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
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.
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