In PCB manufacturing, the design and fabrication of solder mask Bridges are of vital importance, directly affecting the soldering quality and product reliability. The following is an introduction from the aspects of design and production:
The design of the solder mask bridge
The function of the solder mask bridge
A solder mask bridge refers to the connection part formed by solder mask ink between adjacent solder pads. Its main function is to prevent solder from bridging during the welding process and avoid short circuits. Solder mask Bridges are particularly important in the design of fine-pitch components (such as QFP, BGA, etc.) or high-density wiring.
Design considerations
Pad spacing: When the pad spacing is less than a certain value (such as 0.25mm), a solder mask bridge needs to be designed. If the spacing is too small, it will increase the manufacturing difficulty and should be determined based on the process capability.
The width of the solder mask bridge: If the width is too narrow, it may prevent the formation of the solder mask bridge or make it prone to damage. If it is too wide, it may affect the solderability of the pad. Generally, the design is optimized based on the pad spacing and process capacity. For instance, when the spacing is between 0.25mm and 0.35mm, the width of the solder mask bridge is usually 0.05mm to 0.10mm.
The performance of solder mask ink: Select solder mask ink with good adhesion, solder resistance and insulation performance. Its fluidity and curing characteristics will also affect the quality of the solder mask bridge.
Communicate with the manufacturer: Understand the PCB manufacturer's process capabilities, including parameters such as the minimum solder mask bridge width and the alignment deviation of the solder mask layer, to ensure the feasibility of the design.
Avoid the situation of designing solder mask Bridges
IC packaging design: If the open window design is adopted, the solder mask bridge cannot be achieved.
Specific package type: Avoid designing solder mask Bridges at the solder mask openings of contact key packages or PCBS with plug-and-pull gold fingers.
Process limitation: If the pad spacing is too small, forcibly designing a solder mask bridge may lead to a risk of detachment.
The fabrication of solder mask Bridges
Exposure and development processes
Exposure alignment accuracy: The exposure of the solder mask pattern should be highly aligned with the position of the solder pad to prevent the solder mask bridge from shifting or covering the solder pad.
Development time control: Insufficient development may lead to an increase in the width of the solder mask bridge, while excessive development may cause the solder mask bridge to break.
Selection of process technology
Traditional exposure techniques, such as CCD exposure machines, have alignment tolerances. When designing, it is necessary to appropriately expand the solder mask window to reduce the impact on the yield.
Advanced exposure techniques, such as LDI (Laser Direct Imaging) technology, can reduce the deviation caused by the expansion and contraction of the film, improve the alignment accuracy, and are suitable for the production of solder mask Bridges on high-density PCBS.
Quality control during the production process
Control of the width and thickness of the solder mask bridge: Determine the appropriate width of the solder mask bridge based on the copper thickness and process capacity. For instance, when the base copper is ≤1oz, the solder mask bridge is ≥4mil (green and green matte); When the base copper is 2-4oz, the solder mask bridge is ≥6mil (bright black, matte black, white).
Defect detection and correction: After the production is completed, use equipment such as AOI to inspect the graphic quality of the solder mask bridge, check for issues such as fractures, missing parts or covering the solder pads, and promptly correct any defects.
Common Problems and Solutions
The solder mask bridge is broken or missing
Reason: The width of the solder mask bridge is too narrow, or the adhesion of the solder mask ink is insufficient.
Solution: Increase the width of the solder mask bridge, select solder mask ink with better performance, and optimize the exposure and development processes.
The solder pads are covered with a solder mask layer
Reason: Alignment deviation of the solder mask layer or incorrect position of the solder mask bridge in the design.
Solution: Enhance the alignment accuracy of the solder mask pattern and check the window Settings of the solder mask layer in the design documents.
Welding short circuit
Reason: Excessive solder diffusion or unreasonable design of the solder mask bridge.
Solution: Optimize the welding process, such as choosing the appropriate flux and controlling the reflow soldering curve; Redesign the solder mask bridge to ensure its width and position are reasonable.
