Coating in PCBA
PCBA components of electronic equipment operating in extreme harsh working environments, such as high humidity, high salt, dust and vibration, are vulnerable to the effects of salt spray, moisture and mold, causing equipment failure. Therefore, the coating technology of PCB components is attracting increasing attention.
Also known as PCB electronic circuit board protective oil, moisture-proof paint, waterproof adhesive, insulating paint, anti-corrosion paint, anti-salt spray paint, anti-dust paint, protection paint, etc.
The coated PCBA has the properties of waterproof, moisture-proof and dust-proof, cold and hot impact resistance, aging resistance, radiation resistance, salt spray resistance, ozone corrosion resistance, vibration resistance, good flexibility, strong adhesion and so on.Moisture is the most common and destructive factor on PCB.Excessive moisture can significantly reduce insulation resistance between conductors, accelerate rapid decomposition, reduce Q values, and corrode conductors.
The coating refers to the PCB circuit board parts need protection coated with 1 layer thickness uniformity of anti-corrosion paint, it can PCB circuit board parts need protective plate and effectively isolate electronic components and its working environment, protection of PCB circuit board components from damage, so as to improve the reliability of PCB circuit board components, and to further improve the reliability of electronic equipment.
1. Coating material
According to the classification of materials, the common coat at present can be divided into four kinds: acrylic, epoxy, polyurethane and silicone.
In terms of curing methods, there are room temperature curing, heat curing , UV curing and so on.
From the coating method on the classification, there are dip-coating, brush coating and spraying and other different process methods.
2. How to choose the material
The choice of which material to use for coating is depend on the application of the product environment and the application of different production needs.
Ordinary electronic products can choose acrylic anti-paint, it has the advantages of low cost, can withstand a wide range of temperature, from low-temperature -60C to high temperature 130C.
The curing time of the surface layer under normal temperature is less than 10min, and it can be completely cured for 24h. When the temperature reaches 60℃, it can be completely cured for 30min.
For the PCB used in military products, epoxy resin, polyurethane and organosilicin are often used for the three anti-paint due to the extreme harsh environment problems such as high temperature and humidity vibration.These three paints are good for temperature resistance, flame retardancy and dielectric properties. Polyurethane and silicone heat resistance performance is better, so in military, locomotive, industrial control electronic instruments and meters and power communication occasions, will choose polyurethane coating. And in high humidity and high salt spray and other operating occasions can choose silicone coating, it can be protected in the object to generate a layer of elastic film, moisture-proof salt spray corrosion resistance effect is very good, in addition to the temperature tolerance range is also very wide, from -60℃ to 200℃.The surface can be cured in 30min at room temperature and cured completely in 24h.
3. Conditions for Coating
Before coating, it is necessary to ensure that the surface layer of PCB components is clean to ensure good adhesion of the coating.
Coating process has spraying, brush coating, dip coating and other different process methods.
Sincs the coating process is different, viscosity requirements are not the same.
In the use of spray painting process, the viscosity requirement is the lowest to meet the "spray" ability requirements;Dip coating requires the highest viscosity.
The viscosity of conformal coating by the solid proportion in conformal coating to decide, conformal coating manufacturers are usually have their own paint thinner to dilute the solution configuration is not the same concentration of products, applicable to different coating process requirements.
4. Process Requirements
To achieve high quality coating, the following processing requirements must be met during coating:
A. To the components which cannot touch conformal coating, such as connectors, buttons and leds and gold finger, if choose dip coating or manual spraying processing technology, such as covering must be protected with gummed paper, even if is to use automatic coating machine, if the coating area near the position can't be stained with paint so (less than 5 mm), also need to do a good job of barrier precautions;
B. The surface of PCBA shall be cleaned before coating to remove foreign matter on the surface of PCBA.For products with strict requirements, such as automobile, aerospace, navigation and military industry, it is required to wash and dry PCBA before coating can be implemented. For general products with low requirements, dry high-pressure gas can be used to remove the surface dirt(such as dust) of PCBA.
C. Before using selective automatic coating machine for mass production, attention must be paid to the first piece of PCBA inspection. To confirm that there is no leakage, less coating, more coating or bubble, etc. The connector and other prohibited coating devices or areas can not have adhesive paint, coating thickness is uniform and consistent, then the drying can be carried out;
D. The working environment humidity should be controlled below 65%RH during coating. As a composite material, PCB will absorb moisture. In case of moisture absorption, the paint will not fully play a protective role.
Therefore, PCB should carry out coating process as soon as possible after assembly. If PCBA is laid aside for a long time and then be coated, it is better to carry out pre-drying before coating, with the temperature being 60℃ and the time being 24h.
5. Our coating PCBA
Our company can provide coating service for customers.
Customers can request, we will use the materials they choose or according to the product application environment to help customers choose materials and coating process.
This PCBA is used for the controlling of light switch. We use the acrylic anti-paint. The patch components of this board need coating protection. DIP component switches, cables, relays, etc. should not be coated. We use brush coating for this board.
This board is double-sided spray coating, the thickness of coating is 40um. The area of J1~J7, PTH and orange circle area in figure should forbidden coating.