Optimization for Solder Paste Printing Process
During the solder paste printing process, a series of advanced techniques are employed to ensure quality and reduce solder ball formation. Precise control over the solder paste viscosity, printing speed, and squeegee pressure is implemented to ensure the paste is applied evenly and accurately onto the pads.
In the stencil aperture design, laser cutting technology is used to create the aperture, which results in fewer burrs and greater precision. Afterward, the stencil surface is typically polished or etched, and some may undergo a nano-treatment process after polishing. These treatments help achieve a smoother surface, reducing printing resistance. For the aperture design, the apertures are usually cut based on the size of the PCB pads, but for special components, targeted optimizations are made. For example, for SMD (0805 and above packages), to prevent solder paste dispersion during the pick-and-place process, which can lead to solder balls after reflow, the stencil apertures are reduced by approximately 1/3 in length and width, forming a semi-circular groove. This serves as a preventive measure against solder ball formation.
Proper Setting of Reflow Soldering Temperature Profiles
Adjust the reflow soldering temperature profile based on the solder’s melting point and the properties of the PCB material. Use appropriate cooling and heating rates to avoid temperature fluctuations that are too fast or too slow.
Improvement of PCB Layout Design
Increase the pad size properly and optimize the pad layout to avoid excessively tight spacing, thereby reducing the risk of solder ball formation.
- Spacing between Surface Mount Devices (SMDs):
- Same type of SMD: ≥ 0.3mm
- Different types of SMDs: ≥ 0.13mm * H + 0.3mm (where H is the maximum height difference between adjacent components.)
- Hand Soldering and SMD: ≥ 1.5mm
- Spacing between through-hole device (THD) and SMD: Maintain a gap between 1 to 3 mm to avoid pressure that could lead to soldering difficulties.
- Placement between IC and Capacitor: Place capacitors near the power ports, as close as possible, to optimize power performance.
- Component Placement at PCB Edges: Components should be placed parallel to the cutting direction, and no components should be positioned within a certain distance of the edge to avoid damage during cutting.
- Via Placement: Vias should be avoided on pads whenever possible. The VIPPO (Via in Pad Plated Over) process, available from lcsc.com, can be used, where vias are filled with resin and capped with plating to enhance thermal conductivity and electrical performance.
The VIPPO (Via in Pad Plated Over) process, also known as Plated Over for Via (POFV), is a common technique in HDI (High-Density Interconnect) PCB manufacturing. It is used to improve the flatness of the board, reduce solder joint voids, and is particularly suitable for high-density wiring requirements such as BGA (Ball Grid Array) components.
Use of Lead-Free Solder
Lead-free solder has a higher melting point, which helps to reduce the formation of solder balls. By selecting the appropriate lead-free solder, it is possible to improve soldering quality while being environmentally friendly.
Below is a comparison of lead-free solder and leaded solder in preventing solder ball formation:
| Comparison Factors | Lead-Free Solder | Leaded Solder |
| Melting Point | Higher (217°C ~ 227°C), reducing the risk of solder ball formation. | Lower (around 183°C), prone to excessive melting, leading to solder ball formation. |
| Temperature Control Requirements | Requires stricter temperature control to prevent overheating or rapid temperature changes. | More lenient temperature curve requirements. |
| Thermal Stability | Better thermal stability at high temperatures. | Poorer thermal stability under thermal changes |
| Dissolution Rate During Soldering | Slower, preventing premature melting and reducing solder ball formation. | Faster, more likely to melt too early and cause formation of solder balls. |
| Solder Ball Formation Probability | Lower, due to the higher melting point and slower melting process. | Higher, as the lower melting point can lead to premature melting and solder balls. |
| Application | More suitable for high-temperature environments and precision soldering applications. | Suitable for general soldering but may not meet high-precision requirements. |
| Environmental Impact | Environmentally Friendly, less environmental damage | Potentially harmful to environment |
Lead-free solder, due to its higher melting point and better thermal stability, effectively prevents the formation of solder balls during the reflow soldering process, which can occur due to excessively high temperatures. Additionally, its slower dissolution rate helps to minimize the risk further, making lead-free solder a more reliable and environmentally friendly choice in electronic manufacturing.
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