WHAT IS THE PCBA CONFORMAL COATING PROCESS?
PCBA conformal coating is the most common operation for conformal coating for PCB assembly processed products. It can protect the circuit board, modify the appearance of the circuit board, and achieve higher circuit board integration.
Electronic components, large or small, are vulnerable to wear and tear after prolonged use. Consumers expect electronics to last for a long time, so manufacturers have developed ways to protect their components from damage and keep them functional for extended periods of time. One of these methods is called conformal coating. This process protects PCBs from mechanical and chemical damage, extending their lifespan and reliability in multiple ways.
There are mainly the following four common operating processes:
1.Brush
The simplest PCBA conformal coating process can produce excellent coating effects on a smooth surface.
2.Spraying
The spraying method is the most commonly used coating method in the industry, and it is divided into automatic spraying and manual spraying. Machine spraying can realize automatic feeding, save labor and material costs, improve production efficiency, and ensure product consistency, product quality, and surface spraying effects.
3.Dip coating
Dip coating can ensure a complete coating without causing material waste due to overspray.
4.Selective coating film
The coating is accurate and does not waste materials. It is suitable for large-volume coatings, but the requirements for coating equipment are relatively high. It is most suitable for large-volume coatings. Using a prepared XY table can reduce the cover. When spraying PCBA conformal coating, many connectors do not need to be painted. Adhesive paper is too slow and there is too much glue remaining when it is torn. Consider making a combined cover according to the shape, size, and position of the connector, and use the mounting hole to locate it. Cover the parts not to be painted.
5.Matters needing attention during operation:
Clean and bake the board to remove moisture and moisture. The dust, moisture, and oil on the surface of the object to be coated must be removed first so that it can give full play to its protective effect. Thorough cleaning can ensure that the corrosive residues are completely removed, and make the three-proof paint adhere to the surface of the circuit board well. Drying plate conditions: 60°C, 10-20 minutes, take it out of the oven, and apply it while it is hot. The effect is better.
When coating PCBA conformal coating by brushing, the brushing area should be larger than the area occupied by the device to ensure that the device and pads are completely covered.
The circuit board should be placed as flat as possible when brushing the three-proof paint, there should be no dripping after brushing, and the brushing should be smooth, and there should be no exposed parts, preferably between 0.1-0.3mm.
Before PCBA conformal coating and spraying conformal coating, ensure that the diluted conformal coating is fully stirred, and leave it for 2 hours before brushing or spraying. Use a high-quality natural fiber brush and lightly brush and dip at room temperature. If machinery is used, the viscosity of the paint should be measured (with a viscosity agent or a flow cup), and a thinner can be used to adjust the viscosity.
The circuit board components should be immersed in the paint tank vertically. The connector should not be immersed, unless it is carefully covered, the circuit board should be immersed for 1 minute until the bubbles disappear, and then slowly take it out. A uniform film will be formed on the surface of the circuit board. Most of the paint residue should be allowed to flow from the circuit board back to the dipping machine. TFCF has different coating requirements. The immersion speed of the circuit board or components should not be too fast to avoid excessive bubbles.
When using it again after dipping, if there is skin on the surface, remove the skin and continue using it.
After brushing PCBA conformal coating, place the circuit board on the support and prepare for curing. The heating method is to accelerates the curing of the coating. If the surface of the coating is uneven or contains bubbles, it should be placed in a high-temperature oven for more time to be cured at room temperature to allow the solvent to flash out.
Essential Preparation for the Conformal Coating Process
Conformal coating only works well if a component is clean and free of physical defects. There must be no impurities on the component’s surface, including dirt, debris, or moisture, and the coating itself must be uniform. There are many different ways that uneven coating due to a dirty or defective component can cause failures, including corrosion, short circuits, and physical damage. Cracks can appear within the coating where the board and the coating meet, a defect called blistering.
There are also a number of other adhesion problems that can result from a badly prepared board, including discoloration or an uneven finish. One adhesion problem with a memorable name is “fish eyes”, which happens when little craters form on the surface of the coating due to the coating material running unevenly across the board as it sets.
Many conformal coatings are required to meet strict international standards, so creating a conformal coating process that adheres to those standards is essential. The main standard for conformal coatings is IPC-CC-830C, and it contains sections for every variant of conformal coating, along with specific concerns for a variety of environmental issues. These issues include water, fungus, and flammability. The standard also includes guidelines for electrical appearance and functionality, along with guidelines for using different materials and processes when making electronics. The standard was last updated in January of 2019 and is periodically updated to reflect the needs of the electronics industry. Other international standards exist for specialized use cases, including electronics assemblies and soldered electronics. In total, three different international electronics standards deal wholly or partly with the conformal coating process.
Improving Conformal Coatings through Masking
When placing conformal coatings on a component, there are certain areas that it’s best to leave uncoated. These areas can include connectors, holes, LEDs, and certain types of integrated circuits. Preventing these areas from being coated involves a process called masking.
There are multiple masking techniques, including shields, tapes, fluids, and gels. There is no specific technique that is always best for a particular masking scenario, and designers must consider many factors before masking. The shape, geometry, and function of the masked area, along with the level of protection given by the masking, are major factors that should influence how a designer decides to apply masking. Designers must also take into account the expected lifespan of the masking and how difficult it would be to remove the masking if necessary.
Some masking techniques are permanent in preventing adhesion of conformal coatings, while some are temporary. Permanent liquids and gels are common, but some new technologies have opened up more possibilities in temporary masking. Temporary masking liquids that can be cured with ultraviolet light are a recent addition. They are easy to apply and remove without requiring physical media like shields or tape. In addition, they don’t have the same restrictions as solid masking materials on board geometry. Despite these advantages, these temporary liquids require skilled application and removal techniques—although some automation is possible, a skilled technician must direct the application tools, and the process to do that is not trivial.