November 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.
This "Glass for Electronics"
article in a 1965 issue of Electronics World magazine provides some really
interesting information about the properties of glass which I, for one, either never
knew or have forgotten. One such point is that glass is typically defined with a
"softening" temperature rather than a melting temperature. That is because the final
characteristics of the glass is highly dependent on the cooling down time/temperature
profile. Those of us having been in the world of automated printed circuit assembly
(PCA) solder oven operation are familiar with the criticality of time/temperature
profiles, so the concept is not new. In the case of PCA's, profiling is necessary
to accommodate the often widely varying thermal dimension changes over temperature
to prevent fracturing. With glass, it is the final atomic alignment (or misalignment)
that is dependent on the cooling process, akin to tempering of metal (although the
metal is not heated to the point of liquidus flow). At the time of this article,
Corning Glass Works claimed to have >100k unique formulas for glass using various
mixes of elements.
Glass for Electronics
By John R. Collins
New types of glass are playing an increasingly important role in electronics.
They are finding use in delay lines, precision resistors and capacitors, radomes,
and CRT's used for readout.
From the time of the first pioneer efforts in electronics, glass has been prominent
as an insulator, an enclosure, and a dielectric. Special glasses created in recent
years have provided greatly improved electronic products and, in many instances,
have made simplified manufacturing techniques possible.
The variety of glasses now available is staggering. Practically every element
has been utilized in glass-making, and some glasses contain 20 to 30 distinct ingredients.
Corning Glass Works alone reports that it has more than 100,000 different glass
Fig. 1 - Delay lines used in radar, computers, and other electronic
devices can be made from precise polygon of hard glass.
Fig. 2 - High-temperature, high-strength glass-ceramics are used
as combination nose cone/radome in many guided missiles.
Despite this variety, about 90% of all glass produced in the United States is
a type called "soda lime," which is made from approximately 70% silica sand (silicon
oxide) mixed with oxides of sodium and calcium. The alkali oxides act as fluxing
agents and form, with the silica, a mixture that softens and flows at a lower temperature
than pure silica. As the glass cools from the temperature at which it is a true
fluid, its viscosity increases rapidly. The resulting decrease in atomic mobility
is sufficient to prevent the formation of an orderly atomic pattern, and normally
glass does not form a crystalline structure. Instead, a random pattern is frozen
into the glass, a fact which accounts for many of its properties. Since the transition
from a solid to a liquid takes place over a temperature range, glass has no definite
melting point. It is customary, therefore, to refer to the softening temperature
of glass rather than its melting point.
Soda-lime glass is widely used for windows, bottles, drinking glasses, and lamp
bulbs, but its applications in electronics are limited. With a relatively low softening
temperature, it cannot be used where excessive heat is encountered. Surface conductivity,
an important consideration for many applications, is low because glass with high
alkali content tends to absorb moisture. under conditions of high humidity, the
resistivity of soda-lime glass is only about 107 ohms/square, compared
with 1010 ohms/square for glass with low alkali content. This restricts
its electronic usage.
The terms "hard" and "soft" as applied to glass refer to service temperature
rather than mechanical hardness, the dividing point being about 400°C. Aluminosilicate
glass, which has low sodium content and contains a high proportion of aluminum oxide,
has a softening temperature over 900°C and will give useful service up to 650°C.
It has good electrical and chemical properties and is often used as an envelope
for high-performance power tubes and traveling-wave tubes. When melted in optical
quality, it is virtually free of defects and is then useful for faceplates for cathode-ray
The hardest glasses are those containing the highest percentage of silica. A
glass of about 96% silica is made by chemically removing almost all elements except
silica from boro-silicate glass. This leaves a porous structure, so it is then necessary
to fire the glass again - a process that adds to the expense. Nevertheless, such
glass does not begin to soften below 1000°C and can be used regularly at 800°C.
It can be heated to a cherry red, then plunged into ice water without ill effect.
Glass of this kind has excellent infrared transmission properties and has been used
for windows in heat-seeking missiles.
An even harder glass is made by fusing pure silica in which impurity levels are
held to less than one part-per-million. This product is the most transparent glass
ever made and is used in the finest telescopes. It is also used for laser-beam mirrors
and for infrared and ultraviolet windows. Because it has a service temperature from
900 to l100°C, it is used for crucibles needed to grow silicon crystals for
diodes and transistors.
Fused silica glass will transmit an ultrasonic signal with practically no attenuation
or scattering, and this has led to its use in ultrasonic delay lines for radars
and computers. The electronic signal is converted by a transducer into an ultrasonic
signal which travels through the glass until it is picked up by a second transducer
and converted back into electrical energy. Relatively long delay times - 150 micro-seconds
or more - are achieved by means of a folded path whereby the energy is reflected
from facet to facet of a precisely ground polygon. Delay times up to several thousand
microseconds are thus possible in a relatively small physical space.
The advantages of very low acoustical attenuation are offset where temperature
varies widely by the fact that the coefficient of time delay in fused silica amounts
to about 80 PPM/°C. To eliminate the need for temperature control, a special
glass was developed with a coefficient of time delay of only 0.5 PPM/°C. This
material was used in the delay lines (Fig. 1) in the shift registers of the digital
guidance computer for the Gemini manned spacecraft. An important consideration in
the selection of this glass was its insensitivity to vibration and other mechanical
forces encountered in space flight.
Even tougher than fused silica glass is glass-ceramic, which is so hard that
it can be used to make ball bearings for machinery. Glass-ceramic devices are first
formed by ordinary techniques from a special glass batch to which nucleating agents
have been added to promote crystal growth. After the device has been cooled, it
is reheated and subjected to treatment that causes the crystallization of billions
of invisible crystallites throughout the glass body.
Glass-ceramic produced by heat treatment is opaque and far stronger and harder
than the parent glass. It has greater impact and abrasion resistance and improved
thermal and electrical properties. Types have been developed with excel-lent dielectric
properties at microwave frequencies, and these have proved useful for missile radomes.
A radome (Fig. 2) permits passage of radio waves for guidance of the missile and
at the same time protects the internal guidance system from its environment. Such
glass-ceramic radomes have nearly constant dielectric properties over a range from
25 to 500°C; the dissipation and loss factors are nominal.
Fig. 3 - Glass can be fabricated in a number of intricate patterns
as shown by this arrangement of fluid amplifier components.
Fig. 4 - These precision quarter-watt metal-oxide film resistors
use glass bases and can dissipate one-tenth of a watt.
One of the most interesting and useful recent developments is a type of glass
that is sensitive to ultraviolet light. The discovery grew out of the observation
that the windows of old houses sometimes take on a violet coloration. Further investigation
led to the finding that certain types of glass, under the influence of ultraviolet
radiation, form nucleated crystals. Finally, it was determined that such crystals
could be further developed by heat treatment and that they are then 20 times more
susceptible to attack by hydrofluoric acid than glass that has not been exposed
to such radiation.
When exposed to ultraviolet light and heat, photosensitive glass behaves much
like photographic film. An image is formed that is a permanent part of the glass
and which extends in depth throughout the body of the glass. Exposed areas turn
an opalescent white after heat treatment, while unexposed areas remain clear. Because
of the marked difference in sensitivity of the exposed portion, the pattern produced
by photographic means can be etched out with hydrofluoric acid, leaving an exact
copy of the negative through which the exposure was made.
The process of chemically etching or machining glass has proved a most useful
discovery, since glass is difficult to machine by conventional techniques. Fig.
3 shows how photosensitive glass can be chemically machined to form the intricate
pattern needed for a fluid amplifier. Developed by the Diamond Ordnance Fuze Laboratories,
these devices utilize fluids to perform logic functions normally accomplished with
electronic circuits. Other applications for photosensitive-glass etching include
printed-circuit boards, solid-state substrates, optical coding discs, and various
laminated structures. Fine glass screens, containing more than 350,000 precisely
located holes per square inch, are utilized for aperture masks in the manufacture
of color-television picture tubes.
Glass itself is an excellent insulator, but it can be made to conduct electricity
by firing a metallic-oxide film onto its surface. Although such films are usually
less than 0.1 mil thick, they are durable, stable, and-in most cases - transparent.
Through selection of the metallic oxides to be used and control of thickness, electrical
resistance may be obtained anywhere in a range from about 10 to 106 ohms/square.
Coated glass was initially used for residential and industrial heating panels,
self-defrosting windshields and rear-view mirrors, and similar devices. Since the
metallic surface reflects about 50% of the infrared radiation which strikes it,
while letting most of the visible light through, coated glass is also used for windows
that permit viewing of industrial processes involving great heat without discomfort
to the viewer.
If coated glass panels are grounded, they can be used to shield delicate electronic
instruments against r.f. radiation. Used in fluorescent lighting fixtures, they
ground interference but will permit light rays to pass relatively unobstructed.
Tin oxide fired on a glass substrate that matches its coefficient of expansion
forms the basis for a tough resistor with excellent stability under adverse conditions.
Such resistors may vary from tiny devices, shown in Fig. 4, to massive units four
feet long and five inches in diameter which are used as dummy antenna loads for
testing transmitters. They have low noise characteristics and may be hermetically
sealed in glass envelopes for protection against moisture and contaminants.
A related application of the same technique is the production of inductors by
firing metalized silver conductors into special glass coil forms. The thin metal
coatings have almost zero turn-to-turn capacitance, and the inductors can be used
at frequencies to 250 mc. (Fig. 5).
Fig. 5 - Precision variable inductors are fired on glass bases.
Fig. 6 - Cathode-ray tubes with conducting mosaic faceplates
are often used in high-speed electrostatic printing equipment.
Mosaics consisting of a large number of microminiature conductors, precisely
aligned and hermetically sealed in glass, are now being produced, and this has led
to the development of a system of electronic printing. A typical array consists
of conductors one mil in diameter spaced at intervals of four mils, providing a
density of 62,500 conductors per square inch. The mosaics are sealed into the face
of cathode-ray tubes. Some of the tubes presently available are shown in Fig.6.
One of the first applications of conducting mosaics was for the printing of address
labels for magazines. Television-type signals were fed to the cathode-ray tube by
a character generator, and electrostatic patterns of the characters were formed
on a continuous paper web as it passed before the tube. The charged paper was first
processed to make the images permanent and was then cut into individual labels.
This system made it possible to print 36 labels each second.
Similar tubes are used in an arrangement for keeping track of individual railroad
cars. As trains enter a yard and pass by a scanner, images are taken sequentially
of each car in each train, The car numbers showing in the electrostatic print-outs
enable fast pinpointing of the location of any car of any train in the yard.
Cathode-ray tubes with conducting mosaics are also used in certain facsimile
Glass for Capacitors
Although serviceable capacitors were made many years ago from window glass, it
has since been possible to improve the product considerably by careful selection
of the best glass for the dielectric. A number of factors enter into the choice.
Pure silica glass has the lowest dielectric constant - about 3.8. The dielectric
constant is increased by the addition of fluxing agents to the glass mixture, the
largest increases resulting from the heavier ions. Glass containing a high percentage
of lead, for example, has a dielectric constant of 15 to 17. However, such glass
has high power factor and loss and is therefore not used for capacitors.
Even more important than dielectric constant is dielectric strength. This follows
from the fact that the amount of energy that can be stored in a capacitor varies
in direct proportion. to the dielectric constant, but also in proportion to the
square of the dielectric strength. Therefore, a glass with twice the dielectric
strength is as effective as one with four times the dielectric constant.
It has already been pointed out that soda-lime glass has relatively low resistivity
and hence would not be the best choice for capacitors. Instead, a potash-lead glass
is often used. It has a dielectric constant of about 8.8, high dielectric strength,
and low power factor and loss. It can be drawn in the form of a thin, flexible ribbon
about one mil thick, free from holes, cracks, or other imperfections. Furthermore,
it matches the coefficient of expansion of the aluminum foil to which it is mated.
Alternate layers of ribbon and aluminum foil are sealed together at high temperature
to form rugged, monolithic units. Leads are attached, and the dielectric stack is
then fused into a compatible glass case.
Fixed glass capacitors are produced in a range of values from about 0.5 pf. to
about 150 μf. and in working voltages to 6000 volts. They are characterized by
high "Q," good stability, and excellent efficiency at temperatures to 300°C.
Miniature glass trimmer capacitors (Fig. 7) provide a capacitance change of only
0.4 pf. per turn, permitting very precise tuning. The tuning curve is linear, and
the units can be hermetically sealed against moisture.
A very large number of special glasses have been devised to fill particular needs.
For instance, rods made of borate- or silicate-base optical glass and doped with
oxides of the rare-earth elements neodymium or ytterbium now serve as laser elements.
The glass must be of the highest quality, and the rods themselves are tested with
a gas laser source to assure optical homogeneity as well as end flatness and parallelism.
Eastman Kodak Company makes such glass laser rods in sizes up to one inch in diameter
and 36 inches in length. Power output as great as three joules per cubic centimeter
of glass is reported.
High-silica glass is manufactured in a porous form containing billions of microscopic
holes. This product is useful as a moisture-getter, and small slabs of it may be
incorporated in semiconductor cases and relay envelopes for that purpose. The glass
is mechanically strong, non-dusting, and non-flaking. Porous glass is also fabricated
in the form of semi-permeable membranes for isotope separation and is used for chromatography
and diffusion studies.
A non-porous glass membrane is used in the sensing electrode in a pH-measuring
instrument, which determines the acidity or alkalinity of a solution by means of
the hydrogen-ion concentration. A solution of known acidity is placed in the glass
electrode and, when the electrode is immersed in the unknown solution, a voltage
develops across the glass membrane dependent upon the difference in the hydrogen-ion
concentration of the two solutions. Since the membrane is not porous, there is no
transfer of electrons at the electrode-solution interface. However, special glass
is needed to resist the effects of strong acid or alkaline solutions.
Glass to resist nuclear radiation is usually made with high lead content. It
is cast in massive slabs for use in shielding windows in radiation laboratories,
aboard nuclear vessels, and in hospitals where radioactive materials are employed.
Because of its inorganic nature and its random structure, glass offers high resistance
to x-ray and gamma radiation and will not darken with use.
Fig. 7. Precision hermetically sealed glass trimmer capacitors are moisture-proof
and possess linear tuning curves.
A related product is gamma-ray-sensitive glass, a specially developed glass that
will fluoresce upon exposure to ultraviolet light after irradiation by gamma rays.
The intensity of the light emission is proportional to the total gamma-ray dose
received, so badges made from this material serve as sensitive dosimeters. Dosages
from 5 to 1000 roentgens can be measured in this way.
Posted November 28, 2022