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What Are The Important Factors For The Defects Of PCB Solder Paste Printing Machine
Mar 13, 2018

In the surface mount reflow soldering, the solder paste is used for the connection between the pin or terminal of the surface mounting element and the pad, and there are many variables. For example, the application and printing process of the solder paste for the solder paste printing machine. In the process of printing PCB solder paste, the substrate is placed on the worktable, mechanically or vacuum clamping, positioned with the locating pin or vision, and the stencil solder paste is printed with the template (PCB).

In the template PCB solder paste printing process, the printing machine is the key to achieve the desired printing quality.

In the process of PCB printing, the solder paste is automatically distributed, and the printed scraper is pressed down on the template to make the bottom of the template contact the top of the circuit board. When the scraper passes the length of the entire graphic area that is corroded, the solder paste is printed on the plate through the opening of the template / screen. After the solder paste has been deposited, the screen immediately removed (snapoff) after the scraper and returned to the place. This interval or off distance is determined by the design of the equipment, about 0.020'~0.040'.

The separation distance and the scraper pressure are two important variables related to the equipment to achieve good printing quality.

If it is not removed, the process is called on-contact printing.  When all metal templates and scrapers are used, contact printing is used. Non - contact (off-contact) printing is used for flexible wire mesh.

There are three key elements in the PCB solder paste printer,

We call 3S:Solder paste (solder paste), Stencils (template), and Squeegees (silk screen scraper). The correct combination of the three elements is the key to the quality of the continuous screen printing.

Scraper (squeegee)

In the printing process, the scraper can make the solder paste roll the front paste into the template hole, then scrape the excess solder paste, and leave the paste as thick as the template on the PCB pad.

There are two types of scrapers commonly used: rubber or polyurethane (polyurethane) scraper and metal scraper.

The metal scraper is made of stainless steel or brass and has a flat blade shape with a printing angle of 30~55 degrees.  When high pressure is applied, it doesn't dig out solder paste from open hole, and also because it is metal, they are not as easy to wear as rubber scrapers, so they don't need sharpness. They are much more expensive than rubber scrapers and may cause formwork wear. Rubber scraper, use 70-90 rubber hardness tester (durometer) hardness scraper.  When excessive pressure is used, the solder paste that penetrated to the bottom of the template may cause a tin bridge, requiring frequent bottom rubbing. It may even damage the scraper and the template or screen. The high pressure also tends to dig out the solder paste in the wide open hole, causing the solder round angle is not enough. The low pressure of the scraper causes the missing and rough edges. The wear, pressure and hardness of the scraper determine the printing quality and should be carefully monitored. For acceptable printing quality, the edge of the scraper should be sharp, straight and straight.

Template (stencil) type

There are mainly stainless steel templates used at present, and there are three main processes: chemical corrosion, laser cutting and electroforming.

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

In addition, the length and width of the filaments can be reduced by 10% to reduce the area of the solder paste on the plate. Thus, the sealing situation between the template and the pad can be improved due to the incorrect positioning of the pad, and the explosion of the solder paste between the bottom of the template and PCB can be reduced. The number of cleaning times of the bottom of the printing plate can be reduced from every 5 or 10 times to 50 times. Solder paste (solder paste)

The paste is tin powder and rosin (resin) conjugates, rosin is the function of the first stage (reflowing) in the reflow welding furnace, remove the component pins, pads and solder balls on the oxide, at this stage of 150? C for about three minutes.  Solder is an alloy of lead, tin and silver, reflux in the second stage of the reflow furnace, about 220C.

Viscosity is an important characteristic of solder paste. If we want its viscosity to be lower in printing stroke, its mobility is better. It is easy to flow into the template hole and print on the pad of PCB. After printing, the solder paste stays on the PCB weld plate with high stickiness and keeps its filling shape without falling down.

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

Stir the solder paste in the container tank for about 30 seconds with a scraper. Then stir up some solder paste and put it out of the container for three or four inches. Let the solder paste drip down on its own. At the beginning, it should slide down like a thick syrup and then break down into the container. If the solder paste can not slide, it is too thick and the viscosity is too low. If there is no breakage at all times, it is too thin and the viscosity is too low.

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