Failure Mechanism Analysis of Capacitor(III)

Low level failure Mechanism of High Frequency Precision Capacitors.

Mica is an ideal dielectric material for capacitors with high insulation, high temperature resistance, low dielectric loss and thickness of up to 25 microns. The main advantages of mica capacitor are low loss, good frequency stability, small distributed inductance and large insulation resistance, so it is especially suitable for high frequency communication circuits as precision capacitors. However, mica resources are limited and difficult to be popularized. In recent decades, organic film capacitors have been rapidly developed, among which polystyrene film capacitors have the advantages of low loss, large insulation resistance, good stability, high dielectric strength and so on. Precision polystyrene capacitors can be used in high-frequency circuits instead of mica capacitors. Need to be explained is: used in high-frequency circuit in the precision polystyrene capacitor, generally using metal foil plate to increase insulation resistance and reduce loss.

Low level failure of capacitors is a new problem since 1960s. Low level failure refers to the failure of capacitors under low voltage operating conditions, such as open circuit of capacitors or drop of capacity. Semiconductor devices have been widely used since the 1960s. The voltage of semiconductor circuits is much lower than that of electronic tube circuits. The actual working voltage of capacitors is only a few millivolts in some circuits, which results in the low level failure of capacitors. The concrete performance is that the capacitors lose their capacity completely or partially. For the low level impulse, the capacitance of the capacitor is restored to normal.

The main reason for the low level failure is the poor contact between the capacitor outlet line and the capacitor plate, and the increase of the contact resistance, which leads to the complete open circuit of the capacitor or the decrease of the capacitance amplitude.

Aluminum foil is generally used as the electrode plate in the precision polystyrene film capacitor, and the copper elicitation wire is spot welded together with the aluminum foil electrode plate. Aluminum foil is easily oxidized in air; an alumina semiconductor film is formed on the surface of the electrode plate, and the voltage on the oxide film is not enough to break it down at low level, thus the series equivalent capacity of the gap capacitance formed between the aluminum foils. The smaller the gap capacitance, the smaller the series equivalent capacity. Therefore, the low level capacity depends on the thickness of the aluminum oxide layer on the surface of the plate. The thicker the alumina layer, the smaller the capacitance of the capacitor under the low level condition. In addition, when the capacitor works in the AC circuit, its effective capacitance will decrease because the contact resistance is too large, and when the contact resistance is very large, the effective capacitance can be reduced to the degree of open circuit. Even if there is no conductive spacer between the leads of the plate, this effect will occur.