The reliability of wire-bond IGBT modules is limited by the thermal and mechanical fatigue of the inter-bond line connections.

Han Ryong Hwan, a researcher at the Faculty of Electronics, has simulated thermo-mechanical stress distribution in solder joints and wire joints using the ANSYS with an attempt to define a rational linear bond shape for minimum thermo-mechanical stress.

He paid special attention to comparing the stresses in the flip-chip and the wire-bonding interconnect for various wire-bonding types of samples at constant temperature.

The results show that the thermal stress varies with the bending shape of the metal wire in the solders and connecting wires, and the thermal stress decreases with the increase in the number of bending.

You can find the details in his paper “Analysis Of Geometric Effects On The Thermo-Mechanical Stress In Al Bond Wire Interconnects In The Wire-bond IGBT Module” in “Recent Patents On Engineering” (SCI).

Since silicon single crystal is the base material of electronics industry, many electronic devices are based on it. Many electronic sensors are based on this material grown by the floating zone (FZ) process.

The crucial factors for growing silicon single crystal by the FZ process are the current density and frequency applied to the high-frequency (HF) inductor, the pull rate and rotational rate of the feed rod and single crystal and the pressure of the atmosphere. Precise determination of these parameters is very important for improving the quality of the grown crystal, and therefore, a lot of research has been focused on it.

In order to fabricate high-quality single crystals necessary to develop power semiconductor devices, Yu Nam Chol, a section head at the Faculty of Electronics, on the basis of the analysis of preceding studies, has performed three-dimensional transient simulations of the growth of FZ single crystal silicon using ANSYS, analyzed the influence of HF inductor type on the growth of single crystal and determined the optimum morphology.

The results show that stepped HF inductors form better temperature distribution in the melting zone than tilted HF inductors.

For more information, please refer to his paper “Modeling of HF Inductor and Influence on Electro-magnetic Field in Silicon FZ Process” in “Journal of Multiscale Modeling” (SCI).

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).