Conformal coating is a type of coating with special ingredients, used to protect circuit boards and concerning equipment from erosion finally to prolong products' shelf life and guarantee Printed Circuit Boards' (PCBs) security and reliability. Conformal coating features high capability withstanding high temperature and low temperature. It'll become a transparent protective film after solidification, featuring excellent merits including insulation, moistureproof, anti-creeping, shakeproof, dust proof, anti-corrosion, anti-aging, corona resistance etc.
The reason for the existence of conformal coating lies in the fact that PCB boards possibly suffer from issues like erosion, softening, deformation or mould in practical environment that perhaps embraces chemical, vibration, high-dust, salt fog, moisture and extremely high temperature all of which will possibly lead circuit boards to suffer from defects or failure.
Based on the definition and brief introduction on conformal coating, it is easy to summarize that the key role conformal coating plays in PCB manufacturing process lies in its protective function as moisture resistance, waterproof, insulation etc. The reason why conformal coating is a must-be for PCBs is so obvious but a little complex since it is related with so many elements and a couple of them will be discussed in detail in this part.
Moisture is the commonest and destructive factor causing damage to PCB boards. Too much moisture will dramatically decrease insulation resistance and the value of Q, accelerate high-speed decomposition, and get conductors eroded. It's common for us to see metal parts that have been assembled on PCB board is covered with a layer of green copper just because chemical action just takes place between copper and water vapor and oxygen.
It's hardly believe that hundreds of pollutants can be found on the surface of printed circuit boards when conformal coating isn't applied on them. Those pollutants are equivalently hazardous to the reliability and functionality of PCBs. Similar with moisture or high humidity, pollutants contaminated on circuit boards can also lead to bad results such as electronic failure, conductor erosion, or even irretrievable shortcuts. Majority of contaminants left on circuit boards derive from residues during manufacturing process including flux, solvent parting agent, clipped wire and marking ink. Moreover, some contaminants result from manual residue when boards are touched by operating people including human oil, finger print, cosmetics and food residue. Plus, lots of pollutants even come from operating environment, such as salt spray, sand soil, fuel, acid and other erosive vapor and mould.
Nevertheless, those pollutants can be greatly cut by effective measures that can be made during PCB fabrication process. New technologies can be used to optimize manufacturing process to cut down the number of chemical residues on circuit boards. Rigorous operating principles can be set up and conformed to and operation engineers are trained to be compatible with all requirement of scientific manufacturing. For example, when operators are required to wear gloves when holding boards in hand, no fingerprints will be left on them.
When conformal coating is used on the surface of circuit board or components, if negative factors in operation environment take the leading role, conformal coating is capable of reducing or eliminating electronic operating deterioration. As long as conformal coating is able to take hold for such a satisfying time period like longer than life time of final products, conformal coating will be believed to take its full responsibilities. Even though conformal coating is just a thin layer, it is able to excellently withstand mechanical vibration, thermal shock and high-temperature operations.
One point should be noticed that it's a myth that conformal coating is capable of making components being with stronger mechanical strength or sufficient conductivity. Fixation of components depends on mechanical method with joint mixture while conformal coating won't help.
At the very beginning, conformal coating was only used for PCBs belonging to high-tech field. Since electronic products have been increasingly applied in people's daily life, consumers start to pay more attention to the quality and reliability of electronic products. As a result, the application of conformal coating plays an essential role in assisting in effectively improving product quality and decreasing expense to handle breakdowns.
Typical application fields of conformal coating or conformal coating PCBs include:
a. Consumer Electronics. Conformal coating provides protection for circuits serving for home appliances so that they can defeat the following issues:
• Water and Detergent (washing machine, dishwasher, sanitary products, outdoor LED)
• Bad External Environment (monitor, security and fireproof alarm systems)
• Chemical Substances (air conditioner, dryer)
• Contaminants in Office and Home (computer, microwave oven) • Others
b. Automotive Industry. Electronic systems have been constantly called for so the application of conformal coating can meet the fundamental demands in terms of long-term reliability of automotives. Automotive industry calls for conformal coating in order to keep from the following hazards: gasoline vaporization, salt fog or brake fluid.
c. Aerospace. Due to specificity of application environment, aerospace environment sets extremely rigorous needs for electronic device, which is especially true when rapid compression and decompression are concerned. Thus, pressure stability of conformal coating has received wide applications.
d. Navigation. Both fresh water and salty sea-water tend to cause damage to the circuits in the sailing equipment. The application of conformal coating can protect equipment on the surface and below the sea water on the largest extent.
e. Medical Care. Conformal coating is capable of protecting electronic devices from erosion that may be led by external chemical agent or special application environment so that they can run consistently and smoothly.
How to Prepare for Well-Performed Conformal Coating in PCB Design Phase?
To obtain optimal conformal coating on your PCBs or PCBAs, there are some preparations you can make during your PCB design phase.
• Conformal Coating PCB boards that'll go through assembly must contain technical rail that should be at least 5mm.
• The maximum size of Conformal Coating PCB boards that'll go through assembly is 410mm*410mm while that of is 10mm*10mm.
• The maximum height of components assembled on Conformal Coating PCB boards is 80mm.
• The shortest distance between area with conformal coating and without conformal coating is 3mm.
• Complete cleaning enables the complete elimination of eroded contaminants and makes conformal coating solidly stuck to the surface of circuit boards. It's optimal to keep the thickness of conformal coating within the range from 0.1mm to 0.3mm and bake the board for 10 to 20 minutes within 60°C.
During conformal coating application, some components can never receive conformal coating including high-power components with heat delivery surface or heat sink, power resistor, power diode, cement resistor, DIP switch, adjustable resistor, buzzer, battery holder, fuse holder, IC holder, tact switch etc.