Detailed analysis of SMT PCBA three anti-paint coating process
As the size of PCBA components becomes smaller and smaller, the density is higher and higher. The height of the components(the distance between the PCB and the ground clearance) is getting smaller and smaller, and the influence of environmental factors on the PCBA is also increasing. Therefore, we put forward higher requirements for the reliability of PCBA of electronic products.
1. Environmental factors and their impact
Common environmental factors such as humidity, dust, salt spray, mold, etc. will cause various failure problems of PCBA.
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.
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.
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 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.
2 .The main points of the three anti-paint coating process
According to the requirements of IPC-A-610E (electronic assembly testing standard),three anti-paint coating is mainly manifested in the following aspects:
1. Areas that cannot be coated:
Areas that require electrical connections, such as gold pads, gold fingers, metal vias, test holes;
Batteries and battery holders;
Fuses and enclosures;
Lenses of optical devices;
Other areas where performance or operation could be affected by the coating.
2. Areas that must be coated: all solder joints, pins, and conductor parts of components.
3. Areas that can be painted or not painted
Thickness is measured on the flat, unencumbered, cured surface of a printed circuit assembly, or on a coupon that undergoes processing with the assembly. The coupon can be the same material as the printed board or another non-porous material such as metal or glass. Wet film thickness measurement can also be used as an optional method for coating thickness measurement, as long as there is a document indicating the conversion relationship between dry and wet film thickness.
Thickness test method:
1.Dry film thickness measurement tool: a. micrometer (IPC-CC-830B); b. dry film thickness gauge (iron base)
2.Wet film thickness measurement: The thickness of the wet film can be obtained by the wet film thickness gauge, and then the thickness of the dry film can be calculated by the proportion of the glue solid content.
Usually, the edge of the line sprayed by the spray valve will not be very straight, and there will always be certain burrs. We define the width of the glitch as the edge resolution.
The smaller the edge resolution, the better, but the requirements of different customers are different, so the specific coating edge resolution only needs to meet the customer’s requirements.
The glue should cover the product like a smooth and transparent film of uniform thickness, emphasizing the uniformity of the glue covering each area on the product. Then, the thickness must be consistent, and there are no process problems: cracks, delamination, orange lines, pollution, capillarity, and air bubbles.
3. Process of the three anti-paint coating process
Three anti-paint coating process:
Prepare products, glue and other necessary items;
Determine the location of local protection;
Identify critical process details
It should be cleaned in the shortest time after welding to prevent the welding scale from being difficult to clean;
Determine whether the main pollutant is polar or non-polar, so as to select the appropriate cleaning agent;
If you use alcohol cleaning agent, you must pay attention to safety matters: there must be good ventilation and the process rules of drying after washing to prevent the residual solvent from volatilizing and causing an explosion in the oven;
Wash with water, wash the flux with alkaline cleaning solution (emulsion), and then rinse with pure water to clean the cleaning solution to meet the cleaning standard;
3.Masking protection (if selective coating equipment is not used)
Washi tape with self-adhesive film that will not transfer should be selected;
Anti-static paper tape should be used for IC protection;
Shield and protect some devices according to the drawing requirements.
The cleaned and shielded PCBA (component) must be pre-baked and dehumidified before coating;
Determine the temperature/time of pre-baking according to the allowable temperature of PCBA (component).
The process method of three anti-paint coating depends on the PCBA protection requirements, existing process equipment and existing technical reserves, and is usually realized in the following ways:
a.Brushing by hand
Brush coating is the most widely applicable process, suitable for small batch production, products with complex and dense PCBA structures, and demanding masking protection requirements. Since the coating can be controlled at will by brushing, the parts that are not allowed to be painted will not be polluted;
Brushing consumes the least material and is suitable for more expensive two-component coatings;
The brush coating process has high requirements for operators. Before construction, the drawings and coating requirements should be carefully digested, the names of PCBA components can be identified, and eye-catching signs should be posted on parts that are not allowed to be coated;
The operator is not allowed to touch the printed insert with his hands at any time to avoid contamination;
b. Manual dip coating
The dip coating process can get the best coating effect, and a uniform and continuous coating can be applied to any part of the PCBA. The dip coating process is not suitable for PCBA with adjustable capacitors, trimmer cores, potentiometers, cup cores and some components with poor sealing.
Key parameters of the dip coating process:
Adjust the appropriate viscosity;
Control the speed at which the PCBA is lifted to prevent air bubbles. Usually the speed should not exceed 1 meter per second;
Spraying is the most widely used and easily accepted process method, which can be divided into the following two categories:
It is suitable for complex workpieces that are difficult to mass-produce by automated equipment, and it is also suitable for situations where there are many varieties of product lines but small quantities, and can be sprayed to special positions.
The precautions for manual spraying are: paint mist will contaminate some components, such as PCB plug-ins, IC sockets, some sensitive contacts and some grounding parts, and these parts need to pay attention to the reliability of shielding protection. Another point is that the operator should not touch the printed plug with his hands at any time to prevent contamination of the contact surface of the plug.
Usually refers to the use of selective coating equipment for automated spraying. Suitable for mass production, good consistency, high precision, and less environmental pollution. With the upgrading of the industry, the increase of labor costs and the strict requirements of environmental protection, automatic spraying equipment is gradually replacing other coating methods.