Skip to content
LEARN PCB
PCB Design and Layout
PCB Manufacturing
PCB Assembly
PCB Basics
SEPCIFICATIONS
PCB Fabrication
STANDARDS & POLICY
PCB Ordering
Policy

Table of Contents

About The Author

Solder Mask: Critical Role in PCB Manufacturing

Modern electronics are particularly reliant on Printed Circuit Boards (PCBs) since they enable complex networks to function properly in different gadgets from cellular phones to industrial equipment. The solder mask is however less known and yet an important component of PCBs. Solder masks protect copper traces from degradation, avoid short circuits, and make PCBs last longer for their desired purpose without failure of the components. Knowledge of the different classifications and characteristics as well as their application is useful to anyone who is into making or designing PCBs.

what is a solder mask

What is a Solder Mask?

A solder mask (or solder resist) is a protective layer that covers the solderable areas of a PCB that are not intended for soldering. This coating is typically placed on both the top and bottom surfaces of PCB, preventing not just the copper traces but also the inflammation hazards like shorting, catching fire, and others due to the circuitry being exposed. This mask, which is usually green but comes in a variety of colors, helps to prevent electronic components from deterioration by moisture and air, in addition to prolonging the lifespan within electronics.

Why Is Solder Mask So Important in PCB Design?

Solder masks help ensure the electrical and physical properties of modern PCBs. With the miniaturization of electronic parts and Gerber files featuring different layers/stacks of traces/vias adding up to a dense Design, there is little room for errors during the manufacturing stage. In the absence of this mask, chances of shorting and damaging sensitive traces extremely rise, more so in complicated devices such as mobile phones, medical devices, or automotive control units.

Solder masks also help in dissipating heat from PCB to some extent because the soldered copper pads/patterns are better able to endure heat during soldering or from active devices. When instruments are subjected to high-stress operations, this additional covering ensures stable parameters hooks on the use of voltage and current.

Key Characteristics of Solder Mask

Attributes associated with solder masks are:

Material Composition Color Application Thickness
Usually composed of either epoxy or polymer materials they resist chemicals and can endure high temperatures. The most frequently used chemical is epoxy liquid. However, some advanced boards utilize LPI solder masks instead to improve accuracy and reduce labor dependency and time.
Green is the most popular color among the types of colors that can be used in solder masks, however, it can be painted in many such as red, blue, yellow, black, and white. Green is predominately used because it is easily visible even after magnification as opposed to other colors, and it is historically related to the manufacturing of printed circuit boards. However, other colors may also be used for aesthetic or active reasons when producing multilayer printed circuit boards.
Generally, this mask layer has a thickness of between 10 to 50 microns to ensure that the electrical connections or contact pads are not blocked with too much overlap.

Primary Functions of Solder Mask

Solder masks can be applied to PCBs for anyone these reasons:

  • Prevent Solder Bridges

Solder mask prevents the formation of solder bridges which is undesirable since it is the setting of a solder, the bridging of two close conductive paths when the solder flows between them. These lead to short-circuits which can render a PCB useless or cause partial functional defects in it. The function of the solder mask within this context is to prevent solder from wicking into the undesired areas of a PCB, and during assembly, it ensures that solder flows only to the target pads.

  • Protect Copper Traces

PCB solder resist encapsulates copper traces and hence serves its purpose by preventing relative exposure to the outside environment like moisture, dust, and oxidation. During the period of soldering, if there is no solder masking on copper trace, it will certainly suffer from corrosion which will in return make the board poorly function over a period and tend to have reliability problems in the future as its performance deteriorates with time.

  • Mechanical Protection

It acts as a mechanical protector of PCB substrates from physical impacts, cuts, scratches, and/or any damage occurring when the PCB is being manufactured or assembled. It also offers tiny insulation between the parts, so the interference of one component over another due to electromagnetic forces is decreased, thus enhancing the lifetime of the board.

  • Improve soldering process

In the mass production of PCBs, the application of a solder mask is paramount in all the automatic soldering process which includes wave soldering and reflow soldering. The mask layer ensures that solder flows to only the intended areas that have been exposed allowing no falls-on other parts or traces.

  • Improve the look of PCB units:

In addition to functional purposes, solder masks enhance the aesthetic quality of PCBs since they make them look smart and neat when finished.

Types of Solder Masks

Liquid Photo Imageable

Liquid Photo Imageable (LPI) Solder Mask

LPI is in the form of liquid and is usually made to turn solid when subjected to ultraviolet (UV) rays. This method is the best when performing high-density PCB designs which greatly require accuracy. This type of mask has high precision and is suitable for multilayer high-power PCBs, offers good moisture and chemical resistance, and handles small-pitch devices.

LPI masks are primarily used in telecommunications, aerospace, and automotive industries where circuit compactness and susceptibility to failure factors are paramount.

Dry Film Solder Mask

Dry Film Solder Mask

This dry film is a cured mask that is bonded on the PCB surface, it is developed by the application of heat and pressure. This dry film technique is most popular because of its high bonding to the substrate.

This mask is most appropriate for the multilayer types of PCBs. Dry film masks are extensively utilized in most high-end PCBs, which include most industrial and medical electronic applications.

Epoxy Liquid Solder Mask

This is a less sophisticated option, where the mask is applied allowing screen printing and is then cured through heat. Although not as accurate as LPI or dry film, it is cheaper and suitable for backgrounds with a simple layout. The drawback of such masks is their manufacturing cost, so they are suitable for low density. Complex boards and Consumer electronics for which cost is the major aspect.

This mask is typically used in low-cost consumables like cheap consumer electronics, basic circuit boards inside appliances and toys, and other low-end use PCBs.

Application Process of Solder Mask

The process of applying solder resist to a printed circuit board (PCB) is essential to protect the board, its performance, and reliability. This process is critical as the quality of the final product is determined by such as each stage is managed. Here’s a step-by-step guide to the typical application process:

1. Cleaning the PCB Surface

The first part of the process is to ensure that the PCB is free of contaminates that would interfere with adhesion. This step enables the surface to be clean from any interferences such as dust oils oxidation and even fingerprints. The board typically undergoes a process of cleaning with chemical solvents or abrasive brushing after which de-ionized water is rinsed to help wash off any cleaning chemicals left over. After cleaning, hot air is blown directly on the board to dry it.

2. Solder Mask Application

As per the solder mask type (LPI, dry film, or epoxy), the application methods also differ such that:

  • The LPI solder mask is applied as a liquid ink using one of two methods including silk screening and spray coating.
  • A dry film solder mask is supplied in procured sheets, that are laminated onto the PCB using heat and pressure.
  • Epoxy liquid solder resist is applied via screen printing in a similar fashion to LPI but with less precision.

3. Exposure and Development (for LPI and Dry Film Masks)

In the case of LPI and dry film types, after the initial application, it is common for the board to go through UV exposure to set the parts onto which the solder mask was applied.

4. Curing (Final Hardening)

After exposure and development, the still-wet solder mask must be put through a curing cycle appropriately to enhance its properties. The assembled PCB is inserted inside a special cabinet or UV curing unit and is heated up to a selected temperature, which ensures that the solder mask curing process reaches its completion. Proper curing ensures that the solder mask will adhere well to the board and stand up to shielding and harsh physical conditions, moisture, and chemicals. Unclear curing results in a poorly adhered mask that will easily chip or detach itself in time.

5. Final Inspection and Quality Control

After all the steps have been carried out and all the materials have been cured on the PCB further inspection will take place to see to it that was well applied.

Benefits of Using Solder Masks

Solder masks are not just optional add-ons, solder masks are essential for the use, safety, and quality of a PCB. Below are the advantages of applying solder masks while manufacturing PCBs:

  1. Prevention of Solder Bridges
  2. Protection Against Environmental Damage
  3. Improved Aesthetic and Professional Appearance
  4. Increased PCB Durability and Lifespan
  5. Better Thermal Management
  6. Enhanced Electrical Performance
  7. Long-term Cost-Efficiency in Manufacturing

Choosing the Right Solder Mask

Proper selection of the solder mask is paramount in any PCB design and ultimately, it influences the general performance and durability of the electronic product. The selection is determined by other several factors such as the level of design complexity, operating environment, as well as cost constraints.

On dense and compacted surface-mount devices, the solder mask must be very precise to prevent the solder from spreading and causing bridges between adjacent components. In traditional through-hole designs, where components are inserted into pre-drilled holes, the requirements for the solder mask are less strict, but it still needs to protect the board effectively.

In standard through-hole designs where components are placed inside holes that have been drilled previously, the barriers imposed on the solder mask are not so strict, but it still has to serve its purpose and cover the board. If the boards are going to be used in certain extreme conditions like high temperature and chemicals, or physical stress such as automotive or industrial boards then the end use of the solder marine must be of such quality that endure environmental stresses. If cost is important, as in the case of mass production of low-end consumer goods, it could be more rational to go for a cheaper though not-so-good solder mask.

Conclusion

Solder mask plays a vital role in the manufacture of PCBs, ensuring electrical safety, protecting copper traces, and improving the overall durability of the board. It also plays an important role in preventing short circuits, avoiding environmental damage, and improving the aesthetics of the PCB.

The choice of solder mask type depends on factors such as design complexity, operating environment, and cost constraints. By understanding the benefits and considerations of selecting and applying them, PCB manufacturers can optimize their production processes faster and provide you with high-quality, reliable products.

Inspection-of-PCB-assembly
Manufacturing Equipment at FS circuits
lcon
Turn-times as fast as one day.

FS Circuits can manufacture your PCB and have it expedited to you within 24 hours.
Full turnkey boards, with assembly and components in as fast as 5 days.