Bipolar Power Transistor Heat Distribution Based On Flotherm Software

Thermal stress concentration (hot spot) is one of the major causes of failure for bipolar power transistors. With continuously increased power density of devices, bipolar power transistors are faced with the challenge of reducing hot spots and strengthening thermal performance. Researches on power transistors'heat distribution can help improve chip and encapsulation designing as well as optimize chip technology for better thermal performance. By establishing thermal-dynamic models and then employing simulation methods, such researches have been made more quickly and economically.In this paper, a thermal-dynamic model for bipolar power transistors is established and then its heat distribution is analyzed by introducing finite volume method and employing Flotherm softwares. Work done includes:1. analysis of the heat conduct mechanism of a bipolar power transistor of TO-220 type; establishment of its heat distribution model employing FLOTHERM modeling and simulation method; study on the influence of die bonding voids, solder thickness and area, lead frame thickness, etc. on the heat distribution of bipolar power transistors; investigation of the temperature distribution of a transistor upon various conditions; exploration of the relation between steady state thermal resistance and assembly structures.2. study on the influence of bonding layer on transient heat distribution through transient simulation with the model of TO-220 packaging bipolar power transistors; discussion on SOA and heat distribution of the transistor to which a single pulse is applied; discussion of the variation of transistor's temperature distribution when the applied pulse changes; and discussion of the transistor's anti-surge ability under different working conditions.3. verification of the accuracy of steady state and transient state simulation results by experiment with point temperature measurement, infrared temperature measurement, electrical temperature measurement, etc.4. study on thermoelectric feedback effect of the transistor, bringing up a method to model the heat distribution of transistors including thermoelectric feedback effect by conversion of the thermoelectric feedback effect into resistivity variation of Si with temperature and introduction of transistor array.5. discussion on main ideas and methods of reducing hot spots; 4W1E analysis of principal ways and methods of encapsulation optimization employing 4M1E approach to improve the thermal performance and reduce hot spots of bipolar power transistors; bringing-up of main thoughts about improving non-uniform current distribution on chips; analysis of the pros and cons of different chip designs; illustration of cases of successful chip design improvement.