【作者】 任杰；

【导师】 郭福；

【作者基本信息】 北京工业大学 ， 材料工程（专业学位）， 2022， 硕士

【摘要】 随着中国新能源汽车行业的发展,汽车电子正朝着集成化、智能化的方向快速发展。这意味着,汽车电子封装尺寸大幅度减小,通过封装焊点的电流密度迅速增加,同时焊点会经历-40℃至125℃的温度变化,因而对电子产品的可靠性提出了更高的要求。目前国内尚未有能够成熟应用于车载电子的高可靠性无铅钎料,而国际相关企业通过添加多种合金元素来开发商用产品,提高钎料的服役可靠性,因此研究多种合金元素协同影响焊点可靠性的微观机理十分必要。此外,无铅焊点可靠性问题的研究主要集中在单场作用下焊点的电迁移、热疲劳等问题上,而多场耦合作用下焊点的失效行为相对较少。因此开展车载电子钎料电-热耦合场可靠性研究,揭示真实服役环境下焊点的失效行为,对提高我国车载电子钎料的研发设计具有非常重要的理论意义和实用价值。本文以我国自主研发的车载电子钎料LF516(Sn-3.4Ag-0.7Cu-3.2Bi-3.0Sb-0.05Ni-0.006B)为研究对象,以国外已有的商用多元合金钎料Innolot(Sn-3.8Ag-0.7Cu-3.0Bi-1.5Sb-0.15Ni)和广泛使用的SAC305(Sn-3.0Ag-0.5Cu)钎料作为对照。通过研究该钎料的焊接性能,证明该钎料的应用可行性。通过线性对接焊点的电迁移实验,分析金属间化合物在焊点界面处的形成和生长过程,并探究Sb、Bi和Ni等元素的添加对焊点电迁移损伤及失效行为的影响机理。对球栅阵列封装结构焊点进行电-热场加载试验,分析不同晶体取向的焊点的微观组织演变,探究高电流密度载荷和温度循环载荷作用下焊点的失效行为和机理。研究结果表明,LF516和Innolot的熔点和过冷度低于SAC305,Bi降低了钎料的熔点和过冷度,改善了钎料与基板之间的润湿性能。Bi、Ni、Sb元素通过第二相弥散强化和固溶强化作用提高了焊点的硬度。在电迁移过程中,LF516和Innolot焊点正负极界面处的金属间化合物厚度始终小于SAC305焊点,且负极更晚出现空洞和明显的金属间化合物溶解,说明加入Ni、Sb、Bi增强了焊点的抗电迁移性能。LF516焊点负极处出现空洞和金属间化合物溶解早于Innolot焊点,Bi在LF516电迁移过程中产生偏析,难以与Sn形成固溶体,减弱了抑制原子迁移的作用。在电-热场作用下,单晶焊点受到热应力作用,位错沿着小角度晶界滑移,造成晶粒转动。多晶焊点受到的热应力则沿着晶界释放,因此表面凸起沿着大角度晶界变形。加入合金元素以后,LF516、Innolot焊点的Sn基体强度提高,抵抗塑性变形的能力增强,焊点基体承受晶格畸变能的能力提高,导致应力在大角度晶界处发生集中。SAC305中Sn基体强度不足,抵抗塑性变形的能力差,Sn基体通过塑性变形释放晶格畸变能。高电流密度带来的热效应加剧了焊点的疲劳失效。此外,由于电流在晶界处分布不均匀,电流的热效应加速了多晶焊点的失效过程。更多还原

【Abstract】 With the continuous development of China’s new fuel efficient vehicle industry,automotive electronics is developing rapidly in the direction of diversification,integration and intelligence,and the proportion of electronic products in the vehicle has increased significantly.At present,there is no lead-free solder with high reliability that can be maturely applied to automotive electronics in China,while international related enterprises develop commercial products by adding multiple alloying elements to improve the service reliability of solder,so it is necessary to study the microscopic mechanism of multiple alloying elements synergistically affecting the reliability of solder joints.In addition,research on the reliability of lead-free solder joints has mainly focused on the problems of electromigration and thermal fatigue of solder joints under the action of a single field,while the failure behaviour of solder joints under the action of multiple fields is relatively rare.Therefore,it is of great theoretical significance and practical value to carry out research on the electromigration behaviour and thermal fatigue reliability of brazing joints for automotive electronics at high current densities,and to reveal the failure behaviour of solder joints under real service environments,in order to improve the research and design of brazing joints for automotive electronics in China.In this thesis,we take LF516(Sn-3.4Ag-0.7Cu-3.2Bi-3.0Sb-0.05Ni-0.006B),a solder for automotive electronics developed independently in China,and Innolot(Sn-3.8Ag-0.7Cu-3.0Bi-1.5Sb-0.15Ni),the most widely used commercial multi-alloy solder abroad,as the object of study.The SAC305(Sn-3.0Ag-0.5Cu)solder was used as a control to demonstrate the feasibility of the application of this solder by studying its solderability.Electromigration experiments were carried out on one-dimensional linear butt joints to analyze the formation and growth of intermetallic compounds at the interface of the joints and to investigate the mechanism by which the addition of elements such as Sb,Bi and Ni affects the electromigration damage and failure behaviour of the joints.Electrical-thermal coupled field loading tests were carried out on the solder joints of the ball grid array package structure to analyze the microstructure evolution of solder joints with different crystal orientations and to investigate the failure behaviour and mechanism of solder joints when coupled with high current density loading and temperature cycling loading.The results show that the melting point and subcooling of LF516 and Innolot are lower than those of SAC305,and that Bi lowers the melting point and subcooling of the solder and improves the wetting performance between the solder and the substrate;Bi,Ni and Sb elements improve the hardness of the solder joint through second phase dispersion strengthening and solid-solution strengthening.During electromigration,the Intermetallic compounds thickness at the interface between the positive and negative electrodes of LF516 and Innolot joints is always less than that of SAC305 joints,and the cavities and obvious Intermetallic compounds dissolution at the negative electrode appear later,indicating that the addition of Ni,Sb and Bi enhances the electromigration resistance of the joints.cavities and Intermetallic compounds dissolution at the negative electrode of LF516 joints appear earlier than those of Innolot joints,and Bi is present in LF516 electromigration process produced bias,difficult to form solid solution with Sn,weakening the role of inhibiting atomic migration.Under the electric-thermal field,single crystal joints are thermally stressed and dislocations slip along small angular grain boundaries,causing grain rotation.The polycrystalline joints are subjected to thermal stresses that are released along the grain boundaries and the surface bumps are thus extended along the large angular grain boundaries.The addition of alloying elements increases the strength of the Sn matrix in LF516 and Innolot joints,which increases the ability to resist plastic deformation and increases the ability of the solder joint matrix to withstand lattice distortion energy,resulting in a concentration of stress at large angular grain boundaries.The Sn matrix in SAC305 is not strong enough to resist plastic deformation and the Sn matrix releases lattice distortion energy through plastic deformation.The fatigue failure of the solder joint is exacerbated by the thermal effect of the high current density.In addition,the thermal effect of the current accelerates the failure process of polycrystalline solder joints due to the uneven distribution of the current at the grain boundaries.更多还原