Study of degradation by heat and heat‐resistance property in electret condenser microphone

Abstract

SUMMARYThis paper describes improvement of the heat resistance of a fixed electrode electret microphone for mobile equipment. First, hypotheses are presented on the cause of the thermal degradation and the corresponding charge behavior in the fixed electrode electret microphone and a model of the internal electric field is formed. The validity of the model is investigated by the thermal stimulated current (TSC) method. Next, the lifetime of the microphone is estimated by reference to the isothermal charge decay test (ICD method) in addition to the hypotheses. As a model, the charges are assumed to be shifted in the direction of greater depth with reduction of the surface potential. This phenomenon is accelerated by heat. The magnitude of the degradation depends on the process used for fabrication of the electret forming film. However, there is sufficient lifetime margin at room temperature. By applying this margin, the environmental characteristic may be improved if the charges are shifted to the interior or injected deeply to avoid the causes of degradation. As an example, for a general-purpose FEP film with a thickness of 12.5 µm, it is estimated that a lifetime of 1000 hours at 100 °C can be guaranteed by deep injection at depths up to about 8 µm from the electret surface.

Lifetime of electret microphones by thermal degradation analysis via electroacoustic measurements

“…9 represents T(T) VS (fcT) -1 , corresponding to the Arrhenius model. The resulting r function is shown in (3), which enables the time constant r to be calculated at low ambient temperature. T(T) = 3.27-10-18 eTr.…”

“…Therefore, any polarisation loss will, over time, result in loss of both the response and transducer performance. Analyses of the degradation of the electret materials are available in the literature [2][3]; however, identifying studies employing electroacoustic measurements is difficult. Moreover, the authors believe it could be useful to repeat the reliability analyses of consumer electret microphones to investígate how new materials can improve their behaviour despite their low price.…”

“…9. Curve TCT) VS (kT)" 2 as in (3) representing the Arrhenius model (temperature in kelvin). Table 3 shows the time to failure for the three tests, in which the failure times decrease with temperature.…”