PCB conformal coating is a specially engineered polymeric film chemical paints that protects PCB board, electronic components from harmful environmental conditions like moisture,Pollutant, static, vibration, and contamination. The coatings layer is printed to the irregular area of the PCBA board providing increased dielectric resistance, reliability.
The PCBs after manufacturing and assembly are highly prone to contamination which in turn reduce the shelf life of PCBs. The conformal coating is the protective layer made of mixture of different ingredients, to keep PCBs safe from erosion, moisture, contamination, oxides and other particles that can lower the quality of PCBs and greatly decrease performance or even render PCB useless.
The conformal coating can be applied in different ways on PCB surface. The conformal coating will become hardened and transparent layer upon PCB that gives insulation, anti-aging, moist proof and dust proof advantages to PCB. High operating temperature and stringent environmental conditions like shock, vibrations, chemical, inks and acids can badly affect the PCB without conformal coating and can permanently damage or fail the PCB.
Why Conformal Coating Necessary..?
As discussed above, there are many factors that can adversely affect the PCB performance if conformal coating is not applied. Conformal coating actually is a preventive coating that functions as protective measure to protect PCBs from external contaminants that may occur even during the manufacturing of PCBs like flux and other etching chemicals and agents. It prevents the PCB from mechanical vibrations, thermal shocks and high temperature due to environmental conditions or PCB high operating temperature. It increases the reliability and ensures proper functionality of PCBs.
As we all know that moisture is not good for metallic parts, as we observe that areas / locations that are close to sea, have high levels of humidity in air as compared to other areas in cities. So this humidity level will create oxidization in metallic parts and electronic components. PCBs are totally made of metallic parts like copper, silver and aluminum main metals used, hence these metals get reacted with moisture in air hence rendering the PCB to decompose at accelerated pace and conductors get eroded. The copper will turn green due to these water vapors and elevated oxygen levels.
However the conformal coated PCBs are far more protected from this high levels of humidity and water vapors that can be present in our environment where PCBs are used. The conformal coating will prevent the oxidation and deterioration of conductors on PCB hence increasing the life of PCB.
Another most important reason why conformal coating is necessary is pollutant or contaminations. These contaminants can be solid dust particles that can stuck in the tiny spaces in PCB SMT components and can become short-circuit. Other handling pollutants can be human oil, finger prints and food particles. The surrounding where the PCB is being kept or used also contributes to pollutants like spray, grease, oil, fuel spill over, and acid fumes exposure.
The technicians on field must be trained to properly handle electronic equipment’s PCBs by wearing gloves, not to spit and spill liquid and avoid eating food while handling PCBs. PCBs without conformal coating, under these rigorous environmental conditions will get easily damaged and irreversible electronic failure in respective electronic device or equipment.
Pollutants are mainly caused by the imperfection in PCB manufacturing setup like improper cleanliness and chemical residues being uncleansed during electroless plating or etching process.
It is highly recommended to clean the PCB thoroughly before applying conformal coating.
Which PCB Applications Use Conformal Coating:
Nowadays, quality is the best policy. If the quality of the end product is compromised then there is a strong chance that your customer will not order PCB manufacturing or assembly again from you. Hence quality is on top of everything.
Having said that, no matter how expensive is the conformal coating (by the not much expensive), it has become the important part of final finishing stage of PCBs after manufacturing. Almost every industry that has electronic equipment, machines, instrument, devices based on Printed Circuit Boards (PCBs) use conformal coated PCBs.
The consumer electronic products are highly prone to contamination and dust particles that may cause due to bad handling of PCBs like spill-over of water, oil, chemical, detergent and other substances.
- Automotive Industry
The PCBs used in car electronics, audio circuits, AM/FM circuits, front deck, led light circuit are all vulnerable to vibration and shocks. The conformal coated PCB will be protected from harsh environment and will last long
The aerospace environment is full of radiation, shocks and high temperature. Conformal coating on aerospace PCBs is highly recommended.
The electronic devices installed on ships, boats and Navy equipment are all highly affected by elevated humidity conditions. Hence conformal coated PCBs for navigation and said equipment is highly recommended.
- Medical Electronics and Microbiology Instruments
These sensitive electronic instruments are based on delicate PCBs that require to function smoothly and consistently for long life. This is only possible with conformal coating on PCBs of medical electronic devices to protect them from accidental splash of harsh acids or bases or hazardous chemicals used in labs.
How to Apply Conformal Coating:
There are mainly two ways to apply conformal coating at the PCB manufacturing facility.
Manual Brush Coating:
The manual brush coating can be a cost effective solution but can be time taking process. A skilled technician who can smoothly and uniformly brush the conformal coating on PCB surface is recommended.
Manual Aerosol Coating:
A specialized spray room where external contaminant are minimal should be used. A skilled technician with aerosol in hand can spray manually for small scale production of PCBs in cost effective manner. Do not shake the aerosol before spraying as it will cause bubble formation and transferred to board. Keep the PCB at 45O to the surface and distance between the aerosol and PCB should be 20-30 cm, and spray the conformal coating in zig-zag manner from top to bottom, then rotate the board at 90O and repeat until the boars is rotated complete 360O. This process is done to ensure uniformity of conformal coating and that every single point on PCB is coated uniformly
Manual Atomized Spray Coating:
The atomized spray coating is the same process as aerosol coating, but it is much quicker and efficient method. Proper ventilation must be ensured. Proper viscosity and correct ratio of air and material must be ensured.
Automated Dip Coating:
This method is performed by automated robot at very slow pace to ensure proper and uniformly coated PCB. In this method, if you want some parts of PCB not to be coated then you can mask them initially and then initiate the dip coating. After dip coating, the mask layer has to be removed. Since masking and unmasking and process itself is all time taking and material for mask is also wasted hence not recommended for cost effective solution
Automated Selective Coating:
The automatic coating method is performed by robotic hands where the pre-program is fed in the robot memory to tell which portions of PCB to coat and which to not coat. Robots do this task accurately, without getting tired and consistently repeating the same task every time
How to Select PCB Coating for PCBs’ Optimal Performance
As we all are aware of the constantly improving technological advancement in electronic devices and circuits, the pinnacle of engineering art in Printed Circuit Board (PCB) design, development and manufacturing has been witnessed by the people of this world. Our earth is full of highly intelligent machines, automated robots, and scientific miracles and of course lots of PCBs all around each and every corner of earth no matter which country it is or which city it is. However, these PCBs do differ from each other in terms of various aspects like functionality, complexity, and cost of manufacturing, quality and reliability. The topic of discussion of this article is the last two points i.e. quality and reliability of PCBs.
Yes it is right that a high quality electronic product is always desired by potential customers, but it is also very costly and may have complex manufacturing processes involved. Among those complex processes like PCB fabrication, assembly and testing there is a very significant process called “Conformal Coating” of PCBs. This conformal coating is highly important in PCBs in terms of Quality and Reliability of PCB.
What is Conformal Coating and Why it is Important:
The conformal coating the ultra thin protective coating of polymeric film that can be applied on the surface of PCB along with components mounted to protect the electronic components leads, solder joints, exposed traces and other metallic points on the PCB surface from erosion, dust or chemicals arising due to various operational or environmental conditions.
This conformal coating can be as thin as 25 micron and it “conforms” to board shape and component layout. As mentioned, the reason to apply conformal coating on the surface (top and bottom) of PCB is to protect the PCB from external unfavorable environmental conditions so as to increase the operating life of PCB and related electronic device.
Like high temperature in industries, factories, high power electronic machines, these PCBs with conformal coating can sustain extreme temperatures. Likewise electronic devices installed in regions/areas near to sea or oceans can suffer high humidity for example navigation electronics navy equipment can suffer from corrosions/erosions that can result in metal oxidation. Similarly sensitive electronic devices in microbiological labs, medical industry can suffer from toxic chemicals, acidic and basic solvents that can be accidentally spilled upon the PCB but “Conformal coating” on the surface of PCB will protect PCB and components from fatal damage.
How is Conformal Coating Applied?
Actually the method to apply “Conformal Coating” in the proper way is very significant and must be carefully taken into consideration how you are going to apply Conformal Coating. It is as important as the selection of right material for your conformal coating. The main factors that determine the appropriate application of conformal coating are 1- The thickness of coating 2- The level of coverage achieved 3- How well the coating sticks to the board and its components. There are five methods used to apply conformal coating. 1- Hand Coating by brush 2- Aerosol coating 3- Atomized Spray gun coating 4- Automated Dip coating 5- Automated Selective coating
Conformal Coating Curing/Drying Methods:
The conformal coating itself can be classified according to the drying and curing methods being used after conformal coating is done. These methods are
1- Heat/Thermal Cure: where conformal coating is dried in elevated temperature. The speed of drying is much faster than normal room temperature dry/cure.
2- Condensation Cure: in which the PCB’s conformal coating is dried by ambient temperature and atmospheric moisture slows down the curing or drying process.
3- Ultra-Violet (UV) Cure: here the PCB with conformal coating is exposed to UV radiation. The energy of UV light determines the rate of curing of PCB conformal coating
4- Oxidation Cure: In this method, the PCB conformal coating is exposed to open air with high amount of atmospheric oxygen that will help drying/curing due to solvent based conformal coating
5- Catalytic Cure: It is the process of curing conformal coating in which two materials are fused together and one of them is conformal coating. Once the coating is fused with other catalyst material the process of cure is unstoppable until it completes.
Conformal Coating Material Classification:
There are mainly five types of conformal coating materials used. Acrylic Acid resin, epoxy resin, organic silicon, polyurethane (PU) and Parylene coating.
1- Acrylic Acid Resin (AAR):
The Acrylic Acid Resin is highly suitable for (low cost and high volume) ordinary electronic devices because AAR is inexpensive and can easily be applied on the surface of PCB by means of brush, dip and manual or automated spray, thus reducing turnaround time and producing cost effective products.
1- Low Cost
2- Easy to Apply with hand or automated robot
3- Easy to rework
4- Superior Moisture Protection
5- Good Surface elasticity, withstanding static voltage discharge and non-reactive to atmospheric air thus aiding in curing through solvent volatilization
1- As the atmospheric air curing/drying method is used for this material hence a proper ventilation system needs to be ensured
2- Low maintenance of viscosity
3- Low abrasive and chemical resistance
2- Epoxy Resin Conformal Coating (ER):
The conformal coating based on epoxy resin can be done by hand brush, spraying, or dipping. The spray method is recommended for larger production run while brushes are for smaller run or prototype PCBs assembly.
1- High resistance towards moisture and has good dielectric resistance
2- Excellent resistance towards chemicals, abrasives, humidity and high temperature as much as 150OC
1- Epoxy based conformal coatings are very hard and rigid, if tried to peel off or remove then it will cause damage to PCB and its components. By using dangerous solvents to remove this coating it can
2- Bad performance in low temperatures
3- High curing shrinkage
4- They are very difficult to rework
3- Organic Silicon Resin (OSR) Conformal Coating:
The most flexible of the above two types of conformal coating material is Organic Silicon Resin (OSR) conformal coating. These are extensively used in LED lights PCBs without degrading light intensity or color change. Very good for PCBs that are mounted in high humidity open to air external environment. Suitable for PCBs with high operating temperature and high power
1- Good resistance to chemical, moisture, salt fog and high temperature up-to 200OC
2- Good flexibility makes it resilient against vibration stress that can act from external environment on PCB.
3- Good for high humidity outdoor applications of PCB
1- Not abrasion resistant due to rubbery nature
2- Rework is possible but not easy, requiring specialized solvents, long soak time, and agitation like from a brush or an ultrasonic bath
3- Low mechanical strength and weak adhesion to PCB substrate
4- Polyurethane (PU) Conformal Coating:
Suitable for PCBs application in automotive, industrial, instrumentation and telecommunication. Especially in aerospace equipment where fuel vapors are continuously striking the electronic equipment body and thus penetrating inside to affect the PCB board
1- High resistance to moisture, chemical (acid and alkali) and abrasion
1- It goes through a complete curing process for a long time and it tends to become yellow at high temperature due to its high VOC content
2- Like Silicon full removal is not easy
5- Parylene Conformal Coating:
This type of coating is suitable for aerospace electronics, microelectronics, sensors, high frequency circuits, densely populated components based PCBs. It is applied by vapor phase deposition method.
1- Outstanding dielectric strength
2- High resistance to moisture, solvents, extreme temperatures and acidic erosion
3- Very thin coating is possible with evenness.
1- Removal/rework is very difficult
2- High cost is the biggest disadvantage.