Many electronic components are designed on the PCB circuit board, and these components are easy to affect their performance when the PH value is too high, such as insulation deterioration, spontaneous short circuit, power loss and so on. After uniform coating of Conformal coatings, a protective film can be formed on the surface of components, which can improve the stability of circuit board, increase its safety factor and prolong its service life. How to inspect the Conformal coatings after coating?

Before applying the Conformal coatings, the circuit board shall be cleaned first, and then applied with the Conformal coatings after baking dry. The Conformal coatings applied on the board can be seen with the naked eye whether the adhesion is strong and whether the transparency is OK. Whether the brushing is uniform, blistering or whitening. After the Conformal coatings is fully cured, sprinkle a little water on the surface of the board to test whether it can play the effect of waterproof and moisture-proof. In addition, the effect of anti-corrosion and anti salt fog can not be seen immediately. After a certain period of time, the board will be damaged in the regular pollution. Conformal coatings can reflect its protective value.

The test standards after the application of Conformal coatings are as follows:

1. Adhesion (refer to GB / t9286-1998 standard: Cross cut experiment method, Level 0 (maximum)

2. Salt spray test (refer to GB / t1771-2007, GJB150, standard: verify the corrosion resistance of the product. The standard is 72h and the limit test is 1000h)

3. Water absorption (refer to GB / t1738-1979 standard: verify the protection and safety performance of the product)

4. Insulation resistance in humid environment (refer to ipc-tm-6502.6.3.4a standard: verify the insulation safety of products)

5. Mildew resistance (refer to ipc-tm-6502.6.1, GJB150, standard: 28 day mold test, level 0 is the highest)

6. Weather resistance / cold and hot shock test (refer to ipc-tm-6502.6.7.1a, GJB150, standard: IPC Standard 100 cycles, limit test up to 600 cycles)

7. Acid and alkaline resistance test (refer to gb1763-1986 standard: no abnormality of paint film)

8. Breakdown strength (refer to gb1981.2-2003 standard: verify the insulation performance of the product)

9. Corrosion resistance (refer to ipc-tm-6502.6.15 standard: the test sample is free of corrosion)

10. Odor requirements (enterprise defined standards: small odor, no irritating and unpleasant odor; no interference to gas detector parts)

Many customers have bubbles when using the three proofing paint. The reasons for this phenomenon are generally caused by the fact that the circuit board is not cleaned or dried, and there is dust and humidity on the circuit board. If the material has bubbles from the nozzle, factors such as coating process pressure, material viscosity, nozzle model, humidity and so on need to be considered. If bubbles appear after passing through the furnace, please carefully adjust the furnace temperature curve and the traveling route of the nozzle.

If we pay attention to the following five points when using three proofing paint, we can reduce the probability of bubbles.

1. Before using the three proofing paint, remove the dust, moisture and oil on the surface of the object coated with the three proofing paint to ensure that no corrosive residue remains on the surface of the object, so that the three proofing paint of PCB circuit board can adhere to the circuit board well, which is conducive to give full play to the three proofing (moisture-proof, salt fog proof and mold proof) performance of the three proofing paint of circuit board.

2. When applying with brush coating method, the area of three proofing paint brush coating of PCB circuit board should be larger than the area occupied by the device, so as to ensure that the device can be fully covered.

3. When brushing the three proofing paint of PCB circuit board, try to ensure that the objects to be brushed can be placed flat. After brushing, there should be no dew to ensure that the brushing is flat, and there should be no exposed places. The thickness of brushing can be about 25 ~ 50 microns according to different requirements. If there are higher requirements, secondary coating can be carried out.

4. If the immersion method is adopted for coating, ensure that the coated object is vertically immersed in the coating tank (note that the connector cannot be immersed unless carefully covered), immerse it for about 1 minute, and then take it out slowly. The immersion speed should not be too fast to avoid excessive bubbles.

5. After the coating is completed, the heating method can be used to accelerate the curing of the three proofing paint of PCB circuit board. The heating temperature should not be too high, and it is better to be within 80℃.

Epoxy adhesive usually refers to the adhesive made of epoxy resin as the main body. Epoxy resin adhesive usually also includes epoxy resin curing agent, otherwise the adhesive will not cure. After curing in operation, foaming may occur, which may be caused by the following two reasons:

First, bubbles are generated during glue mixing or filling. Due to the viscosity of glue or the difficulty of mixing method, air is brought into the glue solution. The viscosity of the glue is high, and the bubbles are difficult to eliminate. If the glue viscosity is small and the glue solidifies slowly, the bubbles will gradually rise to the surface and disappear automatically.

The second is the bubbles in the curing process. There are other reasons for bubbles in the curing process: too fast curing speed, high exothermic temperature, large curing shortening rate of glue, too many solvents and plasticizers in glue, etc., which will only produce bubbles in the curing process. In order to solve these problems in the curing process, the whole glue formula needs to be adjusted. In order to eliminate bubbles during adjustment and filling, vacuum pump, heating, diluent or defoamer can be used.

The thermal conductivity of ordinary silica gel is poor, and the thermal conductivity is usually only about 0.2w/m · K. However, the thermal conductivity can be improved by mixing thermal conductive filler in ordinary silica gel. Commonly used thermal conductive fillers include metal powders (such as Al, Ag, Cu, etc.), metal oxides (such as Al2O3, MgO, BeO, etc.), metal nitrides (such as sin, AlN, BN, etc.) and non-metallic materials (such as SiC, graphite, carbon black, etc.). Compared with metal powder filler, although the thermal conductivity of metal oxide and metal nitride is poor, it can ensure the electrical insulation performance of thermal conductive potting adhesive.

Another major factor determining the thermal conductivity of potted silica gel is the manufacturing process. The temperature control, pressure, filler and the feeding sequence of various additives in the production process of liquid potting adhesive will also play a decisive role in its thermal conductivity. For example, if the vacuum pressure fails to meet the requirements, too many bubbles will be generated in the raw material, which will directly affect the service performance of potting adhesive. Excessive thermal conductive filler can not only improve the thermal conductivity, but also directly affect the adhesion and fluidity of potting adhesive. Therefore, the potting silica gel with high thermal conductivity and excellent comprehensive performance can be obtained by optimizing the type and amount of filler and the ratio of filler to other additives.

Since the most widely used fields of potting adhesive are high temperature and high pressure environments such as power supply, electronic components and outdoor electrical equipment, the insulation strength and temperature resistance of potting adhesive are highly required. In addition, many electronic industries have UL flame retardant grade requirements for potting adhesive, which is required to be V0 grade. Generally, the working temperature of thermal conductive potting adhesive is -50℃ ~ +240℃, while high temperature resistant potting silica gel can withstand 1200℃ or even higher.