Resistors Improve Performance While Their Size Decreases
May 4, 1964 Electronics Magazine

May 4, 1964 Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Electronics, published 1930 - 1988. All copyrights hereby acknowledged.

When the electronics product world consisted of vacuum tube based circuits, the physical sizes of standard fixed-value passive resistors, inductors, and capacitors were not of much concern in terms of how much volume they consumed. R's, L's, and C's, had wire leads protruding from their molded bodies, or in the case of larger power supply filtering capacitors had solderable tabs. Point-to-point wiring consisted of components and hookup wire suspended in the air between solder terminal strips and tube base tabs. Even with miniature (peanut) tubes, all but the largest passives had no significant impact on overall unit size. Once semiconductors came onto the scene, everything changed. Suddenly, even the standard 1/4 W carbon resistor and tantalum capacitor became a significant factor when attempting to reduce size and weight of electronic assemblies. Component manufacturer research and development departments shifted into high gear to keep up with what would become a rapid paced race to see who could make the smallest, lightest R's, L's, and C's. By the time this article appeared in a 1964 issue of Electronics magazine, significant advances had been made in component miniaturization with metal film and printed thick film resistors. Low power consumption of semiconductor circuits meant that many components no longer needed to handle as much power, so that helped with volume reduction - and reliability. The component industry was still a decade or so away from widespread use of surface mount components, but a paradigm shift had occurred that put them on track to adapt as needed. 

Resistors Improve Performance While Their Size Decreases

Various kinds of miniature resistors are compared in capabilities and in method of production

By Charles L. Wellard

President, American Components, Inc., Conshohocken, Pa.

Resistor technology is moving rapidly into high-density packaging and is looking beyond toward fully integrated circuits.

Widespread use of these components in integrated circuits still seems to be a long way off. Therefore it is well to be familiar with miniature discrete resistors and their improved operation under increasingly severe conditions.

Since the resistor is the most extensively used electronic component, there is a growing demand for small, reliable resistors in modern miniature circuits. Miniature resistors comprise only 5% of present resistor sales, but that share is expected to climb to 20% by 1971 and 50% by 1976.

Materials Compared

The basic materials in miniature resistors are essentially the same as those used in larger sizes. The most common materials are composition carbon, deposited carbon, metal films and glazes.

The table below lists these materials and compares their capabilities. The information is an average of the best capabilities reported by contributing manufacturers, and does not imply that each manufacturer conforms to these standards.

In general, the better type of precision metal films offer the highest performance.

Composition Carbon

The standard-size 1/2-watt composition-carbon resistor is still the workhorse of the radio-television industry. A mass of carbon is molded around two wires, and an insulator is molded around the carbon. This unit maintains a standard tolerance of 20%. It is used by the millions in industrial controls, radio, television, high-fidelity and stereo sets.

One miniature composition-carbon product is of slug-carbon construction. This component has the normal cylindrical shape with axial leads. It is used extensively in hearing aids, where initial tolerance and stability are not severe, and where economic factors are important.

The deposited-carbon resistor, introduced in 1937 or 1938, is a quality pyrolytic-carbon film-type component. The resistance material is deposited on a base in films of varying resistivity. The film characteristics can be controlled over a wide range.

The resistor is usually coated with epoxy or silicone to protect it from moisture. For higher performance levels, the resistor is sometimes molded in an insulating compound. Tolerances of deposited-carbon resistors can be held to 1%. They are being produced by the millions, but they are being gradually replaced by metal films of the evaporated, glazed or oxide types.

Little effort has been made to develop miniature resistors using a deposited-carbon process. This is because a metal film can do practically everything that a deposited-carbon film can do, only better.

While deposited-carbon types are slightly cheaper than metal films, many customers are willing to pay a little more for a far superior product. Deposited-carbon resistors are going into digital computers where stability is more critical than is required for composition-carbon types. They are used in television tuners where a 10% shift in resistor values during usage cannot be tolerated.

More Ohms Per Square

Most of the activity in miniature discrete resistors has been in the use of metal films. Metal-film types take up less space for the same value of resistance than precision wire-wound types. Beyond 25,000 ohms, metal films result in great reductions in size. Manufacturers are delivering a full range of metal-film resistors at prices that are crowding out some carbon-composition types and the trend is expected to continue. The metal-film type is geared primarily to military equipment, space and missile work, ground support equipment, radar, telemetering, communications, and any application that requires low temperature coefficients, high-temperature operation, and tight tolerances. Metal-film types are used in analog and digital computers.

The photographs here show some examples of miniature metal-film resistors. The conformal-coated type are protected by coatings of epoxy or similar material.

The latest military specification covering precision metal-film resistors is MIL-R-10509E. The environmental requirements set forth in this specification are so stringent that only molded or encapsulated types can constantly meet them. At present, the smallest size covered in this specification is the military-style RN55, rated at 0.1 watt at 125°C. This unit is 0.270 inches long with a body diameter of 0.110 inch. RN55 is compared with two miniature molded types.

The noble metal film used in the construction of these resistors has a noise level well below that of carbon-composition and deposited-carbon types. The average level is less than 0.10 microvolt per volt. The advantages of miniature molded resistors include a marked increase in mechanical strength; more environmental protection, especially against moisture; high dielectric strength; and relative immunity from damage in handling, such as damage due to intimate contact with a soldering iron due to carelessness of an operator during insertion.

Although the molded units are somewhat larger than those of the conformal coated types for the same wattage rating, they nevertheless offer tremendous size advantage over the RN55 while maintaining the same performance levels. The military is now proposing additions to specifications to cover miniature resistor types. At least one style is scheduled for coverage during 1964, probably a 0.05-watt style.

Precision miniature resistors can be inserted into custom-made assemblies as individual components. Yield problems, connected with the deposition of multiple components, are thus eliminated.

CTS of Berne.

Glaze resistors are in direct competition with deposited-carbon and tin oxide types. Glaze resistors should cost less than the other two types when they are mass-produced more extensively.

Glazes, or liquid conductive materials, were developed by the Dupont Co. They consist of carbon and powdered metals, such as chromium or molybdenum, dispersed in a liquid glaze. Glazes compete with deposited-carbon and composition-carbon types and fill the performance gap between.

Circuit Boards and Substrates

Another miniature metal-film resistor is the flat type, designed for printed circuit boards. The body is box-shaped, usually longer than it is thick. Leads normally come out parallel to the length for plug-in to circuit boards. In some types the two leads are on the same axis; in others they are offset. A resistor of this type, FE 1/20 is shown.

This precision flat microminiature resistor was designed to eliminate the yield problems connected with direct deposition of multiple resistors on a common base or substrate. The flat resistor can be attached directly to wafers or printed wiring boards. It has special high-conductivity substrates with fired-on terminations of noble metals. A precision metal film is deposited between the termination areas to the range of resistance desired. Two thin coats of a high-temperature silicone are applied over the film for protection and the unit can be supplied for direct soldering or with ribbon leads for soldering or welding.

Another type offering certain mounting advantages, particularly in miniature cord wood techniques is shown above. This miniature metal-film type is available up to 110 K.

Major emphasis has been placed recently on techniques for depositing thin-film resistance elements on a thin substrate. In most cases the substrate plates are 0.032 of an inch thick or less, with an area of one square inch or less. The width is usually within 50% of the length.

The techniques of using the materials shown in the included table are used to produce resistors as an integral part of the base substrate. In such cases, the substrate is usually a high-quality ceramic or glass, onto which has been fired a pattern of noble metal conductors. A portion of these conductive areas supplies the terminations across which the resistive element is applied.

From table 1, it would appear that the material with the most resistance per square would be the most desirable. But other aspects must be considered. Resistive materials with the most ohms per square are usually the least capable of holding the resistance value within required tolerances and of retaining their values with changes in temperature.

On the other hand, evaporated metal films can be applied through precision masks, by photographic techniques, or other pattern or matrix devices, to obtain a large number of squares, and thus a high resistance from their otherwise-limited range of ohms per square.

The ability of metal films to achieve the resistance desired in the first, or blank, stage of resistor construction is within 8% of the desired resistance. A groove or helical pattern is then cut around the blank to achieve the final resistance.

Up to 80% of the units that are produced have an inherent temperature coefficient of resistance within ±50 ppm. per degree centigrade.

Since applying multiple resistors to a common substrate still involves severe technical problems, hybrid circuits are receiving considerable attention.

This year, about 90% of all miniature electronic equipment produced will be built with hybrid circuits. Hybrids combine the solid or integrated circuits and the individual, discrete miniature component. About 20% of these hybrid circuits are made up of the true solid-state circuits that are deposited directly on a substrate or formed in a solid block of silicon. And 80% of these hybrid circuits contain discrete components.

Shrinkage by the Numbers

By 1971, it is estimated that half of the miniature equipment market will use hybrid circuits. Of these, 50% to 70% of the hybrids will be built with solid circuits, the rest will use discrete resistors.

Pellet-film resistors, made of a powder mixture of noble metals and their oxides, are being supplied in production quantities. These resistors are becoming increasingly popular in miniature circuits. The range of values, tolerance and temperature coefficients available in pellet components is approaching that of conventional components.

Pellet circuit elements are adaptable to automatic handling techniques in preassembly testing and circuit fabrication. The pellets show that the basic resistor design consists of a spiral of resistive material.

An alternate design is a fluted pellet, which is used for low resistance values and in very-high frequency applications. The pellet assembly demonstrates a method of high-density pellet packaging using solder-coated terminals and connectors. Resistor pellets and wafers can be packaged with solid-silicon circuits to provide electrical properties with tolerances that would be impossible without individual components.

The miniature resistors are finding considerable use in these hybrid circuits. Metal film resistors are offered in a selection of tolerances and temperature coefficients to match requirements with economy.

Tin Oxide

The tin oxide resistor bridges the gap between composition-carbon and metal-film types. Tin oxide resistors are competitive with composition carbon and are cutting into the deposited-carbon types.

Offering more economy, but with limited properties, the tin oxide resistor fills a need that is less critical than that posed by metal films. Tin oxide resistors contribute an economic advantage, particularly for glassmakers. The stannous chloride used in tin oxide resistors can be deposited on a glass substrate, which is less costly than a ceramic substrate. The tin oxide resistor has a limited range of coverage, compared with metal film types. Tin oxide seldom produces a resistance range over 800 ohms per square. Also, the tin oxide resistor has a temperature coefficient of 200 ppm per degree centigrade. This is high compared with metal film. The table shown above presents a chronology of prices for the various resistor types discussed.

There is some activity among makers of wire-wound resistors to produce miniature resistors. Some wire-wounds are small, but none are as small as the subminiature types we have discussed. Limitations have to do with the wire size, the smallest being about half a mil. Miniature wire-wounds are limited to 5,000 ohms. However, wire-wounds may find some application in the very low values of resistance, particularly under 50 ohms, where most metal-film manufacturers leave off. Applications would be limited, but an assured source of supply would be helpful to the user.

The User's Responsibility

While the component manufacturer has a responsibility to produce a resistor of known reliability, and must state this reliability in common accepted terms, factors beyond the inherent qualities of the miniature resistor itself affect reliability.

The responsibility for a resistor rests not only with its manufacturer, but also with the user. Unreliability in miniature resistors and in other miniature electronic parts is usually caused by handling, misuse, misapplication or abuse from the time the component is received to the time it is put to use. Miniature resistors are more fragile than those of conventional size. Handling requires a softer touch, and soldering temperatures must be controlled more carefully.

A miniature unit does not act as a heat-sink barrier. While larger units are being soldered, the resistor itself acts as a heat-sink to reduce the temperature of the iron tip. It is not uncommon for a miniature unit to reach the temperature of the applying iron in less than eight seconds.

Posted May 6, 2024
(updated from original post on 5/13/2017)