 Generally, solder pastes are made of a tin-lead alloy, with possibly a third metal alloyed, although environmental protection legislation is forcing a move to lead-free solder.Solder paste is thixotropic, meaning that its viscosity changes over time (it becomes less viscous) with applied shear force (e.g., stirring).The "thixotropic index" is a measure of the viscosity of the solder paste at rest, compared to "worked" paste.