Detailed analysis of SMT PCBA three anti-paint coating process

1. Environmental factors and their impact

Common environmental factors such as humidity, dust, salt spray, mold, etc. will cause various failure problems of PCBA.

Humidity
Almost all electronic PCB components in the external environment are at risk of being corroded.

Among them, water is the most important medium for corrosion. Water molecules are small enough to penetrate the molecular gaps of some polymer materials and enter the interior or through the pinholes of the coating to reach the underlying metal and cause corrosion. When the atmosphere reaches a certain humidity, it can cause PCB electrochemical migration, leakage current and signal distortion in high-frequency circuits.

Vapor/humidity + ionic contaminants (salts, flux activators) = conductive electrolyte + stress voltage = electrochemical migration

When the RH in the atmosphere reaches 80%, there will be a water film which is 5-20 molecules thick. All kinds of molecules can move freely, and when there is carbon element, electrochemical reactions may occur.

When the RH reaches 60%, a water film 2 to 4 water molecules thick will be formed on the surface of the device. When pollutants are dissolved, a chemical reaction occurs.

When the RH in the atmosphere is less than 20%, almost all corrosion phenomena stop.

Therefore, moisture protection is an important part of protecting products.

For electronic equipment, moisture exists in three forms: rain, condensation and water vapor. Water is an electrolyte, which can dissolve a large number of corrosive ions to corrode metals. When the temperature of a certain part of the equipment is lower than the “dew point” (temperature), there will be condensation on the surface, structural parts or PCBA.

Dust

There is dust in the atmosphere, and the dust absorbs ion pollutants and settles inside the electronic equipment, causing malfunctions. This is a common denominator for electronic equipment failure in the field.

Dust is divided into two types: Coarse dust is irregular particles with a diameter of 2.5~15 microns. Generally, it will not cause problems such as faults and arcs, but it will affect the contact of the connector. Fine dust is irregular particles with a diameter of less than 2.5 microns. The fine dust falls on the PCBA (single board) and has a certain adhesion force. It must be removed by an anti-static brush.

Hazards of dust:

a. Due to the deposition of dust on the surface of PCBA, electrochemical corrosion occurs and the failure rate increases;

b. dust + damp heat + salt spray damages PCBA the most. In coastal areas, deserts (saline-alkali land), chemical industry in mild rainy seasons south of the Huaihe River, and areas near mining areas, electronic equipment failures are the most frequent.

Therefore, dust protection is an important part of product protection.

Salt fog

Formation of salt fog: Salt fog is caused by natural factors such as ocean waves, tides, atmospheric circulation (monsoon), air pressure, and sunshine, and will fall to the inland with the wind. Its concentration decreases with the distance from the coast, usually 1% of the shore at 1Km from the coast (but it will blow farther during the typhoon period).

Hazards of salt fog:

a. Destroy the coating of metal structural parts;

b. Accelerate the electrochemical corrosion rate, causing metal wires to break and components to fail.

Similar sources of corrosion:

a. There are chemical substances such as salt, urea, and lactic acid in hand sweat, which have the same corrosion effect on electronic equipment as salt fog. Therefore, gloves should be worn during assembly or use, and the coating should not be touched with bare hands;

b. When there are halogens and acids in the flux, they should be cleaned and their residual concentration should be controlled.

Therefore, anti-salt fog is an important part of protecting products.

Mold

Mold is a common name for filamentous fungi, meaning “moldy fungus”. They tend to form a heavily branched mycelium, but do not produce large fruiting bodies like mushrooms. In humid and warm places, some fluffy, flocculent or cobweb-like colonies that are visible to the naked eye grow on many items, which is mold.

Hazards of mold:

a. The phagocytosis and reproduction of mold will reduce the insulation of organic materials, damage and cause failure;

b. The metabolites of mold are organic acids, which affect the insulation and electric strength and generate arcs.

Therefore, anti-mold is an important part of protecting products.

Considering the above aspects, in order to better guarantee the reliability of the product, it must be isolated from the external environment as low as possible, so the three anti-paint coating process is introduced.

Three anti-paint coating refers to coating a thin layer of insulating protective layer on the surface of PCB. It is currently the most commonly used surface coating method after welding, sometimes also called surface coating, conformal coating. It isolates sensitive electronic components from harsh environments, which can greatly improve the safety and reliability of electronic products and extend the service life of products.

Three anti-paint coating can protect circuits/components from environmental factors such as moisture, pollutants, corrosion, stress, shock, mechanical shock and thermal cycle, and can also improve the mechanical strength and insulation properties of the product.

After the coating process, a transparent protective film is formed on the surface of the PCB, which can effectively prevent the intrusion of water droplets and moisture, and avoid leakage and short circuit.