(1) Solder joint shape modeling and forming design
The mathematical model of the SMT solder joint shape is the basis of the solder joint forming design. Whether it is correct or not and whether the modeling method is appropriate is the key to the success of the solder joint shape research. The establishment of the mathematical model is based on the basic theories and mathematical methods of solder joint morphology, such as solder joint wetting theory, capillary phenomenon, and liquid surface expansion theory. The currently proposed SMT solder joint modeling methods mainly include boundary value integral mathematical analysis and finite element numerical analysis based on the principle of minimum energy. The mathematical analysis method is only suitable for solving simple solder joint shapes; the quantitative finite element numerical analysis method can be applied to the solution of various complex three-dimensional solder joints. It has good versatility and subsequent analysis of solder joint quality such as stress/thermal analysis. The continuity of the continuity, its analysis accuracy is good, and it is an ideal method to solve the solder joint shape.
(2) Conversion of solder joint shape model and mechanical analysis model
The solder joint morphology generated by the evolution of the solder joint morphology model is generally a three-dimensional solder joint surface profile and its data file containing the coordinates of the surface node of the solder joint morphology. In order to use CAD technology for further mechanical analysis, it needs to be transformed into three-dimensional solid solder joint graphics and its mathematical model containing internal node coordinates. For simple solder joint shapes, generally relevant general software can be used to complete this conversion; for more complex solder joint shapes, special conversion software or manual assistance is needed to complete the conversion process.
(3) Stress analysis and method of solder joint shape
The failure of SMT solder joints is mainly caused by thermal fatigue. Figure 6.4 is a schematic diagram of thermal fatigue deformation of solder joints. When the electronic assembly is subjected to a thermal load, due to the difference in the thermal expansion coefficient and temperature rise of different materials, a difference in expansion occurs between the component and the substrate, which generates thermal stress. At this time, the deformation of the welded part with the weakest structure tends to be the largest, resulting in the phenomenon of weld joint fracture caused by thermal fatigue.
Solder joint stress analysis technology is a solder joint design technology that combines stress model results and material test results to evaluate and predict the quality of solder joints. Generally, thermal cycle life is used as the target of prediction and analysis. As shown in Figure 6.3, the solder joint stress analysis technology also includes thermal mechanical load conditions, stress evaluation data and evaluation methods, and stress analysis calculation theories and methods. The acquisition and analysis of its related stress parameters involves the mechanical and physical properties of the material, the forming method of the solder joint, the actual load condition of the solder joint, the interaction between materials, etc., and the shape of the solder joint is a complex three-dimensional shape. Therefore, the solder joint stress analysis design has the characteristics of complex geometry, difficult to measure stress parameters, and many mutual influence factors.
