Solder paste printing quality

According to statistics, under the premise of correct PCB design, component and printed board quality, 70% of quality problems in surface assembly quality problems are in the printing process. Whether the printing position is correct (printing accuracy), the amount of solder paste, whether the amount of solder is uniform, whether the paste pattern is clear or not, whether the surface of the printed board is adhered to the solder paste directly affects the soldering quality of the surface assembly board. .

There are many factors that affect the quality of printing. There are mainly the following factors:

1. First, the quality of the template

Stencil printing is contact printing, so the template thickness and opening size determine the amount of solder paste printing. Excessive amounts of solder paste can create bridging, and too little solder paste can result in insufficient solder or inadvertent solder. The shape of the template opening and whether the opening is smooth can also affect the quality of the release. The stencil opening must be flared downwards, otherwise the solder paste will be carried out from the flared chamfer at the time of demolding.

2. Followed by the paste viscosity, printability (rolling, transfer), thixotropy, the service life at room temperature will affect the print quality. If the printability of the solder paste is not good, the serious solder paste only slides on the stencil. In this case, the solder paste cannot be printed at all.

3. Printing process parameters

Solder paste is a thixotropic fluid that is tacky. When the squeegee moves forward at a certain speed and angle, it will exert a certain pressure on the solder paste and push the solder paste to roll before the squeegee to generate the pressure needed to inject the solder paste into the mesh or the leak hole. The viscous friction of the solder paste makes Solder paste produces a shear at the interface between the squeegee and the stencil, and the shear force reduces the viscosity of the solder paste, facilitating the smooth infusion of the solder paste into the mesh or leak.

Squeegee speed, squeegee pressure, the angle between the squeegee blade and the stencil, and the viscosity of the solder paste all have certain restrictions, so only by properly controlling these parameters can the solder paste quality be guaranteed. For example, if the pressure of the squeegee is too large, the solder paste pattern may be blocked during printing. If the printing speed is too fast, the amount of solder paste may be insufficient. If the residual solder paste at the bottom of the template is not cleaned in a timely manner, the solder paste may be contaminated in places other than the pad during printing, and these factors may cause soldering defects such as bridging, soldering, and soldering.

4. Equipment accuracy

When printing high-density, narrow-pitch products, the printing accuracy and repeatability of the printing press will also play a role. If the press is not equipped with a vision centering system, even when the artificial pattern is finely aligned, the PCB pad pattern and the template The hole pattern completely overlaps, but the processing error for the PCB cannot be solved.

5. The use and management of recovered solder paste, ambient temperature, humidity, and environmental hygiene have an impact on solder joint quality. The recovered solder paste and the new solder paste should be stored separately. If the ambient temperature is too high, the viscosity of the solder paste will be reduced. When the humidity is too high, the solder paste will absorb the moisture in the air. If the humidity is too low, the solvent in the solder paste will be accelerated, and the dust in the environment will be accelerated. Mixing into the solder paste will create pinholes in the solder joints.