Stacked Ceramic Capacitors for High Temperatures (≥200°C)

High temperature applications at 200°C or above in electronics for down-hole drilling are driving the development of capacitors with ever more reliable performance. Deeper wells with increased temperatures and pressures have resulted in exposure to harsher conditions for longer times for the electronics used in the extraction tools and deep-well control instrumentation. The capacitor solutions currently available for 200°C operation are reviewed by value and rated voltage. Some key reliability factors attributed of the various technologies are identified. The recent development of stacks made using multi-layer ceramic capacitors (MLCC) of Class-I C0G type dielectric material with nickel inner electrodes are outlined with respect to their performance benefits at ≥ 200°C. Due to its linear dielectric nature this material exhibits highly stable capacitance as a function of temperature and voltage. The development of higher voltages and larger case size capacitors using this technology is discussed together with their incorporation into stacked ceramic capacitors by soldering on lead frames. Stacks of multi-layer ceramic capacitors (MLCC) allow capacitance to be maximized within the volume available. However, the solder interconnects must be evaluated to assess the long term reliability of the stacks at higher temperatures particularly with respect to maintaining mechanical and electrical integrity. The development of a custom high temperature shear test to evaluate the performance of different solder interconnects at temperatures from 200 to 260°C is described. Evaluations of two different HMP Pb-based solders are presented. The high and low temperature shear test data acquired for these solders is analyzed in terms of the strain and strain energy when force is applied. Changes in performance after exposure to temperatures ≥ 200°C are assessed. The results are interpreted with respect to the values required to survive high g-forces and the performance of the different solder compositions. Performance considerations for high shear strength interconnects at 200°C and beyond are discussed.