NTC (negative temperature coefficient) thermistor chips have been adopted effectively as temperature sensing units and temperature compensation units of reference voltage source for their specific characteristics of resistance inversely proportional to temperature.
The NTC consists of several inner electrodes which are enclosed by thermistor ceramic material and three-layer terminations which are connected to the ceramic body.
Many researches on the NTC have developed up to date, and the miniaturization and high speed of electronic devices urgently needs us to improve its reliability and operation characteristics.
Many factors influence the characteristics of the chips and the structures, and material properties are the main factors. The structure and material of solder which is used to mount chips onto the board (PCB) also affect their characteristics. The thermal stress occurs in sintering, soldering and operating processes and the mechanical stress occurs by the bending load of pressure.
Under these processes and conditions, the differences in the material properties such as thermal expansion coefficient and modulus of elasticity of NTC elements focus stresses on the boundary sides and if it lasts for a long time, the NTC might be brought to failure.
O Ryong Jin, a researcher at the Faculty of Electronics, by considering mainly the stress distribution of ceramic bodies, electrodes, boundaries of inner electrodes, soldered regions and three-layered terminations, conducted simulations for optimization of NTC design and analyzed the results.
The simulation results show that when the number of electrodes of NTC increases, the stress distribution in the NTC body changes, which affects the lifetime of NTC.
For more information, please refer to his paper “Residual Stress in Multilayer NTC Thermistors during Bending Test” in “GAS Journal of Engineering and Technology” (SCI).