Typically, the alloy is chosen for an application based on desired mechanical and electrical requirements. The flux type is chosen based on the degree of cleaning required to achieve a good solder joint and the cleaning method used to remove any residual flux. Dispensing small dots of solder paste requires a specific particle size and viscosity.
Generally, the smaller the dot size the finer size particle required to dispense the dot through the tip. While metal particle sizes in a range of -325 to +500 (passes through a 325 screen, but does not pass through a 500 screen, as shown in figure 2) are common, smaller particles ranging from +625 are often used. The smaller the particle size, the greater the surface area per volume as shown in figure 3. This added surface area places more requirements on the flux to flow the solder. Also, while -325 to +500 paste generally contains 85 percent metal by weight, the smaller particle size paste is generally only 70 percent by weight. In order for a dot of solder paste to remain in place during reflow, a relatively high viscosity is desired. A low viscosity material will slump, spreading the particles of metal over a wider area during the reflow process. Drying out the solder paste before adding components and reflowing is a method of managing the spreading problem.