Effect of organic acid in Isopropyl alcohol fluxes on wetting of Sn-Bi solder on Cu surface

Six different organic acids (salicylic acid, benzoic acid, sorbic acid, citric acid, malic acid, lauric acid), served as the active agent in the Isopropyl alcohol flux, were investigated. The wetting test of Sn-52Bi eutectic alloy on Cu substrate was used to evaluate the flux properties such as the maximum wetting force and the active temperature of the organic acids. The results showed that all of the organic acid were ineffective as an active agent below 145°C, when the temperature was more than 165°C, salicylic acid, benzoic acid, sorbic acid, citric acid, and malic acid is effective to promote the wetting of Sn-Bi eutectic solder on Cu substrate. As a comparison, lauric acid is still ineffective. The maximum wetting force for the flux with salicylic acid was 0.18 N/m, benzoic acid 0.17 N/m, sorbic acid 0.14 N/m, citric acid 0.25 N/m, malic acid 0.26 N/m, respectively. Although it is not clear what determines the wetting ability of an active agent in the flux now, the present result shows that both the maximum wetting force and the critical wetting temperature can be used to evaluate the wetting ability of an organic acid, in which the melting point of the organic acids is an important index during the design and choose of an organic acid.