The study of the interfacial reaction between Sn/Ni/Cu substrates of PTH components and Sn-xAg-xCu solder pastes after performing Wave-Soldering process

The experimental samples mainly are the assembled OSP PCB with the Plated-Through-Hole (PTH) components which leads are manufactured with Sn/Ni/Cu substrates and the soldering process is Wave-Soldering (W/S) with two kinds of lead-free SAC solder bars categorized as Sn-3.0wt%Ag-0.5wt%Cu (SAC305) and Sn-0.3wt%Ag-0.7wt%Cu (SC0307). In addition, the test conditions are defined that the experimental temperatures are set at 100°C, 150°C, and 200°C and the baking periods are set for 4, 16, 256, 360, 500, 720, and 1000 hours. The subject of this study aims at the growth behavior, the calculation of activation energy (AE), and the diffusion reaction in Kinetics. In the W/S process (Liquid-Solid status) for both solder pastes, Ni₃Sn₄ phase can´t be found in the IMC layers of SAC/Ni. It can be concluded that the peak temperature in Wave-Soldering process is higher than that in SMT process and the plating Cu layer of PCB is promptly dissolved into liquefied solder pastes to raise Cu concentration. This situation benefits the growth of (Ni_1-yCu_y)₆Sn₅. As the aging temperature rises to 200°C, the thickness of the IMC layers rapidly extends and Ni layer is also dissolved. The reaction behavior of Cu and Sn atoms at the component side also promptly response and it is evident that Cu₆Sn₅ phase is close to the solder side and Cu₃Sn phase is formed near to the component side by EDS instrument. In the reaction of the solid-solid systems, these two intermetallic compounds (IMCs) grow by parabola law and their reaction mechanism is controlled by diffusion. After finishing experiments, the growth velocity, followed by diffusion coefficients calculated at 100°C and 150°Cof the aging temperatures, of SAC0307 is almost equal to that of SAC305. When the aging temperature is raised up to 200°C, Ni layer is rapidly dissolved and the vacancy- - of Ni atom is replaced by Cu atom to form Cu₃Sn phase. Namely, the diffusion reaction at the component side and Ni depletion for SAC305 and SAC0307 don´t have any impact even Cu quantities are within 0.5-0.7wt% of solder pastes or solder bars.