Therefore, the squeegee angle is generally controlled at 45 to 75Â°, and the optimal setting should be within the range of 45 to 60Â°C. In order to avoid deformation and wear of the leading end, it is better to use a harder material, such as a polyurethane having a hardness of 90 or more. Fat rubber and metal scraper.
The best setting for printing pressure
Fig. 9 shows the phenomenon in which the front of the blade is raised during the rolling of the solder paste. If the front of the blade is lifted during the rolling of the solder paste, a gap occurs between the front portion of the blade and the printing screen, and solder paste remains on the printing screen. In order to avoid this phenomenon, the force that is applied to the blade is called the printing pressure, that is, the printing pressure. If this force is too great, the front of the blade is deformed, and the angle of the blade, which plays an important role in the press-in force, is affected.
Therefore, the optimum setting of the printing pressure is the same as the pressure generated by rolling, and is generally 30 N/mm 2 . In addition, since the force generated by rolling varies with the amount of solder paste supplied to the printing apparatus, the operator needs to appropriately adjust the optimum value.
3.2 The process of moving the solder paste to the solder joint surface of the substrate
In order to move the solder paste from the opening portion of the screen printing plate to the solder pad surface of the printed circuit board, it is necessary to fix the printing screen plate and press down the printed circuit board in the vertical direction. The force generated at this time acts on the adhesive force between the solder joint surface of the substrate and the solder paste, and when the solder paste filled in the opening moves from the opening to the solder joint surface of the substrate, it acts on the printed wiring board wall and the solder paste. The friction between the two (this friction is essentially the adhesion of the solder paste). If this frictional force is greater than the adhesive force acting between the surface of the substrate pad and the solder paste, the solder paste does not move at the opening and printing cannot be performed. In contrast, when the friction is small, the solder paste moves smoothly. It can be seen that in this process, the design of the opening portion of the printing screen and the setting of the substrate lowering speed are very important.
The best design of printing screen
The best design of the printed screen from the printing mechanism is as follows:
1. The basic design of the opening part of the printing screen: The area of â€‹â€‹the opening itself is larger than the area of â€‹â€‹the wall surface of the opening, that is, it is necessary to make the adhesive force of the welding spot surface larger than the friction force. Therefore, printing with a large width and a narrow print thickness is very difficult in principle.
2. From the point of view of the friction force with the screen wall surface as small as possible, the wall surface of the opening should be as smooth as possible (the reason for the additional screen plate is used here). In addition, it is designed to have a figure-eight shape in consideration of detachability (in the case of a laser-fabricated screen plate, if it is processed from the substrate side, it will naturally become this shape).
The optimal setting of substrate drop speed
To increase the speed of substrate lowering, due to the adhesive force of the solder paste, the printing screen plate becomes larger and the deflection occurs. If the deflection of the printing screen becomes larger, the screen plate will return to its original position due to the elasticity of the deflection. As a result, in a certain position, the screen plate is rapidly reset due to its elastic force, and the solder paste is lifted around. The two ends form an extremely raised printing shape, and the lifting height is proportional to the deflection of the screen plate. In severe cases, the solder paste is also scraped off so that the solder paste remains in the opening.
Therefore, it is necessary to lower the substrate in accordance with the speed of the solder paste, and the drop speed is usually set to 0.3-3 mm/s.
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