Introduction
With the rise of electronic devices across different settings, the delicate connections within these electronics have come under risk from potential hazards such as extreme temperatures, moisture, and dust. To protect these electronic assemblies, a process known as conformal coating is employed during the production of printed circuit boards (PCBs). This involves the application of a thin, transparent film that adheres to the shape of the circuit board, providing a protective layer for its components.
The conformal coating is composed of a thin layer of non-conductive polymer film. Its purpose is to conform precisely to the uneven landscape of the PCB, offering numerous advantages such as increased dielectric resistance, operational integrity, and reliability. The coating is lightweight and thin, typically 25 to 250 microns. Significantly, it does not add any additional weight or thickness to the PCB assembly.
The primary benefit of using conformal coating lies in extending the lifespan of the circuit board and reducing the failure rates of its components caused by environmental factors. Consequently, conformal coating application has significantly increased, particularly in miniaturized electronics and wearable devices. Industries like automotive and consumer electronics have embraced PCB coating materials to enhance reliability, achieve waterproofing, safeguard their brand reputation and intellectual property, and gain a competitive advantage. This article will equip you with all the essential information to identify the most suitable conformal coating for your specific application requirements.
Types of conformal coatings
PCB Conformal Coatings are the unsung heroes that shield PCBs from harmful elements. Different types of coatings are available, each offering unique benefits based on the operating conditions of the PCB.
Acrylic Coatings:
Acrylic coatings are economical and provide broad-level contamination protection. They have a wide operating temperature range (max 250o F) and excellent adhesion to different substrates. These coatings cure through solvent volatilization, although proper ventilation is necessary. While acrylic coatings offer long-term moisture protection and resistance to UV exposure, they have limited resistance to solvents and are affected by high temperatures. They are suitable for regular electronic devices, providing easy application, good surface elasticity, and superior moisture protection.
Polyurethane Coatings:
Polyurethane coatings are known for their exceptional resistance to moisture and chemicals, making them ideal for harsh environments. They are highly abrasion and solvent-resistant, but their removal can be challenging. These coatings maintain flexibility and cover a wide temperature range, even in low temperatures. Polyurethane coatings find applications in automotive, industrial, and telecommunication PCBs, including aerospace equipment.
Silicone Coatings:
Silicone coatings offer excellent protection across a wide temperature range (max 400o F). They are resistant to chemicals, moisture, and salt spray. Their flexibility makes them suitable for vibrational stresses but lacks abrasion resistance. Adhesion to PCB materials is good, but removing silicone coatings requires specialized solvents and methods. The thermal resistance of silicone coatings can hinder heat dissipation, which may affect the PCBs and their components’ performance.
Epoxy Coatings:
Epoxy-based conformal coatings provide robust protection against chipping, cracking, abrasion, and moisture ingress. They are well-suited for harsh industrial environments and offer excellent resistance to chemical contamination. However, applying epoxy coatings can be complex, requiring specialized equipment and immediate mixing due to rapid curing. Epoxy coatings create a hard coating that is difficult to remove and lacks flexibility. Despite these drawbacks, they offer high humidity and chemical resistance, making them suitable for potting compounds.
UV Cure Coatings:
UV cure conformal coatings are hybrid coatings that combine different chemistry types. They cleverly address shadowed areas during UV light curing, ensuring uniform protection. The advantage of UV cure coatings lies in their rapid curing times, taking only a few seconds under UV light. The secondary cure system will eventually cure areas shielded from direct UV exposure.
Nonetheless, it is worth noting that once these coatings are fully cured, their highly cross-linked structure makes them challenging to remove. Moreover, under certain circumstances, UV cure coatings may exhibit reduced flexibility during thermal changes, necessitating careful testing.
Selection of PCB conformal coating
Selecting the appropriate conformal coating material for circuit boards involves several important factors. By understanding different properties, you can make an informed decision that ensures optimal performance and longevity for your circuitry.
Thermal Properties:
The temperature limitation plays a significant role in determining suitable conformal materials. Coating material influences heat dissipation rates, paths of thermal energy flow, and expected thermal expansion properties. Some coatings are excellent insulators, retaining heat, while others are highly conductive, efficiently dissipating heat.
Environmental Properties:
Factors such as moisture, abrasive particles, chemicals (including solvents), and other substances can impact the board and its components. Accounting for these environmental factors is essential when selecting the appropriate conformal coating.
Physical Properties:
The coating used on the board affects its performance during operation and when subjected to vibrations. A rigid coating that doesn’t flex with the board can create mechanical stresses at connection points like component legs and wires. To address this, adjustments may be required in board design or enclosure.
Application Properties:
Different application techniques, such as dipping, spraying, brushing, or vapor deposition, are available. Choosing the optimal method relies on the dimensions of the board, the types of components and their placement, and the target production volume. Regarding smaller production quantities, it’s frequently advisable to use brush or spray applications as they are the most convenient alternatives available.
Methods of conformal coating
PCB coatings provide distinct qualities, advantages, and possible limitations, rendering them appropriate for diverse applications and varying environmental circumstances. Brushing, spraying, and dipping are the prevalent techniques employed in the conformal coating of PCBs.
Manual Brush Coating:
Best for Low-Volume Production, Rework, and Repair
The brush method offers a straightforward and budget-friendly approach, which proves advantageous for tasks involving limited production, rework, and repair. The conformal coating is manually done using a traditional tool. Due to the manual nature of the process, the coating tends to exhibit increased thickness and may attain a different visual finesse than alternative methods. Even so, individuals needing access to specialized equipment or working on a small number of boards can find this method suitable.
Manual Spray/Aerosol Coating:
Achieving Surface Finish with Cost-Effectiveness
In this method, a skilled technician sprays the conformal coating manually using an aerosol. Before spraying, avoiding shaking the aerosol is important to prevent bubble formation.
The PCB is positioned at a 45° angle, maintaining a safe distance of 20-30 cm from the aerosol. Conformal coating is sprayed in a zig-zag manner from top to bottom, then rotated the board by 90 degrees and repeated to a full 360-degree rotation. This process ensures uniformity of the conformal coating and that every point on the PCB is uniformly coated.
Manual Atomized Spray Coating:
Quick and Efficient
Like aerosol coating, the atomized spray coating method is quick and efficient. However, proper ventilation is crucial, and ensuring the correct viscosity and air-to-material ratio is essential for optimal results.
Automated Dip Coating:
Slow and Uniform Application
Automated dip coating involves using robotic hands to apply the conformal coating slowly, ensuring proper and uniform coverage. If certain parts of the PCB should not be coated, they can be masked before initiating the dip coating process. However, the masking and unmasking process can be time-consuming and result in wasted masking material. Thus, this method is not recommended for cost-effective solutions.
Automated Selective Coating:
Accuracy with Robotic Precision
In this method, robotic hands equipped with pre-programmed instructions apply the conformal coating to specific areas of the PCB while avoiding other areas. This method ensures accuracy, consistency, and repeatability, as robots can perform tasks without getting tired. The automated selective coating is beneficial for achieving precision in coating applications.
Factors to consider when selecting conformal coating application methods
Choosing a suitable coating and deciding how to apply it are crucial steps in the conformal coating process. Following factors that influence coating method:
Production throughput:
Choosing the application method depends on the amount of work before applying, how fast the coating is done, and how quickly the boards can be handled afterward.
Board design:
If boards have many connectors or contain components sensitive to solvents, these factors should be considered when determining the best way to apply the coating.
Equipment:
Consider the frequency of coating needs before deciding regarding equipment.
Pre-coating:
Some processes require masking or taping certain areas to prevent unwanted surfaces from being coated. This preparatory step should be considered when choosing an application method.
Quality:
Automated application methods suit critical electronics and require high repeatability and reliability. This ensures consistent and dependable results.
By considering these variables, you can make an informed decision on the type of coating and the most suitable application method for your specific needs.
Thickness of conformal coating
Conformal coatings are a vital component in safeguarding electronic boards from various hazards. These coatings are incredibly thin, striking the perfect balance between maximum protection and minimal material usage. Being so thin prevents issues such as heat entrapment and unnecessary weight gain. Typically, conformal coatings range from 1 to 5 miles (25 to 127 microns) in thickness, with some coatings even thinner.
Coating Thickness:
Choosing the suitable conformal coating material is essential, and determining the appropriate thickness is crucial, as coating thickness depends on material properties and the PCB’s environment. The thermal expansion coefficient is another factor to consider, as different coating types have varying coefficients. Mismatched coefficients can cause mechanical stress and premature component failure during temperature cycling. Typically, recommended coating thickness ranges from 25 to 250 micrometers.
Measuring Conformal Coating Thickness
To measure the thickness of the conformal coating, there are the following ways:
Wet film Gauge:
This tool consists of notches and teeth with known lengths. One can measure its thickness by placing the gauge directly onto the wet film. The measurement is then multiplied by the percentage of solids in the coating to estimate the dry coating thickness. This technique is efficient and straightforward.
Micrometer:
The measurements are noted both before and after the coating process. By calculating the disparity between the thickness of the cured and uncured coating and dividing it by two, we obtain consistency on a single side of the board. Assessing the standard deviation of these measurements enables us to evaluate the evenness of the coating. It is noteworthy to mention that this technique is particularly well-suited for sturdier coatings that exhibit minimal deformation when subjected to pressure.
Eddy Current Probe Method:
The instruments generate a fluctuating electromagnetic field to assess coating thickness directly. These devices provide exact and non-invasive measurements. However, it is crucial to ensure the presence of a metal backplane or substrate beneath the coating for accurate readings. In cases where the test area needs a metal foundation or the probe cannot be placed evenly on the surface, the measurements may exhibit inaccuracies.
Ultrasonic Thickness Gauge:
This gauge employs ultrasonic waves to measure the coating thickness. Its functioning involves timing the travel duration of sound waves from the transducer through the coating and back to the transducer. A conductive medium like propylene glycol or water is employed to ensure optimal surface contact. Generally, this approach is regarded as non-invasive, except when there are potential concerns regarding the impact of the conductive medium on the coating.
Curing methods
The choice of a curing mechanism directly impacts the application method and throughput of a coating. Different mechanisms offer varying levels of reliability and complexity. Here are the main methods:
Heat/Thermal Cure:
Coating is dried at elevated temperatures, accelerating drying time and increasing production throughput. Care must be taken to avoid damage to sensitive circuit boards and components.
Condensation Cure:
Coating is dried using ambient temperature and atmospheric moisture, resulting in slower curing. Control of ambient humidity is important for consistent results.
Ultra-Violet (UV) Cure:
Coating is exposed to UV radiation, offering rapid throughput. UV-cured coatings are 100% solid systems but are more challenging to repair. Worker safety requires UV curing equipment and protection.
Oxidation Cure:
Coating is exposed to oxygen-rich air, aiding drying and curing, especially for solvent-based coatings. Effective with specific coating formulations benefiting from oxidation.
Catalytic Cure:
Coating is combined with a catalyst material, resulting in an unstoppable curing process. Useful when a single mechanism is insufficient or multiple tools are needed.
Considerations for Curing Methods
While selecting a curing method, it’s crucial to consider the coating’s intended application and the impact on the manufacturing process. Different curing mechanisms have varying levels of complexity and potential for application errors. Here are a few key considerations:
Evaporative Curing Method:
The liquid carrier undergoes evaporation, resulting in the residue of the coating resin. Solvent-based systems offer fast cure times but require proper ventilation. Water-based carriers eliminate flammability concerns but may need longer curing times and are sensitive to humidity.
Moisture Curing:
Relies on ambient moisture to trigger polymerization, often combined with evaporative curing for additional properties.
Heat Curing:
It can be used as a secondary method with moisture, UV, or evaporative curing, but caution is needed to prevent damage to circuit boards and components.
PCB conformal coating process
PCB Conformal Coating must be done carefully as each step is very important for a good quality coating.
PCB Cleaning:
Cleaning PCBs is crucial for optimal performance and to eliminate issues caused by contamination. Problems can arise from flux, poor handling, chemicals, and other contaminants.
Masking:
Masking is essential in conformal coating, reducing process time. The mask reduces process time and improves workflow efficiency.
Coating:
Conformal coating protects PCBs from environmental factors, with various application methods available.
Cure:
Curing solidifies the protective shield using different methods based on coating material and desired characteristics.
De-mask/Rework:
De-masking and rework address production problems, ensuring quality.
Testing:
Testing assesses performance and quality, including surface insulation resistance and visual inspection for defects.
Applications of PCB conformal coating
Though conformal coating may be costly, it has become an essential part of the finishing stage for PCBs after manufacturing. Almost every industry that relies on Printed Circuit Boards (PCBs) in their electronic equipment, machines, instruments, and devices uses conformal-coated PCBs.
Consumer Electronics:
Consumer electronic products are highly susceptible to contamination and dust particles. Protects against contamination and mishandling
Automotive Industry:
PCBs used in car electronics, audio circuits, AM/FM circuits, front deck, and LED light circuits protected from vibrations and shocks.
Aerospace:
Protects against radiation, shocks, and high temperatures
Navigation:
Recommended for electronic devices installed on ships and boards in humid environments.
Medical Electronics Instruments:
Ensures longevity and protects against exposure to harsh chemicals in the laboratory.
Removal of conformal coating
Conformal coating removal is necessary for repairing or replacing damaged circuit board parts. The removal process depends on factors like coating material and treated area size. Standard conformal coating removal methods:
Solvent removal:
Most conformal coatings can be removed using solvents. Acrylic coatings are susceptible to solvents and can be easily removed.
Peeling:
Coatings can be peeled off the circuit board. Works well for silicone-based and flexible coatings.
Thermal/Burn-through:
Effective for most coatings, done with a soldering iron during rework. It’s a quick and efficient way to remove the coating.
Micro-blasting:
Uses soft abrasive materials and compressed air to abrade and remove the conformal coating. Suitable for Parylene and epoxy coatings.
Grinding/Scraping:
Abrasion method for more complex coatings, but can cause damage as a last option.
PCB conformal coating defects
Conformal coating is used to protect electronic components from environmental factors. Common conformal defects include de-wetting, delamination, air bubbles, bubbles and voids, fisheyes, orange peel, and cracking/crazing.
- De-wetting occurs when the coating separates from the substrate due to incompatible contaminants.
- Delamination is caused by contaminated surfaces or insufficient tack time between coats.
- Air bubbles can be introduced during mixing, spraying, or brushing.
- More bubbles and voids can occur due to trapped solvent vaporizing through thick or fast-curing coatings.
- Fisheyes appear as circular areas with a “crater” due to oil or water in the sprayer’s air system.
- “Orange peel” texture is caused by low air pressure, incorrect thinner, or excessive wet coats.
- Cracking and crazing result from excessive film thickness, insufficient time between coats, or high temperatures during curing.
Conformal coating testing
To ensure that the desired characteristics of the coating are met, it is necessary to subject the coating to various environments under suitable testing environments to check its performance range and limits.
Environmental Testing:
Testing the coating in different environments is crucial to reach the required protection level. The testing process aims to replicate or accelerate end-use conditions. To ensure accurate assessments, it is essential to thoughtfully choose accelerated testing methods that align with real-world usage scenarios.
Humidity exposure and salt mist, thermal cycling, aging and shock, and aging tests are conducted individually or in combination, depending on the specific requirements. These tests simulate the effects of corrosion, temperature fluctuations, mechanical shocks, and aging on the coating.
Corrosive Gas Test:
It involves subjecting PCBs to a mixture of gas that includes H2S and SO2, as per the.
Electrical Properties:
Evaluation of electrical properties is vital for all conformal coating applications. Several standard tests are conducted to assess dielectric strength, dielectric constant, dissipation factor, surface insulation resistance, and comparative tracking index (CTI).
Standards used for conformal coating
- MIL-I-46058C a commonly used standard for materials of conformal coating followed in various industries.
- Def Stan 59/47 is a similar standard adopted by the British military to coat high-end electronic devices.
- IEC 61086 relies on supplier self-certification to ensure the quality of coating materials.
- IPC-CC-830B it is an alternative standard to MIL-I-46058C and widely used for qualifying conformal coating materials.
- IPC-CC-830B it is utilized explicitly for material qualifications.
- UL94V0 This standard is employed to evaluate conformal coatings’ self-extinguishing or flammability properties.
- UL746E this standard is responsible for assessing dielectric breakdown and conducting UL94 flammability tests.
Conclusion
In conclusion, conformal coatings protect PCBs from environmental hazards, extend their lifespan, and reduce component failure rates. The different types of coatings, such as acrylic, polyurethane, silicone, epoxy, and UV cure, offer unique advantages and limitations depending on the operating conditions. Understanding the conformal coating process, applications, removal techniques, and common defects is vital for successful implementation.
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