Conformal Coated Guide
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Vishay Sprague
Revision: 11-Apr-16
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Document Number: 40150
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Guide for Conformal Coated Tantalum Capacitors
INTRODUCTION
Tantalum electrolytic capacitors are the preferred choice in
applications where volumetric efficiency, stable electrical
parameters, high reliability, and long service life are primary
considerations. The stability and resistance to elevated
temperatures of the tantalum / tantalum oxide / manganese
dioxide system make solid tantalum capacitors an
appropriate choice for today's surface mount assembly
technology.
Vishay Sprague has been a pioneer and leader in this field,
producing a large variety of tantalum capacitor types for
consumer, industrial, automotive, military, and aerospace
electronic applications.
Tantalum is not found in its pure state. Rather, it is
commonly found in a number of oxide minerals, often in
combination with Columbium ore. This combination is
known as “tantalite” when its contents are more than
one-half tantalum. Important sources of tantalite include
Australia, Brazil, Canada, China, and several African
countries. Synthetic tantalite concentrates produced from
tin slags in Thailand, Malaysia, and Brazil are also a
significant raw material for tantalum production.
Electronic applications, and particularly capacitors,
consume the largest share of world tantalum production.
Other important applications for tantalum include cutting
tools (tantalum carbide), high temperature super alloys,
chemical processing equipment, medical implants, and
military ordnance.
Vishay Sprague is a major user of tantalum materials in the
form of powder and wire for capacitor elements and rod and
sheet for high temperature vacuum processing.
THE BASICS OF TANTALUM CAPACITORS
Most metals form crystalline oxides which are
non-protecting, such as rust on iron or black oxide on
copper. A few metals form dense, stable, tightly adhering,
electrically insulating oxides. These are the so-called “valve”
metals and include titanium, zirconium, niobium, tantalum,
hafnium, and aluminum. Only a few of these permit the
accurate control of oxide thickness by electrochemical
means. Of these, the most valuable for the electronics
industry are aluminum and tantalum.
Capacitors are basic to all kinds of electrical equipment,
from radios and television sets to missile controls and
automobile ignitions. Their function is to store an electrical
charge for later use.
Capacitors consist of two conducting surfaces, usually
metal plates, whose function is to conduct electricity. They
are separated by an insulating material or dielectric. The
dielectric used in all tantalum electrolytic capacitors is
tantalum pentoxide.
Tantalum pentoxide compound possesses high-dielectric
strength and a high-dielectric constant. As capacitors are
being manufactured, a film of tantalum pentoxide is applied
to their electrodes by means of an electrolytic process. The
film is applied in various thicknesses and at various voltages
and although transparent to begin with, it takes on different
colors as light refracts through it. This coloring occurs on the
tantalum electrodes of all types of tantalum capacitors.
Rating for rating, tantalum capacitors tend to have as much
as three times better capacitance / volume efficiency than
aluminum electrolytic capacitors. An approximation of the
capacitance / volume efficiency of other types of capacitors
may be inferred from the following table, which shows the
dielectric constant ranges of the various materials used in
each type. Note that tantalum pentoxide has a dielectric
constant of 26, some three times greater than that of
aluminum oxide. This, in addition to the fact that extremely
thin films can be deposited during the electrolytic process
mentioned earlier, makes the tantalum capacitor extremely
efficient with respect to the number of microfarads available
per unit volume. The capacitance of any capacitor is
determined by the surface area of the two conducting
plates, the distance between the plates, and the dielectric
constant of the insulating material between the plates.
In the tantalum electrolytic capacitor, the distance between
the plates is very small since it is only the thickness of the
tantalum pentoxide film. As the dielectric constant of the
tantalum pentoxide is high, the capacitance of a tantalum
capacitor is high if the area of the plates is large:
where
C = capacitance
e = dielectric constant
A = surface area of the dielectric
t = thickness of the dielectric
Tantalum capacitors contain either liquid or solid
electrolytes. In solid electrolyte capacitors, a dry material
(manganese dioxide) forms the cathode plate. A tantalum
lead is embedded in or welded to the pellet, which is in turn
connected to a termination or lead wire. The drawings show
the construction details of the surface mount types of
tantalum capacitors shown in this catalog.
COMPARISON OF CAPACITOR
DIELECTRIC CONSTANTS
DIELECTRIC
e
DIELECTRIC CONSTANT
Air or vacuum 1.0
Paper 2.0 to 6.0
Plastic 2.1 to 6.0
Mineral oil 2.2 to 2.3
Silicone oil 2.7 to 2.8
Quartz 3.8 to 4.4
Glass 4.8 to 8.0
Porcelain 5.1 to 5.9
Mica 5.4 to 8.7
Aluminum oxide 8.4
Tantalum pentoxide 26
Ceramic 12 to 400K
