PCB Blog - The surface of the fr4 pcb is coated

Conformal Coating is a method of coating on fr4 pcb and forming a protective film, which is usually only a thin layer (about 30 – 210 μ M), which can be used to strengthen the moisture-proof, antifouling, dust-proof and chemical-proof capabilities of electronic products, and can also prevent the continuous corrosion of solder joints and conductors in contact with air, and can play a role of shielding and eliminating some electromagnetic interference, and can also play a role of insulation. This coating also has an additional purpose, which is to increase the wear resistance and solvent resistance of components, and also to release the stress caused by high and low temperature changes. Because of its various advantages, it is often used in products that need to be exposed to extremely harsh environments, such as multi-chemical gas environment, high pollution and dust environment, high humidity environment, extremely high or low temperature environment. In consumer electronic products, it is often used to protect or insulate some motor products or electronic component devices, such as motors, generators, transformers, electric delay valve switches, etc. The coating material of conformal coating basically includes three transparent raw materials, such as Silicone, Acrylic and Urethane, and also uses Epoxy (epoxy resin), but it is opaque, so it is rarely used.

The curing methods of conventional coating can be room temperature curing and heating curing. Here are the characteristics and precautions of these four conformal coating materials:

Silicone material usually presents transparent and elastic rubber state after curing, which is more effective in shockproof, and can also withstand severe stress changes (- 40 ° C~200 ° C) caused by large high and low temperature changes.

Acrylic material usually presents a transparent and hard coating after curing, with low moisture absorption and fast curing time, and good wear resistance and insulation.

Urethane material usually presents a transparent and hard coating after curing, with excellent wear resistance and good moisture-proof ability. Its performance is particularly stable in low temperature environment, but it is less resistant to high temperature.

Epoxy-based coating material is very solid, usually opaque, with good moisture-proof and moisture-proof ability, and its performance of chemical corrosion resistance and wear resistance is also very good. In addition, Epoxy-based coating material also has good dielectric properties. When conformal coating, attention should be paid to avoid coating some parts that need contact or circuit connection during subsequent processing, such as power jack, connector, buzzer, speaker. Because the conformal coating is made of insulating material, attaching to these components that need conductive contact will cause poor electrical properties.

The coating methods of conformal coating can be roughly divided into 3+1:

Dipping: It is more economical for large equipment, but the thickness after coating will be affected by the temperature, time, extraction speed, vertical flow time, whether to add air knife Etc.

Spaying: It is more economical for small and medium-sized equipment. When using spraying, just like painting, the uniformity of coating depends on the relative moving speed, spraying position, spraying pressure, and whether there are higher parts. In addition, the bottom of the part cannot be sprayed, which may require additional treatment.

Brushing: It is more economical for small equipment. When coating, it is easy to cause uneven coating due to the skill proficiency of coating. In addition, pay attention to the problem of hair loss of the brush. The bottom of parts is also difficult to be coated.

Selective Coating: Although there are many benefits of coating, it is often time-consuming and labor-intensive. You can also select the part that needs to be coated. Because most of the conformal coating is transparent and colorless after spraying, it is difficult to check the spraying effect with ordinary naked eyes. Therefore, most conforma coating materials will add a small amount of ultraviolet fluorescent agent, so as long as the ultraviolet light is installed, it can be used to check the spray condition and uniformity.

When selecting the conformal coating material, it is also necessary to pay attention to its heating conditions. Some need to be cured at high temperature, while others can be cured at room temperature. Those cured at high temperature usually have higher hardness and can wear, while those cured at room temperature have more flexible advantages.

1) When using the spraying method, it is necessary to use vacuum equipment, otherwise the sprayed material will be dispersed into the air, affecting human health.

2) During the spraying operation, the fixture can be used to cover some places and parts that cannot be sprayed, so as to prevent the insulating material from being sprayed to some places that need conductive contact.

3) When making conformal coating, you can also use stickers without residual adhesive to stick to some parts that cannot be sprayed.

4) The sprayed fr4 pcb board can be checked by ultraviolet light.