In reflow soldering of surface mount assemblies, solder paste is used for the connection of pins or terminals of surface mount components to pads, with many variations. 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, aligned with a locating pin or visually, and solder paste printed with a stencil .
In the stencil paste printing process, the Screen Printing machine is the key to achieving the desired print quality.

During the printing process, the solder paste is automatically dispensed, and the printing squeegee is pressed down on the stencil so that the bottom surface of the stencil contacts the top surface of the board. As the squeegee walks through the entire length of the etched pattern area, the solder paste is printed onto the pads through the openings in the stencil/screen. After the solder paste has been deposited, the screen is released (snapshot) immediately after the squeegee and returned to its original position. This spacing or disengagement distance is determined by the design of the device, approximately 0.020 “~0.040”.

The disengagement distance and the squeegee pressure are two important equipment-related variables that achieve good print quality.

If not disconnected, this process is called “on contact” printing. Contact printing is used when using full metal stencils and blades .Off contact printing is used for flexible metal screen stencil.

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

1) Squeegee

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

There are two types of squeegee commonly available: rubber or polyurethane scrapers and metal scrapers.

The metal scraper is made of stainless steel or brass and has a flat blade shape with a printing angle of 30 to 55°. When higher pressures are used, it does not dig out solder paste from the openings, and because it is metallic, they are not as easy to wear as rubber scrapers and therefore do not require sharpness. They are much more expensive than rubber scrapers and can cause stencil wear. Rubber scraper, using a scraper with 70-90 rubber durometer .

When excessive pressure is applied, the solder paste that penetrates into the bottom of the stencil may cause a tin bridge, requiring frequent bottom smears. It may even damage the blade and the stencil or screen. Excessive pressure also tends to dig out the solder paste from the wide opening, causing insufficient solder fillet. Low blade pressure results in missing and rough edges. The wear, pressure and hardness of the blade determine the quality of the print and should be carefully monitored. For acceptable print quality, the edge of the blade should be sharp and straight.

2) Stencil

The currently used stencils are mainly stainless steel stencils, which are produced in three main processes: chemical etching, laser cutting and electroforming.

Since the solder paste printed on the metal stencil and the metal squeegee is full, printing with too thick thickness is sometimes obtained, which can be corrected by reducing the thickness of the stencil.

Alternatively, the area of the solder paste on the pad can be reduced by reducing (“fine tuning”) the length and width of the wire opening by 10%. Thereby, the sealing of the frame between the stencil and the pad caused by the misalignment of the pad can be improved, and the “explosion” of the solder paste between the bottom of the stencil and the PCB is reduced. The number of cleanings of the bottom of the printing stencil can be reduced from once every 5 or 10 printing cleanings to once every 50 printing cleanings.

3) Solder paste

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

Viscosity is an important characteristic of solder paste. We require it to have a lower viscosity during the printing process. The better the fluidity, the easier it will flow into the template hole and be printed on the pad of the PCB. After printing, the solder paste stays on the PCB pad and its viscosity is high, maintaining its filled shape without collapse.

The standard viscosity of the solder paste is in the range of about 500kcps to 1200kcps, and a typical 800kcps is ideal for stencil screen printing.There is a practical and economical way to determine if a solder paste has the correct viscosity, as follows:

Stir the solder paste in a container with a spatula for about 30 seconds, then pick up some solder paste, three or four inches above the container, let the solder paste drop down. It should start to slide down like a thick syrup. Then, the segment breaks and falls into the container can. If the solder paste does not slip, it is too thick and the viscosity is too high. If it has been falling without breaking, it is too thin and the viscosity is too low.

(1) Process parameter setting

Snap-off speed and distance between stencil and PCB

After screen printing, the PCB is Snap-off from the screen stencil, leaving the solder paste on the PCB instead of the stencil holes.
For the finest screen printing holes, the solder paste may adhere more easily to the hole walls than to the pads. The thickness of the stencil is important. Two factors are advantageous:

First, the pad is a continuous area, and the inner wall of the screen hole is divided into four sides to help release the solder paste;

Second, gravity and the adhesion to the pad, the solder paste is pulled out of the screen hole and adhered to the PCB within 2 to 6 seconds of printing and Snap-off.

To maximize this beneficial effect, the Snap-off delay can be initiated, with the PCB Snap-off slowly at the beginning. Many machines allow for screening printing delays, and the head 2~3 mm of the table is adjust to be slow.

(2) Printing speed

The speed at which the blade travels on the print stencil during printing is important because the solder paste takes time to roll and flow into the die hole.If the time is not enough, the solder paste will be uneven on the pad in the direction of travel of the blade. When the speed is higher than 20 mm per second, the scraper may scrape through the small die hole in less than a few tens of milliseconds.

(3) Printing pressure 

The printing pressure must be coordinated with the hardness of the blade. If the pressure is too small, the blade will not scratch the solder paste on the template. If the pressure is too high, or the blade is too soft, the blade will sink into the larger hole in the template. The paste is dug out.

(4) Pressure empirical formula

Use a scraper on the metal mesh plate. In order to get the correct pressure, initially apply a pressure of 1 kg per 50 mm of the scraper length. For example, a 300 mm scraper applies a pressure of 6 kg. The pressure is gradually reduced until the solder paste begins to stay on the stencil . The plate is not cleaned and then an additional 1 kg of pressure. Between the solder paste scraping not enough and the scraper blade sinking into the wire hole to dig out the solder paste, there should be an acceptable range of 1~2 kg to achieve a good silk screen effect.

In order to achieve good print results, the correct solder paste material (viscosity, metal content, maximum powder size and lowest possible flux activity), the right tools (printers, stencils and squeegees) and the right process must be available ( Good combination of positioning, cleaning and wiping). According to different products, set the corresponding printing process parameters in the printing process, such as working temperature, working pressure, scraper speed, stencil automatic cleaning cycle, etc., and at the same time to develop strict process management and process.

① Use solder paste in strict accordance with the specified brand within the validity period. The solder paste is stored in the refrigerator. It should be placed at room temperature for more than 6 hours before use. It can be opened and used separately. The solder paste should be stored separately. When using it again, make sure that the quality is qualified.

② Before the production, the operator uses a special stainless steel rod to stir the solder paste to make it uniform, and regularly measure the viscosity of the solder paste with a viscosity tester.

③ After the first printing plate or equipment is printed on the same day, the thickness of the solder paste should be measured by the solder paste thickness tester. The 5 test points are selected on the upper and lower sides , left and right and center of the test surface of the printed board, and the 5 values ​​are recorded. The thickness of the solder paste ranges from -10% of the stencil thickness to +15% of the stencil thickness.

④ During the production process, the solder paste printing quality is 100% tested. The main content is whether the solder paste pattern is complete, the thickness is uniform, and whether there is solder paste tipping.

⑤ Clean the template according to the process requirements after the work is completed.

⑥ After the printing experiment or printing failure, the solder paste on the printed board is required to be thoroughly cleaned and dried with an ultrasonic cleaning device, or washed with alcohol and with high pressure gas ,to prevent the occurrence of solder balls after reflow due to residual solder paste on the board during re-use.