关键词: 混合组装, 焊点, 高温老化, 金属间化合物, 可靠性

Abstract: In order to improve the process compatibility and long-term reliability of Sn/SnPb mixed-assembly sol- dered joints,board-level circuits with daisy chain structures were designed,on which Pb-free QFPs (Quad Flat Packages) were assembled with SnPb solder paste through a typical reflow process.Then,a isothermal aging ex- periment of the assembled samples was carried out for 1500h.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  andCu₃Sn 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 Cu₃Sn 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 per- formance of soldered joints to some extent.

Key words: mixed assembly, soldered joint, isothermal aging, intermetallic compounds, reliability