Analysis And Optimization Of Residual Stress After Reflow Soldering Of Board-level Circuit Modules

During the soldering process of the board-level circuit modules,the material of the solder joint will undergo a change from solid state to melting and condensation under the effect of reflow soldering temperature.After this phase transformation process,a lot of residual stress will be generated and accumulated in the solder joint.The existence of residual stress in the solder joint will cause the solder joint to deform,which will affect the fatigue strength,static strength and corrosion resistance of the solder joint.As a result,the initial process damage in the assembly process of electronic products and the reliability of electronic products are affected.Therefore,enough attention should be paid to the problem of residual stress after reflow soldering in order to ensure the reliability of the solder joint after service.First,established two different board-level circuit module finite element models based on ANSYS,and the after reflow soldering residual stress analysis was performed on the thermal and structural coupling conditions.The results of model 1 and model 2 are consistent.The results show that the solder joint residual stress after reflow soldering is unevenly distributed.The maximum residual stress occurs at the corner where the BGA solder joint contacts the chip and is farthest from the center of the solder joint array.Based on the drilling strain method,a verification experiment for residual stress measurement after reflow soldering is designed and completed.The results show that: the verification experiment results prove the validity of the simulation analysis results.The influence of single factor(solder joint material,BGA solder joint diameter,BGA pad diameter,BGA solder joint height and BGA solder joint spacing)changes of board level circuit module solder joints on residual stress was studied.The results show that the residual stress value of solder 5Sn3.5Ag0.75 Cu is the largest.With the increase of BGA solder joint diameter and solder joint spacing,the residual stress gradually increases.As the diameter of the BGA pad and the height of the solder joint increase,the residual stress gradually decreases.Then,Based on ANSYS PDS module,the sensitivity analysis of the BGA solder joint morphological parameters(solder joint height,solder joint diameter,pad diameter,and solder joint pitch)was performed.The first module result showed that when the confidence was 90%,there are 4 factors that significantly affect the residual stress,and the sensitivity is sorted from large to small as follows: solder joint height>solder joint pitch>pad diameter>solder joint diameter.The second module result showed that when the confidence is 90%,there are 4 factors that significantly affect the residual stress,and the sensitivity is sorted from large to small as follows: solder joint height>pad diameter>solderjoint diameter>solder joint pitch.The factors that have a significant effect on residual stress in the sensitivity analysis results were selected as variables.The response surface-genetic algorithm was used to optimize the structural parameters of the solder joints.Model 1 results showed that the optimal structural parameter combination with the smallest residual stress was the solder joint diameter of 0.31 mm,The solder joint height is0.24 mm,the pad diameter is 0.26 mm,and the solder joint pitch is 0.45 mm.Model 2 results showed that the optimal structural parameter combination with the smallest residual stress is solder joint diameter of 0.38 mm,solder joint height of 0.34 mm,and pad diameter of0.34 mm and solder joint pitch of 0.60 mm.Simulation of the optimized combination shows that the residual stress results are smaller than any combination results in the response surface experiment,which proves the effectiveness of the optimization analysis.Finally,the factors that have a significant influence on the residual stress in the sensitivity analysis results are selected as input,and the BP neural network with momentum prediction model of the unified module of module 1 and module 2 is established through the MATLAB programming language.Based on the prediction model,20 groups of board levels were verified respectively.The maximum difference between the predicted value and the simulated value is 5.83%,and the prediction of the residual stress value of the board-level circuit module after reflow soldering is well realized.