Voiding and printing characteristics of solder paste at different degrees of exposure

Abstract

Solder paste in surface mount technology (SMT), because of operation requirements, is exposed to the atmosphere for different times. The effects of the exposure on the printing and voiding characteristics of the solder paste are presented in this paper. The evaporation of the solvents in the solder paste during exposure was substantiated by weight loss tests and optical examination, and the results show that the weight loss and exposure time relationship can be expressed by an exponential formula. The voids in the joints produced by solder paste with different degrees of exposure were quantified and recorded. It was found that an increase in the exposure was accompanied with a decrease in the void size and content. The printing characteristics of selected solder pastes after exposure to the atmosphere for various times were investigated by studying the scooping and bridging conditions of the prints. The results show that scooping is not only related to the degree of exposure of the solder paste but also to the printing direction and the stencil aperture width. The investigation results further show that the printed division increases in direct proportion to the stencil aperture division, and the exposure time of the solder paste has insignificant effects on the bridging conditions.