PCBA Tech - Principles and parameters of SMT three-proof coating

For electronic equipment devices that operate in extremely harsh working environments such as high humidity, high salt, dust and vibration, their PCB circuit board components (PCBA) are susceptible to the influence of salt spray, moisture and mold to cause equipment failure, so PCB circuit board components The three anti-coating technology is attracting more and more attention. The three-proof coating refers to the coating of a layer of uniform thickness of the three-proof paint on the PCB circuit board components that need to be protected. It can effectively isolate the PCB circuit board components that need to be protected and the electronic components from their working environment. The PCB circuit board components are protected from damage, thereby improving the reliability of the PCB circuit board components and further improving the reliability of the electronic equipment.

1. Three anti-paint materials

According to the classification of materials, the more common three anti-paints at this stage can be divided into four types: acrylic, epoxy, polyurethane and silicone resin; from the curing method, there are room temperature curing, thermal curing and ultraviolet curing, etc.

There are many types; classified from the coating method, there are different process methods such as dipping, brushing and spraying.

The choice of materials for the three-proof paint depends on the application environment of the product and different production requirements. Ordinary electronic products can choose acrylic three-proof paint, which has low cost and can withstand a wide temperature range, from low temperature -60C to high temperature 130C. The curing time of the surface layer at room temperature is within 10 minutes, and it can be fully cured for 24 hours. When it is heated to 60°C, it can be fully cured in 30 minutes.

PCB circuit boards used in military products often face extreme environmental problems such as high temperature and high humidity vibration, so epoxy resin, polyurethane and silicone grease are mostly used for three anti-paints. The temperature resistance, flame retardancy and dielectric properties of these three paints are all ideal. The characteristics of the four commonly used three-anti-paints are shown in Table 1. Polyurethane and silicone have better heat resistance, so in military, locomotives, industrial control electronic instruments and power communications and other occasions, polyurethane three-proof paint will be used. In high-humidity and high-salt-fog operating situations, silicone-based three-proof paint can be used. It can form an elastic film on the object to be protected. The moisture-proof, salt-fog and corrosion-resistant effects are very good. In addition, the temperature The tolerance range is also very wide, from -60°C to 200°C. The surface layer can be cured in 30 minutes at room temperature and fully cured in 24 hours.

2. Three-proof coating technology and application process Before the three-proof paint is used, it is necessary to ensure that the surface layer of the PCB parts is clean to ensure that the three-proof paint adheres well. Three anti-coating processes include spraying, brushing and dipping and other different process methods. The coating process is different, and the viscosity requirements of the three anti-paints are also different. When using the spray paint process, the viscosity of the three-proof paint is the lowest to meet the demand for "spraying" ability, and the viscosity of the three-proof paint is the highest for the dip coating. The viscosity of the three-proof paint is determined by the proportion of solids in the three-proof paint. The manufacturers of the three-proof paint usually have their own paint thinners to dilute products with different concentrations, which are suitable for different coating process requirements.

2.1 The basic coating methods of the three-proof paint are hand spraying, dipping and brushing methods. Although the actual operation is simple and does not require any investment in machinery and equipment, the coating quality is difficult to ensure. There are poor consistency of the coating film thickness and relatively high loss of the three-proof paint. Large, pollute the working environment, easy to splash and contaminate adjacent sockets during coating, selective spraying cannot be completed, manpower is required before coating to cover the components to be protected, the slits between some components and the sides of the components Can't spray to wait for the problem. The emergence of these problems directly affects the quality of the products and urgently needs to be resolved. The selective automatic coating machine is the perfect solution to the above-mentioned problems. It can complete the precise spraying of PCB circuit board components. The spraying head can bypass the non-stick paint components, and the spraying thickness is uniform.

2.2 The performance of the selective automatic coating equipment should ensure the high quality of the three-proof coating, and the selective automatic coating machine should

Meet the following technical parameters and performance requirements: 1) On-line automatic flow operation, reducing manpower turnover procedures; 2) Irregular curve continuous and three-axis linkage functions such as point/line/surface/arc/circle; 3) Additional functions are available Several different spraying heads are suspended, and the equipment can be automatically switched according to different spraying requirements to meet the requirements of different spraying areas; 4) CCD vision centering system can be selected to eliminate the positioning error of PCB circuit board components or central fixtures and improve spraying accuracy; 5 ) Accurate control can be carried out on the spraying volume of three anti-paints.

2.3 Three-proof coating process requirements

To complete high-quality three-proof coating, the following processing requirements must be met during the three-proof coating:

1) For components that cannot be stained with three-proof paint, such as connectors, connectors, buttons, LEDs, and golden fingers, when choosing the processing methods such as dramatic coating, dip coating or manual spraying, cover glue must be applied For paper, even if the fully automatic selective coating machine is used, if the coating area is too close to the non-painting position (less than 5mm), isolation and prevention work must be done;

2) Before coating, it is necessary to clean the PCBA surface layer to remove foreign matter on the PCBA surface layer. For products with strict requirements such as automobiles, aerospace, navigation and military industries, the PCBA must be washed and dried before the coating can be carried out. For general products that are not high, dry high-pressure air can be used to remove dirt such as dust on the PCBA surface layer;

3) Before selecting the optional automatic coating machine for mass production, you must pay attention to the first article inspection. Confirm that there are no problems such as missing coating, less coating, more coating, and air bubbles, connectors and other devices or areas that are forbidden to be coated must not have adhesive paint, and the coating thickness is uniform before baking can be carried out;

4) The humidity of the working environment should be controlled below 65% RH during the three-proof coating. PCB as a composite material will absorb moisture. If moisture absorption occurs, the three-proof paint will not be able to fully protect it. Therefore, the three-proof coating process should be carried out as soon as possible after the PCB circuit board is assembled. If the PCBA is left for a long time and then sprayed with the three-proof paint, it is best to carry out pre-baking before spraying. The baking temperature is 60 degree Celsius and the time is 24h.

3 Common defects and solutions in the coating process of three anti-paints

4 Practical application of selective automatic coating equipment. Selective automatic coating has the advantages of high output, saving three-proof paint, uniform coating thickness, complete selective spraying, and low pollution to the surrounding environment.

The three-proof coating is a very delicate work. There are many factors that affect the quality of the coating. We need to further study and discuss in order to improve the coating quality of the three-proof paint. Compared with traditional coating processes, selective coating equipment has huge performance and cost advantages, and it can be foreseen that it will be widely used in the PCB electronics manufacturing industry in the future.