Conformal Coating Technology for Improved Circuit Reliability

Chapter 1

What is a Conformal Coating? 

Conformal coatings are systems of synthetic resins which provide a secure barrier around a printed circuit board and its components to provide protection from moisture, fungus, dust and other possible contaminants. The coatings also prevent dendrite growth, or oxides growing on the PCB. Dendrite growth can create short circuits and result in product failure.

These coatings enhance dielectric strength, reduce mechanical stress on components and protect a product from thermal shock. A conformal coating can increase the circuit’s reliability and extend the product life. Conformal coatings have become more widely accepted as a means of improving overall product quality and reliability in the electronics manufacturing industry.

Conformal coatings are selected with particular attention to their electrical and thermal properties as well as their method of application. Each coating type has its own special characteristics associated with repairability, toughness, dielectric strength, insulation resistance, solvent resistance, temperature range and stress relief. These attributes have varying degrees of importance depending on processing and the board's end use.

Chapter 2

Why Use a Conformal Coating on PCBs?  

Achieving maximum benefits from coatings requires coordination of the PCB design, coating selection, and application with the proper cleaning protocol. If there was ever a reason to decide to coat or not to coat...

Here are the top 13 reasons to coat:

1. Reliability - which is a must have for electronic devices
2. Inhibit corrosion - protect the assembly against chemical and corrosive attack
3. Provide resistance to fluids and humidity
4. Temperature resistance - protect circuit boards from adverse environmental conditions
5. High abrasion and chemical resistance depending on its chemistry
6. Prevent arcing
7. Provide coverage of sharp edges (components)
8. Simple application in most cases
9. Specialized formulas to provide uniform, thin films (flexibility)
10. Prolong the life span of the product 
11. Breathable protective layer
12. Insulating properties allow a reduction in PCB conductor spacing of over 80%
13. Minimal effect on component weight (lightweight) 

Chapter 3

Techniques to Apply a Conformal Coating to Protect Boards 

A variety of techniques for applying conformal coatings to products are used in the electronics industry. Like most manufacturing processes, conformal coating application methods have evolved from manual to automated systems. Methods for applying conformal coating to printed circuit boards (PCBs) have evolved from dip and spray processes to selective robotic coating processes used today. 

Selecting the application method is based on the end use of the PCB, quantity of boards produced, speed of the coating process needed, cost of the coating process, and more.

Common Application and Dispensing Methods 

Brush - The brushing method is an easy technique used primarily for repair and rework. There is a low cost associated with the brush method however it is labour intensive. With brush application it can be difficult to control the coating thickness, but is still viable and applicable for smaller production runs. Applying coatings by brush is most often done after repairs.

Dip - Dipping consists of most or all of the PCB being "dipped" or immersed into a conformal coating. It is a technique commonly used for high-volume productions and when both sides of the circuit board can be coated. Dipping can coat many PCBs at one time but requires significant masking. 

Spray - Spraying can be applied manually or automated. 

Aerosol cans or a handheld air brush type spray gun are two manual techniques typically used for low-volume production. Because of the nature of this application, areas that don't need coating must be masked. The masking process can become labour intensive. Manually sprayed components usually require multiple applications.

Automated methods apply a conformal coating to a PCB that is moved underneath a reciprocating spray head. This "robotic" method reduces the need for extensive masking and offers a very repeatable process. This type of process deposits coating precisely on the PCB where specified by the system. Processing time is limited by the speed of the robot as coating is applied in a very controlled process. Robotic methods require product design considerations defining ample “keep out” areas and sufficient pathways for conformal coat application.    

While automated methods are often considered the fastest, an extensive evaluation of the design is required to determine the most cost effective solution to applying the specific conformal coating material chosen for your design.

Encapsulation - provides a seal against moisture, and corrosive agents. Its main purpose is to create a "shell" around the assembly and is used to fill cavities or encapsulate components.

Chapter 4

Which Conformal Coating is Right for Your PCB (Advantages and Limitations) 

Guide to Coating Material Selection Considerations 

As a leading electronics manufacturing solutions provider, TT Electronics is asked frequently,

"What coating is available for our project and how do we determine the right coating for our need?"

The way to approach the material selection is to start with your requirements. Key questions that drive those requirements are:

1. What is the operating temperature range?

2. What moisture levels and / or chemicals will the board be exposed to?

3. What is the volume being produced?

4. Do you need to be able to rework your product?

5. What is the intended life of the product?

6. How much physical shock will the board be subject to?

Once you have answered these questions, you can decide the relative merits of the different coating products that will meet your needs. In some cases, you may find that you need to meet a specific military, transportation, or other industry standards, which wraps these requirements together. 

The selection criteria for conformal coatings can be based on a number of things. 

1. Engineering / performance characteristics (electrical, thermal, mechanical, chemical properties and humidity resistance)

2. Printed Circuit Board volume

3. Coating thickness requirements / number of coverage areas

4. Application method (spray, dip, etc.)

5. Material cure mechanism (moisture, heat, UV, etc.)

6. Repairability

Conformal Coating Comparison Chart

In this section, we discuss the most common conformal coatings on the market today with an in-depth comparison of each. 

Acrylics (AR) offer excellent moisture production, but poor resistance to solvents as they can be removed with a weaker solvent such as isopropyl alcohol or xylene. If faced with even stronger solvents, acrylics will not offer the protection that is needed, especially if your product is a mission-critical device.

On the plus side, Acrylic tends to be less expensive and easier to rework. ARs are excellent coating systems from a production standpoint because they are easy to apply. The acrylic coating can also be removed completely or locally by using specifically designed stripping chemistries. The board can then be recoated completely or just in the area needing repair.

Epoxies (ER) provide good humidity resistance and high abrasive and chemical resistance. They are a tough coating, however extremely difficult to remove chemically for rework. Poor repairability creates limitations to its use.

Silicones (SR) Most typical silicone coatings have continuous operating temperature ratings of 200ºC. This is far higher than most urethanes (125ºC). Some silicone coatings are rated as high as 600ºC for ultra-high temperature applications such as the transportation industry, where temperatures can get upwards of 175ºC in the engine compartment.

An additional benefit of silicone conformal coating is its excellent moisture protection. Silicones have been used in cases where there were extreme temperature differences, which resulted in excessive moisture. Other conformal coatings failed within hours or days, where silicone did not, especially when applied thickly. 

Silicone conformal coatings are also among the easiest to apply and rework. Silicone coatings are typically low on solvents, ensuring a smooth coat that cures very quickly (about one hour at room temperature).

Silicones are quite flexible and soft, having a relatively weak resistance to solvents. For assemblies that will require work after coating, this is critical because it will keep labour time down without compromising the integrity of the coating.

Urethanes (UR) These coatings are applied in the same manner as acrylics and will provide better chemical resistance, but have other disadvantages i.e. cure times while acrylics typically cure in as little as 30 minutes. Most urethanes need a longer cure time, so it's a good idea to take into account cure time if you have a high-volume application or tight production schedule.

It should be noted that urethanes are difficult to rework making them more costly. Removal of the coating to rework components or the coating itself can be quite a lengthy process involving harsh strippers, and in some cases during the removal process, the PCB itself or the components soldered to it can be damaged. 

The board can then be recoated completely or just in the area of the repair.

UV Cure - These materials are also more difficult to repair as they are the most chemically resistant and mechanically tough of all conformal coatings.

UV curing of conformal coatings is becoming increasingly important for high-volume manufacturing. This increase in the popularity of UV curable conformal coatings is due to its rapid cure speed, level of processing ease, environmental friendliness and thermal cycling resistance.

Dan Strawn, Application Engineer Ellsworth Adhesives adds that there are typical cost savings with UV curable materials due to its nature (cures in seconds):

 

"It increases throughout (maximum rate of production) by streamlining the manufacturing process. It is simple to dispense with no solvent management (solvent free) or mixing systems required. It adds to worker safety and minimizes environmental impact. There is minimal floor space required. Finally, based on cost coverage calculations, UV curable coatings can be an economical choice compared to solvent-based coatings."

Watch how UV light curing is applied on printed circuit boards that require a conformal coating as the last step in the production process. 

Light Curing Conformal Coating

 Dymax Corporation, Light Curing Conformal Coating, via YouTube

Chapter 5

PCB Cleanliness Prior to Coating

Almost all material suppliers recommend pre-washing and drying the boards before conformal coating. Many companies do not pre-clean prior to coating because of the added cost and time, but they risk reducing the reliability of the board by not washing it.

If there is some conflict, typically cost or time, that prevents cleaning before conformal coating, qualification procedures should be performed to reduce the risk that the application of conformal coating will make things worse, not better. The centre of these qualification procedures is ion chromatography and temperature/humidity/bias testing.

The risk increases as the expected life of the product increases or the environmental factors (temperature, corrosives, humidity) increase. The IPC guidelines on conformal coating (IPC-HDBK-830) addresses cleanliness issue in section 7.3.3.

What can happen if the surface is not cleaned properly before application?

Cleanliness of the area where the conformal coating is applied is essential. Just as critical, is a clean, dry surface free from outside materials. Without the necessary and proper precautions taken, coating defects could occur. PCBs can suffer from a variety of problems if the surface is compromised.

Possible coating defects that may occur:

1. Dewetting takes place when a thin film of liquid ruptures on the printed circuit board. Commonly known as "fisheyes", it is most often caused by a spot of oil, wax, or a drop of silicone rich material. 

2. Cracks and ripples can occur if there was an improper coating mixture. Additional circumstances such as thermal shock could also develop (the rate of change of an environment greater than the material - causing it to expand or contract). A coating that is too thick could also cause a ripple effect. Two thin coats are considered better than one thick coat.

3. Orange peel - is exactly what it sounds like. It can be defined as an uneven textured surface, appearing dull, which looks like the skin of the orange. Reasons for this defective look could appear from applying a second coat before the first is completely dry, improper coating mixture, or the substrate is too hot during application.

4. Bridges or webs - are prime areas for bubbles. Thick coatings between the components can be the root cause of bridge or webs.

5. Dendrite growth - without coating water that is absorbed into the surface of a PCB over time can cause dendrite growth. Since this is a surface cleanliness issue, solder mask plays a key role in dendritic growth. "In addition to design, a product's environment causes ionic contamination to form dendritic growth." [source: PCB Reliability: Cleaning Up Your Act]. 

Chapter 6

Step-by-Step Design for Coating Considerations

What steps are needed for a proper application of a conformal coating?

Every coating has its advantages and limitations, but the main concept is that the conformal coating should "conform" to the board and its components. The following is our 11-Step guide to coating success.

Step 1: When using a liquid coating material try to group all parts requiring coating in the same area. Maintain a minimum clearance of 2.5mm between parts to be coated.

Step 2: Allow 2.03mm (0.080") minimum spacing around components to be prepped for conformal coating.

Step 3: Avoid placing parts close to larger devices that will prevent coating due to accessibility.

Step 4: Interconnect – if at all possible, group all connectors in the same location, or along one side. This helps facilitate dip coating and can completely eliminate masking for the dip coating process.

Step 5: Via holes must be tented to prevent coating material flowing through from one side of the PCB to the other. Via holes should also be located outside the footprint of any parts that could be vulnerable to coating contamination by capillary action e.g. surface mount connectors. If SMT connectors are used that have locating pins, then the guide holes for these should be blind (depth drilled) if possible.

Step 6: SMT connectors wherever possible should be specified as "sealed" e.g. no potential path for coating to penetrate between the insulator, orientation features and contact surfaces. If this is not possible and the assembly requires coating then "gel" type coating material may have to be applied to prevent contamination.

Step 7: Only specify conformal coating for items that really must be coated.

Step 8: Never specify conformal coating as an underfill, and take care where coating could run underneath BGA type parts. If underfill is required then select material specifically designed for this purpose.

Step 9: Avoid coating right up to the edges of PCBs. If required, then specify handling strips to facilitate easier handling.

Step 10: Do not expect the vertical sides of component encapsulation to be fully covered if applying coating using a robot.

Step 11: Mounting holes and grounding contact areas require the same clearances as components if required not to be coated i.e. 2.5mm.  

Chapter 7

Outsource Conformal Coating to Reduce Cost and Maximize External Resources

Outsourcing your coating application allows you to stay focused on your core strengths. With high customer demands, internal capability limitations, and cost control as factors, original equipment manufacturers (OEMs) may need to partner with electronics manufacturing service providers (EMS). By doing so, you can reduce internal operating costs and overhead.

Subcontracting can also reduce the capital equipment needed and unforeseen additional expenditures.

Gaining a partner with expert knowledge for your existing and / or future projects will offer immediate solutions to specific problems. They will have established process controls and procedures already in place to improve quality and productivity levels. 

OEMs looking to save costs and improve their environmental compliance will have no environmental liabilities when outsourcing coatings that require solvent evacuation or curing equipment.

Choosing a coating material can be an exhaustive process if you are not familiar with the marketplace. There are many considerations and requirements that warrant the advice of a professional. Suppliers and other solution providers are able to help you through the selection process and add value during the production process.