Classification and characteristics of PCB coating

Classification and characteristics of PCB coating

The application fields of PCBs are vast and diverse, each facing unique external environments. For instance, PCBs used in industries like automotive, military, aerospace, and maritime must operate in high-temperature and high-humidity conditions, where product damage due to PCB failure is a concern. Additionally, PCB boards and components may suffer from defects such as erosion, softening, deformation, and molding. Therefore, conformal coatings should be sprayed onto the surface of circuit boards to enhance their resistance to environmental corrosion and prolong their lifespan. The extensive use of electronic products has led to a large-scale classification of PCB coatings, with an increasing demand from various industries. Consequently, the primary concern for engineers is selecting a conformal coating with stable protective functionality. This article aims to analyze the classification and performance of PCB coatings and provide a practical guide based on the best conformal coatings.

Classification and Performance of PCB Coatings

• Definition and Functionality

The fundamental goal of applying PCB coatings is to prevent corrosion of circuit boards or PCB components. As a special coating sprayed onto the surface of circuit boards, PCB coatings primarily serve to inhibit molding, humidity, salt spray, vibrations, insulation, and reduce the dimensions of circuit board designs.

Applied alongside other devices on printed circuit boards and PCB components’ surfaces, PCB coatings isolate circuits from harsh environments, ensuring that assembled PCBs are unaffected by adverse conditions. Ultimately, this protects circuits from environmental corrosion, preventing circuit failures or leakage and extending the lifespan of electronic products. In practical scenarios, PCB boards are often subjected to corrosion from harsh environments and operational conditions, such as chemical environments, vibrations, high dust levels, salt spray, humidity, and high temperatures. These factors can lead to numerous defects in circuits, including open-circuit failures, corrosion failures, electronic migration failures, insulation failures, and component failures, to name a few. Spraying PCB coatings effectively isolates PCB boards and components from operating environments, enhancing safety and ensuring longevity. Furthermore, PCB coatings are inherently insulating, preventing leakage. Thus, PCB coatings enable electronic products to achieve higher power and smaller PCB clearances, meeting the increasing demands for component miniaturization.

Classification and characteristics of PCB coating

Classification and Properties

Based on the different chemical elements contributing to PCB coating materials, it is mainly classified into five categories: acrylic resin, epoxy resin, silicone, polyurethane (PU), and parylene coatings. Depending on the solvent type or curing method, it is mainly divided into three categories: solvent-based coatings, water-based coatings, and UV-curable coatings.

a. PCB coatings composed of acrylic resin solvent

Advantages:

  1. Relatively simple curing technique. Additionally, it does not react with air, and its curing process depends on the evaporation of the solvent;
  2. Excellent moisture resistance and flexible viscosity adjustment;
  3. Easy rework;
  4. Relatively low cost.

Disadvantages: However, the protective coating after curing is too thin, and its curing method determines the high content of volatile solvents, requiring a well-ventilated operating environment. Furthermore, it performs poorly in maintaining viscosity and returns to viscosity under high temperature and high humidity.

b. PCB coatings composed of epoxy resin

Advantages:

  1. Resistance to humidity, salt spray, and chemicals;
  2. High-temperature resistance up to 150°C;
  3. Abrasion resistance;
  4. Dielectric properties.

Disadvantages: PCB boards with epoxy resin coatings cannot be reworked unless the epoxy resin film is physically stripped off, which may damage PCB components. Additionally, potential high internal stress may lead to brittle equipment damage, and protective measures should be taken before applying PCB coatings. This type of PCB coating has relatively high peel content and complex two-component ratios, with poor viscosity retention. It also exhibits poor low-temperature and high-curing shrinkage performance.

c. PCB coatings composed of silicone

Advantages:

  1. High flexibility;
  2. Wide operating temperature range up to 200°C, with excellent humidity and UV resistance;
  3. Repairable;
  4. Significant insulation and low surface energy, exhibiting good wettability with substrate materials.

Disadvantages: Low mechanical strength and weak adhesion to substrate materials. Worse still, it cannot withstand scratching.

In conclusion, this type of PCB coating is most suitable for high-heat components, such as high-frequency PCB applications in aerospace, maritime, military, and radar control systems, containing many high-power resistors.

d. PCB coatings composed of polyurethane

Advantages:

  1. Excellent acid and alkali resistance;
  2. High moisture resistance.

Disadvantages: After a long time of complete curing, it tends to yellow at high temperatures due to its high VOC content. Additionally, it often corrodes galvanized screws.

e. Parylene coatings

Parylene coatings polymerize in a vacuum, depositing a uniform coating on the surface of the product. It serves as a special protective coating in the electronics industry.

Advantages:

  1. Uniform thickness;
  2. Excellent acid resistance and corrosion resistance.

Disadvantages: High cost is the primary drawback of parylene coatings.

Parylene coatings are widely used in aerospace, silicon rubber, magnetic materials, microelectronics, sensors, and other fields. So far, parylene coatings are the most effective protective coatings for high-frequency components, high-density devices, and high-insulation devices.

f. UV-curable coatings

Advantages:

  1. Environmentally friendly;
  2. High safety;
  3. High stability;
  4. Excellent humidity and corrosion resistance;
  5. High curing efficiency;
  6. Reworkable.

Disadvantages: If insufficient UV light exposure is applied, blind spots may occur.

Classification and characteristics of PCB coating

How to choose PCB coating correctly?

• Selection Principles

Due to the varying physicochemical properties and processing performances, the optimal PCB coating should be chosen based on comprehensive considerations, including specific operating environments, electrical performance requirements, and PCB layouts.

The ideal PCB coating should meet the following requirements: a. Ideally, it should be a single-component system, easy to handle; b. It should have low viscosity and be easy to spray; c. It should have a relatively long pot life; d. It should cure rapidly without producing by-products; e. It should exhibit excellent flexibility; f. It should have a wide operating temperature range; g. It should be able to protect the PCB from extreme temperatures and mechanical impacts; h. It should have low toxicity and be cost-effective; i. It should be reworkable.

• Verification Methods

The selection of PCB coatings should adhere to certification requirements, such as RoHS, UL, etc., and prioritize PCB coatings with low viscosity, environmental advantages, and non-toxic properties. Once a PCB coating type is selected, its effectiveness should be verified according to the technical specifications provided by the PCB manufacturer.

• Technical Validation

The aim of technical validation is to confirm whether the selected type of PCB coating is suitable for current technologies and products.

Firstly, manual or automatic spraying of PCB coatings onto products can be applied. Once cured, the coating should be observed for smoothness, uniformity, transparency or translucency, and viscosity. Additionally, the presence of bubbles, pinholes, cracks, etc., should be checked.

Secondly, the reworkability of the PCB coating should be considered, and it should be easy to remove from the PCB surface. Physical methods such as heat, mechanical methods, or chemical methods can be employed to verify this aspect. Typically, coating manufacturers provide dispersants, a chemical method used to verify the peelability of PCB coatings without damaging the equipment.

Thirdly, the protective performance should be validated. Since the primary goal of PCB coatings is to provide protection for electronic products, their protective performance should be verified. Dielectric strength tests, insulation resistance tests in humid environments, thermal shock tests, and hydrolysis stability tests should be conducted to validate the performance of PCB coatings.

Spraying PCB coatings onto the surface of PCB boards and components effectively isolates the circuit board, protecting the circuits from corrosion and damage in harsh environments, thereby extending the lifespan of electronic products and ensuring safety and reliability. During the PCB coating selection phase, besides verifying the coating performance, it should also be demonstrated that it is suitable for the customer’s products, technologies, and equipment.

 

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