Thick film circuits were developed for temperature compensating of semiconductor strain gauges and for connecting the gauges to amplifiers in electronic pressure and differential pressure transmitters. In each circuit, ten Au pads for Al wire bonding and thirteen Ag/Pd pads for soldering must be fabricated on a small substrate. The results of the research are shown below.(1) The resistance values and the thermistor constants required for the thermistors are 0.9 ± 0.09 kilo-ohm and 2500 ± 40 Kelvin, respectively. Those characteristics are realized by developing a paste composition composed of a new spinel type oxide (Mn1.6Co0.8Ni0.35Ru0.25O4) RuO2and glass.(2) Accelerated life tests of the thick film thermistors reveal that the resistance drift rates of the thermistors are less than ±0.02 %/year at 120°C.(3) Life tests clarify that the wire bonding pads fabricated by using ESL 8882 Au paste keep more than 4 grams in pull strength, and that the soldering pads fabricated by using Sumitomo CLP 495A Ag/Pd paste are more than 1 kilo-gram in pull strength.(4) The temperature dependence of zero shift and span shift in the pressure transmitter are suppresed within ±0.2% between –40°C and 120°C by using the thick film circuit developed.
Electronic circuits for industrial uses must satisfy the following requirements: low sensitivity to changes in the surrounding temperature, high reliability, and small size. In the amplifier for a pressure transmitter described here, the temperature dependences of its properties, Zero point and Span, are intensively influenced by TCRs of the resistors used, and by a mismatching of the temperature dependences of the off-set voltages between the two operational amplifier IC chips. As forthe thick film resistors, it has been cleared that the DuPont 1700 series resistor pastes are the most suitable:TCRs of less than +80 ppm/°C can be attained, and the change in resistance of the overglazed and laser trimmed resistors is less than +0.1% after 5000 hours at 200°C. As for the operational amplifier IC chips, their temperature dependences of the off-set voltages are checked individually, utilizing a ceramic carrier containing an operational amplifier IC chip. Then a pair of ceramic carriers, with a minimum mismatching of their temperature dependences, is mounted on a thick film circuit substrate by a Pb/Sn solder reflowing. The various transistor chips are divided into three functional blocks, and bonded into three ceramic carriers. Utilization of ceramic carriers in all active devices employed results in sufficient standardization in the characterization of the semiconductor devices and in the hybridizing process.In the thick film hybrid IC amplifier for a pressure transmitter thus developed, the temperature dependences of Zero point and Span are less than –0.5% FS/100°C, respectively. The drifts of Zero point and Span are less than ±0.1% FS in the period of 3000 hours of an operating test with DC 24 V loading at 100°C.
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