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PCBA quality, factors affecting solder paste printing quality in SMT process!


Solder paste printing is the first process of SMT. If it is not handled well, other subsequent links will be affected. SMT is an important link in PCB production, and this level should be controlled well. What affects the printing quality of solder paste?


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1.Quality of solder paste
Solder paste is a paste made by mixing alloy powder with flux. Whether the component can be soldered to the pad well, the quality of the solder paste is critical. There are several factors that affect the viscosity of the solder paste: the amount of alloy powder, the size of the particles, the temperature, the pressure of the doctor blade, the shear rate, and the flux activity. If the quality of the solder paste is not enough, soldering cannot be achieved well, and the final printing effect is naturally not ideal.  

2.Storage of solder paste
In addition to quality, the storage of solder paste is also very important. If the solder paste needs to be recycled, it must pay attention to issues such as temperature and humidity, otherwise it will affect the quality of the solder joint. Excessive temperature will reduce the viscosity of the solder paste, and too much humidity may cause deterioration. In addition, the recovered solder paste should be stored separately from the fresh solder paste, and if necessary, it should be used separately.


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3.Stencil template
Stencil is a solder paste pad that is required to apply solder paste to the PCB. The quality of the stencil directly affects the printing quality of the solder paste. Before processing, the parameters such as the thickness of the stencil and the opening size must be confirmed. Ensure the printing quality of solder paste.
The pitch of the components on the PCB board is approximately 1.27mm. For components with a pitch of 1.27mm or more, the stainless steel plate needs to be 0.2mm thick, and the narrow pitch needs to be 0.15-0.10mm thick. The thickness of the stainless steel plate is determined according to the majority of components on the PCB .

4. Printing equipment
A printing machine is a device that prints solder paste onto a PCB sample board. It is the device that has the greatest impact on process and quality. Printing presses are mainly divided into manual printing presses, semi-automatic printing presses and fully automatic printing presses. These printing presses have a variety of different features and functions. According to different needs, different printing presses are used to achieve the best quality.


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5. Printing method
The printing method of solder paste can be divided into contact and non-contact printing. The printing with gap between screen and printed board is called non-contact printing. When the machine is set up, this distance can be adjusted. Generally, the gap is 0-1.27mm; and the printing method of solder paste printing without printing gap is called contact printing. The vertical printing of the contact printing screen can minimize the impact on printing quality, and it is especially suitable for thin and difficult solder paste printing.

6. Printing speed
A fast scraper speed is beneficial to the rebound of the stencil, but at the same time, it will hinder the transfer of solder paste to the pads of the printed board, and a slow speed will cause poor resolution of the solder paste printed on the pads. On the other hand, the speed of the scraper has a great relationship with the viscosity of the solder paste. The slower the speed of the scraper, the greater the viscosity of the solder paste. Similarly, the faster the speed of the scraper, the smaller the viscosity of the solder paste.



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