Study On Stress State And Reliability Of Large-area Sintered Silver Paste In Power Modules

The service temperature of power semiconductor devices is getting higher and higher,which leads to more and more serious thermal stress caused by thermal mismatch in electronic packaging structures.Especially the large area connection between the substrate and the base plate,which is more likely to be subjected to periodic temperature cyclic loading.The excessive thermal strain accumulates,which in turn causes defects such as cracks or delamination.Therefore,this paper simulates and analyzes the stress state of the connection of large-area substrates sintered with silver paste,and evaluates the connection reliability in combination with the test.First,using ANSYS finite element analysis software to simulate and analyze the residual stress and warpage deformation caused by large-area sintering between the direct bonding copper(DBC)and the copper base plate in the insulated gate bipolar transistor(IGBT)module.The simulation found that the interconnection of the 30×28mm~2DBC substrate and the 40×40 mm~2copper base plate will produce a residual stress of up to 28.9 MPa in the sintered silver layer,and the maximum at the corner of the copper base plate(at a thickness of 3 mm)is 91.1?m warpage,the direction is away from the connection layer.Through the simulation analysis of the influence of the thickness of each component on the residual stress and warpage of the large area base plate connection,it is concluded that the factor that affects the warpage of the large-area substrate connection most is the thickness of the copper base plate.When the thickness of the copper base plate increases from 1 mm to 5 mm,the warpage of a large area substrate connection can be reduced by 190?m.The influence of the thickness of each component on the residual stress of the sintered silver layer is within4 MPa.Secondly,the DBC sinking method and the reverse pre-bending base plate method are used to achieve the purpose of controlling the residual stress of the sintered silver layer within a certain range and reducing the warpage of the sample.Of the above two methods,the DBC sinking method can reduce the residual stress of the sintered silver layer(2.8 MPa),and can also reduce the warpage of the sample(29?m),But this method will increase the processing cost of copper base plate and the printing difficulty of silver paste in practical application.However,the method of reverse pre-bending the base plate can greatly reduce or even eliminate the warpage deformation caused by the sintering of the sample,and it is simple and easy to operate.Finally,the thermal cycling(-55??125?)simulation and test were conducted on the interconnected samples with or without pre-bent base plate.The simulation found that when subjected to thermal cycling loads,the maximum equivalent stress of the connection layer with or without pre-bent base plate interconnection sample occurs at the corner point in each cycle,and the sample without pre-bend is slightly higher.Thus it can be speculated that the reliability of the sample with pre-bent is better than the sample without pre-bent.The thermal cycling test results show that the sample without pre-bent is prone to fracture and delamination at the corner,and the sintered connection layer of the sample with pre-bent shows better reliability than that of the sample without pre-bent,which is consistent with the simulation results.