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Solder paste is the basic element of the SMT process reflow soldering process. It provides the necessary flux for cleaning the surface and the solder that ultimately forms the solder joint. Solder paste is composed of metal powder particles dissolved in a concentrated flux solution. Solder paste has many important uses in the production of surface mount components. Because it contains the flux required for effective soldering, it is not necessary to separately add flux and control the activity and density of flux like interposing devices. Before reflow soldering, the flux also serves as a temporary fixation during the placement and transfer of surface mount components. Obviously, the correct selection of solder paste is essential for producing reliable and defect-free surfaces
Mounting devices is very important. The following points are available for reference when selecting solder paste.
1. Flux activity selection in solder paste
Flux is one of the main components of the solder paste carrier. The solder paste can utilize three different types of flux, namely R flux (resin flux), RMA flux (moderately activated resin) and RA (fully activated resin). The activator in RMA and RA flux can remove oxides and other surface contaminants on the metal surface, and promote the infiltration of molten solder to surface mount pads and component terminals or pins. According to the surface cleanliness of the surface-mounted printed circuit board and the freshness of the device, the medium activity is generally selected, and the high activity or inactive level, and the super active level can be selected when necessary.
2. Viscosity selection of solder paste
The viscosity of solder paste is generally measured with a Brookfield viscometer. The viscosity of the solder paste depends on the characteristics of the application process (screen mesh number, squeegee speed, etc.). For screen printing, the viscosity is usually selected from 400,000 to 600,000 centipoise (cps). For sub-stencil printing, a higher viscosity should be selected, and the range is 800,000 to ¨00000 cps. If a syringe is used for dispensing, its viscosity should be 150,000 to 300,000 cps.
3. Selection of metal content in solder paste
The content of metal in the solder paste determines the size of the weld. The welding seam increases with the increase of the metal percentage, but as the metal content of a given viscosity increases, a slight change in the metal content of the solder will have a great impact on the quality of the solder joint. For example, for the same solder paste thickness, a 10% change in the metal content will change the solder joints from excessive to insufficient. Generally, the solder paste used for surface mount components should be selected with a metal content of 88% to 90%.
4. Selection of solder particle size in solder paste
The shape of the solder particles determines the oxygen content of the powder and the printability of the solder paste. Spherical powder is better than elliptical powder. The smaller the spherical surface, the lower the oxidation ability.
