tantalum capacitors. More than 90% of the today's rare failures in tantalum solid-state electrolytic capacitors are caused by shorts or increased leakage current due to this failure mode. The extremely thin oxide film of a tantalum electrolytic capacitor, the dielectric layer, must be formed in an amorphous structure. Changing the amorphous structure into a crystallized structure is reported to increase the conductivity by 1000 times, combined with an enlargement of the oxide volume. The field crystallization followed by a dielectric breakdown is characterized by a sudden rise in leakage current within a few milliseconds, from nanoamp magnitude to amp magnitude in low-impedance circuits. Increasing current flow can accelerate in an "avalanche effect" and rapidly spread through the metal/oxide. This can result in various degrees of destruction from rather small, burned areas on the oxide to zigzag burned streaks covering large areas of the pellet or complete oxidation of the metal. If the current source is unlimited a field crystallization may cause a capacitor short circuit. In this circumstance, the failure can be catastrophic if there is nothing to limit the available current, as the series resistance of the capacitor can become very low. Impurities, tiny mechanical damages, or imperfections in the dielectric can affect the structure, changing it from amorphous to crystalline structure and thus lowering the dielectric strength. The purity of the tantalum powder is one of the most important parameters for defining its risk of crystallization. Since the mid-1980s, manufactured tantalum powders have exhibited an increase in purity. Surge currents after soldering-induced stresses may start crystallization, leading to insulation breakdown. The only way to avoid catastrophic failures is to limit the current which can flow from the source in order to reduce the breakdown to a limited area. Current flowing through the crystallized area causes heating in the manganese dioxide cathode near the fault. At increased temperatures a chemical reaction then reduces the surrounding conductive manganese dioxide to the insulating manganese(III) oxide (Mn2O3) and insulates the crystallized oxide in the tantalum oxide layer, stopping local current flow. Failure avoidance Solid tantalum capacitors with crystallization are most likely to fail at power-on. It is believed that the voltage across the dielectric layer is the trigger mechanism for the breakdown and that the switch-on current pushes the collapse to a catastrophic failure. To prevent such sudden failures, manufacturers recommend: 50% application voltage derating against rated voltage using a series resistance