Evaluation of unencapsulated ceramic monolithic and MOS thin-film capacitors (25 to 300 C)

Several commercial monolithic ceramic and thin-film MOS chip capacitors were evaluated for use in high temperatures (300 C) geothermal instrumentation. Characteristics of the commonly used dielectric materials (NPO, X7R, BX) and temperature dependence of the insulation resistance are briefly discussed. Some ceramic capacitors with NPO dielectric materials had insulation resistances above 10 megohms at 300 C and less than 2% change in capacitance from 25 C to 300 C, while the X7R and BX dielectric materials exhibited insulation resistances below 10 megohm and changes in capacitance greater then 50%. The thin-film capacitors showed good stability at 300 C. However, during aging, bonds and bond pads presented a problem causing intermittently open circuits for some of the devices.