The purpose of applying the solder paste is to apply a proper amount of solder paste uniformly on the pcb pads to ensure that the pads of the chip components and the pcb pads are in good electrical connection and have sufficient mechanical strength. Applying solder paste is a key process in the SMT reflow process. There are three methods of applying solder paste, such as drop coating, screen printing and metal stencil printing. In recent years, non-contact solder paste printing technology has been introduced. Among them, metal stencil printing is the most common method currently used.
Solder paste printing is a key process to ensure SMT quality. According to statistics, under the premise of PCB design specifications, components and printed board quality ensured, 60%~70% of the quality problems are in the printing process.
The requirements for applying solder paste are as follows (see Figure 1).
1 The amount of solder paste applied is uniform and the consistency is good. The solder paste pattern should be clear, and try not to stick between adjacent patterns. The solder paste pattern should be consistent with the land pattern and try not to be misplaced.
2 Under normal circumstances, the amount of solder paste per unit area on the pad should be about 0.8mg/mm2; for narrow-pitch components, it should be about 0.5mg/mm.
3 The solder paste printed on the substrate can be allowed to have a certain deviation compared with the desired weight value, and the area of the solder paste covering each pad should be more than 75%. When using no-clean technology, the solder paste is required to be completely on the pad, and the lead-free solder paste completely covers the pad.
4 After solder paste printing, there should be no serious collapse, the edges are neat, the misalignment is no more than 0.2mm; for the narrow-pitch component pads, the misalignment is not more than 0.1mm. The surface of the substrate is not allowed to be contaminated by solder paste. When using no-clean technology, the solder paste can be placed on the pad by reducing the size of the template opening.
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