The modern electronics market is experiencing a trend towards globalization. Overseas production not only provides high-quality service but can also reduce the overall project cost through cheap labor. To strike a balance between speed and cost, sea transportation is a common choice. However, during long-distance transportation, PCB is often left unattended, which can lead to a range of additional risks. For instance, the journey may involve bumps, causing friction between the boards, or corrosion due to high levels of moisture in the air. Regardless, it’s essential to take protective measures to mitigate potential risks. Among these measures, conformal coating is an effective solution.
What is Conformal Coating
Conformal coating, also known as conformal paint, is a specialized protective chemical coating or polymer film used in the electronics industry. It forms a conformal, gel-like layer that covers the surface of a PCB to provide complete isolation from the external environment. This protective measure helps prevent threats such as corrosion and abrasion to solder joints, component pins, and traces. Typically, the coating thickness falls within the range of 25 to 75 micrometers.
Users can choose from various types of coatings based on their specific requirements, each with its own advantages and drawbacks. For instance, acrylic coatings dry quickly and do not shrink when used but have limitations in terms of physical resistance. In contrast, epoxy resins are more effective in resisting physical and chemical threats but are challenging to rework. On the other hand, silicone coatings offer excellent wetting and adhesion properties but have longer curing times and higher costs.
Although conformal coatings do not impact the performance of electronic components, they are still required to meet stringent international standards, such as IPC-CC-830C. This standard includes sections on different variants of conformal coatings and addresses various environmental concerns, including issues related to water, fungi, and flammability.
How to Apply Conformal Coating to a PCB
Improper procedures when applying conformal coating can result in various defects like cracks, fisheyes, bubbles, and more. These defects not only affect the appearance but can also lead to adhesion issues, potentially causing peeling due to adhesion problems. Ensuring the correctness of the conformal coating process is crucial. Here are some key points to consider:
- If you use different manufacturers for manufacturing and assembly, it’s advisable to apply the conformal coating after assembly.
- Before applying the coating, make sure to protect the components properly to avoid the coating from seeping into gaps or covering component pins.
- Thoroughly clean the PCB surface before starting, ensuring there are no contaminants, including dust, moisture, or fingerprints. Impurities not only affect the appearance but can also lead to coating delamination or even malfunctions.
Brush Application
This is the most straightforward method, where operators use soft brushes to evenly apply the coating on the PCB surface. However, it’s not recommended as it can be prone to human-induced irregularities, even with experienced operators.
Spray Application
Coating materials are stored in aerosol form, making it easy to use. You simply aim the nozzle at the board and press the valve to spray. To ensure effective spraying, maintain the nozzle at the appropriate distance from the board and keep it at a 45° angle. A common mistake when using aerosols is shaking the can. Unlike other solutions, conformal coating remains stable and does not separate or precipitate. Shaking the can is not only unnecessary but can also introduce air bubbles. It’s recommended to use these aerosols in a well-ventilated area while wearing a mask and maintaining a windward position.
Dip Coating
This involves fully immersing the PCB into the conformal coating solution. This process is typically performed using equipment and is highly effective for batch production. However, it’s crucial to control the speed at which the equipment lifts the board. Rapid lifting or agitation during the gel-like phase of the coating can result in fisheye formation.
Selective Coating
This is the most precise and efficient method but requires costly equipment. Selective coating allows for accurate and rapid application of conformal coating and is highly recommended for manufacturers.
How to Remove Conformal Coating
For various reasons, such as troubleshooting or upgrades, you may need to remove existing components from a PCB. However, the conformal coating on the PCB surface can be a hindrance at this point. Therefore, it’s essential to prioritize the removal of the coating. There are several methods to remove it, including physical methods, chemical methods, or laser methods, depending on factors such as the material and the area to be cleared.
One relatively straightforward method is physical removal, suitable for most types of coatings. Before you start, it’s recommended to heat the PCB to reduce the adhesion between the coating and the PCB. Then, using tools like toothpicks, needles, or other sharp implements, gently insert them into the coating and gradually lift it.
For conformal coatings sensitive to chemicals, it’s advisable to use a chemical solution. Before purchasing the solution, identify the type of coating material and buy the appropriate solvent; this is crucial for a successful removal process. Simply apply the solution to the coating surface and wait for the coating to soften. During this process, keep a close eye on the board and add a neutralizing agent at the appropriate time to prevent any solvent damage to the PCB.
After the coating has been removed, be sure to thoroughly inspect all aspects of the PCB to ensure there are no residues. Pay special attention to the coating on the bottom of the components, as any remaining coating in this area may lead to damage when removing the components, potentially affecting the entire PCB pad. It’s recommended to examine the board in a well-lit area and tilt it to view hidden areas.
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