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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.
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| 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. | |||||||||||||||||||||||||||||||||||||
