Conformal coating is a precautionary measure applied to the surface of PCB to protect them from threats such as dust, chemicals, and moisture. However, due to process or other oversights, thin film defects such as bubbles, fisheyes, orange peel, and waves may occur. These are not just cosmetic flaws; severe cases may directly impact the electrical performance and reliability of the PCB. Therefore, if there are any defects in the conformal coating, it needs to be removed and reapplied.
So, how to remove conformal coating from a PCB?
In addressing this issue, TechSparks provides this tutorial, introducing four techniques for removing conformal coating: Heating, Mechanical, Chemical, and Micro-Abrasion. Choose one based on your specific needs.
Heating
As the first method introduced, what does this entail?
Most not recommended!
As the name suggests, the heating method involves using devices such as soldering irons to heat the surface of the PCB, melting the conformal coating film, and eventually removing it. Clearly, this approach poses a series of issues:
- Firstly, some conformal coating compounds can produce toxic gases at high temperatures, posing health risks to operators.
- Secondly, for assembled PCB, high temperatures may lead to component damage.
- Additionally, cured conformal coatings exhibit high-temperature resistance, requiring higher temperatures and longer heating times. If temperature control is not precise, it may result in PCB burning or delamination.
- Finally, even if the above issues do not arise, heating will inevitably cause discoloration and leave residues, which is detrimental to re-soldering.
Mechanical
The most straightforward method for removing conformal coating, may have some tutorials suggesting various tools, but since you’ve decided to use mechanical, why make it so complicated?
- Start by understanding the conformal coating compound being used, ensuring it has low adhesion for mechanical removal.
- It is recommended to begin removing from the film around the component solder joints, where the film is often thickest (≤20 mil).
- Then, use a small knife to cut a V-shaped incision on the surface of the film.
- Use a toothpick, tweezers, or other tools to gently lift the incision until it is completely removed.
- It is advisable to use a solvent in conjunction to reduce the adhesion of the film.
- Avoid using brute force!
Chemical
The most common method for removing conformal coating from the surface of a PCB is using chemical solvents. This approach is not only applicable to multiple boards simultaneously but also ensures thorough cleaning. However, it’s essential to be aware that no solvent is universally effective. The key to this method lies in the compatibility between the solvent used and the conformal coating compound. For certain special materials, this method may be ineffective.
When choosing a solvent, it’s best to seek recommendations directly from customer service. It is not advisable to use solvents that are too strong or may pose a threat to the PCB and components. In most cases, solvents do not corrode or dissolve the film; instead, they expand or soften it. Once the film has expanded or softened sufficiently, components can be removed first, and then a brush can be used to thoroughly remove the coating. For both sides of the board, the entire board needs to be immersed in the solvent.
The entire expansion and softening process may take several hours, varying depending on the materials used. To expedite this process, you can heat the chemical solvent and film. If possible, ultrasonic agitation is recommended.
Micro-Abrasion
Micro-abrasion is a physical method that involves using fine particles to scrub the surface of a PCB and remove conformal coating. Initially, fine particles are introduced into an internal airflow, and under compressed air pressure, they are directed to scrub the PCB surface, ultimately removing the conformal coating.
The challenge in this entire process lies in controlling the equipment because, at high speeds, fine particles can generate electrostatic charges, leading to potential damage to components. To prevent fine particles from entering the grinding area surrounding the flexible film is crucial. Currently, some common protective measures include ion wind knives, custom ESD handles, and antistatic dissipation substrates. There are also considerations for the fine particles used:
- Walnut Shells: A type of large-particle fine particle that can effectively remove conformal coating. Due to their softness, they are less likely to cause serious consequences even in case of operational errors. However, this method may produce residues and generate a very high ESD voltage.
- PE Microspheres: Similar to walnut shells but smaller, which may require more time to remove the conformal coating. The advantage lies in lower electrostatic discharge.
- Baking Soda: Softer than any other fine particles, reducing physical damage to the board. This is advantageous for sensitive PCBs. However, as it is an alkaline material, immediate cleaning is required after micro-abrasion to prevent corrosion.
Additionally, there are more options such as aluminum trioxide, nylon microspheres, PS microspheres, PP microspheres, each with its own advantages and disadvantages.
Compound Combined with Removal Technology
| Compound | Heating | Mechanical | Chemical | Micro-Abrasion |
|---|---|---|---|---|
| Polyurethane | Poisonous gas may be released | Too hard, difficult to remove | Methanol-based/alkaline activator solution, ethylene glycol ether-based/alkaline activator solution, etc., | Remove 0.004-inch thick film in less than 1min |
| Silicone | Operation requires 200°C and produces smoke | Good elasticity, fast and effective | Dichloromethane/acidic activator solution or hydrocarbon-based/acidic activator solution., | Suitable for films up to 0.020-inch thick |
| Acrylic Resin | Fast and effective | Although fast, may cause damage | Dichloromethane, chloroform, ketones, butanone, ethyl acetate, etc., | Simple and fast |
| Epoxy Resin | Poisonous gas may be released | Too hard, difficult to remove | Dichloromethane/acidic activator or DYNALOY products., | Remove 0.004-inch thick film in less than 1min |
| Poly(p-xylene) | Fast and effective | Although fast, may cause damage | Soak in tetrahydrofuran solvent for 2-4h, then wash with ethanol. | Simple and fast |
| UV Curing | Poisonous gas may be released | Depends on coating thickness | Different chemical solvents are used based on the type of pre-polymerized base resin | Simple and fast |
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