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.
Continuing with the series
on capacitor types, particularly dielectric material, this July 1965 Electronics
World magazine article reports on glass materials used by
Corning Glass Works. Glass dielectrics are popular for aerospace and space
applications because of their tolerance for high radiation levels found in
regions not protected by the Earth's atmosphere. Glass compound consistency
provides for mass producing values with tight tolerances and exceptional
parameter tracking over temperature. High "Q" values and low loss at extreme
temperature and high frequency (at the time) made them the component of choice
by missile and satellite designers. 0.5 pF through about 0.01 μF is the typical
value range for glass dielectric capacitors. Author Archer Martin mentions
radiation exposures of 1018 NVTth, which appears to be a measure of
neutron flux exposure, but I could not find a good definition of the term
("NVT," without the "th" is used
A typical glass capacitor such as those discussed in the text.
By Archer N. Martin / Corning Glass Works
Known best for its aerospace applications, this capacitor is now being used in
other high-quality circuitry. Various types of glass capacitors, with their characteristics,
The history of the glass capacitor began in World War II, when the sources of
natural mica were threatened. Because glass is electrically and physically stable
and chemically inert, the Signal Corps and Bureau of Ships suggested that Corning
Glass Works begin development of a glass dielectric to replace capacitor mica. An
optical-grade, lead-potash composition was produced with electrical properties equal
to and in some cases better than those of mica. A process was then developed to
form the glass in an extremely thin ribbon of practical width. With this dielectric,
the company introduced a commercial glass capacitor in 1951 that was a direct substitute
for the mica capacitor.
The capacitors are made by stacking alternate layers of aluminum foil and glass
ribbon until the desired capacitance is obtained, then fusing the assembly into
a monolithic block as shown in Fig. 1. The same glass composition is used for both
the case and dielectric, insuring that the electrical properties of the capacitor
are entirely those of the dielectric. Leads are welded to the electrodes to form
a glass-to-metal seal where they enter the case, creating a truly hermetic capacitor.
Military specification MIL-C-11272 was written to describe only glass capacitors
but has been broadened in recent years to include other types.
Fig. 1 - Basic construction of the glass-dielectric capacitor unit.
Fig. 2 - (A) Quality and dissipation factor and (B) percent capacitance
change with frequency for glass dielectrics.
Fig. 3 - (A) Insulation resistance and (B) capacitance change
with frequency for glass-dielectric capacitors.
Plastic-potted glass capacitors are made for printed boards.
Because of interest generated by the exotic aura of aerospace missions, the glass
capacitor is known mostly for its application in missiles and spacecraft. With the
glass capacitor, Corning was the first company to meet the Minuteman component-reliability
research and development goals of the Autonetics Division of North American Aviation.
This singular success helped make the glass capacitor famous, but it also may have
tended to obscure its usefulness in fields aside from the specialized one of reliability.
A creditable market exists for the component in such equipment as high-quality test
sets, measurement instrumentation and process controls, in such functions as high-"Q"
filtering, reliable bypassing in r.f. circuits, and peripheral computer applications,
and in other systems where circuit parameters cannot be allowed to wander.
The singular advantage of glass-dielectric capacitors is the way they help circuit
designers predict the operation of their circuits. There are few components possessing
as sturdy and consistent a history of stability and reliability. The manufacturer
states, for example, that any two of its units, regardless of capacitance value
or size, exhibit temperature coefficients within 10 PPM/°C of each other.
For a designer, this means that his circuits containing many capacitors will
track accurately and predictably during temperature variations. In addition, retrace
of the TC of glass capacitors is essentially absolute, assuring negligible hysteresis.
Capacitance drift with load life is less than 0.5%. Voltage coefficient is stated
as zero, so capacitance can be expected to remain the same despite voltage levels
Other specs that characterize the glass capacitor are extremely low losses and
high "Q", even at elevated temperatures and high frequencies. With such low losses,
designers an realize higher "Q" and the resultant narrower bandwidths in filters
and tuned circuitry.
Most glass capacitors range in capacitance from 0.5 to 10,000 pf. (0.01 μf.)
but can come as high as 150,000 pf. (0.15 μf.). The capacitance range below 10,000
pf. makes up about 15 to 20% of the total capacitor dollar. Tolerances for glass
capacitors range from 1% to 20%, d. c. working voltage is generally between 300
and 500 volts but can be made as high as 6000 volts, and operating temperature range
is from -55 to +125°C.
Fig, 2A shows the range of quality factor ("Q") and dissipation factor with frequency,
while Fig. 2B demonstrates the percent capacitance change with frequency. Fig. 3A
shows the insulation resistance with temperature curve for glass capacitors, while
Fig. 3B illustrates the capacitance change with temperature.
Temperature coefficient (TC) of these capacitors is +140 ±25 PPM/°C
at 100 kc. with essentially absolute retrace. At any given temperature, the TC will
not deviate from the curve by more than 5 PPM.
Load life is less than 0.5 % capacitance change after 2000 hours at 125°C
with 150% of rated voltage applied. The change in resistance with moisture is negligible
with the hermetically sealed units, and the plastic-potted types show an insulation
resistance in excess of 1010 ohms after standard military tests.
Radiation resistance, except for the plastic-potted units, shows no significant
change in capacitance from exposure to levels of 1018 NVTth.
The plastic-potted unit (Type TYO) was developed to provide a glass capacitor
at prices comparable to those of ceramic units. Potted in a plastic shell, this
device has parallel radial leads spaced symmetrically with the square-cornered case
on 0.1-inch grids. This unit lends itself to fast, high-density, upright installation
on printed-circuit boards.
Two other types of glass capacitors are designated CYFR and CYFM styles by the
company. Both are fusion-sealed, thus making them virtually impervious to environment.
The CYFR capacitors have an established reliability figure, proven under Minuteman
stress conditions, of 99.9996% per thousand hours at a 60% confidence level and
at full-rated voltage at 125°C. Stated another way, this figure means that no
more than four failures (deviation from specified performance) will occur in a billion
part-hours of operation.
The CYFM style is electrically and environmentally interchangeable with the CYFR.
However, the former units provide cost savings up to 60%, since they do not undergo
the expense of extensive reliability testing and documentation.
Another type of glass capacitor is represented by medium-power transmitting units,
designated CY60 and CY70 rated at 1 kva. and 7.5 kva., respectively.
Power ratings significantly higher than listed can be easily obtained merely
by making sure that the operating temperature of the capacitor remains below 125°C.
This can be done with terminal and/or body heat sinks. Terminal heat sinks are very
efficient because of the many layers of solid aluminum that extend into the capacitor.
The CY60 and CY70 capacitors are used for r.f. applications such as power amplifiers,
low-power transmitters, and many electronic devices in grid, plate, coupling, tank,
and bypass functions. Their relatively small size and low weight make them suitable
for airborne equipment and mobile transmitter applications.
Performance of the glass capacitor is a direct function of its dielectric. The
composition is closely controlled by the manufacturer from day to day, month to
month, and year to year, and unlike mined dielectrics does not have to be graded
or culled. During more than a decade of commercial experience, this control is what
has given the glass capacitor its reputation for stability, reliability, and performance
Posted September 12, 2022