Long term stability of polymer based resistors tested by noise, non-linearity and electro-ultrasonic spectroscopy

The long term stability of polymer based thick film resistors was correlated with the results of standard testing methods: the low frequency noise measurements, non-linearity and new proposed method: Electro-Ultrasonic Spectroscopy. The samples were made using different resistive and conducting pastes. The resistive pastes were made using one type of conducting grains - carbon and graphite particles, suspended in different polymer vehicles. Contacts were made by polymer based dipping silver. The results of standard measuring methods are compared with those of the electro-ultrasonic spectroscopy. This method is based on the phonon interaction with conducting electrons and on the change of the contact area among the conducting particles. The ultrasonic signal changes the contact area between the conducting grains in the resistor structure and then the resistance is modulated by the frequency of ultrasonic excitation. Resultant intermodulation voltage appearing on the sample depends on the value of AC current varying with frequency f_E and on the ultrasonic excited resistance change DeltaR varying with frequency f_U. Ultrasound-excited resistance change in polymer based thick film structures is in the range 0.1 to 1.1 mOmega, which corresponds to the relative resistance change of the order from 10^-6.