Factors Affecting Quality in Solder Paste Printing (III)

3 Process control factors in the solder paste printing process

Solder paste printing is a very crafty process, and it involves a lot of process parameters. Improper adjustment of each parameter will have a very big impact on the quality of placement products.

3.1 screen printer printing parameter setting adjustment

This article will discuss the process control factors that affect solder paste printing quality in several ways:

(1) Squeegee pressure

The change of the squeegee pressure has a great influence on the printing. Too little pressure will make the solder paste unable to effectively reach the bottom of the template. The cut cannot be well deposited on the pad; too much pressure will cause the solder paste to be printed too thin and may even damage the form. The ideal state is just to scrape the solder paste from the surface of the stencil. In addition, the hardness of the scraper also affects the thickness of the solder paste. A too soft blade will cause the solder paste to dent, so it is recommended to use a harder scraper or metal scraper.

(2) Print thickness

The thickness of printing is determined by the thickness of the template. Of course, the settings of the machine and the properties of the solder paste also have a certain relationship. The slight adjustment of the printing thickness is often achieved by adjusting the speed of the doctor blade and the pressure of the doctor blade. By appropriately reducing the printing speed of the doctor blade, the amount of solder paste printed on the printed board can be increased. It is clear that reducing the speed of the blade is equivalent to increasing the pressure of the blade; on the contrary, increasing the speed of the blade is equivalent to reducing the pressure of the blade.

(3) Printing speed

Squeegee speed is conducive to the rebound of the template, but at the same time will hinder the transfer of solder paste to the PCB pad, and the speed is too slow will cause the pad printed on the solder paste resolution is poor. On the other hand, the speed of the doctor blade has a great relationship with the viscosity of the solder paste. The slower the doctor blade speed, the greater the viscosity of the solder paste. Similarly, the faster the doctor blade speed, the lower the paste paste consistency. Usually for fine-pitch printing speed range 12mm/s∽40mm/s

(4) Printing method

The stencil printing method can be divided into on-contact and off-contact printing. Printing with a gap between the template and the printed board is called non-contact printing. In the machine setting, this distance is adjustable, the general gap is 0∽1.27mm; and the stencil printing without printing gap (zero gap) is called contact printing. The vertical printing of the contact printing template minimizes the impact on the print quality. It is particularly suitable for fine-pitch solder paste printing.

(5) parameters of scraper

The parameters of the scraper include the material, thickness and width of the scraper, the elasticity of the scraper with respect to the tool holder, and the angle of the scraper with respect to the template. These parameters influence the distribution of the solder paste to varying degrees. When the angle θ of the doctor blade relative to the template is 60o∽65o, the quality of the solder paste printing is the best.

The relationship between the size of the opening and the direction of the squeegee must be taken into account when printing. The traditional printing method of the solder paste is that the blade runs at an angle of 90o along the X or Y direction of the template. This often leads to different solder paste amount in different directions of the opening, and the experimental authentication shows that the direction of the opening is parallel to the direction of the doctor blade. When the thickness of the solder paste is scraped, the thickness of the solder paste is about 60% more than the thickness of the solder paste. The squeegee is printed in the direction of 45o, which can significantly improve the imbalance of the solder paste on the opening direction of the different stencils, and can also reduce the damage of the squeegee to the fine-pitch stencil opening.

(6) Demoulding speed

The detachment speed of the printed board and the template also has a great influence on the printing effect. For a long time, it is easy to leave solder paste on the bottom of the template. The time is too short, which is not conducive to the erection of the solder paste and affects its clarity. The ideal demoulding speed is shown in Table 4.

Table 4 Recommended release speed

Pin spacing

Recommended speed

Less than 0.3mm

0.1~0.5mm/sec

0.4~0.5mm

0.3~1.0mm/sec

0.5~0.65mm

0.5~1.0mm/sec

More than 0.65mm

0.8~2.0mm/sec

(7) Template cleaning

In the solder paste printing process, every 10 boards need to be cleaned once at the bottom of the template in order to eliminate the attachment on the bottom of the board. Usually, anhydrous alcohol is used as the cleaning liquid.