Influences of Organic Acids in the Soldering Flux System on Wettability and Self-alignment Ability of Sn–3.0Ag–0.5Cu Solder Paste during Reflow Process

In this work, the effects of different components in the soldering flux with adding respectively six organic acids on the self-alignment performance of Sn–3.0Ag–0.5Cu solder paste were studied systematically. Starting from a conventional soldering flux formulation, six different organic acids were added respectively into the flux to prepare the modified fluxes. The wetting balance tests were conducted to evaluate the wettability of six fluxes in the same alloy (Sn–3.0Ag–0.5Cu solder powder). Then, the passive components (R1206 and R0603) were placed on the solder pastes printed on PCB pads with an initial offset for a half of the pad distance and a rotation angle of 45 degree, and reflowed using an automatic SMT machine. A large amount of components offset data before and after reflow was obtained and analyzed statistically. It has been shown that the fluxes with adding different organic acids exhibit distinct wettability, and thereby having different component self-alignment performances. The wetting force of the flux is proportional to the self-alignment ability of the solder paste, i.e., the larger the wetting force, the higher the self-alignment ability. The activity of organic acids in the solder paste is closely related to the substituent of organic acids. Different substituents have distinct electron absorption ability, leading to the difference in ionization ability of organic acids in the soldering flux. Statistical analysis of offset data for the components indicates that the self-alignment ability of the solder paste is strongly related to the presence of organic acids.