV TEMF coefficient, there would be a 0.7 μV error generated in the sense circuit, or about 14%. And since the TEMF voltage is impressed upon the sensed voltage - not as a part of the carrier signal---it is not rejected in the common mode rejection (CMR) function of the differential amplifier. Similarly, when the TEMF is in the op amp section, the TEMF error is amplified along with the desired signal, and retains the same relative error.

Another key consideration is to have the leads of your precision components made from alloys which have sufficiently low and inconsequential thermal EMF. Since there are at least two interconnections inside the component, any thermal gradient within the component itself will generate an internal thermal EMF voltage. Anything that will cause thermal gradients in a component should be avoided.

Thermal gradients can also be caused by the improper dissipation of internally generated heat from the component. Some manufacturers provide good internal thermal EMF design. For example, the internal thermal EMF of VPG’s Bulk Metal® Foil resistors is specified to be less than 0.1 μV/°C.

It is always a good idea to shield or keep low-level circuits away from high-power areas. A cooling fan can heat an area as well as cool it because of the way air-convection currents set up within the electronic package.

Low power-consuming op amps and higher-resistance voltage dividers always work better in low-level circuits. In general, good results can be obtained if the resistance values are kept between 10 kΩ and 150 kΩ.

It is important to understand the sources of thermal EMF in resistors. Since resistor leads or terminations are usually made from a material which is different from the resistive material, thermal EMFs in these components are created by external sources of heat, or by heat generated when power is applied to a resistor, or by both effects. In a resistor comprised of two leads or wraparound terminations to a resistive element, the thermal EMF voltage polarities at either end of the resistor oppose each other. If the ends are at different temperatures, this generates an error voltage that affects the DC circuits.

If the two points on the printed circuit board, where the resistor leads or SMD terminations are connected, are at different temperatures, the heat flow through the resistor will create temperature gradients inside the resistor. As a consequence, a differential thermal EMF will be created. Its magnitude will depend on the materials used, the construction of the resistor, and the temperature difference between the leads or terminations ΔT.

The resistor’s internal connections can also see different temperatures due to other external sources of heat, such as radiation from neighboring devices or air currents. The magnitude of the above effects is expressed in microvolts per degree centigrade. Another source of thermal EMF is the heat generated when power is applied to a resistor. Due to the dissymmetry of power distribution over the resistor surface and dissymmetry in heat conducting paths, the internal joints will see different temperatures, giving rise to thermal EMFs. In this case, the magnitude will depend on power applied and is expressed in microvolts for a given power level.

Of course, all the sources of Thermal EMF can be additive.

Different resistor types have different thermal EMF properties, as shown in the graph below, which compares thermal EMF performance for foil, thick film, and two metal-strip technologies. The graph (Figure 3) shows thermal EMF versus time when the same low-level thermal gradient is induced across each of four SMD 2512 size 0.05-Ω resistors.

Figure 3: The thermal EMF result for foil, thick film, and two metal-strip technologies, each with the same induced thermal gradient

Conclusion

Reducing circuit area introduces new design challenges associated with thermal management and its unintended consequences. Thermal ElectroMotive force (TEMF) introduces error voltages wherever temperature differentials exist at the junction of two dissimilar metals, such as where internal resistive elements are joined to the external terminations of a resistor.

Temperature differentials are developed across a resistor from uneven internal power dissipation, terminations heated by heat-radiating components, and from thermal dissipation paths running along the circuit board in both the conductive paths and the base board material itself. The foil, which was originally devised specifically for its innate TCR and its linear coefficient of expansion, also has a very low TEMF of only 0.05 to 3 μV/°C, depending upon the configuration. To achieve the optimum results in analog-circuit designs from a thermal EMF point of view:

• Locate DC circuits in an area of constant temperature.
• Use low thermal EMF components, such as Bulk Metal Foil resistors.
• Dissipate the internally-generated heat of the component in a way that reduces thermal gradients.
• Avoid convection and heat-radiation effects in DC areas.
• Use low-power components.

About the author
Yuval Hernik holds a BSc. in electrical engineering from the Technion (Israel Institute of Technology). He has been a director of application engineering at Vishay Precision Group–Bulk Metal Foil resistors since 2008.