Reliability and Failure Mode in Solid Tantalum Capacitors

Failure Modes – Overview

The first solid Tantalum capacitors with a tantalum powder anode sintered in vacuum, an anodic oxide film of tantalum as the dielectric and a manganese dioxide (MnO₂) cathode were invented in the early 1950s at the Bell Telephone Laboratories in the USA.¹  Initially these capacitors were called Dry Electrolytic Device, and later the name was changed to Solid Electrolytic Tantalum capacitors. Similar to the liquid electrolyte cathodes in Wet Tantalum capacitors developed earlier, the MnO₂ cathode supplies oxygen to the tantalum pentoxide (Ta₂O₅) dielectric maintaining its chemical composition under applied voltages and elevated temperatures. The chemical and phase transformations in Solid Electrolytic Tantalum capacitors are shown in Fig. 1.

According to Fig. 1, tantalum anode extracts oxygen from the Ta₂O₅ dielectric enriching tantalum with oxygen Ta(O) and leaving oxygen vacancies Ta₂O_5-x in the dielectric in vicinity of the anode-dielectric interface (1). Under the gradient of concentration oxygen vacancies diffuse to the dielectric-cathode interface (2) where they are compensated by oxygen from the MnO₂, leaving oxygen vacancies MnO_2-x in the cathode (3). These oxygen vacancies are compensated by the oxygen diffusion from the distant from the dielectric part of the MnO₂ cathode toward the cathode-dielectric interface (4). This sequence of the oxygen migration processes provides dynamic equilibrium in the chemical composition of the dielectric and thus stable DC characteristics, particularly DC leakage (DCL) during long-term testing and field application of these capacitors.

Additionally, the MnO₂ cathode also provides strong self-healing properties to the Solid Electrolytic Tantalum capacitors due to the phase transformation into low manganese oxides (Mn₂O₃, Mn₃O₄, MnO) under the heat released in the defect sites in the dielectric that have high leakage current density (Fig. 1). The unique property of the manganese oxides is that they sharply increase resistivity as they release oxygen and transform from higher into lower manganese oxide (Fig. 2), while oxides of almost all the other metals reduce resistivity as they transform from higher to lower oxides.

Resistive low manganese oxides block current flow through the defect sites in the dielectric, which is critical for the reliability of the Solid Electrolytic Tantalum capacitors.

The AC characteristics of Solid Electrolytic Tantalum capacitors are superior to these in Wet Tantalum capacitors where low mobility of ions in the liquid electrolyte cathode cause high equivalent series resistance (ESR) and related rapid capacitance loss with frequency, especially at low temperatures. A combination of the stable DC and AC characteristics with high volumetric efficiency in terms of charge (Q/cc) and energy (E/cc) and high reliability (the cumulative failure rate is decreasing with time) provided steady increase in applications of Solid Electrolytic Tantalum capacitors since their mass manufacturing was initiated in the US by Sprague Electric in 1954 and until early 2000s.

Polymer Tantalum capacitors with conductive polymer cathode were developed by NEC Corporation, Japan, and introduced to the market in middle 1990’s.² Similar to the Solid Electrolytic Tantalum capacitors, Polymer Tantalum capacitors have a sintered tantalum powder anode and anodic oxide film of tantalum as the dielectric; however, their cathode is made of  inherently conducting polymer typically made of poly(3,4-ethylenedioxytheophene) (PEDOT).  The conductivity of PEDOT is higher than conductivity of MnO₂, providing lower ESR, and thereby better capacitance stability with frequency and ripple current capability in Polymer Tantalum capacitors in comparison to these characteristics in Solid Electrolytic Tantalum capacitors.   

The time-to-failure distribution at accelerated testing of Polymer Tantalum capacitors is different fromthat in Solid Electrolytic Tantalum capacitors.^3,4 Typically, there are no or very few failures at the earlier stages of the testing and then majority of the capacitors fail within relatively short period of time (wear-out region). Though as presented in Refs.^3,4 median times to failure in Polymer Tantalum capacitors at normal application conditions were lengthy, the wear-out performance of these capacitors was concerning, especially, in high reliability applications.

The typical failure mode inSolid Electrolytic and Polymer Tantalum capacitors is low insulation resistance or a short.  The hypothesis about ignition and burning tantalum failure mode in SMD type Solid Electrolytic Tantalum capacitors was initially presented by Prymak⁵ after more than 40 years of broad applications of these capacitors without any ignition and burning reported.  According to Ref.⁵ the heat released at breakdown of these capacitors causes microcracks in the Ta₂O₅ dielectric where exposed tantalum anode is ignited by oxygen from the reducing MnO₂ cathode.  Since PEDOT cathode doesn’t have active oxygen in its molecular structure, no ignition and burning of tantalum anodes takes place at the breakdown event in Polymer Tantalum capacitors (“benign” failure mode).  

To prove the hypothesis about ignition and burning tantalum failure mode in Solid Electrolytic Tantalum capacitors and no ignition failure mode in Polymer Tantalum capacitors, the reverse polarity voltage was continuously increased on these capacitors until Solid Electrolytic Tantalum capacitors ignited at reverse voltage about the twice rated voltage and the voltage was disconnected.  It is well known that Tantalum capacitors are polar with breakdown voltage at reverse polarity much less than the rated voltage. In this case the ignitions reported in Ref.⁵ could be caused by flammable epoxy compound in external encapsulation and could be observed on Polymer Tantalum capacitors as well with slight additional increase of the reverse voltage and related current and heat. 

To reduce the risk of failure in Solid Electrolytic Tantalum capacitors, 50% derating (Va/Vr = ½) was recommended.⁶ and accepted for most applications. The 50% derating causes about 10x loss in volumetric efficiency due to a combination of the thicker dielectric and lower anode surface area since larger tantalum particles, coarser tantalum powders, are needed to form the thicker dielectric. As an example, Fig. 3 shows Tantalum capacitors B-case 4.7 uF – 25 V (no de-rating) and D-case 4.7 uF – 50 V (50% de-rating).

Besides the loss in volumetric efficiency, increasing thickness of the dielectric due to de-rating can cause larger density and size of the defect sites in the dielectric (Fig. 4) that affect the reliability of Tantalum capacitors [7].

The loss in volumetric efficiently and fear of ignition and burning tantalum failure mode, which now dominates online publications, resulted in decline in general applications of Solid Electrolytic Tantalum capacitors including the applications where high reliability and environmental stability of these capacitors are most needed. At the same time, the applications of Polymer Tantalum capacitors were increasing; however, at much lower rate in comparison to that in Ceramic and Film capacitors. The wear-out failure mode was one of the major factors slowing applications of Polymer Tantalum capacitors.

In this paper the reliability and failure mode in surface mount Solid Electrolytic and Polymer Tantalum capacitors were investigated on the parts manufactured with conventional technology and flawless technology (F-Tech).⁷  Flawless technology suppresses typical defects such as crystalline inclusions in the amorphous matrix of the Ta₂O₅ dielectric that continue growth during the capacitor testing and field applications eventually causing cracks in the dielectric and capacitor failures. The critical part of the F-Tech is reducing bulk oxygen content in tantalum anodes that promotes formation of the crystalline seeds at the anode-dielectric interface. Reducing the oxygen content in tantalum anodes in F-Tech is achieved either by treatment of the sintered in vacuum tantalum anodes in deoxidizing atmosphere, such as magnesium vapor, or by sintering of the pressed tantalum powder pellets in a deoxidizing atmosphere (F-Tech with deox-sintering).