Influence of Popular Science Scraper on PCBA Processing

Influence of Popular Science Scraper on PCBA Processing

1. Included angle of scraper:

In the electronic processing and production process, the included angle of the scraper affects the magnitude of the vertical force of the scraper on the solder paste. The smaller the included angle is, the greater the vertical component force is. If the pressure generated by changing the scraper angle is greater than 80 °, the solder paste can only move forward without rolling. At this time, there is almost no component force in the vertical direction, and the solder paste will not be pressed into the opening of the printing template window. The best setting of the scraper angle should be 45 °~60 °. At this time, the solder paste has good rolling property.

2. Speed of scraper:

The speed of the scraper is fast, and the force on the solder paste becomes larger. Considering the actual situation of solder paste pressing into the window, that is, the time for solder paste pressing becomes shorter. If the scraper speed is too fast, the solder paste cannot roll but only slide on the printing template. Because it takes time for solder paste to flow into the window, it can be clearly felt when printing thin pitch QFP graphics. When the scraper runs along one side of QF, the solder paste graphics on the bonding pad perpendicular to the scraper are fuller than the other side. Therefore, some printers have the function of rotating the scraper by 45 ° to ensure that the amount of solder paste on all sides is uniform when printing thin pitch QFP yuan. The maximum printing speed should ensure that the solder paste printing on the FQFP bonding pad is uniform and full in both vertical and horizontal directions, Generally, when the scraper speed is controlled at 20~40mm/s, the effect of the brush is better.

3. Pressure of scraper:

When the solder paste rolls, it will exert a positive pressure on the vertical balance of the scraper device, which is commonly referred to as the printing pressure. When the printing pressure is insufficient, the solder paste will not be clean. If the printing pressure is too large, it will lead to leakage behind the template and scratches on the steel plate surface. Therefore, the pressure of the scraper is generally set at 5~12N/(25mm). The ideal scraper pressure should be based on just scraping the solder paste from the steel plate surface.

4. Scraper width:

If the scraper is too wide relative to the PCB, more pressure and more solder paste are required to participate in its work, which will result in a waste of solder paste. Generally, the width of the scraper is the length of PCB (printing direction) plus about 50mm, and the scraper head should be ensured to fall on the metal template.

5. Printing clearance:

Generally, the zero distance between PCB and template is maintained (it is also required to be controlled at 0~0.5mm in the early stage, but it should be zero distance when there is FQFP). Some printing machines also require that the PCB plane be slightly higher than the template plane. After adjustment, the metal template of the template is slightly propped up, but the height of this propping up should not be too large, otherwise the template will be damaged. From the perspective of the scraper operation, the scraper can operate freely on the template, which requires that all solder paste in the path of the scraper be scraped away, No excess solder paste shall be left, and the scraper shall not leave scratches on the template.

6. Separation speed:

After solder paste printing, the instantaneous speed of the steel plate leaving the PCB is a parameter related to the printing quality, and its adjustment ability is also a parameter reflecting the quality of the printing machine, which is particularly important in precision printing. The early printing presses were separated at a constant speed. The advanced printing presses had a tiny stay process when the steel plate left the solder paste pattern to ensure the best printing pattern.

7. Shape and material of scraper:

The material and shape of the scraper head has always been a hot topic in printing solder paste. There are many shapes and materials of scraper, which can be divided into two categories: polyurethane hard rubber and metal scraper. At present, with the widespread use of stainless steel plate, the printing press scraper is made of metal scraper, which is made of high hardness alloy and has high fatigue resistance, wear resistance, bending resistance and other properties. When the cutting edge runs on the template, the solder paste can be easily pushed into the window, and the printing from larger and deeper windows to ultra-fine spacing has excellent consistency; In addition, the metal scraper has a long service life, no need to modify, and the formwork is not easy to be damaged; There is no sag and fluctuation of solder during printing. The printing quality of metal scraper is obviously better than that of rubber scraper.

Shenzhen PCB manufacturer will explain to you.