There are three key elements in the printing of automatic solder paste printing machine, which is usually called printing 3S: solder paste, stencils, and squeegees. The correct combination of the three elements is the key to continuous automatic solder paste printing.

1、 Printing paste of automatic solder paste printing machine

Solder paste is a combination of tin powder and rosin. The function of rosin is to remove oxides on component pins, pads and tin beads in a stage of reflowing soldering furnace. This stage lasts for about three minutes at 150 ℃. Solder is an alloy of lead, tin and silver. It refluxes at about 220 ℃ in the second stage of reflow furnace.

Viscosity is an important characteristic of solder paste. During the printing process, the lower the viscosity, the better the fluidity. It is easy to flow into the template hole and print on the PCB pad. After printing, if the solder paste stays on the PCB pad and its viscosity is high, it will maintain its filled shape without collapse.

The standard viscosity of solder paste is about 500kcps ~ 1200kcps, and the typical 800kcps is ideal for template screen printing. There is a practical and economical method to judge whether the solder paste has the correct viscosity, as follows:

Use a spatula to stir the solder paste in the container for about 30 seconds, then pick up some solder paste, three or four inches higher than the container, and let the solder paste drop by itself. At first, it should slide down like thick syrup, and then break and fall into the container in sections. If the solder paste cannot slip, it is too thick and the viscosity is too low. If it falls all the time without breaking, it is too thin and the viscosity is too low.

2、 Steel mesh template type of automatic solder paste printing machine

At present, the solder paste printing template mainly includes stainless steel template, and its production mainly includes three processes: chemical corrosion, laser cutting and electroforming.

Because the solder paste printed by the metal template and metal scraper is full, sometimes the printing with too thick thickness can be obtained, which can be corrected by reducing the thickness of the template.

In addition, the area of solder paste on the pad can be reduced by reducing ("fine tuning") the length and width of the wire hole by 10%. Thus, the sealing of the frame between the template and the pad caused by the inaccurate positioning of the pad can be improved, and the "explosion" of the solder paste between the template bottom and the PCB can be reduced. The cleaning times of the bottom surface of the printing template can be reduced from once every 5 or 10 times to once every 50 times.

3、 Automatic solder paste printing machine solder paste printing scraper

The function of solder paste printing scraper is to make the scraper roll the solder paste in front and make it flow into the template hole when printing by the full-automatic solder paste printer, and then scrape off the excess solder paste and leave the solder paste as thick as the template on the PCB pad.

There are two common types of scraper: rubber or polyurethane scraper and metal scraper.

The metal scraper is made of stainless steel or brass, with a flat blade shape and a printing angle of 30 ~ 55 °. When using higher pressure, it will not dig out the solder paste from the opening, and because they are metal, they are not as easy to wear as rubber scrapers, so they do not need to be sharp. They are much more expensive than rubber scrapers and may cause formwork wear. Rubber scraper, a scraper with 70-90 durometer hardness. When excessive pressure is used, the solder paste infiltrating into the bottom of the formwork may cause tin bridge, which requires frequent bottom wiping. It may even damage the scraper and formwork or wire mesh. Too high pressure also tends to dig out the solder paste from the wide opening, resulting in insufficient solder fillet. The low pressure of the scraper causes omissions and rough edges. The wear, pressure and hardness of the scraper determine the printing quality, which should be carefully monitored. For acceptable printing quality, the edge of the scraper should be sharp, straight and straight.