Discussion on the influence of scraper on PCBA processing

1. Angle of scraper:

In the process of PCB processing, the Angle of the scraper affects the vertical force of the scraper on the solder paste, the smaller the Angle of the scraper, the greater the vertical component of the force. By changing the Angle of the scraper, the pressure generated can be changed. If the Angle of the scraper is greater than 80°, the solder paste can only keep its original state and do not roll. The solder paste will not press into the stencil window opening. The best setting of the Angle of the scraper should be 45°~60°, at which time the solder paste has good rolling property.

2. Speed of scraper:

The speed of the scraper increases the force on the solder paste. Considering the actual situation of solder paste pressed into the window, that is, the time of solder paste pressed into the window becomes shorter. If the speed of the scraper is too fast, the solder paste can not roll but only slide on the printing template. Because it takes time for the solder paste to flow into the window, this can be obviously felt in the printing of fine-spaced QFP graphics. When the scraper runs along the QF side, the solder paste graphics on the pad perpendicular to the scraper are fuller than on the other side, so some printers have the function of the scraper rotation 45°. In order to ensure the fine spacing QFP printing, the maximum printing speed should ensure that the FQFP pad solder paste printing vertical and horizontal direction is uniform, full, usually when the scraper speed is controlled in 20~40mm/s, the brush effect is better.

3. Pressure of scraper:

As the solder paste rolls, it exerts a positive pressure on the vertical balance of the scraper device, commonly known as printing pressure. Insufficient printing pressure will cause the solder paste to scratch unclean, if the printing pressure is too large, it will lead to leakage behind the template and scratches on the surface of the steel plate. Therefore, the pressure of the scraper is generally set at 5~12N/ (25mm). The ideal pressure of the scraper should be just to scrape the solder paste clean from the surface of the steel plate.

4. Width of scraper:

If the scraper is too wide relative to the PCB circuit board, then it needs more pressure, more solder paste to participate in its work, resulting in a waste of solder paste. Generally, the width of the scraper is PCB length (printing direction) plus about 50mm is the best, and to ensure that the scraper head falls on the metal template.

5. Printing gap:

Usually, the zero distance between PCB and template is kept (it is also required to control the distance between 0~0.5mm in the early stage, but it should be zero when there is FQFP). Some printing machines also require the PCB plane to be slightly higher than the plane of the template. After adjustment, the metal template of the template is slightly propped up, but the height of the propped up should not be too large, otherwise the template will be damaged. The scraper runs freely on the template, not only requires the solder paste to be scraped away wherever the scraper goes, leaving no excess solder paste, but also requires the scraper to leave no scratches on the template.

6. Separation speed:

After solder paste printing, the instantaneous speed of steel plate leaving PCB board is a parameter related to the printing quality, and its adjustment ability is also a parameter reflecting the quality of the printing press, especially important in precision printing. While early presses had constant speed separation, advanced presses had a tiny pause as the plate moved away from the paste pattern to ensure optimal print pattern.

7. Scraper shape and making material:

The material and shape of the blade head have always been a hot topic in printing solder paste. Scraper shape and production materials have a lot, from the production of materials can be divided into polyurethane hard rubber and metal scraper two categories. At present, with the widespread use of stainless steel plate, metal scraper is used for printing press. It is made of high hardness alloy, with high fatigue resistance, wear resistance, bending resistance and other properties. When the cutting edge is run on the template, the solder paste can be easily pushed into the window, from large and deep Windows to ultra-fine spacing printing has excellent consistency; In addition, the metal scraper life is long, without correction, the template is not easy to damage; Printing without solder sag and ups and downs. The printing quality of metal scraper is obviously better than that of rubber scraper.