Introduction

Conventional tantalum capacitors with a

MnO

second electrode system have proven to be a

highly reliable solution with high volumetric efficiency

for over 50 years. Continuous improvements have led

to the development of higher CV (capacitance times

voltage constant) powders that have enabled the

creation of capacitors with higher capacitance in

smaller case size dimensions. Three main features of

tantalum capacitors have been the focus for a key

development direction, i.e. the reduction of ESR

(equivalent series resistance), the reduction of

ignition risk and the flexibility of the supply chain. The

evolution of DC/DC converters and power supplies

has required a significant reduction of and major

improvements in safety. Supply chain flexibility issues

grew during the business upturn in 2000 where a

slow response time from the tantalum metal supply

chain resulted in a serious shortage of tantalum

capacitors. Despite the fact that the supp ly chain has

now brought in additional capacity to prevent a re-

occurrence of year 2000 it has remain a significant

issue in the minds of purchasing and design

managers as a restriction to using and even a reason

to remove tantalum capacitors from their boards.

Two new steps in technology have been successfully

introduced to the market in order to meet the

requirements from designers for the very latest

electronic devices:

1] tantalum capacitors with polymer electrodes

2] niobium oxide capacitors

Major reductions in ESR are achievable by replacing

the MnO

electrode by a conductive polymer. ΜnΟ

conductivity is 100 times less than that of metals and

it represents a significant part of the total capacitor’s

ESR. Replacement of oxygen rich MnO

has also

helped to reduce the potential for ignition of

capacitors with a polymer cathode.

Niobium is a sister metal to tantalum in the periodic

table and it has many similar features. AVX has been

a pioneer in the development of niobium capacitors.

This intial development was focused mainly on

resolving issues with the supply chain & availability

as niobium is much more abundant in nature.

Niobium oxide has been identified as a niobium

based material with the best features for capacitor

production. It exhibits a metal like conductivity and

can be produced with simpler and higher yielding

powder manufacturing techniques. Niobium oxide

also provides a high ignition resistance and safety

through its efficient self-arresting failure mechanism.

Additionally a capacitor made from NbO material

improves steady state reliability. The NbO capacitor

has been commercialized under the AVX trade name

OxiCap

TM.

One limitation during the early stages of introduction

of these new technologies, is a relatively lower CV in

comparison to conventional tantalum capacitors with

MnO

. However the process of high CV introduction

for these new technologies is much faster based on

the knowledge already gained from “base” tantalum

technology. Figure 1 shows the evolution of

downsizing on a very popular rating 100μF 6.3V.

Evolution of 100uF 6.3V Downsizing

100

120

1985 1990 1995 2000 2005 2010

OxiCapTM NbO

Tantalum Poly mer

Tantalum MnO2

Volume [mm3]

D case

2mm D case

C case

B case

A case

Fig.1. Time evolution of 100

F 6.3V downsizing

It is possible to see that even today conventional

technology offers the highest CV solution in an A

case (1206), however both polymer and NbO

technologies are moving towards higher CV faster

and will provide an equivalent solution in the near

future. It should be also noted that higher derating

(e.g. use of capacitor at a lower voltage than rated) is

recommended for tantalum MnO

capacitors in low

impedance circuits and from the application point of

view the offering in polymer technology today is

already equivalent. It suggests that the role of

polymer and NbO technologies will grow in the future

as a way to increase CV of capacitors.

Key Features

The following chapter will describe the key

features of tantalum polymer and NbO capacitors.

The scope of this paper is not to give a detailed

overview of all the differences but to explain the key

features that make these capacitors popular for some

circuit applications. More detailed descriptions can be

found in references 1] – 5].

Tantalum Capacitors with MnO

Electrode

The conventional technology with MnO

was

introduced to the market 30-40 years ago as a major

improvement over the existing wet tantalum

electrolyte, electrode system. Since that time

tantalum MnO

capacitors have established a strong

position as a highly reliable, stable and high CV

capacitor. Voltage range is typically from 2.5 to 50V

and the case size offering has grown from basic four

EIA case sizes (A,B,C,D) to more than fifteen in order

to better meet specific height or space limited design

requirements. The other case sizes includes larger

cases to offer capacitance up to 1500μF, small case

sizes with high CV in a minimum footprint and low

profile packages with heights as low as 0.6mm. The

unique flexibility of the powder technology to provide

thin & flat capacitors is very important for applications

where height is critical such as cellular phones and

MicroHardDrives. The strength of the conventional

MnO

technology is in the robustness a gainst thermo-

mechanical load, temperature and DC Bias stability,

voltage range up to 50V and very good steady state

reliability. That is why the most popular applications