What Is Solder Mask Dam? — NextPCB

What is a Solder Mask Dam?

A solder mask bridge, also known as a solder mask dam, or sometimes referred to as a "solder resistance bridge"  or "dam PCB" , is a small section of the solder mask layer on a printed circuit board that is left open or exposed around the perimeter of each pad or through-hole. The purpose of a solder mask bridge is to prevent the flow of solder or other conductive materials from one electrical connection to another during the assembly process.

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By creating a small barrier, the solder mask dam ensures that the solder joints are formed only where intended, preventing electrical shorts and other types of damage that can cause electronic devices to malfunction or fail. Solder mask dams are important for ensuring the reliability and performance of electronic devices, as they protect the copper traces and other metal features from oxidation, contamination, and other forms of damage.

Solder Mask Material Composition and Types

Solder mask is a complex mixture primarily composed of resin (such as epoxy, polyurethane, or acrylic), hardener, fillers, dyes, and UV-reactive substances. The choice of solder mask material is crucial:

1. Liquid Photoimageable (LPI) Solder Masks: LPI is the most common type, offering versatility and cost-effectiveness for standard applications. It is excellent for detailed PCBs with small parts and tight spaces, often used on flexible, rigid-flex, and BGA boards.
2. Dry Film Solder Masks: This type involves vacuum lamination and is generally preferred for high-precision, high-density wiring boards due to its uniform thickness and better resolution, although it is typically more expensive than LPI.
3. Epoxy-based Solder Masks: These are the most common and economical, offering good chemical and thermal resistance for general-purpose use.
4. Polyimide Solder Masks: These provide superior thermal endurance, making them highly suitable for demanding applications like aerospace and military use where higher temperatures are expected.

Overview of PCBs and Solder Mask Layers

Printed circuit boards are the backbone of many electronic devices, providing a platform for electronic components to be connected and communicate with each other. The solder mask layer is a thin, uniform layer of polymer material that is applied to the surface of the board to protect the copper traces and other metal features from oxidation, contamination, and other forms of damage. The solder mask layer also serves as a barrier to prevent the flow of solder or other conductive materials from one electrical connection to another.

Role of Solder Mask Dams

Solder mask dams are an essential feature of the solder mask layer. By creating a small barrier around each pad or through-hole, solder mask dams prevent the flow of solder outside of the intended area during the assembly process. This ensures that the solder joints are formed only where intended, preventing electrical shorts and other types of damage that can cause electronic devices to malfunction or fail.

Solder mask dams are particularly critical for:

• Fine-Pitch Components: For integrated circuits (ICs), Ball Grid Arrays (BGAs), and Quad Flat No-leads (QFNs), where pads are closely packed, dams are necessary to avoid solder spillover and maintain precision.
• High-Density Interconnect (HDI) PCBs: In miniaturized designs with very tight spacing, solder mask dams are the primary defense against bridging between adjacent traces and pads.
• Improving Manufacturing Yield: By minimizing soldering defects like bridging and cold joints, proper solder mask dams significantly contribute to higher production yields and reduce costly rework.

How Solder Mask Dams Work？

Solder mask dams function by preventing the solder from flowing outside of the intended area during the assembly process. Without solder mask dams, molten solder can flow onto adjacent conductive features, causing electrical shorts (solder bridges) or other types of damage.

During fabrication, the solder mask is applied over the entire board and then selectively removed from areas where soldering is required (pads, vias). The dam is the intentional, intact strip of solder mask material left between two closely spaced pads or pins. By acting as a physical "dam," this strip ensures the surface tension of the molten solder confines it to the intended pad, improving the reliability and quality of the PCB assembly. Solder mask dams are crucial for preventing these types of problems and ensuring the reliability and performance of electronic devices.

Types of Solder Mask Dams

There are several types of solder mask dams that can be used, including open, hatched, and round dams. Each type of solder mask dam has its advantages and disadvantages, and the appropriate type should be selected based on the specific requirements of the PCB design.

Open Solder Mask Dams

Open solder mask dams are the simplest and most common type of solder mask dam. They are created by leaving a small open space around each pad or through-hole, allowing solder to flow onto the pad or through-hole without spreading to adjacent conductive features.

Hatched Solder Mask Dams

Hatched solder mask dams are similar to open solder mask dams, but they use a pattern of diagonal lines to create a barrier around each pad or through-hole. The diagonal lines create small gaps in the solder mask, which allow the solder to flow onto the pad or through-hole without spreading to adjacent conductive features.

Round Solder Mask Dams

Round solder mask dams use a circular shape to encircle each pad or through-hole. The circular shape creates a barrier around the pad or through-hole, preventing solder from flowing onto adjacent conductive features.

Solder Mask Defined (SMD) vs. Non-Solder Mask Defined (NSMD) Pads

The way a pad is defined relative to the solder mask is a critical design decision directly impacting dam performance and assembly reliability.

NSMD is generally preferred for large components and BGAs where high solder joint reliability is paramount, while SMD is often chosen for ultra-fine-pitch designs where preventing solder bridging is the top priority. However, it is important to note that the copper etching process is generally capable of a tighter tolerance than the solder masking process, leading to a general preference for NSMD pads.

How to Design Solder Mask Dam

Designing an effective solder mask dam requires careful attention to material properties, dimensions, and manufacturing tolerances.

PCB Layout

The PCB layout is an essential consideration when designing the SMD. The designer must ensure that there is enough space for the dam and that it doesn't interfere with other components or traces on the board.

Component Placement

Additionally, the placement of the components on the board is also important when designing the solder mask dam. The designer must consider size and shape and protect the component without obstruction.

Solder Mask Material

The type of solder mask material used is also a significant consideration. The designer needs a compatible material for insulation and protection that fits the manufacturing process.

• Thermal Management: For boards subject to high temperatures, such as during reflow soldering (up to 260°C) or continuous operation, designers should specify High-Tg (glass transition temperature) solder mask materials to prevent delamination or loss of insulation performance. > Learn High TG PCBs
• Chemical Resistance: The material must resist common surface finish processes like Electroless Nickel Immersion Gold (ENIG) and Immersion Tin (Sn).

Setting up the Design Rules

The first step in designing an SMD is to set up the design rules. We determine the minimum size and clearance for the dam and actively avoid any interference it may have with other components or traces on the board.

• Minimum Dam Width: The dam must be wide enough to physically block solder flow while remaining narrow enough for manufacturability. Standard dam widths typically range from 4 to 8 mils (0.1–0.2 mm).
• Fine-Pitch Requirement: For high-density designs, the recommended minimum solder dam width is 100 μm (4 mils). High-reliability IPC Class 3 boards specifically require a minimum of 4 mil dam between adjacent pads.
• Color Impact: Solder mask color affects the minimum printable width. Green solder mask can reliably adhere at 4 mils (0.10 mm), while colors like black or white often require a minimum width of 6 mils (0.15 mm). Green is often preferred for very fine pitch due to this capability, as well as providing good contrast for Automated Optical Inspection (AOI).
• Design Rule Checks (DRC): Utilize PCB design software (like HQDFM, KiCad, or, Altium) to run DRCs that specifically check the minimum distance between any two solder mask openings (pads or vias) to ensure the dam width meets the specified minimum value. These checks are vital as narrow sections (slivers) of solder mask under 75 microns tend to peel or lift during processing.

Placing the Solder Mask Dam

After setting up the design rules, the designer can actively begin placing the solder mask dam on the board. We must place the dam adequately to protect the component while ensuring it functions properly.

Advanced Manufacturing Techniques for Solder Mask Dams

LDI technology has largely replaced traditional phototooling for high-accuracy solder mask application.

• Precision and Resolution: LDI systems use high-precision lasers to directly image the solder mask material, offering enhanced imaging accuracy. The LDI process, particularly when used with LPI masks, can reliably achieve registration accuracies as tight as ±10 microns.
• Ultra-Fine Pitch Capability: The precision of LDI allows manufacturers to reliably achieve minimum solder dam widths as small as 2 mils (0.05 mm) in high-density production environments, though 4 mils (0.1 mm) remains the standard recommendation for robust design. This high resolution makes it suitable for complex applications like HDI and multi-layer boards.

Tolerances and Cost

While tighter tolerances (such as narrower dam widths or more precise registration) improve performance, they inherently impact the fabrication process:

• Increased Cost: Tighter tolerances often require specialized equipment, more meticulous processes, and reduced throughput, leading to increased overall production costs.
• Extended Lead Times: The complexity involved in manufacturing ultra-high-density designs with strict solder mask requirements can extend the overall fabrication cycle time. Designers must balance the need for reliability with the cost and time constraints of the project.

> Recommend reading: Custom PCB and PCBA Cost Analysis: How to Optimize Design and Reduce Expenses

Conclusion

In short, it is significant in making PCBs for electronic devices. Additionally, it also adds a layer of protection by covering the copper traces to prevent any accidental short circuits. Furthermore, this protective layer also prevents oxidation and corrosion. 

> Recommend reading: IPC-2581: The Unified Data Standard for Smart PCB Manufacturing

It's part of the PCB fabrication process that involves several steps, including designing the PCB layout, printing the solder mask layer, and curing it. The solder mask's thickness, color, and material type affect the PCB's performance. It is significant to consider these factors when designing the solder mask layer. Proper design rule implementation, including the selection of appropriate dam widths and pad types (SMD vs. NSMD), is essential to leverage the full benefit of this critical PCB feature.

About the Author

Stacy Lu

With extensive experience in the PCB and PCBA industry, Stacy has established herself as a professional and dedicated Key Account Manager with an outstanding reputation. She excels at deeply understanding client needs, delivering effective and high-quality communication. Renowned for her meticulousness and reliability, Stacy is skilled at resolving client issues and fully supporting their business objectives.

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