The wetting ability of solder paste is crucial for both the printing quality and the soldering effectiveness. To enhance the solder paste's wetting performance during the soldering process, the following aspects should be considered:
1. Solder Paste Selection and Handling
- Solder Paste Composition and Quality: Choose solder paste that contains highly active flux, as this helps to improve wetting performance. Ensure the solder paste is not expired, as expired solder paste may lead to decreased flux activity.
- Solder Paste Viscosity and Particle Size: The viscosity of the solder paste should be moderate. Both too thick or too runny solder paste can negatively affect its wetting ability. The uniformity and size of the solder paste particles also impact the printing performance. It is generally recommended that the diameter of the solder paste particles be about one-fifth of the steel mesh opening size, with the maximum particle diameter not exceeding 0.05mm.
- Solder Paste Temperature and Stirring: Before use, solder paste should be fully warmed up and stirred to ensure uniformity and proper flow characteristics.
2. Printing Process Optimization
- Stencil Design: The aperture diameter and shape of the stencil should be adjusted according to the pad design. The aperture diameter should generally be smaller than one-third of the pad size to ensure the solder paste can easily flow and cover the pad. After experimentation, it is found that a stencil aperture diameter subtracted by the foot diameter between 0.3mm to 0.5mm works best.
- Printing Pressure and Speed: The printing machine’s pressure should be balanced. The pressure and speed of the squeegee should be set according to the product’s characteristics. Generally, the printing cycle time for one PCB should be controlled between 5 and 10 seconds. Insufficient pressure or slow printing speed can result in excessive solder paste being printed, which may affect the wetting ability.
- Printing Temperature and Humidity Control: The printing temperature and humidity should remain stable. The temperature variation should not exceed 5°C, and an optimal environment would be between 20°C to 25°C with 40%-60% RH. This helps prevent changes in the solder paste's performance due to temperature fluctuations.
3. PCB and Component Preparation
- PCB Flatness: The flatness of the PCB plays a vital role in the solder paste printing quality. Ensure the PCB is free of deformation or warping.
- Components and Pads: Components and pads should be clean, and free of oxidation or contamination. The pad design should be reasonable. The pad on top of the hole should be smaller, while the bottom pad should be larger. Avoid designing pads that are entirely grounded, as this ensures the solder paste can smoothly wet the pad edges.
4. Environmental Control and Operational Standards
- Environmental Humidity and Temperature: The printing workshop should maintain a constant temperature and humidity to prevent solder paste from absorbing moisture or oxidizing.
- Operational Standards: Operators should be proficient in the printing process and follow established operational standards to avoid human errors that can affect printing quality.
5. Subsequent Soldering Process Coordination
- Soldering Temperature and Time: The soldering temperature should exceed the melting point of the solder paste to ensure proper melting and wetting of the pad. The soldering time should be long enough, generally recommended to be at least 3.5 seconds, to ensure the solder paste can fully flow to the pad edges and create a solid solder joint.
- Soldering Atmosphere: Maintain a good atmosphere during soldering to avoid harmful gases, such as oxygen, that can negatively impact soldering quality.
Overall, improving the wetting ability of solder paste during the printing process requires addressing multiple aspects, including solder paste selection and handling, optimization of the printing process, preparation of the PCB and components, environmental control and operational standards, and coordination with subsequent soldering processes. By comprehensively considering these factors and implementing the corresponding measures, the solder paste's wetting ability can be significantly improved, leading to better printing quality and soldering results.
At PCBYES, with over 5 years of experience in PCB prototype and fabrication, we understand the critical nature of solder paste performance and its impact on final product quality. Our team of skilled engineers can assist you in optimizing every step of the process, ensuring your products meet the highest standards of quality and reliability.
