TAJB335016 datasheet(29/51 Pages) AVX

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Tantalum capacitors are manufactured from a powder of

pure tantalum metal. The typical particle size is between 2

and 10 µm.

4000µFV

10000µFV

20000µFV

Figure 1.

The powder is compressed under high pressure around a

Tantalum wire to form a ‘pellet’ (known as the Riser Wire).

The riser wire is the anode connection to the capacitor.

This is subsequently vacuum sintered at high temperature

(typically 1500 - 2000°C). This helps to drive off any impuri-

ties within the powder by migration to the surface.

During sintering the powder becomes a sponge like

structure with all the particles interconnected in a huge

lattice.

This structure is of high mechanical strength and density, but

is also highly porous giving a large internal surface area

(see Figure 2).

The larger the surface area the larger the capacitance. Thus

high CV (capacitance/voltage product) powders, which have

a low average particle size, are used for low voltage, high

capacitance parts. The figure below shows typical powders.

Note the very great difference in particle size between the

powder CVs.

By choosing which powder is used to produce each capac-

itance/voltage rating the surface area can be controlled.

The following example uses a 22µF 25V capacitor to

illustrate the point.

C = o r

where

o is the dielectric constant of free space

(8.855 x 10-12 Farads/m)

r is the relative dielectric constant for Tantalum

Pentoxide (27)

d is the dielectric thickness in meters

(for a typical 25V part)

C is the capacitance in Farads

and

A is the surface area in meters

Rearranging this equation gives:

A =

o r

thus for a 22µF/25V capacitor the surface area is 150 square

centimeters, or nearly half the size of this page.

The dielectric is then formed over all the tantalum surfaces

by the electrochemical process of anodization. To achieve

this, the ‘pellet’ is dipped into a very weak solution of phos-

phoric acid.

The dielectric thickness is controlled by the voltage applied

during the forming process. Initially the power supply is kept

in a constant current mode until the correct thickness of

dielectric has been reached (that is the voltage reaches the

‘forming voltage’), it then switches to constant voltage mode

and the current decays to close to zero.

The chemical equations describing the process are as

follows:

Anode:

2 Ta

→ 2 Ta5+ + 10 e

2 Ta5+ 10 OH-

→ Ta

2O5 + 5 H2O

Cathode:

10 H

2O – 10 e

→ 5H

↑ + 10 OH-

The oxide forms on the surface of the Tantalum but it also

grows into the metal. For each unit of oxide two thirds grows

out and one third grows in. It is for this reason that there is a

limit on the maximum voltage rating of Tantalum capacitors

with present technology powders (see Figure 3).

The dielectric operates under high electrical stress. Consider

a 22µF 25V part:

Formation voltage

Formation Ratio x Working Voltage

4 x 25

100 Volts

Technical Summary and

Application Guidelines

Figure 2. Sintered Tantalum

INTRODUCTION

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