What is conformal coating for circuit boards?
Conformal coating is a special polymer film-forming material used to protect circuit boards, components, and other electronic components from adverse environmental conditions caused by corrosive air, humidity, heat, fungi, and pollutants such as dust and soil.
What are the classifications of conformal coatings?
Generally speaking, there are 5 common conformal coatings on the market.
1. Acrylic adhesive
Acrylic adhesives are easily soluble in many organic solvents, making them convenient for repairing boards and typically only providing selective chemical resistance properties.
Acrylic adhesive has the advantages of fast drying, good mold resistance, no shrinkage during curing, and good moisture resistance. But the disadvantage is low wear resistance, easy to be scratched, broken, and peeled off.
2. Epoxy resin
Epoxy resin is usually a two-component compound that begins to cure after mixing.
Epoxy resin has good wear resistance and chemical resistance, as well as reasonable moisture resistance. But this type of coating is difficult to remove and rework. Due to the occurrence of film shrinkage during the polymerization process, it is recommended to use buffer solution around precision components. Curing at low temperatures can minimize the shrinkage rate.
3. Polyurethane
Polyurethane has good moisture resistance and chemical resistance properties. Due to its good chemical resistance, a stripping agent is required to remove the coating (which may leave ionic residues). These residues may need to be thoroughly cleaned to prevent corrosion of the bottom plate. Polyurethane can be reworked through welding, but it usually produces brown residue that affects the appearance of the product.
4. Silicone
Silicone is usually a single compound that begins to solidify when exposed to moisture in the air and at a certain temperature. Silicone has good wettability and adhesion after curing on all surfaces of electronic materials or modules. It can be widely used in environments with high temperature (>120 ℃), humidity sensitivity, chemical resistance, corrosion resistance, and antifungal properties.
5. Ethyl carbamate
Ethyl carbamate has strong protection, hardness, and high solvent resistance, providing excellent wear resistance and low moisture permeability. In addition, it also has good low-temperature adaptability, but cannot work well in high-temperature environments, and most cannot be repaired or reworked.
What kind of product needs conformal coating?
A: Use the environment with more serious pollution, such as outdoor, oil pollution, iron dust, high humidity (dust).
For example: air conditioning outdoor unit, refrigerator, water heater.
B: There are many small components (0402, 0201), BGA, etc., and the device density is high, easy to short circuit.
C: Poor process and residues on the PCBA board (such as small tin beads, etc.).
D: Factory anti-static measures are not in place, etc.
The advantages of conformal coating
It has the function of preventing short circuit fault caused by wet dust, oil pollution, iron dust and other pollutants.
It can fix the residue on the board (such as tin beads) in the production process to prevent the factory test from being normal, but the tin beads move and cause a short circuit during use.
Electrostatic prevention, conformal coating has a certain protective effect on static electricity in soldering, assembly and test etc.
It increases the difficulty for competitors to copy boards.
Can prevent PCBA pollution and failure caused by insects and eggs. (Early computer, socket failure quite many this situation).
A certain anti-corrosion effect, such as: sulfide pollution, halogen pollution, some glue accessories on PCBA are especially easy to adsorb these pollutants, resulting in the corrosion of the pad and other failure.
Disadvantages of conformal coating
Increase costs.
If the production pass-through rate is low, it will increase the difficulty of production line maintenance.
The market repair of products will also increase the difficulty of repair.
