The process of conformal paint

1. Definition Conformal paint is a special formula of coating, which is used to protect circuit boards and related equipment from environmental erosion, and has good high and low temperature resistance. The conformal paint is coated on the surface of the circuit board, and after curing, it will form a transparent conformal protective film (anti-moisture, anti-salt spray, anti-mildew).

2. The main application of conformal coating The PCB circuit board that has used conformal coating has the "three-proof" performance of anti-fog, moisture-proof and mildew, and also has the properties of cold and thermal shock, aging resistance, radiation resistance, salt spray resistance, ozone corrosion resistance, vibration resistance, good flexibility, strong adhesion, etc., so it is extremely widely used. Typical application scenarios include the following:

Civil and Commercial Applications Conformal coatings (common lamination) provide protection for electronic circuits in household appliances with:

Water and detergents (washing machines, dishwashers, sanitary products, outdoor electronic LED screens);

External unfavorable environment (display, anti-theft, fire alarm device, etc.);

chemical environment (air conditioners, dryers);

harmful substances in offices and homes (computers, induction cookers);

All other circuit boards that require three-proof protection.

Automotive Industry The automotive industry requires conformal coatings to protect circuits from hazards such as gasoline vapors, salt spray/brake fluid, etc. With the rapid growth of electronic systems in automobiles, the use of conformal coatings has become a fundamental requirement to ensure long-term reliability of automotive electronics.

Aerospace Due to the special use environment, the aviation and aerospace environment have strict requirements for electronic equipment, especially under the condition of rapid pressurization and decompression, it is still necessary to maintain good circuit performance, so the pressure stability of conformal paint is widely used.

Navigation Whether it is fresh water or salty seawater, it will cause harm to the electrical wiring of ship equipment, and the use of conformal paint can maximize the protection of equipment on the water surface and even submerged underwater.

Medical conformal coatings protect electronic devices from external chemicals and special use environments, ensuring their continuous and stable operation.

outdoor electronics, etc

3. Conformal paint process specifications and requirements "Painting requirements"

Painting thickness: The thickness of the paint film is controlled from 0.05mm to 0.15mm, and the thickness of the dry film is 25um - 40um.

Secondary coating: In order to ensure the thickness of products with high protection requirements, secondary coating can be carried out after the paint film is cured (whether to apply a second coating according to the needs).

Inspection and repair: Visually inspect whether the coated circuit board meets the quality requirements and repair the problem. For example, when the needle and other protected areas are dipped in conformal paint, you can use tweezers to clip the degreased cotton ball or clean cotton ball dipped in washing board water to scrub it clean, and be careful not to wipe off the normal paint film when scrubbing.

Component replacement: After the paint film is cured, if you need to replace the component, you can do as follows:

Directly weld the components with electric ferrochrome, and then use a cotton cloth dipped in washing board water to clean the substances around the pads;

soldering replacement components;

Use a brush dipped in a conformal paint brush to paint the weld and cure the paint film.

"Coating process"

Get ready

Prepare products, glue, and other necessary items;

Determine the site of local protection;

Identify key process details.

cleaning

It should be cleaned in the shortest time after welding to prevent welding residues from being difficult to clean.

Determine whether the main contaminant is polar or non-polar in order to choose the appropriate cleaning agent;

If alcohol cleaning agents are used, safety precautions must be paid to: there must be good ventilation and drying process rules after washing to prevent the residual solvent from volatilizing and causing explosion in the oven;

When cleaning with water, rinse the flux with an alkaline cleaning solution (emulsion), and then rinse the cleaning solution with pure water to meet the cleaning standard.

Masking protection (if no selective coating equipment is used), i.e. masking

paper tape should be selected that the self-adhesive film will not transfer;

Anti-static paper tape should be selected for the protection of ICs;

Some devices are shielded according to the drawings.

Dehumidification The PCBA (component) that has been cleaned and shielded must be pre-baked and dehumidified before application; The temperature/time of pre-baking is determined according to the temperature that the PCBA (component) can allow.

The temperature that the PCBA (component) can allow determines the temperature/time comparison table for pre-baking

Coating The coating process method depends on the PCBA protection requirements, existing process equipment and existing technical reserves, and usually has the following implementation methods: [manual brushing]

Hand painting method. Brushing is the most widely applicable process, suitable for low-volume production, products with complex and dense PCBA structures, and products with strict masking protection requirements. Since the coating can be controlled at will, the parts that are not allowed to be painted are not polluted;

Brushing consumes the least amount of material and is suitable for higher-priced two-component coatings.

The brushing process has high requirements for the operator, and the drawings and coating requirements should be carefully studied before construction, and the names of PCBA components should be recognized, and the parts that are not allowed to be painted should be marked with eye-catching markings;

The operator is not allowed to touch the printed insert with his hands at any time to avoid contamination.

【Handmade Dipping Coating】

Hand dipping method. The dip coating process can achieve the best coating effect, and can apply a uniform and continuous coating to any part of the PCBA. The dip coating process is not suitable for PCBA with adjustable capacitors, trim cores, potentiometers, cup cores, and some devices with poor sealing.

Key parameters of the dip coating process:

adjust the appropriate viscosity;

Control the speed of lifting the PCBA to prevent bubbles, usually not more than 1 meter per second.

Spraying is the most widely used and easily accepted process method, which is divided into the following two categories: (1) Manual spraying  is suitable for the situation where the workpiece is more complex and difficult to produce in large quantities by automatic equipment, and it is also suitable for the situation where there are many varieties of product lines but the quantity is small, and can be sprayed to a more special position.

Manual spraying method. Manual spraying should be noted that paint mist will contaminate certain devices, 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. In addition, the operator should not touch the printed plug with his hands at any time to prevent contamination of the plug contact surface.

(2) Automatic spraying usually refers to the use of selective coating equipment for automatic spraying, which is suitable for mass production, with good consistency, high precision and low environmental pollution. With industrial upgrading, labor costs and strict environmental protection requirements, automatic spraying and spraying equipment is gradually replacing other coating methods.

Automatic spraying method.

【Key points of coating process】 According to the requirements of IPC-A-610E (Electronic Assembly Testing Standard), it is mainly manifested in the following aspects: [Area]

Areas that cannot be applied:

Areas that require electrical connections, such as gold pads, gold fingers, metal vias, test holes;

batteries and battery holders;

Connector;

fuses and housings;

heat dissipation device;

Jumper;

lenses for optical devices;

Potentiometer;

Sensor;

Switches without seals;

Other areas where the coating can affect performance or operation.

Areas that must be applied:

All solder joints, pins, and conductor parts of components.

Areas that can be applied or not

Device-free green oil coverage area.

【Thickness】 The thickness range of each type of coating material is standard.

【Thickness test method】

Dry film thickness measurement tool:

千分尺(IPC - CC - 830B);

Dry film thickness gauge, micrometer, dry film meter.

Wet film thickness: the thickness of the wet film can be obtained by the wet film thickness gauge, and then the thickness of the dry film is calculated by the proportion of glue solid content, and the thickness of the wet film is obtained by the wet film thickness gauge, and then the dry film thickness is calculated.

【Quality Requirements】

The surface of the circuit board should not have flowing paint or dripping, and pay attention to not dripping into the locally isolated parts when painting with a brush;

The conformal paint layer should be flat, bright and uniform, and the surface of the pad, patch element or conductor should be well protected;

The surface and components of the paint layer should not have bubbles, pinholes, ripples, shrinkage holes, dust and other defects and foreign objects, no powdering, no peeling, note: before the surface of the paint film is dry, do not touch the paint film at will;

Locally isolated components or areas should not be coated with conformal paint.

4. Common problems and solutions in the use of conformal paint

Bubble problems

Problem manifestation: After coating, bubbles appear on the surface of the conformal paint layer, which will affect the protective performance and appearance of the paint layer.

Causes:

In the dipping process, if the lifting speed exceeds 1 meter per second, it is easy to produce bubbles;

If the surface of the circuit board is not thoroughly dried before coating, if the dehumidification is not thorough, the residual moisture will form bubbles after coating;

Too vigorous agitation of the paint can bring in air to form bubbles.

Workaround:

For the dipping process, the speed of lifting the PCBA is strictly controlled to ensure that it does not exceed 1 meter per second;

Before coating, ensure the effectiveness of the dehumidification process, and carry out pre-baking and dehumidification according to the temperature and time allowed by PCBA;

Stir the paint in a gentle way, such as using a low-speed agitator, and let it sit for a while after stirring to allow air bubbles to escape.

nsufficient adhesion problem

Problem manifestation: The conformal paint is easy to peel off from the surface of the circuit board and cannot play a good protective role.

Causes:

The surface of the circuit board is not thoroughly cleaned, and there are contaminants such as oil stains and flux residues, which affects the combination of conformal paint and the circuit board.

The type of conformal paint chosen does not match the board material, and different circuit board materials may require different formulations of conformal paint to ensure good adhesion.

Workaround:

To ensure the quality of the cleaning process, choose the appropriate cleaning agent according to the polarity or non-polarity of the main contaminants, such as using a suitable emulsion solution for polar pollutants, and then rinse them with pure water;

When choosing conformal paint, conduct adaptability tests according to the material of the circuit board and choose the appropriate product.

Local miscoating problem

Problem manifestation: Conformal paint is applied to areas that should not be coated (such as gold pads, gold fingers, etc.), resulting in circuit connection problems or component performance affected.

Causes:

The operator made a mistake during manual brushing or spraying, and did not effectively shield and protect the areas that should not be coated or accidentally applied to these areas;

When using automatic spraying equipment, the wrong program setting of the equipment or equipment failure leads to incorrect coating.

Workaround:

For manual operation, improve the skills and sense of responsibility of operators, carefully confirm the areas that should not be coated before coating and make conspicuous markings, and operate carefully during construction;

For automatic spraying equipment, carefully check the equipment program settings before use, and regularly maintain the equipment to ensure the normal operation of the equipment.

5. The future development trend of conformal coatings

Research and development of environmentally friendly conformal paint

With the increasingly stringent environmental protection requirements, the use of traditional conformal paints containing volatile organic compounds (VOCs) will gradually be restricted. The development of low-VOC or even VOC-free environmentally friendly conformal paints has become an inevitable trend, which not only meets the existing protective performance requirements, such as moisture-proof, salt spray, mildew-proof, etc., but also minimizes the impact on the environment and human health during production and use.

18.  High-performance multi-functional conformal paint

The future conformal coatings will develop in the direction of high performance and multi-function. For example, in addition to the existing protective properties, it may have better thermal conductivity to meet the increasing thermal needs of electronic devices, while also having a self-healing function that can automatically repair when the paint layer suffers minor damage, maintaining the integrity of the protective performance.

19.Intelligent coating process

In terms of coating technology, intelligence will be the development direction. Using artificial intelligence and automation technology, coating equipment can automatically adjust coating parameters such as coating thickness, coating area, etc. according to the structure and coating requirements of the circuit board, which can not only improve the accuracy and consistency of coating, but also reduce human errors and improve production efficiency.