高温时效下Sn/SnPb混装焊点的微观组织研究

A Probe into Microstructure of Sn/SnPb Mixed-Assembly Soldered Joints Under Isothermal Aging

针对Sn/Sn Pb混合组装焊点在工艺兼容性和长期可靠性方面存在的问题,设计了带菊花链结构的板级电路,采用回流焊接工艺对无铅方形扁平封装(QFP)器件和Sn Pb焊料实现混合组装,对组装样品进行1500 h的高温老化实验。通过对高温老化前后混装焊点显微组织的分析和电、力学性能的研究,探讨混装焊点两侧焊接界面金属间化合物(IMC)的生长规律及其对焊点电、力学性能的影响.结果表明:Cu6Sn5和Cu3Sn金属间化合物厚度均与老化时间的平方根呈线性关系,混装焊点界面的Cu6Sn5分解反应是Cu3Sn化合物的主要生长机制;老化过程中富铅相在焊接界面的聚集,切断了焊点内Sn原子的扩散通路,形成阻碍IMC层进一步生长的抑制区;焊点基体β-Sn的尺寸粗化、Pb的富聚以及具有本质脆性的IMC层状生长降低了焊点的抗拉强度,层状IMC的厚度在一定程度上反映了焊点的力学性能.

In order to improve the process compatibility and long-term reliability of Sn / Sn Pb mixed-assembly soldered joints,board-level circuits with daisy chain structures were designed,on which Pb-free QFPs（ Quad Flat Packages） were assembled with Sn Pb solder paste through a typical reflow process. Then,a isothermal aging experiment of the assembled samples was carried out for 1 500 h. Moreover,the microstructure,electric performance and mechanical performance of the mixed-assembly soldered joints before and after the aging were analyzed,through which the growth mechanism of IMC（ Intermetallic Compounds） at both sides of soldering interfaces and its effect on the electric and mechanical performances of soldered joints were explored. The results show that（ 1） the growth of Cu6Sn5 and Cu3Sn IMC at soldering interfaces exhibits an excellent linear relationship with the square root of aging time,and the decomposition reaction of Cu6Sn5 is the primary growth mechanism of Cu3 Sn IMC at soldering interfaces;（ 2） the Pb phase gathering in soldering interfaces in the aging process cuts off the diffusion pathway of Sn atom and thereby inhibits the further growth of interface IMC; and（ 3） the coarsening of β-Sn size,the accumulation of Pb-rich phase and the quick growth of interface IMC layer with intrinsic brittleness all result in the decrease of tensile strength of soldered joints,and the thickness of layered IMC reflects the mechanical performance of soldered joints to some extent.