Solder paste is a mixture of alloyed solder powder and a flux medium. The flux medium is a gel type material which suspends the powder to avoid separation. It has rheological additives for good printability. The flux medium also contains activators. The activators' key function is to perform “fluxing” which is the cleaning of oxides from metal surfaces. Oxides are found not only on the solder powder surface, but also on components and PCB metalizations. Oxides need to be removed in order for proper melting and coalescing of solder particles, in addition to wetting the pad and component surfaces, forming a reliable solder joint. The activators are typically corrosive, which is why solder paste has a specified shelf life and requires refrigeration.