Scraper selection and single-sided PCB screen printing effect

The squeegee is a tool for scratching the ink on the screen to make it leak on the printing surface. The squeegee is composed of a squeegee and a doctor blade. The screen printing squeegees of single-sided printed circuit boards are commonly used polyurethane squeegee. Its high strength, wear resistance and solvent resistance are very good, thickness is 8~10mm, Shore hardness is 65~75°, scraper The shape of the choice of right angles.

Usually the hardness of the squeegee is indicated by color, and the color is gradually deepened with the increase of its hardness. At present, most of the materials are imported materials. Generally, the red color represents the hardness of 60~65°. It is a soft squeegee and is suitable for PCB solder resist patterns and characters. Graphics and other printing; 70 ~ 75 ° for the hardness of scratch, some with green, and some with milky white, mostly used for the printing of resist patterns and conductive printing materials. Targeted selection of the appropriate hardness of the scraper will help improve screen printing quality. If it is not properly selected, it will cause poor printing failure. For example, in the printing process of the PCB solder resist pattern, a scraper with a hardness of 75° or more is selected. When encountering a circuit pattern with a relatively dense conductor spacing, it is difficult for the hardness scraper to completely print the printing material on the screen to the circuit. Sometimes, in order to completely print the printing material on the screen, Increased printing pressure causes screen damage or small nicks on the squeegee blade, which can occur later in printing. There will be stripe marks along the direction of movement of the squeegee.

When installing the squeegee, it is necessary to select a straight, non-defective squeegee bar. The height of the squeegee squeegee blade exposed by the squeegee is 10 to 25 mm. The softer the exposed squeegee is, the less exposed it becomes, making the squeegee harder and less likely to bend. If a pneumatic screen printing machine is used to print PCB graphics, some of the adhesive strips are inserted into a part of the height of the scraper clip, and the height of the outside stripper strips exceeds 25mm. The thickness of the stripper strip can be used according to the thickness of the stripper, lmm or 1. The 6mm thick copper clad board trim is clamped on the exposed squeegee as a support plate to make it rigid. The height of the support plate should be 10~25mm shorter than the height of the squeegee bar so that it will not be printed. Due to the pressure applied on the softer squeegee, the printed image is in contact with the printing surface, resulting in blurred printed images, especially in the process of printing PCB solder resist patterns, due to the pressure increase, the surface contact, the resistance of the pad The position is easily eroded, and the ink layer is thickened, which affects the curing effect of the solder resist ink and at the same time greatly reduces the service life of the squeegee.

The length of the squeegee should be selected as short as possible according to the principle of application. It is 40-50mm longer than the entire image on the screen. If it is too long, deformation will occur on both sides of the blade of the squeegee. Naturally, during the process of screen printing Bending is uneven, causing the entire ink coating to be uneven; if the squeegee is too short, when the resist pattern is printed, shadows are likely to occur on both sides of the pattern. When printing the solder mask pattern and the insulating layer and protective layer pattern of the carbon paste, both ends will Printed material sometimes cannot be printed or the ink layer on the edge is thick. In addition, after the proper length of the squeegee is cut, the sharp edges of the squeegee are cut off with a tool and sanded to make it blunt so as not to scratch the surface of the photosensitive film or the capillary photosensitive film coated on the screen. The printing process leaks ink.

In order to obtain a good printing effect, check whether the blade edge is straight before printing. The scraper should be washed immediately after use, especially when screen printing PCB solder resist pattern and character mark graphics, etc. The solvent is strong and the time immersed for a long time will make the squeegee squeegee strip softer and accelerate the wear and tear. When printing, the film layer is very thick and the position of the resist pad is eroded, which affects the quality of the product.

Reasonably selecting the scraper helps to improve the screen printing quality of each graphic of the PCB. At present, the more popular scraper “wheel rest” (ie, rotation) can better improve the service life of the scraper. The above are just some of my experiences in the online printing industry over the years. I have discussed the exchanges with my colleagues here. We have some deficiencies in the text.

Source: China PCB Manufacturing Network