PCBA Tech - Understand the causes of SMT solder paste printing defects

In SMT PCB production, solder paste printing is a key step. Since solder paste is used to directly form solder joints, the quality of solder paste printing will affect the performance and reliability of surface mount components. The high-quality solder paste printing guarantees high-quality solder joints and final products. Statistics show that 60% to 90% of soldering defects are related to solder paste printing defects. Therefore, it is very important to understand the causes of solder paste printing defects.

The analysis of solder paste printing defects is listed below:

1 Solder paste The irregular shape of solder powder formed by the powder can easily block the steel mesh holes. This will result in a sharp decline after printing. Reflow will also cause defects in solder balls and short-circuit bridges.

Spherical is best, especially for fine pitch QFP printing.

Particle size If the particle size is too small, the result will be poor adhesion of the paste. It will have a high oxygen content and cause solder balls after reflow.

In order to meet the requirements of fine-pitch QFP welding, the particle size should be controlled at about 25~45μm. If the required particle size is 25~30μm, ultra-fine solder paste-pitch IC of less than 20μm should be used.

Flux Flux contains a thixotropic agent, which can make the solder paste have pseudoplastic flow characteristics. Since the viscosity of the paste decreases when it passes through the template hole, the paste can be quickly applied to the PCB pad. When the external force stops, the viscosity will recover to ensure that no deformation occurs.

The flux in the solder paste should be controlled between 8% and 15%. Lower flux content will result in excessive application of solder paste. Conversely, a high flux content can result in insufficient amount of solder applied.

2 Template thickness If the template is too thick, the solder bridge will be short-circuited.

If the template is too thin, it will lead to insufficient welding.

Aperture size When the aperture of the template is too large, a solder bridge short circuit may occur.

When the stencil aperture is too small, insufficient solder paste will be applied.

Aperture shape It is best to use a circular template aperture design. Its size should be slightly smaller than the PCB pad size to prevent bridging defects during reflow.

3 Printing parameters Blade angular velocity and pressure The blade angle affects the vertical force exerted on the solder paste. If the angle is too small, the solder paste will not be squeezed into the template hole. The best blade angle should be set at about 45 to 60 degrees.

The higher the printing speed means the less time it takes to apply solder paste through the surface of the stencil hole. Higher printing speed will result in unsuitable solder.

The speed should be controlled at about 20~40mm/s.

When the blade pressure is too small, it will prevent the solder paste from being cleanly applied to the template.

When the blade pressure is too high, it will cause more paste to leak. The blade pressure is usually set at about 5N~15N/25mm.

4 PCB humidity control during printing If the PCB humidity is too high, the water under the solder paste will quickly evaporate, causing the solder to splash and produce solder balls.

If the PCB was manufactured six months ago, please dry it. The recommended drying temperature is 125 degrees for 4 hours.

Paste storage If the solder paste is applied without temperature recovery time, the water vapor in the surrounding environment will condense and penetrate the solder paste; this will cause the solder to splash.

The solder paste should be stored in a refrigerator at 0 to 5 degrees Celsius.

Two to four hours before use, put the paste in a room temperature environment.

The above is the analysis of the solder paste printing defects listed in the smt patch processing for your reference.