PCBA Tech - SMT basic knowledge of solder paste printing

In the reflow soldering of SMT surface mount assembly, the solder paste is used for the connection between the pin or terminal of the surface mount component and the pad, and there are many variables. Such as solder paste, screen printing machine, solder paste application method and printing process. In the process of printing solder paste, the substrate is placed on a workbench, mechanically or vacuum clamped and positioned, aligned with positioning pins or vision, and solder paste printing is performed with a stencil.

In the stencil paste printing process, the printer is the key to achieving the desired printing quality.

During the printing process, the solder paste is automatically dispensed, and the printing squeegee is pressed down on the template so that the bottom surface of the template contacts the top surface of the PCB circuit board. When the squeegee travels through the entire length of the pattern area that is corroded, the solder paste is printed on the pads through the openings on the stencil/screen. After the solder paste has been deposited, the screen will snap off immediately after the squeegee and return to the original place. This separation or disengagement distance is determined by the equipment design, about 0.020"~0.040".

Disengagement distance and squeegee pressure are two important equipment-related variables to achieve good printing quality.

If it is not disengaged, this process is called on-contact printing. When using an all-metal template and squeegee, use contact printing. Off-contact printing is used for flexible metal screens.

There are three key elements in solder paste printing, which we call 3S: Solder paste, Stencils, and Squeegees. The correct combination of the three elements is the key to continuous printing quality.

Squeegee

When the squeegee is printing, the squeegee rolls the solder paste in front to make it flow into the template hole, and then scrapes off the excess solder paste, leaving solder paste as thick as the template on the PCB pad.

There are two common types of scrapers: rubber or polyurethane (polyurethane) scrapers and metal scrapers.

The metal scraper is made of stainless steel or brass, has a flat blade shape, and uses 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 a rubber scraper, so they do not need to be sharp. They are much more expensive than rubber squeegees and may cause template wear. Rubber scraper, use 70-90 durometer hardness scraper.

When using too high pressure, the solder paste that penetrates into the bottom of the template may cause solder bridges, requiring frequent bottom wiping. It may even damage the squeegee blade and template or screen. Excessive pressure also tends to dig out the solder paste from the wide openings, causing insufficient solder fillets. The low pressure of the squeegee causes omissions and rough edges. The wear, pressure and hardness of the squeegee blade determine the printing quality and should be carefully monitored. For acceptable printing quality, the edge of the squeegee blade should be sharp and straight.

z template (Stencil) type

The templates currently in use are mainly stainless steel templates, which are produced in three main processes: chemical corrosion, laser cutting, and electroforming.

Since the solder paste printed by the metal template and the metal squeegee is full, sometimes too thick printing may be obtained. This can be corrected by reducing the thickness of the template.

In addition, the length and width of the wire hole can be reduced ("fine-tuned") by 10% to reduce the area of solder paste on the pad. Thereby, 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 bottom of the template and the PCB can be reduced. The cleaning frequency of the bottom surface of the printing template can be reduced from once every 5 or 10 printings to once every 50 printings.

Solder paste

Solder paste is a combination of tin powder and resin. The function of rosin is to remove oxides on component pins, pads and tin beads in the first stage of reflowing soldering furnace. This stage is at 150°C. Lasts about three minutes. Solder is an alloy of lead, tin, and silver, and is reflowed at approximately 220°C in the second stage of the reflow oven.

Viscosity is an important characteristic of solder paste. We require that the lower its viscosity during the printing process, the better its fluidity, which can easily flow into the template hole and be printed on the PCB pad. After printing, the solder paste stays on the PCB pads, and its high viscosity maintains its filled shape without collapsing.

The standard viscosity of solder paste is approximately in the range of 500kcps~1200kcps. The typical 800kcps is ideal for stencil screen printing.

There is a practical and economical way to determine 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. Let the solder paste drip down on its own. At the beginning, it should slide down like a thick syrup, and then break in sections and fall into the container. If the solder paste does not slip off, it is too thick and too low in viscosity. If it keeps falling without breaking, it is too thin and the viscosity is too low.

(1) Process parameter setting

The separation speed and separation distance between the template and the PCB (Snap-off) After the screen printing is completed, the PCB and the screen printing template are separated, leaving the solder paste on the PCB instead of the screen printing hole.

For the finest screen-printed holes, the solder paste may be easier to adhere to the hole wall instead of the pad. The thickness of the template is very important. Two factors are beneficial:

First, the pad is a continuous area, and the inner wall of the wire hole is divided into four sides in most cases, which helps to release the solder paste;

Second, together with gravity and adhesion to the pad, the solder paste is pulled out of the wire hole and adhered to the PCB within 2 to 6 seconds for the silk screen and separation.

In order to maximize this beneficial effect, the separation can be delayed. At the beginning, the separation of the PCB circuit board is slower. Many machines allow a delay after silk-screen printing, and the stroke speed of the falling head of the worktable can be adjusted to be slower than 2~3 mm.