#

Back to blog

How Solder Masks Improve PCB Assembly and Reliability

As the electronics industry advances, the demand for circuit boards in terms of reliability and high density continues to soar. To meet these evolving needs, manufacturers strive to optimize high-density interconnection (HDI) technology and solder mask technology. In this insightful article, we will delve into the critical role of PCB solder masks, their types, applications, and the impact they have on PCB assembly.

What is Solder Mask?

The solder mask layer, also known as the window layer or green oil layer, plays a pivotal role in the PCB manufacturing process. It serves as a protective layer that covers specific areas of the circuit board, allowing for precise patterning during subsequent finishing and soldering processes. By utilizing a negative film output technique, the solder mask is designed to reveal the exposed copper skin while leaving the designated non-green oil areas untouched.

The negative output of the PCB solder mask layer implies a complete inversion of the actual situation. Areas marked for green oil application during the design stage are, in fact, non-green oil regions known as window openings. Thus, the solder mask layer serves the critical purpose of creating windows within the overall green oil surface, enabling proper soldering of the non-green oil components.

Why Use Solder Mask on PCBs?

The use of solder mask on printed circuit boards (PCBs) offers several important benefits that justify its inclusion in the PCB manufacturing process. Solder mask, also known as solder resist, solder stop, or solderable mask, serves multiple critical functions:

  1. Reduces Solder Bridging: During the assembly of PCBs, particularly with surface mount components, there is a risk of solder bridging, where solder unintentionally connects two adjacent pads or traces. Solder mask provides a protective barrier that prevents solder from spreading between closely spaced pads, reducing the likelihood of bridging and short circuits.
  2. Insulates Conductors: Solder mask serves as an electrical insulator. It prevents electrical connections or shorts between different conductive traces on the PCB, ensuring that current flows only through the intended circuit paths. This insulation is vital for the proper functioning and reliability of electronic devices.
  3. Protects Surfaces: Exposed copper traces on PCBs are vulnerable to oxidation and environmental corrosion. Solder mask acts as a protective shield, preventing the copper surfaces from coming into direct contact with the environment. This protection helps maintain the integrity of the circuitry and prevents deterioration over time.
  4. Enables Selective Coating or Plating: Solder mask is selectively applied to cover specific areas of the PCB while leaving others exposed. This selective coating allows for targeted processes such as selective soldering or selective gold plating. It also enables the application of solder paste only to designated areas during assembly.
  5. Improves Aesthetics: Solder mask adds a finishing touch to PCBs by providing a colored and uniform surface. The most common color for solder mask is green, but it is available in various colors. This aesthetic improvement not only makes PCBs visually appealing but also helps in identifying different boards and their functions.
  6. Strengthens Pads: Solder mask reinforces and strengthens component pads on the PCB. This reinforcement is particularly important for flexible PCBs, where the pads may be more susceptible to damage or wear and tear. The solder mask adds durability to these critical areas.
  7. Enhances Assembly: During PCB assembly, solder paste or adhesives are applied to the board. Solder mask helps these materials adhere properly to the designated areas, ensuring that components are accurately placed and securely attached during the soldering process.

In summary, the use of solder mask on PCBs is essential for improving the reliability, manufacturability, and functionality of electronic devices. It provides protection against various environmental factors, prevents solder-related issues, and contributes to the overall quality of PCB assemblies. While it adds some cost and complexity to the manufacturing process, the benefits it offers far outweigh these considerations, making solder mask a crucial component in modern PCB production.

Importance of Solder Mask

The solder mask on a printed circuit board (PCB) serves a vital role in protecting the PCB from damage during manufacturing processes involving high temperatures, such as soldering and leveling. Without the solder mask, the PCB’s dielectric material, made of materials like glass fiber and epoxy resin, could degrade, leading to electrical issues. Additionally, the solder mask prevents solder bridges from forming between closely spaced pads and traces, ensuring the integrity of electrical connections. It also acts as a shield against environmental factors, preserving the PCB’s performance and preventing corrosion or oxidation. Ultimately, the solder mask’s role in safeguarding the PCB’s functionality and reliability is paramount.

Furthermore, the solder mask enables selective application to specific areas of the PCB, allowing for targeted processes like selective soldering or gold plating. While aesthetics may not be its primary function, the solder mask enhances the PCB’s appearance, provides color-coding for identification, and contributes to a finished and professional look. Despite the complexities it introduces to PCB manufacturing, the solder mask’s multifaceted benefits make it an indispensable component in the production of high-quality electronic devices.

Solder Mask Layer vs. Solder Flux Layer

Let’s examine a two-layer PCB as an illustrative example. A basic two-layer PCB configuration consists of two breadboards (top and bottom) forming the core, a prepreg layer in the middle, and two solder mask layers (top and bottom) along with two silkscreen layers (top and bottom).

During PCB manufacturing, the entire solder mask is not uniformly coated with green oil due to factors like soldering and heat dissipation. Consequently, we commonly refer to these exposed areas as “windows” where the copper is visible.

The soldering layer, also known as the paste mask, pertains to the specific layer on the PCB that remains uncoated with green oil. In essence, it refers to the PCB stencil used in the manufacturing process. Many engineers consider it synonymous with the top layer, aligning with the data presented on the top layer. The purpose of the soldering layer differs from that of the solder mask layer. Its existence is not primarily for soldering purposes but rather to facilitate SMT assembly. During the SMT process flow, precise application of solder paste onto the pads is achieved by accurately dispensing the paste through the stencil apertures, hence the term “upper tin layer” for the soldering layer.

While both the paste mask and solder mask are involved in tinning processes, there are fundamental distinctions between them. In terms of internal circuit board functionality, the solder mask PCB layer serves the dual purpose of preventing salt spray and moisture, in addition to enabling soldering. Conversely, the paste mask primarily focuses on SMT stencil production, specifically for surface-mount package assembly. Typically, the solder mask layer is applied uniformly with green oil, leaving areas without the solder mask layer exposed. On the other hand, the paste mask is utilized for stencil design, with the specific objective of facilitating component placement, particularly in high volume PCB assembly scenarios.

Solder Mask Design Considerations

Proper solder mask design is essential for enhancing both the ease of assembly and the overall reliability of a printed circuit board (PCB). Here are some crucial considerations to keep in mind when designing the solder mask:

  1. Specify Final Finished Copper and Mask Fiducials: To ensure the best alignment during manufacturing processes, such as soldering and component placement, it’s important to specify final finished copper and mask fiducials on the PCB. These fiducials act as reference points for precise alignment, contributing to the accuracy of the assembly process.
  2. Keep Clearances Between Mask and Pads Consistent: Consistency in the clearances between the solder mask and pads is crucial. Inconsistent clearances can lead to soldering issues, shorts, or unreliable connections. Maintaining uniform clearances helps ensure a reliable assembly process.
  3. Follow IPC Guidelines: Adhering to industry standards, such as those defined by the IPC (Association Connecting Electronics Industries), is essential for reliable PCB design. IPC guidelines often recommend a 0.2mm pullback from pads, which helps prevent solder mask encroachment on critical areas, maintaining the integrity of the electrical connections.
  4. Verify Pullbacks Don’t Fully Expose Traces: While pullbacks from pads are essential, it’s equally important to verify that these pullbacks don’t fully expose traces. Excessive exposure can lead to potential shorts or damage to the traces. Ensuring a balanced approach to pullbacks is key to successful solder mask design.
  5. Mask-Defined Land Pads Improve Registration: Mask-defined land pads offer improved registration during assembly processes. These defined pads facilitate accurate placement of components, reducing the chances of misalignment and assembly defects.
  6. Specify Sizes in Both Copper and Mask Layers: To avoid discrepancies and errors, it’s advisable to specify sizes in both the copper and mask layers of the PCB design. This consistency helps ensure that the intended dimensions are maintained throughout the manufacturing process.
  7. Mask-Over-Pads Options for Easier Assembly: Consider using mask-over-pads options when appropriate. Mask-over-pads involve applying solder mask over the pads, leaving only specific areas exposed for soldering. This method simplifies the assembly process and ensures that solder is applied precisely where needed.
  8. Select Suitable Mask Dam Sizes to Control Paste Volumes: The size of the mask dams (the areas where solder mask covers the copper traces) plays a crucial role in controlling solder paste volumes during assembly. Properly sized mask dams help prevent excessive solder paste, reducing the risk of solder bridging or other solder-related defects.
  9. Specify Mask Tenting Over Drilled Vias Where Needed: When vias (plated through-holes) are present on the PCB, specifying mask tenting in certain areas can be beneficial. Mask tenting involves covering the vias with solder mask to prevent solder from wicking down the holes during assembly. This helps maintain the integrity of the PCB and prevents solder-related issues.

Influence of Solder Mask on PCBA

The solder mask, also known as solder resist or solder stop, is a crucial component in PCB (Printed Circuit Board) manufacturing, and its impact on PCBA (Printed Circuit Board Assembly) is profound. One key influence lies in accurate component placement and soldering. The solder mask defines precise areas for components, and well-designed openings are vital to ensure soldering accuracy and reliability during assembly.

Another critical role of the solder mask is providing protection against environmental factors. It acts as a shield, safeguarding the PCB’s copper traces and conductive elements from moisture, dust, and contaminants. Without it, environmental damage could lead to corrosion and electrical short circuits, reducing the PCB’s reliability and lifespan.

Furthermore, the solder mask offers electrical insulation and prevents unintended electrical connections. It plays a crucial role in maintaining consistent impedance characteristics, particularly in high-frequency applications. Additionally, it acts as a protective shield during soldering processes, preventing solder from adhering to unintended areas, which is crucial in densely populated PCBs. These factors underscore the solder mask’s significance in ensuring the functionality, reliability, and longevity of PCBA.

Solder Mask Process Guidelines

The proper application of solder masks and adherence to design guidelines are essential for ensuring the reliability and functionality of printed circuit boards (PCBs). Let’s delve into the key solder mask process guidelines and design considerations:

Design Guidelines

In practice, the use of solder masks in PCB designs is a discretionary choice, and designers can easily incorporate them by specifying a few parameters. Some software tools even offer automated solder mask generation capabilities. However, it is crucial to engage in a thorough discussion with the chosen PCB manufacturer before commencing the design process. This ensures a comprehensive understanding of their specific capabilities regarding solder mask thickness and minimum copper pad spacing, as these considerations are not universally applicable to all PCBs.

Neglecting or mismanaging simple solder mask issues, such as inadequate or excessive openings or an imbalance between the number of openings and the number of copper pads in the circuit plane, can result in the failure of a circuit board. Identifying whether such problems arise from negligence or inadvertent modifications in the design files may require time and careful investigation. Failing to address these issues diligently can potentially lead to catastrophic consequences. Consequently, meticulous examination of your design files is of paramount importance.

Via Cap Oil

Via cap oil, also known as via cover oil, is a technique used in circuit board manufacturing where the PCB vias are covered with solder mask to prevent them from being exposed. Unlike via filling, via cap oil only covers the ring circumference of the vias. When the entire via is covered, it is referred to as filling or plugging.

Manufacturers commonly employ via cap oil as a solder mask process to protect circuit boards. This method is often used in conjunction with epoxy filling or mask plugging, taking into consideration the PCB cost of manufacturing. Among the various via bumping techniques, LPISM bumping is considered the most cost-effective approach.

Solder Mask Dam

Solder Mask Dam, also known as solder mask gap, is a crucial component in PCB design. Its primary purpose is to ensure an adequate spacing between solder surface features to prevent the occurrence of solder bridges. Generally, the solder dam distance is set at half the width of the conductor pitch. However, in cases where fine conductive patterns below 100µm are utilized, this rule can be relaxed to accommodate the specific requirements of the design.

Solder Mask Opening

Solder Mask Opening is a critical feature in PCB design that allows the circuitry to be exposed for the application of solder paste during the soldering process. It is typically implemented by removing the solder mask layer on the outer surface of the PCB in specific areas. The accuracy of these openings is of utmost importance, as any inaccuracies can result in the unintended exposure of copper that should not be printed with solder paste. This can lead to issues such as circuit board short, corrosion, or damage to the circuit traces.

Solder Mask Coverage or Extension

This specification, also known as solder mask swell, can have positive, zero, or negative values:

  • Positive Solder Mask Extension: When there is a distance between the edge of the solder mask and the exposed outer perimeter of the pad, it is referred to as positive solder mask extension or swell. This ensures that the pad is adequately covered by the solder mask, providing protection and preventing unintended solder bridging.
  • Zero Solder Mask Extension: When there is no gap or spacing between the solder mask and the pad, it is considered a zero solder mask extension. This means that the solder mask precisely aligns with the pad boundaries.
  • Negative Solder Mask Extension: In some cases, the solder mask may extend beyond the pad boundaries, overlapping a portion of the pad. This is known as negative solder mask extension. It is typically used to provide additional solder mask coverage in areas where increased protection is required, such as high-density circuitry or areas prone to potential solder bridging.

By adhering to these design guidelines and solder mask process considerations, designers and manufacturers can ensure the successful production of high-quality PCBs that meet the specific requirements of their applications.

Solder Mask Properties

When it comes to selecting the right solder mask for your printed circuit boards (PCBs), it’s crucial to consider several key properties. These properties play a vital role in determining the performance, reliability, and longevity of your PCBs. Let’s delve into the important properties to evaluate when choosing a solder mask:

Dielectric Strength

Dielectric strength refers to the insulation resistance and breakdown voltage ratings of the solder mask. It’s essential for the solder mask to provide effective electrical insulation between conductive elements on the PCB. High dielectric strength ensures that the solder mask can withstand voltage differences without allowing electrical breakdown or leakage. A robust dielectric strength is vital for preventing short circuits and ensuring the integrity of the circuit.

Adhesion

Adhesion is the ability of the solder mask to bond securely to the PCB substrate and copper traces. Strong adhesion is essential to prevent delamination or peeling of the solder mask, especially during thermal cycling and exposure to various environmental conditions. Good adhesion ensures that the solder mask remains firmly attached to the PCB, protecting it from moisture, contaminants, and mechanical stress.

Resolution

Resolution refers to the line widths and spacing that the solder mask can reliably reproduce. This property is crucial for precise and accurate application of the solder mask, particularly in cases where fine-pitch components or intricate PCB designs are involved. High-resolution solder masks enable the creation of well-defined solder mask openings and patterns, enhancing the overall quality of the PCB.

Flexible and Thermo-Mechanical Properties

The flexibility and thermo-mechanical properties of the solder mask are important for PCBs that may undergo bending or experience temperature fluctuations. Flexible solder masks are ideal for applications involving flexible PCBs or devices that require resilience against mechanical stress. Additionally, understanding the coefficient of thermal expansion (CTE) of the solder mask is essential, as it affects the dimensional stability of the PCB when exposed to temperature variations.

Solder and Chemical Resistance

Solder masks need to withstand the rigors of soldering processes, including reflow soldering, flux application, and cleaning. Effective solder and chemical resistance ensure that the solder mask remains intact and unaffected by the soldering environment. It prevents solder mask degradation, discoloration, or the formation of defects that could compromise the PCB’s performance.

Flame Retardance

Flame retardance is a critical property for safety compliance, especially in applications where PCBs are used in environments where fire hazards are a concern. Flame-retardant solder masks are designed to resist combustion and inhibit the spread of flames. Meeting flammability standards ensures the safety and reliability of electronic devices.

Temperature Range

The functional temperature range of the solder mask is essential, as it determines the operational limits of the PCB. The solder mask should be capable of withstanding both high and low temperatures without experiencing degradation, loss of adhesion, or changes in its properties. Understanding the maximum and minimum functional temperatures is crucial for selecting the right solder mask for specific applications.

Moisture and Corrosion Resistance

Moisture and corrosion resistance are vital for protecting the PCB from environmental factors, such as humidity and exposure to corrosive substances. An effective solder mask should create a hermetic seal, preventing moisture ingress and the formation of corrosion on the PCB’s copper traces. This property ensures the long-term reliability of the circuitry.

To make an informed choice, it’s essential to verify the solder mask vendor’s reported property test results. By doing so, you can ensure that the selected solder mask material is suitable for your particular application and operational environment. Proper consideration of these solder mask properties contributes to the overall quality and reliability of your PCBs, making them well-suited for their intended purpose.

Solder Mask Types

1. Epoxy Liquid Photoimageable Solder Masks (LPC/LPI)

Epoxy liquid photoimageable solder masks are the most commonly used variety in the world of printed circuit boards (PCBs). Their popularity stems from being both cost-effective and versatile. These solder masks are cured through exposure to ultraviolet (UV) light, ensuring precise and accurate patterning. What’s more, they offer flexibility in color choices, with options that include the traditional green, as well as red, blue, and more. Due to their cost-effectiveness and adaptability, epoxy liquid solder masks find extensive use in a wide range of PCB applications, making them a popular choice for many electronic devices.

2. Epoxy Powder Coating

Epoxy powder coating solder masks are a distinct variety applied to PCBs as dry powder. Their curing process involves the application of heat, which transforms the dry powder into a solid solder mask. While epoxy powder coating solder masks are cost-effective, they may have limitations in terms of their capabilities compared to other types. As a result, they are primarily employed in the manufacturing of consumer electronics and devices, where cost efficiency is a critical factor.

3. Silicone-Based and Urethane Acrylate Solder Masks

Silicone-based and urethane acrylate solder masks are known for their ability to withstand high temperatures, making them suitable for applications where elevated heat is a concern. Additionally, these solder masks offer flexibility, which can be advantageous in applications involving flex boards or high-temperature boards. However, their unique properties come at a higher cost compared to epoxy-based options. As such, they are often chosen for specialized applications where their combination of high-temperature tolerance and flexibility is required.

4. Flexible Solder Masks

Flexible solder masks are designed to maintain their flexibility even after curing. This property is particularly important for applications involving flexible circuits. However, the production of flexible solder masks can involve more complex processing techniques to achieve the desired level of flexibility. These solder masks are predominantly used in applications where the PCB itself needs to be bendable without compromising functionality.

5. Flame Retardant Solder Masks

Flame retardant solder masks are specifically formulated to meet flammability standards, ensuring safety compliance in environments where fire hazards are a concern. They are commonly found in consumer goods where adherence to safety standards is crucial for user protection. Flame retardant solder masks are preferred in applications where fire safety is a priority, such as appliances and consumer electronics.

Each of these solder mask formulations has its unique set of properties and benefits, making them suitable for particular environments and applications. The choice of solder mask type should be based on the specific requirements and constraints of your PCB project, ensuring that it aligns with the performance and safety expectations of the final electronic device. Proper selection of the solder mask type contributes to the overall success and longevity of the PCB in its intended application.

How is solder mask applied to your PCB?

Applying solder mask to a PCB is a crucial step in the PCB manufacturing process. Solder mask is a protective layer made of a polymer material that covers the metal traces on the PCB, ensuring proper soldering and preventing short circuits. The application of solder mask involves several methods and materials, depending on the specific requirements of the PCB and the desired outcome. Here’s how solder mask is applied to a PCB:

  1. Selection of Solder Mask Material: The first step is to choose the appropriate solder mask material based on the application, cost considerations, and desired properties. There are various types of solder mask materials available, including epoxy liquid, liquid photoimageable solder mask (LPSM) inks, and dry film photoimageable solder mask (DFSM). Each material has its advantages and is selected based on the PCB’s requirements.
  2. Solder Mask Printing: The most common method for applying solder mask is through a process similar to printing. For epoxy liquid solder masks, a liquid epoxy is applied to the PCB’s surface using a silkscreen or stencil. This process is akin to airbrushing face paint using a stencil. The solder mask is applied uniformly, covering the entire PCB surface.
  3. Liquid Photoimageable Solder Mask (LPSM): LPSM is another option where the solder mask material is applied to the PCB through methods such as silkscreening or spraying. After application, the PCB is exposed to the desired pattern using UV light. The areas exposed to light harden, while the unexposed areas remain in liquid form. Subsequently, the unexposed solder mask is developed, leaving openings for components to be soldered to the copper pads.
  4. Dry Film Photoimageable Solder Mask (DFSM): DFSM is applied differently; it involves vacuum lamination. A dry film solder mask material is laminated onto the PCB surface. Once applied, the film is exposed to the pattern using UV light and then developed. This process results in a solder mask with openings aligned to the PCB’s design.
  5. Thermal Cure: Regardless of the method used, after the solder mask pattern is defined, the PCB goes through a thermal curing process. This step helps solidify the solder mask material, making it durable and resistant to high temperatures and environmental factors.
  6. Integration into PCB Design: In the realm of electronic design automation (EDA), the solder mask is treated as an integral layer of the PCB and is described using Gerber files, just like other layers such as copper and silkscreen layers. These files provide the necessary instructions for the PCB manufacturing equipment to apply the solder mask accurately and precisely according to the design specifications.

In summary, applying solder mask to a PCB involves selecting the appropriate material, applying it to the board’s surface through methods like printing, exposure to UV light for pattern creation, development to create openings, and finally, thermal curing. The solder mask layer is a critical component that contributes to the reliability and functionality of the PCB, ensuring proper soldering and protection of the underlying copper traces.

Conclusion

The seemingly unassuming green lacquer-like surface finish of solder mask on PCBs serves a multitude of vital functions that go beyond aesthetics. When approached with the right design, selection, and application, high-performance solder masks tailored for specific applications significantly enhance PCB assembly yields, reinforce solder connections, and extend the operational lifetimes of electronic devices.

Collaboration and communication with your PCB fabrication partners are essential to ensure that the solder mask is optimized based on their capabilities and the specific requirements of your product. By recognizing the importance of solder mask and harnessing its potential, electronics manufacturers can continue to meet the demands of the ever-evolving industry and deliver reliable, high-quality PCBs for a wide range of applications.

Recommended Posts

Take a Quick Quote

Discover how our expertise can help with PCBA project.