Investigation On The Packaging Of Power Module Interconnected By Silver Sintering And Ball Grid Array

Power module is the core of the inverter system in new energy vehicles.The power density of the vehicle-orited power inverter unit can reach over 10KW/L,and it also faces a service life of more than 15 years.Therefore,its requirements for the electrical and thermal performance as well as reliability of the power module are much higher than those of other power systems.Traditional power modules use wire bonding and reflow soldering processes to achieve chip interconnection.However,this interconnection method brings about large parasitic parameters and junction-to-case thermal resistance,which limits the power density and service temperature of the power module and causes fatigue failure problems such as wire breaking and solder fatigue.Due to the limitations of traditional packaging materials and structures,the performance of the power chip is difficult to be fully exerted.New interconnection technologies have become the key to solving current problems.The representative solutions include silver bonding and wireless interconnection structures.This packaging method involves complex interdisciplinary issues and urgently needs in-depth research for practical applications.Therefore,this dissertation focuses on the research and development of the silver sintering mechanism,silver sintering joint performance,design and manufacturing of ball grid array（BGA）interconnection of IGBT power module.The main work and achievements are as follows:（1）A sintering dynamic model of silver particles was constructed using phase field method,revealing the mechanism of the effects of sintering temperature and particle geometric characteristics on neck growth and grain boundary migration.The local curvature size of silver particles and the sintering temperature determine the diffusion direction and rate of mass diffusion.The higher the temperature and the larger the local curvature difference,the faster the neck growth and grain boundary migration.Multi-scale and multi-shaped mixed particles are conducive to accelerating the sintering process.The interfacial angle between two particles was defined by decomposing the equilibrium dihedral angle,and the grain boundary migration mechanism of circular particles was analyzed.It was found that the interfacial angle showed dynamic balance during the process of grain coarsening,and the difference in interfacial angle between particles led to grain boundary curvature.The tension generated by the grain boundary curvature promoted the grain boundary migration and caused gain boundry straightened,and the larger the difference in interfacial angle,the faster the evolution of grain boundary curvature,which promoting the particle coarsening process.（2）Sintered silver pastes with multi-scale and multi-shape particle features was investigated,and variation rules of microstructure and macroscopic performance under different sintering conditions was revealed.Due to the large curvature difference of the initial silver particles and the low temperature decomposition of organic shell,sintered microstructures can quickly grow necks at low temperatures（200℃）,thereby rapidly entering the middle stage of sintering.With the increase of sintering temperature and time,the sintered microstructures become denser with reduced porosity,decreased volume conductivity thermal resistance,and enhanced interface shear strength.By optimizing the patch parameters,sintering for 50 minutes at 250℃can achieve the highest shear strength up to 76.9MPa,and the thermal resistance can be decerased by 27%compared with traditional solder material SAC305.（3）Several typical wireless package structure for double-sided DBC soldering are simulated and analyzed by finite element method.It is found that the package form of the ball array with underfill shows better thermomechanical performance in the simulation of constant temperature,constant power density and temperature cycle.The IGBT module based on BGA technology was then trially produced.The preliminary electrothermal characterization showed that the thermal resistance was reduced by 27.4%and the parasitic inductance was reduced by 33.3%compared with the traditional package.（4）Packaging structure and manufacturing process of the BGA-type IGBT power module was optimized.Based on the sintered silver and BGA technology,IGBT module with a specification of 500A/600V was manufactured.Electronic and thermal performance of IGBT module was thoroughly evaluated,and its reliability and failure mechanisms were analyzed through power cycling experiments and crack propagation simulations.The experiments showed that under the condition of single-side cooling,the thermal resistance of the junction to case can be as low as0.052K/W,which is 48%lower than that of the wire bonding module,and the saturation on-state votage drop V_CESAT is 29.6%lower than that of the wire bonding module.When the junction temperature is close to 150℃,junction temperature difference is set to 120℃,the power cycle life can be reach to 29074 cycles,which is more than 5 times longer than the life of wire bonding module.The simulation showed that the critical solder ball is subject to both tensile and compressive stress,with pre-existing cracks at the area dominated by ₁K where tensile stress prevails,and those at the area dominated by K₂ where compressive stress prevails.