SMT printing process control points

2022-07-28 204

Production management SMT automatic printing machine 1) Graphic alignment: The optical positioning point (MARK point) of the substrate and steel mesh on the workbench is aligned by the printing press camera, and then the X, Y, Θ of the substrate and the steel mesh are finely adjusted, so that the substrate pad graphic and the steel mesh opening pattern completely coincide. 2) The angle of the scraper and the steel mesh: 

The smaller the angle between the scraper and the steel mesh, the greater the downward pressure, which is easy to inject the solder paste into the mesh, but it is also easy to make the solder paste be squeezed to the bottom surface of the steel mesh, causing the solder paste to stick. At present, most of the automatic and semi-automatic printing machines use 60 °.The angle of the scraper and the steel mesh 3) the amount of solder paste input (rolling diameter): The rolling diameter of the solder paste is ∮ h≈ 13 ~ 23mm is more appropriate. ∮ h is too small, it is easy to cause solder paste to leak printing and less tin. ∮h is too large, too much solder paste in the case of a certain printing speed, easy to cause the solder paste can not form a rolling motion, the solder paste can not be scraped clean, resulting in poor printing demoulding, printing after the thick solder paste and other printing defects; And too much solder paste is exposed to the air for a long time to the quality of the solder paste. In production, the operator checks the height of the solder paste strip on the mesh plate every half an hour, and moves the solder paste on the mesh plate beyond the length of the scraper to the front end of the mesh plate with a wood scraper every half an hour and distributes the solder paste evenly. The amount of solder paste invested 4) scraper pressure: scraper pressure is also an important factor affecting the quality of printing. The scraper pressure actually refers to the depth of the scraper's descent, the pressure is too small, and the scraper does not adhere to the surface of the steel mesh, so it is equivalent to increasing the printing thickness. In addition, too little pressure will leave a layer of solder paste on the surface of the steel mesh, which is easy to cause printing defects such as printing molding bonding. 5) Printing speed: Because the speed of the scraper is inversely proportional to the viscosity of the solder paste, there is a narrow spacing, and the speed is slower when the high-density graphic is used. The speed is too fast, the time for the scraper to pass through the steel mesh opening is relatively short, and the solder paste cannot fully penetrate into the opening hole, which is easy to cause printing defects such as the solder paste molding is not full or missing the printing. There is a certain relationship between the printing speed and the scraper pressure, the reduced speed is equivalent to increasing the pressure, and the appropriate reduction of the pressure can play an effect of improving the printing speed. The ideal scraper speed and pressure should be just right to scrape the paste from the surface of the steel mesh. The influence of scraper pressure and speed on printing: the influence of scraper pressure and speed on printing 6) Printing gap: Printing gap is the distance between the steel mesh and the PCB, which is related to the retention of solder paste on the PCB after printing. 7) Separation speed of steel mesh and PCB: After solder paste printing, the instantaneous speed of the steel mesh leaving the PCB is the separation speed, which is a parameter related to printing quality, and is the most important in dense spacing and high-density printing. Advanced presses whose stencil leaves the solder paste pattern with 1 (or more) tiny dwell processes, i.e. multi-stage demoulding, ensures optimal printing molding. When the separation speed of the steel mesh and the PCB is large, the adhesion of the solder paste is reduced, and the cohesion between the solder paste and the pad is small, so that part of the solder paste sticks to the bottom surface of the steel mesh and the open hole wall, resulting in printing defects such as less printing and tin collapse. When the separation speed slows down, the viscosity and cohesion of the solder paste are large, so that the solder paste is easy to break away from the perforated wall of the steel mesh, and the printing state is good. 8) Cleaning mode and cleaning frequency: cleaning the bottom surface of the steel mesh is also a factor to ensure the quality of printing. The cleaning mode and cleaning frequency should be determined according to the solder paste, steel mesh material, thickness and open hole size. (Set dry cleaning, wet washing, one-time reciprocating, wiping speed, etc., the cleaning frequency can be used with reference to the steel mesh), steel mesh pollution is mainly caused by the spillage of solder paste from the edge of the opening hole. If it is not cleaned in time, it will contaminate the surface of the PCB, and the residual solder paste around the steel mesh opening will harden, and in severe cases, it will also block the steel mesh opening.