What are the main steps of the SMT solder joint stress analysis method
Establish material stress-deformation relational database
The material stress-deformation relational database is an important content of the input data for stress analysis design, which is directly related to the accuracy of stress analysis. The establishment of a material stress-deformation relational database is a necessary part of the solder joint stress analysis design, and its workload is also relatively large. The strain process of the solder joint is a small stress creep process. The stress-deformation data can be obtained by a formalized test, that is, the hysteresis loop of the relationship between stress and deformation is obtained corresponding to the actual thermal load (thermal cycle) state, and the deformation range is obtained from it, that is Determine the thermal fatigue life, in order to reduce the workload, the creep effect of solder joints can also be dealt with in a simplified way. This method uses the yield stress corresponding to the thermal load to perform elastoplastic analysis to obtain the deformation range. The yield stress A% is the sum of the highest temperature yield stress pot* and the lowest temperature yield stress.
Simplified analysis of solder joint stress
The stress analysis of the solder joints is first carried out by a simplified method, and then the weak links of the strength are analyzed in detail to improve the design efficiency.
Refined analysis of local deformation of solder joints
Refined analysis can only obtain the average stress and deformation of the solder joint joints. In order to obtain the accurate value of local deformation in local areas, especially weak areas, local deformation refining analysis must be performed. The detailed analysis generally adopts the finite element method to assist the design with the aid of a computer.
The detailed analysis should first consider the trend of creep according to the characteristics of the material, which can be carried out by the simplified elastoplastic analysis method described above. Then, the finite element segmentation and refinement analysis is performed on the deformation-specific parts such as the joint end of the solder joint and the joint interface of different materials. Finally, the corresponding geometric shape evaluation or reconstruction is carried out. In order to further improve the accuracy of the analysis, the introduction of correction parameters can also be used to evaluate and correct the stress-specific parts.
Analysis of solder joint crack development
The stress analysis of solder joints generally takes the cracks of the research object as the target life standard, which is correct from the perspective of reliability and selection of reasonable solder joint shapes. But in fact, solder joint cracking is the limit of life, which is much longer than the life of solder joint cracks. Therefore, where cracks are allowed, crack progress analysis should be carried out during design. The easiest way to analyze the crack progress is to assume that the local deformation of the solder joint is directly related to the final fracture, and that the crack generation life and the fracture life are proportional. Since this ratio varies with the shape of the solder joints, it is difficult to obtain an accurate value. A more accurate method is to investigate the relationship between the local deformation range and the crack progress rate, and predict the crack progress life. There is also an analysis of the relationship between the crack progress rate and the J-integral range based on inelastic failure mechanics.
Analysis and evaluation of results
The solder joint stress analysis technology generally takes the thermal cycle life as the prediction and analysis target, and its comprehensive design results are evaluated by the length of the solder joint life. For this reason, the CAD process of SMT solder joint morphology is an optimized selection design process of different morphologies of the same type of solder joints under specific assumptions and environments, and the optimal result (reasonable morphology) corresponds to the formation of the weld Point is the solder point with the longest theoretical life. However, because the actual solder joint formation conditions and working environment are different, there may be errors in theoretical analysis, so intelligent analysis and evaluation systems with expert knowledge are often needed in the analysis and evaluation.
