Classification and Performance of PCB CoatingPosted on 2020-07-22
Definition and function
PCB coating is a special coating sprayed on the surface of the circuit board. It is mainly responsible for preventing vibrations such as molding, humidity and salt spray, insulation and other functions such as reducing the design size of the circuit board.
Its main functions are:
- The PCB coating isolates the circuit from the harsh environment. So that the assembled PCB will not be affected by the harsh environment.
- Ultimately protect the circuit from environmental erosion. The PCB coating itself is insulated to prevent circuit failure or leakage. Prolong the service life of electronic products.
Therefore, PCB coate makes electronic products have higher power and smaller PCB spacing. The requirements for miniaturization of components are getting higher and higher.
Classification and attributes
Based on the different chemical elements in PCB coating materials, it is mainly divided into five categories:
- Acrylic resin.
- epoxy resin.
- polyurethane (PU) .
- parylene coating.
According to different solvent types or curing methods, it is mainly divided into three categories:
- Solvent-based paint.
- Water-based paint.
- UV light curing coating.
(1) Solvent acrylic resin
Advantage: Curing technology is relatively simple. Moreover, it does not react with air, and its curing process depends on the volatilization of the solvent; Excellent moisture resistance and flexible viscosity adjustment; Easy to rework and repair; The cost is relatively low.
Disadvantages: The protective coating is too thin after curing. The curing method determines the high content of volatile solvents, so the operating environment must have a good ventilation system. In addition, it does not perform well in maintaining viscosity, and returns to viscosity at high temperature and high humidity.
In short, this PCB coating is suitable for ordinary electronic products.
(2) Epoxy resin
Advantage: Humidity, salt spray and chemical resistance; High temperature resistance up to 150°C; Abrasion resistance;
Disadvantages: Has dielectric properties. PCB boards with a PCB coating coated with epoxy resin cannot be reprocessed. Unless the epoxy film is physically peeled off, this may damage the PCB components. In addition, potential high internal stress may also cause damage to fragile equipment, and protective measures should be taken before PCB coating is applied. This type of PCB coating has a relatively high peel content and a complex two-component ratio, with low viscosity maintenance. It also has poor low temperature and high curing shrinkage properties.
In short, PCB coatings composed of epoxy resins are rarely used.
(3) Organic silicon
Advantage: High flexibility; Operating temperature range up to 200°C, with excellent humidity and UV resistance; Can be repaired; It has obvious insulation and low surface energy, and has good wettability with the base material.
Disadvantages: The mechanical strength is low, and the adhesion to the base material is weak. Worse, it cannot withstand scratches.
In short, this type of PCB coating is most suitable for high-heat components. For example, high-frequency PCBs containing many high-power resistors are used in shipping, military and radar control systems.
Advantage: Excellent acid and alkali resistance; High humidity resistance.
Disadvantages: After a long time to complete the curing process. Due to its high VOC content, it tends to turn yellow at high temperatures. In addition, it tends to corrode the galvanized screws.
In short, this PCB coating has a wide range of applications, including automobiles, industrial control, electronic instruments, power and telecommunications.
(5) Parylene coating
Parylene coating separates polymers in vacuum. On the surface of the product is a uniform deposited coating. It can be used as a special protective coating in the electronics industry.
Advantage: Uniform thickness; Excellent acid resistance and corrosion resistance.
Disadvantages: High cost.
In short, parylene coatings are widely used in silicone rubber, magnetic materials, microelectronics, sensors and other fields. So far, parylene coating is the most effective protective coating for high-frequency components, high-density devices and high-insulation devices.
(6) UV curing coating
Advantage: Environmental protection; High security; High stability; Excellent humidity and corrosion resistance; High curing efficiency; can be reworked.
Disadvantages: If the application of UV light is insufficient for imaging, blind areas will occur.
Selection principle of PCB coating
Due to different physical and chemical properties and processing properties, the best PCB should be based on comprehensive considerations to choose the coating. Including the specific operating environment, electrical performance requirements and PCB layout.
The best PCB coating should meet the following requirements:
- Preferably a single component, easy to operate;
- Should have the characteristics of low viscosity and easy spraying;
- Has a long shelf time;
- It should be cured at high speed without producing by-products;
- Has excellent flexibility;
- Has a wide operating temperature range;
- Can protect the PCB board from extremely high/low temperature and mechanical shock;
- It has the characteristics of low toxicity and low cost;
- Can be modified.
PCB coating should be selected according to certification requirements. For example, RoHS, UL, etc. and PCB coating with low viscosity, environmental advantages and non-toxic performance should be selected. After determining a PCB coating, its effectiveness should be verified according to the PCB manufacturer’s technology.
The goal of technical verification is to prove whether this type of PCB coating is suitable for current technologies and products.
First, manual spraying or automatic spraying can be used to spray PCB coating on the product. Once cured, it should be observed whether the coating is flat, uniform, transparent or translucent and has no viscosity. In addition, you should observe whether you can see bubbles, pinholes, cracks, etc.
Second, the reworkability of the PCB coating should be considered, and the PCB coating can be easily peeled off. In order to verify the properties of this aspect, you can rely on physical methods, such as electric irons, mechanical methods or chemical methods. Usually, paint manufacturers provide dispersants. This chemical method is used to verify the peeling ability of PCB coatings and the equipment is not damaged.
Third, the protection performance should be verified. The basic goal of PCB coating is to provide protection for electronic products, so the protection performance should be verified. Dielectric withstand voltage test, insulation resistance test in humid environment, thermal shock test and hydrolysis stability test should be carried out to verify the performance of PCB coating.
The PCB coating sprayed on the surface of the PCB board and PCB components can effectively isolate the circuit board. Protect the circuit from the erosion and damage of the harsh environment. This can extend the service life of electronic products and ensure safety and reliability. In the PCB coating selection stage, in addition to verifying the performance of the coating, it should also be proved that it is suitable for the customer’s products, technology and equipment.
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