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 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.
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.