July 1965 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Mica capacitors of the
dipped type were used quite a bit in the military electronics assemblies I used
to build in the early and middle 1980's while working at Westinghouse Electric
Corporation's Oceanic Division in Annapolis, Maryland. They had the
characteristic medium brown color and were shiny. According to Arco Electronics
author E. M. Rothenstein, mica is one of a very few natural materials directly
adaptable for use as a capacitor dielectric. "Mica, being a natural mineral and
adapted to use without physical or chemical alteration, is completely inert both
dimensionally and electrically. As a dielectric, it will not exhibit aging or
deterioration nor subtle variants in electrical properties." In 1965
when this article appeared in Electronics World magazine, mica capacitor
tolerances were in the realm of ±0.25%, which is amazingly good, and were
useable over the full -55 to +150°C MIL-SPEC range. Values from 1 pF through
1 μF were available. Most likely modern processes have found a way to
improve on the natural characteristics. In fact, this 2022 Cornell-Dubilier
datasheet for their line of
surface mount mica
capacitors suggests (IMHO) they are the world's most perfect capacitor.
Vintage Arco capacitors can be found on eBay.
Mica Capacitor Color Code
By E. M. Rothenstein
Executive Vice President, Arco Electronics
Mica capacitors are widely used because of their high stability and good reliability.
This article covers the toil, silvered, reconstituted, molded, and dipped types.
Mica is one of a very few natural materials directly adaptable for use as a capacitor
dielectric. Its physical and electrical properties, plus its rare characteristic
of perfect cleavage, elevate mica to a position of the very best natural capacitor
dielectric known today. Its position is further strengthened in comparison with
organic synthetics, such as plastic films, and multiple-composition dielectrics,
such as ceramics, by reason of its inherent stability. Mica, being a natural mineral
and adapted to use without physical or chemical alteration, is completely inert
both dimensionally and electrically. As a dielectric, it will not exhibit aging
or deterioration nor subtle variants in electrical properties.
The property of perfect cleavage enables mica fabricators to split blocks of
mica into sheets as thin as 0.0001 inch. The surfaces of the split sheets are parallel,
and the splitting along natural crystalline structure lines can be accomplished
with relative ease and uniformity.
The best of several varieties of mica is muscovite, a particularly clear form
with the best electrical performance. It is found primarily in South America and
the Orient, with the latter being the principal source. In listing the properties
of muscovite mica which pertain to its use as a capacitor dielectric, examination
of the figures will provide an immediate explanation of its popularity:
Dielectric strength - 3000 to 6000 volts per 0.00 1 inch.
Volume resistivity - greater than 2 x 1013 ohm centimeters.
Dielectric constant - 6.5 to 8.7.
Dissipation factor - about 0.0001 at radio frequency to about 0.001 at 60 cycles.
Operating temperature - approaching 500°C.
Mica, being a natural product, exhibits a non-uniformity of chemical composition
and purity. Gross variants are screened out, but variability is still present, as
evidenced by the figures just mentioned. Some synthetics have inherently uniform
characteristics which will sometimes prove to be a design advantage.
Fig. 1 - Mica capacitors are made in "sandwich" fashion.
As the frequency of the current passing through the capacitor
increases, a self-resonant condition is approached. The capacitive reactance, dominant
at normal operating frequencies, decreases, while inherent inductive reactance increases
with a rise in frequency. The inductance of the mica capacitor is found in its leads,
lead assembly, and the metallic internal structure of the active capacitive section.
Actual tests confirm the inverse relationship between resonant frequency and square
root of the capacitance value. The inductance of the capacitor remains relatively
constant for a given lead length. It is also very nearly the same for all case-size
configurations. Shortening the wire leads will effect a modest increase in resonant
frequency. As self-resonance is approached, dissipation factor rapidly increases
as the resistance of the capacitor becomes a greater percentage of the total impedance.
At resonance, capacitor is purely resistive.
The use of mica sheets in capacitors is accomplished by the classic technique
of interleaving alternate layers of insulating and conducting material, as shown
in Fig. 1. Each "sandwich" of insulation between two conductors creates the capacitor,
with capacitance being a function of area and spacing of the plates, and dielectric
constant of the insulation. Each sandwich of capacitance is paralleled by using
tin-lead foil conductors extending alternately from opposite edges of the mica stack.
All foils from one side are shorted together and connected to one capacitor termination.
The other foils, also interconnected, form the second electrode. Parallel capacitors
are additive in capacitance.
The conducting foil does not reach to the limits of area of the mica sheet, except
on the edge over which the foil extends for electroding. This area between the edge
of conductor and insulator is commonly called the "margin." This margin is a safety
area to prevent conduction from one plate to another around the edge of the mica
insulation. There is danger of a current path through impurities on the surface
of the mica or of flashover in air or gaseous material which may exist within the
capacitor structure. This is eliminated through provision for adequate areas of
The earliest mica capacitors in common usage were of a foil structure as described.
There were two primary classes, identified as receiving and transmitting. Both these
basic styles are still widely used today. The receiving mica is a smaller device
for low-power, low-voltage circuits with electrodes clamped to pigtail wire leads.
Molded-foil mica capacitors are limited to the order of 0.01 μf., a voltage rating
of 2500 v.d.c., and current-carrying capacity measured in milliamperes. The later
development of a silvering technique for capacitor plates has expanded these limits.
The transmitting mica capacitor, as its name implies, is used in higher power
circuits-voltages as high as 30,000 v.a.c. and currents in excess of 100 amperes
at radio frequency. To allow mica capacitors to withstand such electrical extremes,
it is necessary to increase the mica thickness to gain sufficient resistance to
voltage break-down, increase the margin area, pot the capacitor in insulating material
to eliminate gaseous elements, reducing the possibility of "flashover" around the
edges of the mica and corona, and increase the thickness of the conducting foil
to accommodate heavy current. Ribbons of foil are used to connect each plate to
the capacitor terminals.
When electrical stress levels are too great for the simple parallel capacitor
structure illustrated (as in the case of excessive voltage for a single sheet of
mica, or an excessive current requirement despite the low dissipation factor of
mica as a dielectric material, whose current-carrying capability is limited mainly
by the heat generated within the capacitor due to I2R losses), the capacitor
sections are connected in series. This provides a voltage-dividing effect, easing
the dielectric burden on the individual mica films. Since series connection of capacitors
provides a divisive capacitance effect, greater plate area is required per unit
of capacitance, and the volume of the capacitor per unit of capacitance is greatly
increased. Heat generation within the capacitor is diminished, and the larger surface
area of the capacitor enclosure affords faster dissipation of heat to the surrounding
For purposes of physical comparison with each other and with other types of capacitors,
the molded receiving-type capacitors vary in volume from well under 0.1 cubic inch
to somewhat less than a cubic inch. The potted transmitting styles vary from 3 to
more than 500 cubic inches in volume.
Although some paper and plastic-film dielectric capacitors have made inroads
into the mica transmitting field, the latter style of capacitor is still pre-eminent
in its area. However, the foil-electrode, molded receiving-type mica capacitor has
been largely supplanted by silvered mica capacitors and ceramic dielectric units.
The foil mica structure had excellent and unmatched stability by early standards,
and there was no suitable substitute for low-capacitance values until the advent
of ceramic capacitors. The ceramics eventually replaced foil mica to a large extent
in general-purpose, broad-tolerance use where capacitance change with temperature
was not a design factor. For the burgeoning market of precise frequency-selective
and timing circuits, foil mica did not fill the bill and was superseded by silvered
mica and temperature-compensated ceramics. So the original foil mica capacitor today
has a very small share of the capacitor market.
The silvered mica differs from the foil in that the conductive capacitor plate
is a thin layer of silver which is screened and fired onto the surface of the mica.
The following advantages are obtained:
1. The exact screening process provides an area on the surface of the mica which
is predictable in size and position to a degree not possible in a foil construction.
2. Since the plate is physically bonded to the dielectric, no relative motion
occurs in the presence of physical, electrical, or environmental stresses, making
the capacitor extremely stable and its variation with temperature closely repeatable.
3. The direct contact of plate and dielectric eliminates the potential presence
of air or other foreign matter in the active dielectric area of the capacitor. This
preserves the dielectric characteristics of the mica and eliminates instability
created by expansion and contraction of air pockets in foil mica capacitors.
4. The obvious instability created by warping or wrinkling of foil during fabrication
Slips of conducting foil are still used to provide an electrical path from the
silvered plates to the terminals of the capacitor. As in the case of pure foil capacitor
construction, the foils are brought out alternately at opposite edges of the stack,
folded to the top of the stack, and clinched together with the clamp-lead assembly.
Fig. 2 shows a silvered mica capacitor before it is encased in its protective enclosure.
In this case, the capacitor is a dip-coated unit.
Although mica capacitors are enclosed by means of a wide variety of methods,
there are four principal styles. Low-power receiving types are usually molded or
dipped; high-power transmitting types are potted in molded plastic cases or in large
ceramic tubes with metal terminations.
The molded-style receiving mica capacitors retained practical exclusivity in
the marketplace for some 30 years. The dipped style is a relatively recent innovation.
For some time, the mold material was a phenolic resin, and it remains as the case
material in a large percentage of molded micas made today. The phenolic resin has
excellent electrical properties, good moisture resistance, is hard, and will not
flow extensively during molding to contaminate the active capacitance field. However,
its high-temperature performance leaves much to be desired, and it is necessary
to use an impregnant (usually wax) to obtain a good moisture seal. The phenolic
will not bond to the pigtail lead when molding takes place, so a possible moisture
path exists at the interface of wire lead and mold material. This must be filled
by an impregnant to serve as a moisture barrier.
In recent years, softer plastics with greater ease of flow, even at substantially
lower molding temperatures and pressures, and better high-temperature characteristics
have been used as mold materials. A bond of sorts is made between the material and
lead, prompting some manufacturers to eliminate the impregnating process. However,
the electrical characteristics of the softer resins are not as good as those of
the phenolic, principally affecting dissipation factor, and resin moisture resistance
is not as good. Investigation into newly developed plastics for use as enclosure
materials continues today, and it is likely that markedly improved mold materials
for mica capacitors will appear in the near future.
Another facet of molding effects on the capacitor is the tendency toward reduction
of the life expectancy and increase in the failure rate of the mica section. The
heat and pressure of molding apparently produce a fatigue effect which influences
the reliability of the capacitor. This effect is noticeable in phenolic-molded units
and manifests itself to a lesser degree in mica capacitors molded in the softer
resins. Much higher reliability is obtained in capacitors which have been dip-coated
instead of molded.
The improvement in reliability, change in lead configuration to increase its
adaptability to printed circuitry, reduction of fabrication cost, and some saving
in size are all factors in development and rapid industry acceptance of the dipped
unit. The reduction in stresses related to en-closure also tends to decrease capacitance
change during the finishing process and makes for a more stable capacitor in operation.
The dip is usually a combination of resins applied in a series of coats to build
up necessary dielectric insulation and a moisture barrier. An excellent bond can
be obtained between case material and wire lead, completing the moisture seal. A
proper blend of resins will exhibit excellent moisture resistance, electrical properties,
and satisfactory performance at a temperature of 150°C.
If the dipping is performed at less than atmospheric pressure, a near monolithic
structure is obtained, giving the capacitor great strength and physical stability
and excluding corona-producing air and other impurities. A minor disadvantage of
this technique is the introduction of the case material into the dielectric field
of the capacitor. Since the electrical properties of the dip material do not approach
those of mica, higher dissipation factor and temperature coefficient is the result.
However, the increase in these characteristics is slight, not enough to change the
relative merit of the mica capacitor when compared with other dielectric materials.
The dipped mica capacitor is still capable of far exceeding minimum published standards
in all respects. Incidentally, these published standards are MIL-C-5C for military
applications and the Electronic Industries Association Standard RS-153-A.
Fig. 2 - An assortment of mica capacitors. Silvered mica capacitor
(upper right) is shown before application of coating.
Mica capacitance insulation resistance varies with the environmental
temperature present during its operation.
The higher power potted transmitting types are enclosed in molded plastic cases
specifically designed for the purpose or in large ceramic tubes. The mica capacitor
stack is clamped under pressure for retention of physical integrity and minimum
spacing between plates. The capacitor is inserted in the case with proper electrical
connections made to screw terminals, and the structure is potted. In the case of
the large tubular styles, the large metal end caps also serve as terminals, and
a compressive force during closure adds to the assembly's strength and rigidity.
It might be well to point out that some mica capacitors have been made in tubular
forms, using the rolling technique normally associated with paper and plastic-film
capacitors. Even the thinnest natural mica sheets are not flexible enough to withstand
rolling; but in this case, the rolls of dielectric are not mica in natural form
but reconstituted mica. Reconstituted mica is made by flaking the mica into very
small pieces and then reorienting the flakes into paper-like thin sheets. usually
incorporating a small percentage of organic binder material. The binder material
has an appreciable effect on electrical characteristics. However, in one instance,
a firing process has enabled re-constitution of the mica flakes into sheets without
the use of binders so that most of the basic properties are retained. The use of
reconstituted mica capacitors is limited to certain applications for which they
are particularly adaptable.
Whether for receiving or transmitting applications. the principal characteristics
of mica manifest themselves in the ultimate decision as to the type of capacitor
to be employed. In the case of a high-power application, the decisive factors are
the fundamental security inherent in use of this natural material, coupled with
electrical excellence when the dictates of application clearly demand mica. A properly
designed high-power mica capacitor will operate year after year, withstanding high
dielectric stresses and passing heavy a.c. current without noticeable deterioration.
During this period, the capacitor may be expected to maintain its original capacitance
value within narrow limits, and the effect of changing temperatures is minimal.
When we consider low-power applications, again the inherent stability of the dielectric
and its relative insensitivity to electrical or environmental variants is decisive.
A silvered mica capacitor may be manufactured to extremely close tolerances; but
more important, it can be expected to retain the original setting throughout operating
life. To verify this fact, it is well to call attention to the fact that when size
limitations eliminate the use of air-dielectric capacitors as standards, mica dielectric
is used for the fabrication of primary capacitance measurement standards.
After establishing the key points of stability and reliability (there is more
actual documentation on verified reliability of micas than any other), certain comparisons
can be made with other dielectrics. These may demonstrate a narrow superiority of
other materials within limited spheres of performance, but for all-around use, mica
still plays a most important role.
1. Temperature-compensating ceramic capacitors (specifically NPO zero-temperature
coefficient) may have temperature coefficients closely controlled to a point where
capacitance variation with temperature may be less than that for a silvered mica
capacitor, but the mica will have better long-term aging characteristics, a much
lower dissipation factor, less sensitivity to frequency variation, less sensitivity
to voltage variation, and smaller size in high-capacitance values.
2. Polystyrene dielectric capacitors may have lower dissipation factor and dielectric
absorption, but again, mica will have superior long-term stability, will operate
reliably at much higher temperatures, have a much lower temperature coefficient
(percentage change in capacitance value per unit change in temperature), and physically
be a more rugged dielectric material.
3. Polyester dielectric capacitors may rival mica in reliability (with conservative
design), and they may be physically rugged and provide greater capacitance per unit
volume. But in all other respects, from a standpoint of electrical and environmental
performance, mica remains superior.
Mica Capacitor Characteristics
Mica-Capacitor Color Code
The electrical value of a mica capacitor can be determined from an examination
of the six-dot color code imprinted on the body of the capacitor.
When the capacitor is being read, the direction indicator (an arrow on the body
in some form) should point to the right of the reader. The dots are read from left
to right across the top row and from right to left across the bottom row, in that
The only exception which may be made occurs when three significant figures are
to be found in the characteristic. In this case the upper three dots represent the
three significant figures and the identification color is eliminated.
If the color code is to be used to identify a MIL molded or dipped mica capacitor,
a nine-dot sequence is used. The six dots on one face of the capacitor remain and
have the same identification as the six-dot code previously discussed. There are
three additional dots all the reverse side of the capacitor and these are identified
Mil 9-Dot Code
Posted September 19, 2022