Microstructure Evolution And Electrical Performance Degradation Of Al Metallization In IGBT Module

Insulated gate bipolar transistor(IGBT)is the core power devices to achieve power control and conversion,which are widely used in electric traction and other occasions,such as hybrid vehicles,subways and locomotives.In IGBT module,the Al metallization forms a bondable metal layer to connect the IGBT chip and Al bond wire,and realize the electrical interconnection between IGBT chip and substrate circuit.The power loss of IGBT module can change with the variation of the service conditions,resulting in the fluctuation of junction temperature of IGBT module,which makes each interconnection structure layer of IGBT module endure repeated thermal stress.Al metallization is located on the surface of heat source(IGBT chip),which endures high temperature.Furthermore,the difference between the thermal expansion coefficient of Al metallization(25 ppm/?)and the silicon chip(2 ppm/?)is about12 times.These make the Al metallization layer endure repeated impact of thermal stress.Thus,the Al metallization can undergo microstructure evolution such as the surface roughening and internal crack,which induce the increase of resistance.These can affects the performance of IGBT module,even leads to the failure of the application system,and brings serious economic losses.In order to ensure the IGBT module operates safely,it is necessary to study the influence of different service conditions on the temperature of IGBT module,and evaluate the stress and microstructure evolution mechanism of the Al metallization inside IGBT module under temperature load,and investigate the correlations between the reconstruction of Al metallization and its electrical performance for optimizing the packaging structure and electrical performance of IGBT module and improving the reliability of IGBT module.In this paper,a power cycling test bench was built to simulate the state of IGBT in service.The test bench was built from the main circuit design,water-cooling system design,drive and protection system design,control system design and data acquisition and processing system design and other aspects.The test bench was verified from the angles of function and reliability.The results show that this power cycling test bench can carry out the DC power cycling experiment with the current level of 100?400 A and the duty cycle of 0.2?0.8,and PWM power cycling experiment with switching frequency of 1?10 k Hz and can satisfied the demand of subsequent experiments.The power cycling experiment of IGBT module was carried out by using the test bench.The influence of service conditions(current,duty cycle and switching frequency)on temperature of IGBT module was studied.Then,basing on ABAQUS finite element software,the temperature and stress distribution of each interconnection layer of IGBT module under the temperature cycling load is analyzed,and the temperature and stress distribution of the Al metallization are analyzed.The results show that when the current increases from 200 to 400 A,the maximum temperature and temperature difference of IGBT module increase rapidly.As the duty cycle increases from 0.3 to 0.7,the temperature difference of IGBT module decreases rapidly.As the switching frequency increases from 2 to 10 k Hz,the maximum temperature of IGBT module increases.In each interconnection layer of IGBT module,the Al metallization has plastic deformation.The plastic deformation of the Al metallization is the most serious in the center of the chip,followed by the area near the foot of the bond wire,and the weakest in the edge of the chip.According to this conclusion,the following experiments focus on the analysis of these three positions of the Al metallization.The microstructure evolution of the Al metallization in IGBT module during power cycling was studied.DC power cycling tests with 80?140?temperature variations were performed on four IGBT modules.The cycle times are 0 kcycles,100kcycles,164 kcycles and 193 kcycles,respectively.The microstructure(surface morphology and internal crack)of Al metallization in IGBT module was observed.Three roughness values,the arithmetical mean roughness of the surface,S_a,the root mean square roughness of the profile,R_q,and the average distance between two neighboring asperities,?_ave,were used to characterize the surface morphology of Al metallization.The cracks were characterized by measuring crack geometric parameters.The results show that the microstructure of Al metallization evolves with the increasing of power cycling times.Under 0 to 100 kcycle,the surface roughness of Al metallization increases dramatically.When the cycling times increase from 100 kcycles to 164 kcycles,the surface roughness of the Al metallization is accompanied by cracks.Under 164 kcycles to 193 kcycles,these cracks propagate rapidly down to inner of Al metallization.The microstructure evolution mechanism of Al metallization was dislocation slip.The three-dimensional polycrystalline geometric models of Al metallization with different grain diameters and shapes were established in ABAQUS by Voronoi algorithm.Moreover,the simulation of these models under temperature cycling load was carried out.The influence mechanism of grain diameters and shapes on the mechanical properties and surface morphology of Al metallization was analyzed.The results show that the grain size and shape of the Al metallization cause the uneven stress distribution and plastic deformation,and the surface of the Al metallization becomes uneven.The results show that the larger the grain diameter is,the stronger the slip degree of each slip surface in the Al grain is,and the greater the roughness of the Al metallization is.The influence of grain shape irregularity on the surface morphology evolution of Al metallization is different.The slightly irregular grain shape of Al can slow down its roughness.However,the very irregular grain shape can aggravate the roughness of Al metallization.The influence of microstructure evolution of Al metallization on its electrical properties was studied.The resistance of the Al metallization in IGBT module was measured by four probe resistance test methods at four power cycling times of 0,100kcycles,164 kcycles and 193 kcycles.Combined with the surface morphology and crack evolution degree of Al metallization in different power cycling stages,several groups of Al metallization simplified analytical models containing different roughness parameters and crack geometric parameters were established.The resistance and current density of these models were quantitatively analyzed.The results show that the increasing of the root mean square roughness of the Al metallization can cause the increase of the current flow path,thus induces the increase of resistance.With the increase of the average distance,the current flow path of the Al metallization decreases,and causes the resistance decreases.In addition,the parameters of crack geometry have different effects on the electrical parameters of Al metallization.The results indicate that the crack depth has the largest effect on the Al metallization resistance compared with the crack length and width.With increasing crack depth and length,the maximum current density in the crack vicinity increases continuously.However,increasing crack width reduces the maximum current density.