Ceramic capacitors are rapidly increasing in
capacitance and volumetric efficiency (CV/cc)
due to higher dielectric constants and smaller
dielectric thickness as well as higher layer
counts. To compete with ceramic capacitors and meet demands
for miniaturization, tantalum (Ta) capacitors also need to
increase their volumetric efficiency.
Traditionally, the only way to increase CV/cc in Ta capacitors
was to reduce particle size in the Ta powder, thereby increasing
the surface area of the anode. This leaves aside packaging efficiency
as a common issue for all types of capacitors.
Although Ta powder manufacturers continue to increase
powder CV, the application of newly developed, high-CV
powder is limited to low-working-voltage (WV) capacitors
with very thin dielectrics. Higher-voltage capacitors cannot
use high-CV powder because they require a thicker dielectric
than that in the low-voltage parts.
The thicker dielectric grows through the “necks” between
the powder particles and clogs fine pores between particles,
reducing anode surface area and, thereby, CV. This means that
for mid-voltage and high-voltage capacitors, which constitute
the bulk of Ta capacitors, the applicable powders have been in
use for some time.
High reliability and stable leakage current are critical for
demanding applications. Issues such as difficult or impossible
accessibility for repair, the high cost of equipment, and the
potential for personal injury require reliability of the highest
level. Additionally, stable dc-leakage (DCL) characteristics are
necessary to ensure that designers can develop power supplies
that serve their target purposes.
Recently developed techniques do improve the use of Ta
in anodes for the creation of additional capacitance, providing
higher CV/cc for Ta capacitors while simultaneously
improving their stability and reliability. To meet the growing
demands of critical applications, newly developed processes
show great potential.
ANODE CV VS. POWDER SINTERING CONDITIONS
Press density (d) and sintering temperature (Ts) of Ta powder
are the two major parameters that influence utilization of Ta
in Ta anodes. Figure 1 shows CV/cc as a function of d and Ts
in anodes sintered with 23k CV/g Ta powder. This data shows
that it’s possible to increase CV/cc at high press density and
low sintering temperature.
But with conventional sintering in a vacuum, low sintering
temperature doesn’t provide sufficient bonding between
the powder particles and between the particles and lead wire,
affecting the mechanical and electrical properties of sintered anodes. This is due in part to oxygen, which dissolves in Ta particles
from natural surface oxide during sintering in a vacuum and
acts as a sintering inhibitor.
SINTERING IN REDUCING ATMOSPHERE
Sintering in a reducing atmosphere such as magnesium (Mg)
vapor results in the removal of oxygen from Ta particles. This
intensifies the diffusion of Ta atoms, allowing the growth of
“necks” between powder particles at lower temperatures than
when sintering in a vacuum.
Our alternative process allows low-temperature sintering in
a deoxidizing atmosphere, the initial results of which appear in
Figure 2 for 50k CV/g Ta powder. The figure 2 shows CV/cc, oxygen
content, and delta volume in Ta anodes sintered in a vacuum
(Sintering) and sintered with deoxidizing (D-sintering).
D-sintering increases CV/cc by approximately 35% and radically
reduces oxygen content in sintered anodes versus sintering
in a vacuum. Increases in CV/cc with D-sintering incur anode
expansion, while sintering in a vacuum results in anode shrinkage
(Fig. 2b). This difference in volume change between regular sintering
in a vacuum and D-sintering is the result of a change in the
dominant sintering mechanism.
With low-temperature D-sintering, the dominant sintering
mechanism is surface diffusion of Ta atoms. This results in open
pores and an expansion of the anode volume, providing the highest
possible volumetric efficiency. The reduction in oxygen content
also improves dc-leakage current behavior.
Continue on Page 2