Comprehensive Guidelines for PCB Solder Mask in 2024

In the realm of electronic manufacturing, the production of printed circuit boards (PCBs) involves intricate processes and meticulous inspections. PCB is composed of essential components such as solder mask, silk screen layer, wiring, and core, with each manufacturing step requiring precision and thorough scrutiny. 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, FS Technology sheds light on the critical role of PCB solder masks.

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.

 

As the PCB substrate comprises glass fiber and epoxy resin, it lacks the sufficient heat resistance required for processes such as hot air solder leveling (HASL) and surface mount soldering. Without a solder mask layer, prolonged exposure to high temperatures can lead to surface degradation of the dielectric material. Therefore, the solder mask layer acts as a protective barrier, safeguarding the circuit from potential damage caused by long-term exposure to elevated temperatures during these manufacturing processes.

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). Refer to the diagram below:

Internal structure of double-layer PCB

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 following diagram provides a visual representation:

Details of PCB soldering layer

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. By carefully examining the structural diagram of the double-layer PCB above, you will notice the absence of a dedicated soldering layer within the circuit structure. 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.

The following image demonstrates the soldering layer (paste mask):

PCB flux 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.

Influence of Solder Mask on PCBA

SMT Soldering Difficulty

SMT soldering poses various difficulties, particularly when dealing with SMD components featuring small pin spacing. Inadequate minimum solder mask spacing design can significantly impact the assembly process, leading to increased factory damage rates and higher assembly costs for customers.

Solution: To mitigate these challenges, it is crucial to optimize the component packaging process based on the actual capabilities and PCBA processes of the manufacturing facility. This involves conducting a thorough review of the customer’s design documents and strengthening the internal design department within the company.

Solder Mask Thickness Control

Precise control over the thickness of the solder mask is a critical aspect of PCB manufacturing. Insufficient solder mask thickness can result in copper leakage in line traces and IC false soldering issues. Conversely, an excessively thick solder mask layer may lead to suspension bridges and open circuits during reflow soldering.

Solution: To address this, it is essential to carefully manage the solder mask thickness. It should be thinner than the copper foil pad’s thickness, with a solder mask thickness of at least 10 µm at sharp corner positions on the circuit board. Additionally, the solder mask thickness on traces and copper foil should be kept below 35 µm.

Solder Mask Processing and Pad Alignment

During solder paste printing in non-standard SMT assembly facilities, contamination on the pad surface may occur, resulting in poor solder joints or solder balls. These issues stem from mismatched solder mask and pad alignment.

Solution: When designing the solder mask, strict adherence to PCB design guidelines is vital. Designer should aim to minimize spacing or air gaps around pad features to ensure optimal solder paste printing and prevent contamination.

Solder Shrinkage

In situations where an excessive number of SMDs are present on the PCB, along with imprudent board design, adjacent SMDs may share a common wire. This can cause stress during solder shrinkage due to heat, leading to displacement or breakage of the components.

Solution: Effective communication with the PCB assembly service provider is essential to address component and board design concerns. Through collaboration, issues can be resolved by implementing reasonable component installation practices and ensuring appropriate design considerations are taken into account.

4 Types of PCB Solder Mask

Top and Bottom Side Masks

Top-side and bottom-side solder masks are commonly employed by electronic engineers to identify issues in the green solder mask layer. These PCB layers are applied using film or epoxy methods. The component pins are then soldered onto the board through the openings registered by the masks.

The top-side mask corresponds to the conductive trace pattern on the board’s top side, while the bottom-side mask is used for the opposing side of the board.

Epoxy Liquid Solder Masks

Epoxy liquid solder masks offer a cost-effective option for PCB project. This technique, known as silk-screening, utilizes a woven mesh to support ink-blocking designs. The mesh enables the identification of areas where ink can be transferred.

Liquid Photoimageable Solder Masks (LPI/LPISM)

Liquid Photoimaging Soldermask (LPI or LPISM) is a liquid soldermask material that is applied to the surface of the circuit board and undergoes a curing process. This advanced soldermask material enables precise application of solder and ensures accurate electrical connections by selectively leaving openings only in areas where soldering is required.

Dry Film Solder Mask-DFSM

Vacuum lamination is employed in the application of dry film solder masks on printed circuit boards. Following the lamination process, the dry film is exposed and undergoes subsequent processing. Openings are strategically located to create a pattern once the film has been processed. Afterward, the components are soldered to the copper pads. Electrochemical processing is utilized to deposit copper layers onto the board.

Copper is layered both on the trace areas and inside the holes, forming the desired circuitry. To protect the copper circuits, a layer of tin is applied. In the final phase, the film is removed, revealing the etched marks. Thermal curing is also employed to ensure proper adhesion and stability.

Dry film solder masks are commonly employed in high-density wire boards due to their advantageous properties. They prevent the solder mask from seeping into the through holes, ensuring precise circuit formation.

SolderMask Process Guidelines

Design Guidelines

In practice, the use of soldermasks in designs is a discretionary choice. Designers can easily incorporate a solder mask by specifying a few parameters, and certain 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 soldermask 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.

Tented vias created with soldermask
Tented vias created with soldermask

SolderMask Dam

SolderMask 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 dam
Solder Mask DAM

SolderMask Opening

SolderMask 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 opening
Solder mask opening

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.
Solder Mask Coverage

Application Notes

When applying a solder mask to a printed circuit board, it is crucial to ensure proper isolation between groups of contact pads, such as those located under microcircuit leads, and other conductive elements like vias, contact pads, and conductors.

This isolation serves to minimize the time and effort required during the soldering process. Without proper isolation, tiny solder bridges may form between adjacent contact pads, leading to complications and additional time needed for detection and elimination. Failure to detect such solder bridges can result in short circuits, causing operational issues or even component failure.

In addition to facilitating proper soldering, the solder mask serves as a protective layer for the surface of the PCB. It shields the board from the potentially corrosive effects of aggressive chemical coatings applied during soldering processes, including chemical and chemical-technological methods.

It is important to note that while the solder mask provides some protection, it does not safeguard the PCB from moisture in harsh operating conditions due to its hygroscopic nature. For moisture protection in such cases, specialized organic coatings, often referred to as conformal coatings in technical literature, are used.

Dry or Liquid

Choosing between dry and liquid solder mask depends primarily on the requirement for hole tenting, which refers to the overlapping or covering of the holes on a PCB.

The objectives of hole tenting are as follows:

  • Insulation during assembly to prevent contact between the conductive pattern and components with conductive (metal) surfaces.
  • Protection of the copper column of plated vias from aggressive environmental factors, such as pickling solutions, washing liquids, fluxes, etc., particularly if the vias are not protected by finishing coatings.
 

Liquid Solder Mask Advantages:

  • Cost-effectiveness of the manufacturing process due to the low specific material consumption, enabling the formation of thin protective layers of approximately 30 µm (even on relatively high relief areas up to 70 µm).
  • Comparatively higher adhesion compared to dry solder masks, as the liquid material is applied in a liquid state.
  • Possibility of creating narrow jumpers with a width of up to 0.15 mm or even less.
  • Resistance to hot and concentrated alkaline solutions after final polymerization, primarily to solutions used in the immersion gold plating process at all stages.

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