Study on temperature dependence of viscosity of halogen-free dispensing solder paste

A dispensing process has been used to apply solder paste in surface mount technology (SMT) production. To stabilize the dispensing process, it is necessary to understand the influence of storage conditions on the viscosity of solder paste as one of various factors. In this paper, halogen-free dispensing solder paste is treated at a constant temperature and the variation of its viscosity is examined using a cone-plate-type viscometer, which can measure the viscosity of even a small amount of solder paste. Also, the tin salt is extracted from the solder paste after thermal treatment and the amount of tin in the tin salt is measured, from which the mechanism of increasing viscosity is discussed. In addition, the activation energy for increasing the viscosity of the halogen-free dispensing solder paste is calculated from an Arrhenius equation, and a predictive formula for the viscosity of solder paste is derived. Using this formula, it is possible to prevent dispensing failures by optimizing the storage conditions of halogen-free dispensing solder paste.