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PCB Tech

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PCB Tech

PCB Tech

Detailed analysis of the process flow of SMT PCBA three anti-paint coating
2021-09-29
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Author:Frank

Detailed analysis of the process flow of SMT PCBA three anti-paint coating
As the size of PCBA components is getting smaller and smaller, the density is getting higher and higher; the height of the supporting height between the devices and the device (the distance between the PCB and the ground clearance) is also getting smaller and smaller, and the environmental factor has an impact on the PCBA. The influence is getting bigger and bigger, so we put forward higher requirements for the reliability of electronic products PCBA.
PCBA components change from large to small, from sparse to dense

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

Electronic PCB components in the external environment with humidity are almost at risk of corrosion. Water is the most important medium for corrosion. The water molecules are small enough to penetrate the mesh molecular gaps of some polymer materials and enter the interior or Corrosion occurs to the underlying metal through the pinholes of the coating. When the atmosphere reaches a certain humidity, it can cause PCB electrochemical migration, leakage current, and signal distortion in high-frequency circuits.

pcb board

Steam/humidity + ionic pollutants (salts, flux activator) = conductive electrolyte + stress voltage = electrochemical migration

When the RH in the atmosphere reaches 80%, there will be a water film with a thickness of 5-20 molecules, and various molecules can move freely. When carbon is present, electrochemical reactions may occur; when the RH reaches 60%, the surface of the equipment The layer will form a water film with a thickness of 2 to 4 water molecules. When pollutants are dissolved, a chemical reaction will occur; when the RH in the atmosphere is less than 20%, almost all corrosion phenomena will stop;

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

For electronic equipment, moisture exists in three forms: rain, condensation and moisture. 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 adsorbs ion pollutants and settles inside the electronic equipment, causing malfunctions. This is the common cause of failure of electronic equipment in the field.

There are two types of dust: coarse dust is irregular particles with a diameter of 2.5-15 microns, which generally will not cause problems such as faults and arcs, but affect the contact of the connector; fine dust is irregular particles with a diameter of less than 2.5 microns, fine Dust falling on the PCBA (single board) has a certain degree of adhesion and can only be removed with an anti-static brush.

Dust hazards: a. As the dust settles on the surface of PCBA, electrochemical corrosion occurs, and the failure rate increases; b. Dust + damp heat + salt spray cause the greatest damage to PCBA, coastal, desert (saline-alkali land), southern Huai River, mild rain season chemical industry, The areas near the mining area have the most electronic equipment failures.

Therefore, dust prevention is an important part of protecting products.

Salt spray

The formation of salt fog: The salt fog is caused by natural factors such as ocean waves, tides, atmospheric circulation (monsoon), atmospheric pressure, sunshine, etc. It will drift inland with the wind, and its concentration decreases with the distance from the coast. Usually, the shore is 1Km away from the coast. 1% (but the typhoon period will blow further).

Harmfulness of salt spray: a. Destroy the coating of metal structural parts; b. Accelerate the electrochemical corrosion rate and cause metal wires to break and components to fail.

Similar sources of corrosion: a. There are chemical substances such as salt, urea, lactic acid and so on in hand sweat, which have the same corrosive effect on electronic equipment as salt spray. Therefore, gloves should be worn during assembly or use, and the coating should not be touched with bare hands; b. There are halogens and acids in the flux, which should be cleaned and its residual concentration should be controlled.

Therefore, preventing salt spray is an important part of protecting products.

Mold

Mold is a common name for filamentous fungi, which means "mold fungus". They can often form luxuriant mycelium, but they do not produce large fruit bodies like mushrooms. In damp and warm places, some visible fluffy, flocculent or cobweb-like colonies grow on many items, that is, mold.

PCB mold
Harm of mold: a. The phagocytosis and reproduction of mold reduce the insulation of organic materials, damage and fail; b. The metabolite of mold is organic acid, which affects the insulation and electric strength and generates electric arc.

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

Considering the above aspects, the reliability of the product must be better guaranteed, and it must be isolated from the external environment as low as possible, so the conformal coating process is introduced.


, The PCB after the coating process, the shooting effect under the purple light, the original coating can also be so beautiful!

Three-proof paint coating refers to the application of a thin insulating protective layer on the surface of the PCB. It is currently the most commonly used method of surface coating after welding. It is sometimes referred to as surface coating and conformal coating (English name coating , Conformal coating). It isolates sensitive electronic components from the harsh environment, which can greatly improve the safety and reliability of electronic products and extend the service life of the products. Three-proof paint coating can protect circuits/components from environmental factors such as moisture, contaminants, corrosion, stress, impact, mechanical vibration, and thermal cycling. At the same time, it 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.