Interfacial Reactions Among Sn-based Solders And Substrates

During electronic packaging and subsequent service of devices, interfacial reactions among solders and substrates may occur and affect reliability and long-term performance of solder joints. Thermodynamics and kinetics of solder/substrate interfacial reactions are then theoretically and practically significant to design of solder compositions and process. Supported by "Thermodynamic and kinetic interfacial ractions among lead-free solders and substrates (No.50371104)" and "Kinetics of formation sequence of intermediate phases in interfacial reactions between two pure metals (No.50671122)" by National Natural Science Foundation of China, phase equilibria, phase formation in Sn-based solder/substrate interfacial reactions were studied through incorporation of diffusion couple technique, alloy preparation and calculation of phase diagram (CALPHAD) method, with main results as the following:(1) Phase equilibria of Sn-Zn-Co system at650℃and750℃were determined through x-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Sn-Zn-Co system was optimized based on thermodynamic data and information of phase equilibria obtained from this work and literatures. A set of self-consistent thermodynamic parameters were achieved.(2) By using CALPHAD approach, Ag-Co binary system was thermodynamically assessed. Combining previously optimized Ag-Sn and Co-Sn binary systems, Sn-Ag-Co ternary system was extrapolated. By adopting thermodynamic parameters of Sn-Ag-Co system and results from Scheil-Gulliver model, solidification behaviors of3ternary Sn-Ag-Co alloys were simulated. The simulation can explain experimental microstructure evolution very well.(3) Interfacial reactions in Co/Sn couples at300,400,500and600℃were studied by using diffusion couple technique, SEM and EPMA. For Co/Sn couples at300℃, CoSn3was detected to form first. While CoSn2was the first compound to form at400℃and500℃. And CoSn was observed to form first at600℃.(4) Interfacial reactions in Sn-3.5wt.%Ag/Co couples at300℃were studied. CoSn3was detected to form first. Parabolic growth of CoSn3layer as a function of time was observed, indicating that it is Sn diffusion responsible for its growth. In comparison with interfacial reaction of Co/Sn at300℃, we found that the addition of Ag inhibiting the growth of CoSn3.(5) Interfacial reactions between Sn-Zn alloys (with35or60at.%Zn) and Co substrate at360℃were studied. The compound γ1-CoZn7was observed to form in Sn-35at.%Zn/Co, while the compound y2-CoZn13was formed in Sn-60at.%Zn/Co.(6) Combined considering nucleation and growth kinetics, one quantitative kinetic criterion to predict phase formation at interfaces between two metals was proposed. Formation of intermetallic compounds in binary couples (Cu/Sn, Ni/Sn, Ag/Sn, Au/Sn, Co/Sn, Al/Cu, Al/Ni, Al/Au, Al/Co etc.) and simple ternary couples (Sn-Ag/Co and Sn-Zn/Co) can be explained reasonably well with the present criterion with thermodynamic parameters of relevance.