Causes and Solutions of Bad Phenomenon of Solder Paste on Scraper

Solder paste printing is a very important process and an important link in SMT chip processing. The beginning of the whole PCBA one-stop service is the printing of solder paste. Now let's learn some problems about the poor release of solder paste from the scraper.

Solder paste printing is a very important process and an important link in SMT chip processing. The beginning of the whole PCBA one-stop service is the printing of solder paste. Now let's learn some problems about the poor release of solder paste from the scraper.

During printing, the solder paste often creeps slightly along the scraper, resulting in a slightly larger contact area with the scraper than with the template. When the right printing is finished, lift the left scraper, and a lot of solder paste sticks to the scraper. The right auxiliary knife is ready for the next printing to the left. However, very little solder paste remains in front of the scraper for printing.

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Reasons and solutions for poor release of solder paste on the scraper:

① Solder paste too sticky, too thick: If the solder paste is very sticky or very thick. Relative to adhesion. The factor of gravity can be omitted. This large knife contact surface area controls the distribution of solder paste. The result is that most of the solder paste will be on the scraper. If the solder paste gradually dries and therefore adds new additive, this phenomenon will also occur. However, the low island is easy to release from the force. However, the ability to control components will be lost when mounting, so this method should not be used. Solder paste shall have moderate viscosity and be non-volatile solvent. Although there is only a low metal content, reducing the viscosity value can also help improve the scraper release, but this method will lead to higher collapse, so it can only be a substitute solution. In general, for fine pitch SMT applications, the metal content should be 90% or slightly higher.

② The solder paste gradually dries on the template

③ Insufficient amount of solder paste added on the template: if the volume and weight of the added solder paste are small, the scraper has a larger contact surface area than the template. If there is no accident, it will lead to poor release of the scraper. Therefore, depending on the type of solder paste, it is recommended that the diameter of solder paste ripple should be greater than 0.5 inch. For pastes with higher viscosity values, the ideal rolling size is not less than 0.75 inch. If there is a hole in the solder paste curtain at the initial stage when the scraper picks up, it indicates that the amount of solder paste is insufficient and it is time to add solder paste. After the first printing, in order to promote the release of solder paste from the knife, the scraper can be held at an appropriate position for 10-20s before the next printing.

④ The scraper handle protrudes too much and the auxiliary knife height is low: the printing press scraper height and the scraper handle extends too much to the template are easy to cause the solder paste to contaminate the knife handle during printing, resulting in a large contact area between the solder paste and the scraper system. Therefore, the problem of scraper release inevitably arises. Such problems can be corrected by using a scraper with large height or a thin fixed plate on the side of the template

⑤ The contact angle between the scraper and the template is too small: the large contact angle between the scraper blade and the template can help reduce the end climbing height of the solder paste, thus reducing the release of the bad scraper.

⑥ Formwork surface is too smooth: in principle, smooth surface and low surface can produce low adhesion, so there is satisfactory scraper release. Generally speaking, the surface of all auxiliary knives, including rubber scraper and metal scraper, has been treated smoothly. In addition, the addition of polytetrafluoroethylene layer on the surface has no effect on improving the release of the scraper. However, on the other hand, the stencil sketching the surface will increase the adhesion between the solder paste and the stencil, which has been proved to be quite effective.

The above are some problems that the PCB manufacturer explained to you about the poor release of solder paste from the scraper.