Introduction
Conformal coating refers to a thin protective material applied over the surface of a printed circuit board (PCB). The coating conforms to the contours of components and traces, providing insulation, chemical resistance, and environmental protection. Read on to learn more about conformal coatings, their properties, application methods, design considerations, and benefits for improving PCB reliability.
What is Conformal Coating?
A conformal coating is a thin polymer film deposited and cured over a circuit board. Coatings are applied in liquid form then harden to produce a uniform protective layer. The material conforms closely to board topography, providing insulation and sealing out contaminants.
Conformal coatings are usually 25-75μm (1-3 mils) thick when cured. This allows them to cover complex and dense board geometries. Common materials used include polyurethane, silicone, acrylic, epoxy, and parylene. Additional filler compounds can be added to modify properties like thermal conduction or dielectric constant.
The primary functions of conformal coating include:
- Preventing electrochemical migration between traces
- Isolating high voltage circuits
- Shielding components from moisture and chemicals
- Protecting boards from dust, dirt and debris
- Providing mechanical support for fragile components
- Electrically insulating boards and trace conductors
Applying a suitable conformal coating enhances the reliability and lifespan of PCB circuitry in demanding environmental conditions.
Why Use Conformal Coatings?
Conformal coatings provide important protections that improve manufacturing yields and extend operating life of circuit boards. Key benefits include:
Contamination Resistance
The conformal coating isolates boards from dust, dirt, and debris. This prevents contaminant particles from causing short circuits or corrosion.
Moisture and Chemical Resistance
Coatings block access of water, solvents, or cleaning agents that could lead to electrochemical corrosion and dendritic growth between traces. This is critical for boards exposed to condensation or fluids.
UV Protection
UV-blocking conformal coatings shield components and boards from solar radiation damage. This extends service life for products used outdoors.
Fungus Resistance
In hot, humid environments, fungus can grow on boards and cause shorting or signal leakage. Fungicidal coatings prevent fungal contamination.
Thermal and Mechanical Shock Protection
The coating layer provides a cushion from sharp temperature changes or impacts from vibration, dropping, etc. This reduces physical damage like cracked solder joints, broken traces, or delaminated pads.
Electrical Insulation
Conformal coatings isolate closely spaced conductors. This allows increased component density by preventing arc-over and shorting between traces. It also insulates boards from accidental contact with other objects.
Corrosion Protection
Moisture trapped against metal component leads and pads creates electrolytic corrosion cells. The coating forms a barrier that blocks electrochemical reactions.
Proper conformal coating improves product reliability and prevents field failures in harsh operating environments. The process is considered mandatory for high reliability electronics like military, automotive, aerospace, and medical.
Conformal Coating Materials
Various material types and formulations are used for circuit board coatings. Key properties include:
Acrylic
- Most common and low cost material
- Provides good humidity and chemical resistance
- Hard but somewhat brittle film when cured
Silicone
- Excellent temperature resistance and flexibility
- Resists humidity, chemicals, radiation, fungus
- Softer coating more prone to cuts and abrasion
Urethane
- Tough, abrasion resistant flexible coating
- Good humidity and chemical resistance
- Can be applied thick without running or sagging
Epoxy
- Hard, rigid, durable coating layer
- Excellent chemical and moisture resistance
- Withstands high temperatures up to 200°C
Parylene
- Applied via vapor deposition so coats all surfaces uniformly
- Excellent penetration into tight spaces
- Inert, biocompatible, transparent, and flexible
Other
- Paraxylylene (parylene variant)
- Silicones blended with epoxy or urethane
- Addition of fillers like Teflon or silicone dioxide
Material selection depends on needs for flexibility, temperature resistance, dielectric properties, thickness, etc. Products are available in many formulations optimized for specialized applications.
Application Methods
Conformal coatings can be applied to assembled boards manually or using automated equipment. Common application methods include:
Manual Brushing
The coating is brushed onto the board using an ordinary paint brush or specialized soft brush. Low volume applications are often brushed by hand.
Dipping
The completed board is dipped into a bath of coating material then withdrawn to leave a layer coating all surfaces.
Conformal Coating Spray
An air atomizing spray gun is used to apply coating in a fine mist over the board surfaces. This produces a thin, uniform coating layer and is a common manual application method.
Selective Coating Spray
A specialized spray system uses programmable X-Y tables and masking fixtures to only coat specific areas of a board. This allows selective application on high volume production lines.
Auto-Jet Spray
Similar to inkjet printing, piezoelectric nozzles propel tiny droplets of coating onto the board in programmed spray patterns. This provides controlled, precise application.
Vapor Deposition
Parylene coatings are applied by vaporizing a precursor liquid and depositing the resulting polymer uniformly over all board surfaces in a vacuum chamber. No solvents are used.
Choosing a suitable application method involves tradeoffs around coating performance, throughput requirements, capital cost, and process complexity.
Design and Assembly Considerations
Applying a conformal coating during board assembly impacts certain design and manufacturing factors:
Coverage Requirements
Determine if specific areas like connectors or testpoints require mask-off from coating. Manual coverage is difficult for dense designs.
Component Height Clearances
Tall components could interact or shadow areas from coating spray. Design spacing for spray nozzle access.
Board Fixtures
Masking or holding fixtures that secure boards in coating equipment can obstruct overlay. Include keepouts in board outline.
Vias or traces under large components require coating gaps to prevent thermal expansion damage during soldering. Add cutouts in coating layers.
CTE Mismatch
Coating and board materials expand differently when heated. Size connections and anchors to withstand shear forces during thermal cycling.
Component Compatibility
Verify coatings will not damage component materials or inhibit proper electrical or mechanical function. Test for incompatibilities.
Coating Viscosity
Thin, low viscosity coatings flow better around dense components but cover less evenly. Match coating thickness to clearances.
Repairability
Coatings make it harder to probe testpoints or replace components. Ensure repair access if needed.
Conformal coatings add requirements that must be addressed during both the design and manufacturing stages to ensure coverage, performance, and serviceability.
Design Techniques for Conformal Coatings
Several design techniques should be considered when planning for conformal coating:
- Increase spacing slightly between tight component placements
- Avoid capillary gaps that can wick and trap coating
- Include fiducials for optical inspection of coating coverage
- Place vent holes for outgassing when coating over cavities
- Minimize tall components that may shadow smaller parts
- Define “keep-out” zones that are masked from coating
- Ensure adequate access space for coating tools around perimeter
- Add undercoating layers to level surface and improve adhesion
- Specify extended PCB tabs for holding during coating process
Careful design and process planning is required to successfully integrate conformal coating into board fabrication.
Conformal Coating Inspection
Verifying coating quality and coverage is important to ensure protection. Typical inspection methods include:
Visual Inspection
Examining boards under magnification to confirm uniform, complete, and acceptable coating.
Coating Thickness Measurement
Using a coating thickness gauge or microscope measurement to validate thickness meets specifications.
Cross-Section Analysis
Microscope inspection of coating cross-section reveals voids, contaminants, coverage uniformity, and adhesion.
X-Ray Imaging
X-ray images can reveal shadowing, bridging, bubbles, and voids hidden under coating layers.
Microsectioning
Cutting, mounting, and polishing coated coupons to expose cross-section for inspection under a microscope.
Destructive Testing
Testing cuts, bends, or pulls on coated traces verify coating adhesion and protective strength.
Automated optical inspection systems are often used. They capture board images that are digitally analyzed for coverage using programmed algorithms.
Conformal Coating Repair and Removal
Despite best efforts, problems with conformal coating may still occur requiring rework:
Coating Removal
Stripping products dissolve, swell, or mechanically peel away coatings for removal and reapplication. Harsh chemical strippers completely remove coatings.
Recoating
Screen printing or dispensers locally apply additional coating layers to address thin spots or voids found during inspection.
Thermal Curing
Running boards through a curing oven can remove moisture or cure under-exposed areas to improve performance.
Component Replacement
Coatings complicate component removal if repairs are needed. Manual coating removal around a component may be required.
Cleaning Residue
Cleaners remove coating residue, runs, or drips from previously coated boards prior to recoating.
Reworking and repairing coated boards requires specialized tools, techniques, and training to avoid damaging boards.
Conformal Coating FQA
Below are some frequently asked questions regarding the use of conformal coatings on PCBs:
How long does conformal coating add to PCB assembly time?
Manual spray coating adds 1-2 minutes per board. Automated selective coating systems typically take 5-15 seconds per board depending on size.
What is the typical cure schedule for conformal coatings?
Standard acrylic and urethane coatings fully harden within 4-6 hours at room temperature. Thermoset materials require heating like 1 hour at 90°C.
How do conformal coatings affect PCB thermal performance?
The low thermal conductivity of coatings somewhat insulates boards from airflow cooling. This must be considered in thermal designs.
How thick can conformal coatings be applied?
Standard coating thickness is 25-75μm. Some urethanes and silicones specially formulated for thick coatings can apply up to several millimeters thick.
What inspection technology best detects coating voids?
X-ray imaging can reveal hidden voids and shadowed areas undetectable by visual inspection. This provides the most thorough coverage analysis.
Conclusion
Applying conformal coating is an inexpensive process that enhances PCB reliability, extends operating life, and prevents premature failures in harsh environments. Matching the coating material and application method to the design requirements is critical to gaining the benefits of protection while minimizing manufacturing impact. As PCB technology advances, conformal coatings have become a standard manufacturing step for improving the robustness of electronic circuit boards.
How to Select PCB Coating for PCBs’ Optimal Performance
Introduction:
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.
Advantages:
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
Disadvantages:
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.
Advantages:
1- High resistance towards moisture and has good dielectric resistance
2- Excellent resistance towards chemicals, abrasives, humidity and high temperature as much as 150OC
Disadvantages:
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
Advantages:
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
Disadvantage:
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
Advantages:
1- High resistance to moisture, chemical (acid and alkali) and abrasion
Disadvantages:
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.
Advantages:
1- Outstanding dielectric strength
2- High resistance to moisture, solvents, extreme temperatures and acidic erosion
3- Very thin coating is possible with evenness.
Disadvantages:
1- Removal/rework is very difficult
2- High cost is the biggest disadvantage.
