【作者】 宋奎晶；

【导师】 魏艳红；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 硕士

【摘要】 有限元数值模拟是进行服役环境温度循环载荷下SMT焊点应力应变状态分析以及寿命预测的最有效方法之一。其中,钎料本构模式和寿命预测方法的选取直接决定着模拟结果的准确性,具有重要的理论研究价值。本文综合考虑钎料的应变硬化、蠕变变形和高温软化等特征,建立钎料的粘混合硬化本构关系,研究热循环加载下焊点内的应力应变场演变规律并进行寿命预测。首先,采用混合硬化本构关系综合描述热循环加载下钎料的各向强化特征和循环加载的包申格效应,建立钎料的基于幂指数硬化模型的等向硬化增量本构关系和基于Ziegler双线性硬化模型的随动硬化增量本构关系,并利用MARC二次开发接口和Fortran语言开发用户子程序,实现了等向、随动、混合硬化三种本构关系的有限元计算。同时基于钎料的Darveaux蠕变方程,开发用户子程序定义钎料的粘性性质,得到了蠕变问题的有限元求解过程。其次,在Darveaux焊点寿命预测模型的基础上,选取焊点整体的平均应变能密度作为主控力学参量对SMT焊点进行寿命预测,考虑了微小焊点的高约束度及网格密度对应力应变场的影响,利用二次开发接口提取单元应变能密度、单元体积等信息。最后,研究钎料本构关系的选取及焊点形态对焊点力学行为和疲劳寿命的影响。焊点力学行为的模拟结果表明,元件与基板间隙高度处的焊根内侧产生应力集中,是焊点的薄弱部位;焊点内的应力应变分布具有温度历史相关性,但应变能密度分布基本不随时间变化,将其作为主控力学参量判断裂纹萌生及扩展位置具有代表意义。焊点寿命预测结果表明,钎料本构关系的选取会对寿命预测结果产生重要影响。采用理想弹塑性本构关系的寿命预测结果最高;进一步考虑粘性时,非弹性变形增大,寿命略低;采用粘混合硬化钎料本构关系时,焊点具有明显的硬化特征,导致蠕变变形增大,进而使应变能密度显著增大,焊点寿命最低。由于粘混合硬化钎料本构关系考虑了钎料变形的多种影响因素,因此可将其寿命预测结果作为焊点可靠性设计的依据。随着焊点圆角形态从凹变凸,应力应变集中程度降低,焊点的寿命提高。更多还原

【Abstract】 Numerical simulation is proved to be one of the best tools to analyze the stress and strain evolution in SnPb solder joint and predict its fatigue life under environment cycling temperature. In the simulation, the selection of constitutive model of SnPb and life prediction method determine the accuracy of finite element simulation results directly, so it needs deeply theoretic study. In this paper, a viscous-mixed hardening constitutive model of SnPb is established, which can describe the strain hardening, creep deformation and high temperature softening characteristics of SnPb.Firstly, the mixed hardening constitutive equation has been used to describe the isotropic hardening characteristic and Bauschinger Effect under cyclic loading. The incremental constitutive relations are built based on Power Exponential hardening model and Ziegler bilinear hardening model, respectively. The three kinds of constitutive relations, including isotropic hardening equation,kinetic hardening equation and mixed hardening equation have been completed by users’subroutine of MARC written in Fortran. In addition, User’s subroutine has been compiled to define the creep deformation performance of SnPb based on Darveaux steady creeping equation. Afterward, FEM calculation process to solve creep deformation problem is given.Then based on Darveaux life prediction model of solder joint, the average strain energy density of the whole solder joint is chosen as the dominant mechanical factor for fatigue life prediction of SMT solder joint. The high constraint of the tiny joint and the effect of meshing density on the stress-strain field have been considered, and information of unit such as strain energy density, volume is abstracted by subroutine program.Finally, the influence of constitutive models of SnPb and shape of the solder joints on the mechanical behavior and fatigue life is studied. The simulated results of stress-strain field evolution in SMT solder joint show that the root of solder joint between the component and substrate gap is the weakest zone with a high stress concentration. The distribution of stress and strain within the solder joint under temperature cycling is dynamic and relative with the temperature history. As the strain energy density field remains about the same, it’s proved to be an appropriate dominant mechanical factor to judge the location where the crack begins and expands.It shows that the constitutive model of SnPb has a great impact on the life prediction results of solder joint. When the ideal elastic-plastic constitutive model is used to define the mechanical behavior of SnPb, life of the solder joint is the longest. When creep deformation is taken into account, the nonlinear deformation is much larger. Accordingly, the calculated life of solder joint is slightly lower. When the viscous-mixed hardening constitutive model is used, the solder joint shows an obvious strain hardening characteristic and the improved stress leads to much larger creeping deformation. Synthetically the strain energy density increases significantly that life of the solder joint is comparatively the shortest. With most factors affecting SnPb deformation considered, the simulated result can be used as reference for reliability design of solder joint. With quantity of solder increasing and the shape changing from concave to convex, the average strain density gets lower and fatigue life increases.更多还原