Reliability Enhancement Of Power Modules By Restricting Junction Temperature Fluctuation Through Increased Transient Thermal Capacity

Power modules are widely used in traction applications and renewable power generation,which require high reliability.In the harsh environment,the power modules must experience deep power cycling which causes large temperature fluctuations and hence leads to fatigue stresses.Reducing the chip junction temperature fluctuation can significantly improve power module reliability.This paper proposes a power module design with considerably reduced junction temperature fluctuation using phase change material(PCM)as part of the thermal management which is contained by a copper framework above semiconductor chip without sacrificing thermal resistance in cooling path.Cyclical power is applied to PCM module and experiment results shows that the restriction of junction temperature fluctuation is effective in a certain range of power period.Mechanism of PCM in restricting junction temperature fluctuation is demonstrated by simulation.With the significantly reduction of junction temperature fluctuation,the reliability of the PCM module is increased.It’s meaningful to bring a new approach to improve the reliability of power module in abominable power fluctuating conditions.The main innovations of the thesis include:(1)Aiming at the problem of the junction temperature fluctuation of the power module,a method is proposed to fill the power module with PCM to increase the heat capacity and reduce the junction temperature fluctuation.Power fluctuations will cause junction temperature fluctuations in power module.Long-term junction temperature fluctuations will cause the aging of power module,resulting in solder fatigue and bond wire lifting,which will affect the reliability of the power module.Combined with domestic and foreign research status,according to the Cauer thermal network model of the power module,two methods for restricting junction temperature fluctuations are proposed: reducing thermal resistance and increasing thermal capacity.Then,the equivalent heat capacity properties of PCM are proposed.PCM can be filled inside the module to increase the equivalent heat capacity.Through the thermal network model of power module,aiming at periodic power fluctuations,it is theoretically analyzed that increasing the heat capacity can effectively reduce the junction temperature fluctuation of the power module.(2)For restricting the junction temperature fluctuation,a power module that can be filled with phase change material inside is designed.First,the equivalent heat capacity properties of the phase change material are analyzed,and the theoretical analysis is that in the case of power fluctuations,the way of filling the phase change material inside the module can effectively reduce the junction temperature fluctuation.Secondly,discuss the location of the phase change material in the module.In order to avoid additional thermal resistance in the heat dissipation path,the phase change material is filled above the chip.Based on this idea,a copper framework with a special structure is designed,which can be filled with phase change materials and can be used as a part of the internal connection of the power module.A hole is opened in the middle of the copper frame to expose part of the chip to facilitate observation of the temperature change.The module topology is a rectifier circuit,and a larger size diode chip is used to simplify the design and production of the copper framework.In order to evaluate the effect of PCM power module,according to the 3D model of the phase change module,make every parts required by the module,including copper frame,DBC,soldering pad and other materials.Use a vacuum welding furnace to realize the connection between the copper framework,chip,and DBC,and use X-ray equipment to exam the void ratio of the welding layer to ensure the welding quality.Compare the latent heat,thermal conductivity,phase change temperature and other properties of various types of phase change materials,and select the phase change materials suitable for the power module,and fill them in the trenches of the copper frame.An experimental platform was built to enable the phase change module to conduct periodic large currents,and use the temperature rise generated by the heat loss of the chip to measure the junction temperature changes under different power fluctuation cycles,further verifying the effect of the phase change material in restricting junction temperature fluctuations.The finite element simulation software Comsol is used to build a simulation model of the phase change module,and the junction temperature changes of the phase change module under different power fluctuation cycles are simulated by the software,and the junction temperature fluctuation changes of the chip with and without filling phase change materials are compared.Then,the process of the phase change material changing with the junction temperature under different cycles reveals the principle of restricting junction temperature fluctuations.Experiments and simulations jointly verify the effect and principle of phase change materials on the restriction of junction temperature fluctuations.(3)In view of the cyclical fluctuation of junction temperature,the life evaluation model of phase change power module is established.Assess the effect of the phase change module on the improvement of the lifetime of the module after restricting junction temperature fluctuation of power module.First,different materials inside the module have different thermal expansion coefficients,which will cause thermal stress between adjacent layers under the fluctuation of junction temperature.Thus it will lead to the failure mechanism of the module: solder layer fatigue and bonding wire lift-off.Secondly,the Coffin-Manson-Arrhenius fatigue life model of the power module is derived.The corresponding cycles to failure are calculated according to the experimental results,and the influence of the phase change material on the lifetime of the power module is analyzed.The calculation results show that the number of failure cycles can be increased by a maximum of 5 times after filling the phase change material.Finally,corresponding to the irregular junction temperature fluctuations,it can be sorted and classified by the rain flow counting method,and then the Miner fatigue damage accumulation method is used to calculate the service life of the module under this working condition.When the phase change module is under irregular load changes,through simulation or experiment,the junction temperature change data is measured,and the module life is calculated using the above method.Finally,the improvement effect of the phase change material on module lifetime under this working condition can be verified.The research results of this paper have important academic value and technical support for the restriction mechanism and method of junction temperature fluctuation of power modules,and have certain reference significance for improving the reliability of power modules.