With the rapid advancement of electronic technology, miniaturization, BGA packages, and high-density chips with pitches as small as 0.3mm to 0.5mm are becoming increasingly common. This has led to higher demands on soldering techniques in PCB assembly. While automated placement machines have improved efficiency, numerous factors still influence the quality of solder joints. This article explores key considerations during PCB design from the perspective of surface mount technology (SMT) to ensure better soldering outcomes. If these guidelines are not followed, it can result in poor solder quality, bridging, or even damage during rework.
**Factors Affecting PCB Soldering Quality**
Achieving a high-quality PCB involves strict control throughout the entire process—from design to component placement and soldering. Key influencing factors include: the PCB layout, board quality, component quality, oxidation levels of component leads, solder paste quality, printing accuracy, placement machine precision, reflow oven temperature profile settings, and more. The most challenging issue often lies in the PCB design stage, where designers may lack hands-on soldering experience, while factory workers may not fully understand the design intent. This disconnect can lead to suboptimal results.
**Recommendations for PCB Design**
Below are practical suggestions for PCB designers to avoid issues that could affect soldering quality:
1. **Positioning Holes**: Four holes (minimum diameter 2.5mm) should be placed at the corners of the PCB for accurate alignment during solder paste printing. These holes should be aligned along the X or Y axis.
2. **Mark Points**: Mark points are essential for the placement machine to identify the PCB's position. They should be placed diagonally on the board—either round or square pads—and must not interfere with other component pads. Both sides of the board should be marked if components are present on both sides.
3. **Side Clearance**: At least 3mm of space should be left along the long edge of the PCB to allow for proper handling by the placement machine. No components should be placed within this area to prevent misalignment or damage during reflow.
4. **Avoid Vias on Pads**: Placing vias directly on pads can cause solder paste to flow into the via, leading to insufficient tin coverage and weak solder joints. This should be avoided to maintain good solderability.
5. **Polarity of Components**: Diodes and tantalum capacitors must be clearly marked with correct polarity to prevent incorrect mounting by workers.
6. **Silkscreen and Logo Placement**: Silkscreen text and logos should be minimized or hidden, especially on densely populated boards, to avoid interfering with the placement of components and the visual inspection of solder joints.
7. **IC Pad Extension**: For ICs like SOP, PLCC, and QFP, extending the pad length to 1.5 times the IC leg length improves manual soldering and ensures better adhesion between the component and the PCB.
8. **Pad Width Consideration**: The width of the PCB pad should match the IC’s lead width to prevent excessive solder bridging between adjacent pins.
9. **Component Orientation**: Components should not be rotated at arbitrary angles. Most placement machines only support 90°, 180°, or 270° rotations. Misaligned components can lead to poor solder joint formation.
10. **Shorting Adjacent Pins**: Avoid using direct shorting methods that make it difficult to distinguish connections after soldering. Instead, use solder mask to isolate pins and ensure clean, safe connections.
11. **Middle Pad Issues**: Chips with central pads can cause short circuits if not properly spaced. Reducing the size of the central pad increases the gap between it and surrounding pads, reducing the risk of shorts.
12. **Spacing Between High-Profile Components**: Components with different heights should not be placed too close together, as this can cause physical contact during reassembly and trigger safety mechanisms in the placement machine.
13. **BGA Package Considerations**: Since BGA packages are invisible after soldering, it is recommended to add two 30-mil locating holes on the PCB for easier rework and stencil alignment.
14. **PCB Color Choice**: Avoid using red-colored PCBs, as they appear white under the camera light used by placement machines, making it harder to detect and place components accurately.
15. **Small Components Under Large Ones**: Placing small components under large ones can complicate rework. It is better to place them on the bottom side to avoid damage during disassembly.
16. **Copper Pad Connection**: Directly connecting component pads to large copper areas can cause thermal imbalance, leading to incomplete solder melting. Isolating pads from large copper regions improves soldering quality.
17. **Process Edges**: Adding process edges to the PCB can help with handling and rework without affecting the main board layout.
**Conclusion**
As more engineers become proficient in PCB design software, it’s crucial to consider the practical implications of their designs on the soldering process. By following these guidelines and maintaining open communication with manufacturing teams, engineers can significantly improve the quality and reliability of their PCB assemblies. Developing good design habits and understanding the constraints of the production environment is essential for successful electronics manufacturing.
ELCB 3 Phase
ZHEJIANG QIANNA ELECTRIC CO.,LTD , https://www.traner-elec.com