Detailed analysis of SMT PCBA three anti-paint coating process

As the size of PCBA components is getting smaller and smaller, the density is getting higher and higher; The height between the devices and the devices (the pitch/ground clearance between the PCB and the PCB) is also getting smaller and smaller, and the influence of environmental factors on the PCBA is also increasing, so we put forward higher requirements for the reliability of electronic products PCBA.
PCBA components from large to small, from sparse to dense change trend
Environmental factors and their effects
Common environmental factors such as humidity, dust, salt spray, mold, etc., cause various failure problems of PCBA
Humidity in the external environment of electronic PCB components, almost all there is the risk of corrosion, of which water is the most important medium for corrosion, water molecules are small enough to penetrate the mesh molecular gap of some polymer materials into the interior or through the coating pinholes to reach the underlying metal corrosion. When the atmosphere reaches a certain humidity, it can cause PCB electrochemical migration, leakage current and signal distortion in high-frequency circuits.
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Vapor/humidity + ionic contaminants (salts, flux active agents) = conductive electrolyte + stress voltage = electrochemical migration
When the RH in the atmosphere reaches 80%, there will be 5 to 20 molecules thick water film, all kinds of molecules can freely move, when there is carbon, may produce electrochemical reaction; When RH reaches 60%, the surface layer of the equipment will form a water film with a thickness of 2 to 4 water molecules, and chemical reactions will occur when pollutants dissolve into it. When RH < 20% in the atmosphere, almost all corrosion phenomena stop;
Therefore, moisture protection is an important part of product protection.
For electronic devices, moisture comes in three forms: rain, condensation, and water vapor. Water is an electrolyte that can dissolve large amounts of corrosive ions that corrode metals. When the temperature of a certain part of the equipment is below 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 to settle inside the electronic equipment and cause failure. This is a common feature of electronic failures in the field.
Dust is divided into two types: coarse dust is irregular particles with a diameter of 2.5 to 15 microns, which generally does not cause problems such as failure, arc, but affects the contact of the connector; Fine dust is irregular particles with a diameter of less than 2.5 microns. Fine dust has a certain adhesion on PCBA (veneer) and can be removed by anti-static brushes.
Hazards of dust: a. Due to dust settling on the surface of PCBA, electrochemical corrosion is generated, and the failure rate is increased; b. Dust + damp heat + salt spray has the greatest damage to PCBA, and the electronic equipment failures are the most in coastal, desert (saline-alkali land), and the chemical industry and mining areas near the Huaihe River during the mildew and rain season.
Therefore, dust protection is an important part of the protection of products.
Salt spray
The formation of salt spray: salt spray is caused by natural factors such as waves, tides and atmospheric circulation (monsoon) pressure, sunshine, and will fall inland with the wind, and its concentration decreases with the distance from the coast, usually 1Km from the coast is 1% of the shore (but the typhoon will blow further).
The harm of salt spray: a. damage the coating of metal structural parts; b. Accelerated electrochemical corrosion rate leads to metal wire breakage and component failure.
Similar corrosion sources: a. There are salt, urea, lactic acid and other chemicals in hand sweat, which have the same corrosive effect on electronic equipment as salt spray, so 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 controlled.
Therefore, salt spray prevention is an important part of product protection.
mold
Mildew, the common name for filamentous fungi, means “moldy fungi,” which tend to form luxuriant mycelium, but do not produce large fruiting bodies like mushrooms. In moist and warm places, many items grow some visible fluff, flocculent or spider colonies, that is mold.
PCB mold phenomenon
The harm of mold: a. mold phagocytosis and propagation make the insulation of organic materials decline, damage and failure; b. The metabolites of mold are organic acids, which affect the insulation and electrical resistance and produce arc.
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Therefore, anti-mold is an important part of the protection of 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 shape coating process is introduced.
After the coating process of the PCB, the shooting effect under the purple lamp, the original coating can also be so beautiful!
Three anti-paint coating refers to the PCB surface coated with a thin layer of insulation protective layer, it is currently the most commonly used post-welding surface coating method, sometimes known as surface coating, coating shape coating (English name coating, conformal coating). It isolates sensitive electronic components from harsh environments, greatly improving the safety and reliability of electronic products and extending the service life of products. Tri-resistant coatings protect circuits/components from environmental factors such as moisture, contaminants, corrosion, stress, shock, mechanical vibration and thermal cycling, while also improving the mechanical strength and insulation properties of the product.
After the coating process, the PCB forms a transparent protective film on the surface, which can effectively prevent the intrusion of water beads and moisture, avoid leakage and short circuit.
2. Main points of coating process
According to the requirements of IPC-A-610E(Electronic Assembly Testing Standard), it is mainly manifested in the following aspects
Complex PCB board
1. Areas that cannot be coated:
Areas requiring electrical connections, such as gold pads, gold fingers, metal through holes, test holes; Batteries and battery mounts; Connector; Fuse and housing; Heat dissipation device; Jumper wire; Lenses of optical devices; Potentiometer; Sensor; No sealed switch; Other areas where coating can affect performance or operation.
2. Areas that must be coated: all solder joints, pins, component conductors.
3. Areas that can be painted or not
thickness
Thickness is measured on a flat, unimpeded, cured surface of the printed circuit component, or on an attachment plate that undergoes the manufacturing process with the component. The attached board may be of the same material as the printed board or other non-porous material, such as metal or glass. Wet film thickness measurement can also be used as an optional method for coating thickness measurement, provided that the conversion relationship between dry and wet film thickness is documented.
Table 1: Thickness range standard for each type of coating material
Thickness test method:
1. Dry film thickness measuring tool: a micrometer (IPC-CC-830B); b Dry Film Thickness Gauge (Iron base)
Micrometer dry film instrument
2. Wet film thickness measurement: The thickness of the wet film can be obtained by the wet film thickness gauge, and then calculated by the proportion of the glue solid content
Thickness of dry film
The wet film thickness is obtained by the wet film thickness gauge, and then the dry film thickness is calculated
Edge resolution
Definition: Under normal circumstances, the spray valve spray out of the line edge will not be very straight, there will always be a certain burr. We define the width of the burr as the edge resolution. As shown below, the size of d is the value of the edge resolution.
Note: The edge resolution is definitely the smaller the better, but different customer requirements are not the same, so the specific coated edge resolution as long as it meets customer requirements.
Edge resolution comparison
Uniformity, the glue should be like a uniform thickness and smooth transparent film covered on the product, the emphasis is on the uniformity of the glue covered in the product above the area, then it must be the same thickness, there is no process problems: cracks, stratification, orange lines, pollution, capillary phenomenon, bubbles.
Axis automatic AC series automatic coating machine coating effect, uniformity is very consistent
3. The realization method of coating process and coating process
Step 1 Prepare
Prepare products and glue and other necessary items; Determine the location of local protection; Determine key process details
Step 2 Wash
It should be cleaned within the shortest time after welding to prevent welding dirt from being difficult to clean; Determine whether the main pollutant is polar or non-polar in order to select the appropriate cleaning agent; If alcohol cleaning agent is used, safety matters must be paid attention to: there must be good ventilation and cooling and drying process rules after washing, to prevent residual solvent volatilization caused by explosion in the oven; Water cleaning, wash the flux with alkaline cleaning liquid (emulsion), and then wash the cleaning liquid with pure water to meet the cleaning standard;
3. Masking protection (if selective coating equipment is not used), that is, mask;
Should choose non-adhesive film will not transfer paper tape; Anti-static paper tape should be used for IC protection; According to the requirements of drawings, some devices are shielded;
4.Dehumidify
After cleaning, the shielded PCBA (component) must be pre-dried and dehumidified before coating; Determine the temperature/time of pre-drying according to the temperature allowed by PCBA (component);
Table 2: PCBA (components) can be allowed to determine the temperature/time of pre-drying table
Step 5 Apply
The process method of coating depends on the PCBA protection requirements, the existing process equipment and the existing technical reserves, which are usually achieved in the following ways:
a. Brush by hand
Hand painting method
Brush coating is the most widely applicable process, suitable for small batch production, PCBA structure is complex and dense, need to shield protection requirements of harsh products. Because brushing can control the coating at will, the parts that are not allowed to be painted will not be polluted; Brush consumption of the least material, suitable for the higher price of two-component coatings; The brushing process has high requirements for the operator, and the drawings and requirements for coating should be carefully digested before construction, and the names of PCBA components can be identified, and eye-catching marks should be affixed to the parts that are not allowed to be coated. The operator is not allowed to touch the printed plug-in by hand at any time to avoid contamination;
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b. Dip by hand
Hand dip coating method
The dip coating process provides the best coating results, allowing a uniform, continuous coating to be applied to any part of the PCBA. The dip coating process is not suitable for PCBA components with adjustable capacitors, trimmer cores, potentiometers, cup-shaped cores and some poorly sealed devices.
Key parameters of dip coating process:
Adjust the appropriate viscosity; Control the speed at which PCBA is lifted to prevent bubbles from forming. Usually no more than 1 meter per second increase in speed;
c. Spraying
Spraying is the most widely used and easily accepted process method, which is divided into the following two categories:
① Manual spraying
Manual spraying system
It is suitable for the situation that the workpiece is more complex and difficult to rely on automated equipment for mass production, and it is also suitable for the situation that the product line has many varieties but the amount is small, and it can be sprayed to a special position.
Manual spraying should be noted: paint mist will pollute some devices, such as PCB plug-ins, IC sockets, some sensitive contacts and some grounding parts, these parts need to pay attention to the reliability of shielding protection. Another point is that the operator should not touch the printed plug by hand at any time to prevent contamination of the plug contact surface.
② Automatic spraying
It usually refers to automatic spraying with selective coating equipment. Suitable for mass production, good consistency, high precision, little environmental pollution. With the upgrading of the industry, the improvement of labor costs and the strict requirements of environmental protection, automatic spraying equipment is gradually replacing other coating methods.