Conformal coating for PCB

Engineers who worked on hadware boards which have to operate very low temperatures might not have come across conformal coating requirement. But who work on marine, defence, military, aerospace domains and hardware board designers who worked on board designs which have to work up to high temperatures and humidity might have used conformal coating. Now a days we can see conformal coating being used very extensively across other domains also (Telecomm, industrial, automotive, instrumentation). We can say that where there is a need for reliability and high protection conformal coating of PCB is a must.

Conformal coating is a chemical coating on top of the assmebled PCB which helps electronics to get shielded against harsh environments. If you have seen some relays, transformers available in the market, they have a plastic enclosure for internal electronics with a gel compound filled for protection against shock, vibration, stress, thermal, corrosion, moisture. This is what we call "plotting". You can see the huge black ics in the below figure.

Plotting is an expensive technique and conformal coating helps to achieve the protection simpler. The also disadvantage is that specific temperatures are used to form the protective layer which may cause the electronics to loose bonding. Conformal coating also adds dielectric strength to the PCB. Also, conformal coating ensures board reliability. Conformal coating can protect electroincs boards against the following problems:

• Temperature (which we call thermal cycling in environmental tests)
• Humidity
• Moisture
• Mechanical stress
• Dust accumulation
• Fungus formation
• Corrosion because of salt spray
• Shock & Vibration
• Chemical contamination

There are specifications available forconformal coating guidelines especially in military and aerospace. For example, MIL-1-46058 is one such sytndard. Silicone is the most commonly used conformal coating material used for protection against high temperatures.

What are the various other conformal coating materials available?

Silicone, Epoxy, Acrylic, Urethane, Parylene. Urethane is the most widely used.

Once conformal coated, can we remove it?

Yes, it can be removed and there are several techniques available. Approach the manufacturer to get to know the type of material used and technique to remove coating. Acrylics are the easiest to remove.

What care need to be taken for resoldering a component on already conformally ciated PCB?

Once conformally coated, very high temperature must be applied by soldering machine to remove that coating. This may cause SMT pads to get lifted. In susch cases, careful attention must be provided while aplying temperature beyond certain range. Also, once rework is done take care that conformal coating residue is not present on the board which can give a problem in the long term. Take care of ESD which performing such procedures for removing conformal coating.

What can be the thickness of conformal coating?

Have seen a conformal coating of thickness up to 50um.

What are the advantages of conformal coating?

• Eliminates need for costly mechanical enclosures
• Less weight
• Easy removal

What are the techniques used for conformal coating?

• Brush
• Spray
• Dip
• Automated

What care need to be taken for conformal coating?

Eliminate any surace defects. This means you have to equally spray without any gaps

Chose the conforming material as per the application and requirement

Choose the conformal coating technique as per expertise available

The drying process after coating must be simple. Can be air dired, normal dried or UV dried.
Coating material complies to industrial standards and is environmental friendly

How can suraface defects be detected in conformal coating?

Using UV light inspection