Composition
A solder paste is essentially powdered metal solder suspended in a thick medium called flux. Flux is added to act as a temporary adhesive, holding the components until the soldering process melts the solder and makes a stronger physical connection. The paste is a gray, putty-like material. The composition of the solder paste varies, depending upon its intended use. For example, when soldering plastic component packages to a FR-4 glass epoxy circuit board, the solder compositions used are eutectic Sn-Pb (63 percent tin, 37 percent lead) or SAC alloys (tin/silver/copper, named for the elemental symbols Sn/Ag/Cu). If one needs high tensile and shear strength, tin-antimony (Sn/Sb) alloys might be used with such a board. 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 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. Depending upon the formulation of the paste, it may be very important to stir the paste before it is used, to ensure that the viscosity is appropriate for proper application.
Read more about this topic: Solder Paste
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