What is Solder Mask and How to Apply it on PCB

In electronics, current flows through the circuits on a PCB to each component. However, external circuits are exposed, which means they may be subject to threats such as friction, moisture, chemicals, and more. Regardless of the source of these threats, the ultimate performance of the circuit is affected. To protect the surface circuitry, it is necessary to apply a solder mask on the top of the board.

What is Solder Mask

Solder mask layer on PCB surface

Printed circuit boards are formed through the lamination of multiple layers, with the solder mask positioned at the topmost layer of the PCB. Without the solder mask, the color of the PCB is determined by the substrate material. However, in the majority of electronics, you will find that most PCB are green. In fact, this is due to the use of green soldermask ink. Therefore, the solder mask is also referred to as the green oil layer.

Further Reading: Why Are Circuit Boards Green and What’s the Significance?

The solder mask layer is typically designed as a negative image to aid the designer in creating the layout. This means that areas, where the solder is not required, are covered, while the areas where it is needed are left exposed. The soldermask is not applied to the entire surface of the board during manufacturing but only leaves a thin layer of silver-white tin plating on the necessary pads and traces. For this reason, it is often referred to as an open window layer.

Further Reading: Guide to PCB Solder Mask Opening Design

Function of Solder Mask

Firstly, due to process or equipment issues, the surface of the PCB may exhibit some irregularities. The soldermask ink smoothens these uneven areas, not only enhancing visual appeal but also facilitating subsequent assembly processes.

In the PCBA process, components are mounted onto the board, and solder paste adheres to exposed copper pads through openings in the solder mask. Subsequently, during reflow soldering, a robust metallurgical bond is formed between the leads and pads. Throughout this process, the solder mask serves as a protective barrier, ensuring that solder adheres only to designated areas, preventing unintended solder bridges or PCB short circuits between closely spaced pads or traces.

In practical applications, the solder mask, being applied over the copper layer, prevents certain contaminants and threats from directly impacting the circuit. As the device operates, current flows through the PCB towards the components, generating heat in the process. The solder mask acts as an insulating barrier on the circuit board, reducing heat transfer.

In addition to these aspects directly affecting circuit performance, you’ll find markings on the solder mask known as silkscreen. This not only aids in assembly but also provides visibility and identification, enabling users to perform maintenance or repairs.

Types of PCB Solder Mask

TypeDescriptionFeatures
ThermosettingMade of thermosetting resin, it is cured by high temperature to form a hard film-like structure and is the most common solder mask;High temperature resistance, corrosion resistance, insulation;
UVIt is formed by curing with UV light and is usually transparent or light yellow;Easy handling and short cure times for quick turnaround projects;
CoatingApply the solder resist material evenly on the entire PCB surface by coating to form a protective layer;Low cost, easy repair, suitable for special shape PCB
Thermal PeelableRelies on hot stripping tools or heating equipment for processing when in use;Easy to repair and modify, suitable for frequent replacement of components;

Solder Mask Design Guidelines

Tenting Vias

Tent vias and vias fill are two concepts that can be easily confused in the field of PCB assembly. A tent vias refer to a via that is fully covered by a solder mask to prevent exposure, with the annulus covered while the inside of the hole remains empty. On the other hand, via fill means that the inside of the via is completely filled, either referred to as a filled via or a mask-blocked via.

Tented vias have gained popularity in modern PCBA processes due to their various benefits:

  • Prevention of solder bridging: By covering the via with a solder mask, the likelihood of solder bridging during assembly and subsequent short circuits is reduced.
  • Mitigation of solder migration: Tented vias help minimize the risk of solder migrating from SMD pads to vias, especially when the vias are located at pad edges or in standard BGA patterns.
  • Protection against environmental factors: By covering the vias, tenting reduces the possibility of damage to the vias resulting from exposure to the operating environment.

Depending on the specific requirements of your project, different options are available:

  • For optimal project profitability, TechSparks recommends considering either epoxy filling or mask plugging techniques.
  • If through-hole augmentation is necessary, LPI (Liquid Photo-Imageable) soldermask augmentation is recommended.
  • When prioritizing quality, it may be advisable to use relatively more expensive resins as filler materials.
Tenting Vias
Tented vias created with soldermask

Solder Mask Gap

This aspect pertains to the spacing or gap between the solder mask of a PCB and its components. The purpose of this spacing is to ensure sufficient distance between the solder and prevent any contact, also known as a Solder Mask dam. If the gap is too small, it can lead to solder bridges, whereas a larger gap may result in exposed pads or wires. Here are two recommendations for designing the PCB solder mask gap:

  • (a) Keep the gap at approximately half the width of the conductor pitch. When drawing conductive patterns with widths less than 100 μm, the solder resist gap can be less than 50 μm.
  • (b) Consider the coefficient of expansion during the design process. Thermal expansion and contraction can cause slight changes in the dimensions of the PCB, which may affect the solder resist gap.

Pads

A solder pad is a metal area, either round or square, used to establish connections with electronic components such as chips, resistors, and capacitors. It is closely associated with the solder resist layer, which can be categorized into two types: SMD and NSMD.

In SMD pads, the solder mask surrounds the pad’s edges and acts as a barrier, preventing solder from flowing beyond the designated pad area. During the manufacturing process, the size of the pad used in a BGA is determined by the mask opening in the solder mask. If the mask opening is smaller than the size of the solder pad, the solder mask limits the final pad size. Therefore, adjusting the size of the mask opening allows control over the solder pad dimensions.

On the other hand, NSMD pads refer to areas on the PCB that are not covered by the solder mask. This includes the gaps between pads and the mask. Since these pads lack the protective solder mask, special attention is required during the soldering process.

Pads​

Solder Mask Opening

The exposed portion of the PCB, where the solder mask is etched or stripped away, is referred to as a solder mask opening. This opening creates a pad or solder area that exposes the underlying metal, typically a copper layer.

Solder mask openings are commonly incorporated in PCB designs for areas that require manual or specialized soldering operations. These areas may include soldering points for high-power components, heat sink parts, or special connectors. By creating openings in the solder mask, soldering operations can be facilitated, allowing for better direct solder contact. Furthermore, openings can serve specific design requirements, such as placing openings near signal leads or test points to ease measurement or debugging processes.

Solder Mask Expansion

Extension is a technique employed in PCB design to extend the solder mask into non-solder areas. This technique provides additional protection or enables the fulfillment of specific design requirements. Designers utilize design software to define the spacing between the board surface features and the solder mask, which can have positive, negative, or zero values.

  • Positive value: This indicates that there is a gap between the end of the solder mask and the outer circumference of the exposed pad.
  • Negative value: In this case, the solder mask overlaps with a portion of the pad area.
  • Zero value: A zero spacing value means there is no gap or spacing between the solder mask and the pad.

How To Solder Mask PCB

Steps to Implement Solder Mask

Step 1: Cleaning

Before applying solder mask, it is essential to thoroughly clean the PCB to remove any contaminants, oils, or residues that may hinder proper adhesion of the solder mask. Common practices include using deionized water, alcohol, and acetone. After cleaning, the PCB should be allowed to dry.

Step 2: Solder Mask Application

The two methods that we use for the solder mask application is: Screen Printing and Curtain Coating

Screen Printing

The most common method is screen printing, where a solder mask material (usually an epoxy-based liquid) is applied using a stencil or mask. The stencil allows the solder mask to be precisely applied only to areas where it’s needed, leaving other areas exposed.

Curtain Coating

In some cases, especially for larger-scale production, solder masks can be applied through curtain coating methods. This method involves flowing the solder mask material across the entire PCB surface and then curing it to leave only the desired areas covered.

Step 3: Curing

After applying solder mask, immediate heat curing may lead to defects such as cracks. Therefore, it is common to perform a pre-cure to remove some solvents from the coating. Subsequently, the board is exposed to heat or ultraviolet light to achieve overall curing.

Step 4: Exposure and Development

If the solder mask is applied photo imageable, a photolithography process is used. The PCB is exposed to UV light through a solder mask film with the desired pattern. This is followed by a development process where the unexposed areas are washed away to reveal the defined solder mask pattern.

Step 5: Final Inspection

Once the solder mask is applied and cured, a final inspection is carried out to ensure proper coverage, absence of defects, and adherence to design specifications.

Improve Solder Mask Quality

Inspection

Inspection techniques in solder mask application involve various methods to verify the quality and adherence of the applied mask.

Visual inspection, often aided by magnification or automated optical systems, identifies defects like incomplete coverage, registration errors, or thickness variations.

Microscopic examination ensures finer details, detecting issues at a more granular level. Additionally, precise instruments measure solder mask thickness, ensuring uniformity across the PCB.

These techniques, coupled with rigorous adherence to customer specifications, ensure the solder mask application meets stringent quality criteria, guaranteeing the reliability and functionality of the printed circuit boards.

Standard and Criteria

IPC Standards

The IPC provides standards such as IPC-SM-840 for solder mask materials and IPC-A-600 for acceptability of printed boards. These standards outline criteria for solder mask quality, including coverage, registration, adhesion, and dimensional tolerances.

ASTM International Standards

ASTM standards, like ASTM D7091/D7091M, might be referenced for testing adhesion strength of the solder mask material on the PCB substrate.

Some clients or industries may have specific requirements or additional criteria for solder mask quality, necessitating adherence to their specified standards or guidelines.

Conclusion

Understanding and implementing solder masks are fundamental in ensuring the integrity, reliability, and manufacturability of PCB designs. Adherence to precise solder mask specifications, coupled with meticulous design considerations and manufacturing optimizations, guarantees robust protection, mitigates potential defects, and ultimately results in high-quality printed circuit boards ready for reliable deployment in electronic systems.

Related Articles:

Flex PCB Coverlay Guide to Assist You Apply It Better

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