Detailed Process Description of Solder Mask Bridge in PCB

Detailed Process Description of Solder Mask Bridge in PCB

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The surface of a PCB is coated with a layer of lacquer known as solder mask ink. This is one of the most common and essential types of ink used in PCB manufacturing. While approximately 90% of solder mask is green, other colors such as red, blue, black, white, and yellow are also available.

The primary function of solder mask ink is insulation. During the soldering process, it helps prevent short circuits caused by solder bridging and physical damage to conductive traces. It also protects against insulation degradation or corrosion caused by environmental factors such as dust and moisture.

A solder mask bridge refers to the section of solder mask between the solder pad openings of closely spaced component leads—typically ICs with fine-pitch pins. The role of the solder mask bridge is to prevent solder flow and bridging between pads during soldering. To ensure reliable solder joints and avoid shorts, it is essential to maintain solder mask bridges between adjacent pads wherever possible.

Types of Solder Mask Bridges

Each PCB layout and component density imposes specific requirements. PCB designers must choose the appropriate type of solder dam based on these needs. Different types of solder mask bridges offer various advantages, improving reliability and reducing the risk of solder bridging.

1. Open Solder Mask Bridge

An open solder mask bridge exposes designated areas of copper on the PCB. It provides controlled spacing between components to allow selective electrical connection. This technique prevents solder from migrating through the structure while still maintaining stable electrical conductivity. Open solder dams are especially suitable for high-frequency and RF (radio frequency) applications, where precise solder flow control is critical due to the performance sensitivity of the circuits.

2. Mesh (Grid) Solder Mask Bridge

The mesh solder mask bridge employs a grid-like structure in its design. This configuration helps to minimize thermal stress during soldering. The mesh layout enhances adhesion strength and prevents cracking of the solder mask material. Mesh-type solder dams are particularly effective for large copper areas. By incorporating these design improvements, the mechanical reliability and stability of the circuit board are significantly increased.

3.Circular Solder Mask Bridge

When using a circular solder mask bridge, a ring-shaped pattern is formed around the pad. This design helps prevent bridging between components in tightly packed areas of the circuit board. Due to its structural layout, circular solder mask bridges enable more precise and efficient soldering operations, while also minimizing the risk of short circuits.

Circular dams are particularly well-suited for fine-pitch and high-density PCBs, where maintaining solder joint integrity is crucial. With this design, solder joints are kept clean and uniform, contributing to consistent overall board quality.

Solder Mask Bridge in PCB

Design Rules and Standards for Solder Mask Bridge

1. Minimum Width Requirements

  • Standard Design:
    The typical minimum width of a solder mask bridge is ≥ 6 mil (0.152 mm), suitable for general consumer electronics and industrial control PCBs.

  • High-Density PCBs (e.g., HDI boards):
    The width can be reduced to 3.2 mil (0.08 mm) or even smaller, depending on the fabricator’s capabilities. For example:

    • JLCPCB, using LDI (Laser Direct Imaging) technology, can achieve a minimum solder dam width of 2 mil (0.05 mm).

  • Ultra-High-Density Packaging (e.g., FC-BGA):
    A width range of 0.05–0.08 mm is used, suitable for high-end server boards and AI processors.

2. Pad Spacing Consideration

The solder mask bridge width should be calculated based on pad spacing. A typical empirical formula is:

Solder Mask Bridge Width = Pad Spacing − 2 × Pad Width − 2 × Fabrication Tolerance

Example:
If pad spacing = 0.5 mm, pad width = 0.25 mm, tolerance = 0.05 mm, then:

Solder mask bridge width ≥ 0.05 mm

3. Thickness Requirements

  • Standard Thickness:
    The solder mask layer is typically 8–15 μm thick.

    • If too thin: risks of soldering failure due to insufficient insulation.

    • If too thick: may negatively affect assembly precision.

  • Recommended Practice:
    Use multiple screen printing passes or spray coating to achieve a thickness ≥ 15 μm, especially for high-reliability applications.

Manufacturing Considerations for Solder Mask Bridges

The process capability of solder mask bridges is affected by ink color, copper thickness, and board layout:

  • Green solder mask ink offers better process control and can accommodate smaller solder dams compared to colored inks.

  • Thicker copper requires wider solder mask bridges, while thinner copper allows for narrower and more stable solder dams.

1. For Base Copper Thickness ≤ 1 oz:

  • Green & Matte Green: Solder mask bridge ≥ 4 mil

  • Other Colors: Solder mask bridge ≥ 5 mil

  • On large copper pour areas: Solder mask bridge ≥ 8 mil

2. For Base Copper Thickness 2–4 oz:

  • Glossy Black, Matte Black, White: Solder mask bridge ≥ 6 mil

  • On large copper pour areas: Solder mask bridge ≥ 8 mil

3. Solder Mask Bridges Between Large Copper Areas (HASL Surfaces):

  • To prevent solder bridging between large copper zones (especially with HASL finish), solder dam width must be ≥ 8 mil.

Solder Mask Bridge in PCB-1

Main Application Scenarios of Solder Mask Bridge

1. Fine-Pitch Packages

For ICs with fine-pitch packaging such as QFP (Quad Flat Package), LGA (Land Grid Array), and BGA (Ball Grid Array), the pad spacing is typically less than 0.5 mm.
Solder mask bridges are essential to prevent solder bridging between adjacent pads, which could otherwise lead to short circuits.
This is especially critical in automated soldering processes, where solder paste tends to spread under heat; without solder mask bridges, solder bridging becomes more likely.

2. High-Density Routing Areas

In HDI (High-Density Interconnect) PCBs, where routing is dense, solder can easily flow between adjacent pads if solder mask bridges are not implemented.
Solder dams act as physical barriers, enhancing electrical insulation and ensuring the reliability and safety of the circuit.

3. Prevention of Solder Ball Spatter

Solder mask bridges help seal gaps between pads, reducing the likelihood of solder balls splashing into unintended areas during reflow soldering.
This significantly improves soldering quality and reduces potential post-soldering defects.

4. Multilayer Boards or Bottom-Side Soldering

In double-sided PCB assemblies, when components are soldered on the bottom layer, solder mask bridges can be applied between pads on the top layer to prevent flux or solder contamination of areas that should not be soldered.
This is particularly useful in wave soldering or selective soldering applications.

Conclusion

Solder mask bridges are a critical structure in ensuring the reliability of PCB soldering. Their design must take into account pad spacing, manufacturing capability, and soldering process.
By selecting appropriate solder dam widths, optimizing solder mask material properties, and closely collaborating with PCB manufacturers, one can significantly improve PCB yield and long-term reliability.

As electronic products continue to evolve toward higher density and finer pitch designs, solder mask technologies will continue to advance to meet the growing demands of miniaturization and performance.