PCB Tech - Common process problems and solutions in SMT electronics manufacturing

Common process problems and solutions in SMT electronics manufacturing
For electronic equipment devices that work in harsh environments such as high humidity, high salt, dust and vibration, the circuit board components (PCBA) are susceptible to the influence of salt spray, moisture and mold and cause system failures. Therefore, the three-proof coating of circuit board components Overlay technology is receiving increasing attention. Three-proof coating refers to coating a layer of uniform thickness of three-proof paint on the area that the circuit board component needs to protect. It can effectively isolate the area and electronic components that the circuit board component needs to protect from the working environment and protect the circuit board. The components are protected from damage, thereby improving the reliability of the circuit board components and further improving the reliability of the electronic equipment device.

1. Three anti-paint materials

According to the classification of materials, the current common three anti-paints 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.

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

The choice of the three-proof paint depends on the application environment of the product and adapt to different production needs. 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 surface curing time at room temperature is within 10 minutes, and it is fully cured within 24 hours. When heated to 60°C, it can be fully cured within 30 minutes. For circuit boards used in military products, epoxy resins, polyurethanes and silicone greases are often used in three anti-paints because they often face harsh environmental problems such as high temperature, high humidity and vibration. The temperature resistance, flame retardancy and dielectric properties of these three paints are all ideal. The characteristics of the four commonly used three-proof 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 workplaces, silicone-based three-proof paint can be used. It can form a layer of elastic film on the object to be protected. The tolerance range is also very wide, from -60°C to 200°C. Silicone conformal coatings can be cured on the surface 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 of the circuit board components is clean to ensure that the three-proof paint adheres well. Three anti-coating processes include spraying, brushing and dipping. The coating process is different, the three anti-paint viscosity requirements are also different. When using the spraying process, the three anti-paints require the lowest viscosity to meet the "spraying" capability requirements, while the dip coating requires the highest viscosity of the three anti-paints. The viscosity of the three-proof paint is determined by the solid content of the three-proof paint. The manufacturers of the three-proof paint generally have their own thinners to dilute products with different concentrations to meet the requirements of different coating processes.

2.1 Basic coating methods of three-proof paint: hand spraying, dipping and brushing. Although the operation is simple and does not require any equipment investment, the coating quality is difficult to guarantee. There are poor consistency of coating film thickness and large loss of three-proof paint., Pollution to the environment, easy to splash and contaminate adjacent sockets during coating, inability to achieve selective spraying, manual sticking of adhesive paper before coating to cover the devices that need to be protected, and some slits between the components and the side of the components cannot be sprayed. The existence of these problems has affected the quality of the products and needs to be solved urgently. The selective automatic coating machine is the perfect solution to the above problems. It can realize the precise spraying of the circuit board components, the spraying head can avoid 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 manual turnover procedures;

2) It has the functions of continuous irregular curves such as point/line/surface/arc/circle and three-axis linkage;

3) Multiple different spraying heads can be hung at the same time, and the equipment can be automatically switched according to different spraying requirements to meet the requirements of different spraying areas;

4) CCD visual centering system can be selected to eliminate the positioning error of circuit board components or central fixtures and improve spraying accuracy;

5) Accurate control of the spraying volume of the three-proof paint.

2.3 Three-proof coating process requirements To achieve high-quality three-proof coating, the following process requirements need to be met during the three-proof coating:

1) For components that cannot be stained with three anti-paints, such as connectors, connectors, buttons, light-emitting diodes, and golden fingers, when using dramatic coating, dip coating, or manual spraying, masking tape should be attached. 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 surface of the PCBA to remove foreign matter on the surface of the PCBA. 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 applied. For general products, dry high-pressure air can be used to remove dirt such as dust on the PCBA surface;

3) Before adopting the selective automatic coating machine for mass production, it is necessary to pay attention to the first article inspection work. Confirm that there are no problems such as missing coating, undercoating, overcoating and air bubbles, connectors and other devices or areas that are forbidden to be coated must not have sticky paint, and the coating thickness can be uniform before baking;

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 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 pre-bake before spraying. The baking temperature is 60°C and the time is 24h.