It is used in PCB assembly, and including prototype PCB assembly, especially when using reflow soldering techniques.
When solder paste is used in mass PCB assembly as well as prototype PCB assembly there are a number of stages that are undertaken. First solder paste is applied to the printed circuit boards. The solder paste is only applied to the areas where solder is required. - tutorial, information, article about what is solder paste, how to use solder paste and gain the most from the PCB assembly process. Soldering Processes Tutorial Includes. Summary. Solder is basically metal wire with a low melting point, where low for our purposes means low enough to be melted with a soldering iron. For electronics, it is traditionally a mix of tin and lead. Lead has a lower melting point than tin, so more lead means a lower melting point.  Flux paste is a cleaner and corrosivepaste to work with copper and other metals to help the solder attach to the other two pieces that your soldering. Source(s): 20+ yrs in bldg maint field. Solder flux deos a couple things, it helps keep the guns element clean and it helps the solder bond to the part in question. You can buy rosin core solder with the flux already inside the solder, this is much easier to use then to use solder flux. Solder paste viewed under a microscope. A solder paste is essentially powder 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. 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. Solder paste is typically used in a screen-printing process, in which paste is deposited over a stainless steel or polyester mask to create the desired pattern on a printed circuit board. 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. Solder paste is a form of solder that is used in PCB assembly, and including prototype PCB assembly, especially when using reflow soldering techniques. The solder paste is a mixture a solder spheres and a specialized form of flux. - tutorial, information, article about what is solder paste, how to use solder paste and gain the most from the PCB assembly process. Soldering Processes Tutorial Includes. Solder paste is a material used in the manufacture of printed circuit boards to connect surface mount components to the copper traces of the board. 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. To apply flux to copper pipes, use a small brush, often provided with the flux when you buy it. For electronic circuits, dip the soldering iron tip, the solder itself and/or the wire directly into the can of flux. Soldering paste serves three purposes. It helps clean the copper as it heats, it helps the solder flow more evenly and it helps the solder adhere to the copper.
http://www.radio-electronics.com/info/manufacture/soldering/solder_paste/how-to-use-solder-paste.php http://electronics.stackexchange.com/questions/3/what-are-the-different-types-of-solder-used-for https://answers.yahoo.com/question/index?qid=20070829130320AAoTpwS https://en.wikipedia.org/wiki/Solder_paste https://en.wikipedia.org/wiki/Solder_paste http://www.radio-electronics.com/info/manufacture/soldering/solder_paste/how-to-use-solder-paste.php https://en.wikipedia.org/wiki/Solder_paste http://www.ehow.com/facts_5871555_soldering-paste_.html
what is solder paste used for