Numerical Simulation And Reliability Analysis Of Printed Circuit Board Assembly Fabrication Process

A printed circuit board assembly(PCBA)consists of a PCB and various components mounted on the surface,which is a core product in the integrated circuit industry.It has been widely used in 5G communications,automotive electronics,computers,medical devices,aerospace,etc.However,the curing shrinkage of the resin and the mismatch of thermal expansion coefficients between different materials lead to warpage of the PCB and its components when subjected to temperature loads during the fabrication process,which in turn affects the quality and reliability of the product.Therefore,it is of great scientific significance and engineering application to study the stress,strain and warpage evolution rules of PCBA during the fabrication process,which can help improve the reliability and product yield of PCBA products and reduce the design and fabrication cost.In this paper,a PCB model is established with layering in the thickness direction and precision partitioning within the copper layer plane,which better reflects the structural characteristics of PCB thickness direction lamination and copper trace distribution within the copper layer.The fabrication process of PCBA involves heat transfer,curing heat generation,thermal expansion,chemical shrinkage and stress relaxation,etc.In this paper,based on the thermochemical model and 3D viscoelastic constitutive equation,the finite element model of PCB is established in ABAQUS software by combining subroutines USDFLD,HETVAL,UMAT and UEXPAN,and the reliability of the model is validated by comparing the simulation results with experiment results.First,a coupled thermal-chemical-mechanical multi-physical field method is used to simulate and analyze the PCB lamination process.That is,the temperature and curing degree fields of PCB during the lamination process are first calculated in the heat transfer analysis,and then the temperature and curing degree fields are imported into the mechanical analysis in the form of predefined fields to calculate the stress,strain and displacement fields of PCB during the lamination process.The results show that the in-plane non-uniform and the non-symmetric in the thickness direction curing shrinkage of the resin in the copper layer are the root causes of PCB warpage during the lamination process;after opening mold,the main rule of PCB warpage shape is convex toward the side with high copper content in the thickness direction;the warpage value of PCB is maximum at the time of mold opening and decreases with temperature after mold opening.The experimental results are in good agreement with the simulated PCB warpage shape,and the relative error of warpage value is within 20%.Secondly,the validated PCB finite element model is used to research the effects of process factors such as holding temperature,maximum pressure and holding time at high temperature and high pressure on the PCB warpage value after opening mold.The results show that the holding temperature has a great effect on PCB warpage value after opening mold,and PCB warpage value after opening mold increases with the increase of holding temperature;the maximum pressure and holding time at high temperature and high pressure have less effect on PCB warpage value after opening mold.Finally,a finite element model of the PCB processing-reflow process is established based on lamination process.The reliability of the above model is validated with experiment results,realizing the integrated finite element simulation of PCB lamination-processing-reflow soldering.On this basis,a PCBA multi-structure model containing such as PCB,solder joint array and package chips is established.The residual stress and strain of the PCB laminationprocessing process are also inherited as the initial stress and strain of the reflow soldering,and multi-structure reflow simulation is performed.The results show that the mismatch of thermal expansion coefficients between different materials is the root cause of warpage of PCB and its components during reflow soldering,where no curing reaction of resin is involved.Therefore,the warpage value of PCB A,PCB and package chip all increases with increasing temperature and decreases with decreasing temperature;the PCB and package chips are free warping before soldering and constrained warping after soldering;the warpage of package chips will be similar to that of corresponding soldering areas on the PCB due to their mutual influence after soldering;compared with PCBA without initial stress and strain,PCBA with initial strain has initial warpage feature at the initial moment of reflow,while initial stress increases the warpage value of PCBA in the first reflow process,but has less effect on the warpage of PCBA in the second reflow process.