Solder paste is arguably one of the most highly engineered materials. What is usually called solder paste “flux” is a combination of activators that dissolve oxides off metal surfaces, rheological additives that give the paste the desirable thixotropic properties to print well, and solvents to dissolve all of the chemicals. See Figure 1.
Figure 1. The many required functions of solder paste flux.
What many people miss is that the flux must also have rosin that acts as an oxygen barrier to protect the surfaces of the solder powder from oxidation during reflow. This rosin also provides tack to hold the components in place. In addition, the rosin works with the rheological agents to give the solder paste the best rheological properties.
A modern solder paste prints well, has good response-to-pause printing, provides good tack to hold the components on the PWB after printing, has a broad reflow process window, and after reflow has good surface insulation resistance properties.
In addition, many solder pastes are designed to minimize defects like voiding, graping, tombstoning, the head-in-pillow defect, and many others. So, we ask a lot from our solder pastes, but don’t forget the oxygen barriers as being one of the many required properties.