In the last issue, we focused on the knowledge of the classification method of solder paste, so how should we choose solder paste in practical applications? The good selection of solder paste will greatly improve the production process and efficiency, and the quality of solder paste will also Regarding the quality of SMT processed products, how to choose solder paste can refer to the following points according to the performance and usage requirements of solder paste:
1. The activity of the solder paste can be determined according to the cleanliness of the surface of the printed board. At present, the RMA grade is generally used, and the RA grade is used if necessary.
2. During fine pitch printing, solder paste with appropriate particle size should be selected according to its production and process requirements. Domestic solder paste manufacturers, including SMT manufacturers, associate the particle size of tin powder with "mesh". The basic concept of mesh refers to the sieve. The number of meshes per square inch of the mesh; in the actual tin powder production process, several layers of screens with different meshes are mostly used to collect tin powder. The tin powder and particle size are also different. The particle size of the finally collected tin powder particles is also a regional value, and the solder paste is selected based on this standard.
3. Solder paste with low melting point should be used when soldering thermal components or when components cannot withstand high temperature.
4. When using the no-clean process, use a no-clean solder paste that does not contain chloride ions or other strong corrosive compounds.
5. The viscosity of the solder paste is also a very important factor in the selection of solder paste. In the SMT workflow, since the solder paste is printed (or dotted) and the components are pasted, to the reflow heating process, there are A process of moving, placing or handling the PCB; in this process, in order to ensure that the printed (or dotted) solder paste is not deformed and the components that have been attached to the PCB solder paste are not displaced, the solder paste is required to enter the PCB There should be good adhesion and holding time before reflow heating.
The above suggestions for the selection of solder paste are for reference only. In the actual SMT application, we still have to choose the solder paste suitable for the soldering requirements of components according to the actual situation.
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