How to Remove Protective Coatings from PCBs

How to Remove Protective Coatings from PCBs

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Before production and processing, a protective conformal coating is typically applied to the surface of a PCB to safeguard it from environmental damage. This coating helps prevent water, dust, salt, and dirt from coming into contact with sensitive components, thus preserving the performance of the mainboard.

Removing conformal coatings can be challenging due to their durability and resistance to wear. If rework is needed, alcohol is not the ideal choice for removing these coatings. While alcohol is inexpensive and readily available, it lacks the solvent strength to effectively dissolve the coating and often requires extended soaking to have any effect. In this article, we will explore effective methods for removing protective coatings from PCBs.

Types of Conformal Coatings

There are five common types of conformal coatings available on the market:

  1. Acrylic Resin
    Acrylic resins dissolve easily in many organic solvents, making them convenient for board rework. They offer selective chemical resistance, dry quickly, resist mold, do not shrink during curing, and provide good moisture resistance. However, they have low abrasion resistance and are prone to scratching, cracking, and peeling.

  2. Epoxy Resin
    Typically composed of two parts that begin to cure upon mixing, epoxy resins offer excellent abrasion resistance, chemical resistance, and decent moisture protection. However, they are difficult to remove and rework. Because film shrinkage occurs during polymerization, a buffer solution is recommended around precision components. Curing at lower temperatures can help minimize shrinkage.

  3. Polyurethane
    Polyurethane coatings provide strong moisture and chemical resistance. Due to their robust chemical properties, removing them usually requires strippers, which may leave ionic residues behind. These residues must be thoroughly cleaned to avoid baseboard corrosion. Although rework through soldering is possible, it often results in brown discoloration that can affect the product’s appearance.

  4. Silicone
    Silicone is typically a single-component compound that begins to cure when exposed to moisture in the air and a certain temperature. Once cured, it forms a uniform, well-adhering layer across all surfaces of electronic components or modules. It is suitable for high-temperature environments (>120°C), as well as settings that require moisture sensitivity, chemical resistance, corrosion protection, and antifungal properties.

  5. Urethane (Polyurethane Carbamate)
    Urethane offers strong protection, hardness, and high solvent resistance. It provides excellent abrasion resistance and low moisture permeability. While it performs well in cold environments, it is not suitable for high-temperature applications. Most urethane coatings are difficult or impossible to rework or repair.

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Common Types of Protective Coatings and Removal Methods

  1. Chemical Solvent Method

Applicable Types:

  • Polyurethane: Methanol/ethylene glycol ether with an alkaline activator, or toluene/xylene.

  • Acrylic: Methylene chloride, chloroform, ketones (e.g., acetone), γ-butyrolactone, or butyl acetate.

  • Silicone: Methylene chloride or specific hydrocarbon solvents.

  • Epoxy: Difficult to remove once cured; for small areas, methylene chloride with an acidic activator and a cotton swab may be used.

Procedure:
Apply the solvent to the coating surface. Once the coating swells, gently wipe with a cotton swab or soft cloth. Avoid letting the solvent spread to unintended areas.

  1. Physical Removal Methods

Heat Method:

  • Tools: Soldering iron or hot air gun.

  • Note: Carefully control the temperature (do not exceed component tolerance). Suitable for high-temperature resistant components. Work quickly to avoid damaging the laminate.

Micro-abrasion Method:

  • Tools: Specialized abrasive equipment (e.g., walnut shell or glass bead media).

  • Note: Mask surrounding areas to prevent electrostatic buildup. Should be performed by trained personnel.

Mechanical Scraping:

  • Tools: Razor blade or small knife.

  • Steps: Cut a V-groove at the solder point, apply solvent, then lift the coating. Best for localized rework.

  1. Specialized Cleaning Agents

Recommendation: Use eco-friendly cleaning agents (e.g., Kyzen ES125A).
Method: Ultrasonic cleaning or soaking. Suitable for large areas or complex PCBs.

  1. Localized Replacement Method

Use Case: When only specific components need replacement.
Steps: Use a soldering iron to heat and remove the coating on the component, replace the part, clean the area, and reapply conformal coating.

Summary

PCB protective coatings are surface-applied materials designed to protect circuit boards from moisture, dust, chemicals, and high temperatures, thus improving product reliability. Common types include:

  • Acrylic (easy to apply, requires specific solvents for removal),

  • Polyurethane (strong protection, difficult to remove, may emit toxic fumes when heated),

  • Silicone (heat-resistant and reworkable), and

  • Epoxy (very tough once cured, hard to remove).

Removal methods should be selected based on coating type and may include chemical solvents (e.g., methylene chloride, methanol), physical methods (heat, abrasion), special cleaning agents, or localized replacement. Always prioritize safety and environmental responsibility, and take care to avoid damaging the PCB or its components.