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
Charles L. Wellard is the author of Resistance and Resistors,
McGraw-Hill, 1960. He received a Bachelor of Science degree from the Massachusetts
Institute of Technology in 1946 and a Master's from Carnegie Tech in 1947, both
in electrical engineering. For the past three years he has been president of American
Components. Inc. He also worked for three years at the Clifton Precision Products
Co. as technical director of the systems division.
Various kinds of miniature resistors are compared in capabilities and in method
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.
Resistor types with size comparisons.
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.
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.
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.
Resistive materials and their capabilities.
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
Flat cermet microplanar resistor. Electra Manufacturing Co.
Spiral and fluted resistor pellets. P.R. Mallory & Co.
Evaporated R-C network.
Construction of flat type metal resistor, FE-1/20, not to scale.
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
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.
Comparison of metal film specification limits.
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.
Resistor wafer in substrate.
Mounting advantages for cordwood.
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.
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
Chronology of resistor price and performance.
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 13, 2019