As we all know, the continuous progress of electronic equipment and circuit technology, the peak of engineering art in the design, development and manufacture of printed circuit board (PCB) has been witnessed by people around the world. Our planet is full of highly intelligent machines, automated robots and scientific marvels, and of course, there are plenty of PCBS in every corner of the planet, no matter what country or city it is. However, these PCBS do differ in functionality, complexity, cost of manufacture, quality and reliability. This article focuses on the last two points, the quality and reliability of PCBS.

Yes, potential customers always want high quality electronics, but this is very expensive and can involve complex manufacturing processes. In the complex process of PCB manufacturing, assembly and testing, there is a very important process called “conformal coating” of PCB. This conformal coating is very important in PCBS in terms of quality and reliability.

What is conformal coating and its importance:

Conforming coating, an ultra-thin protective coating of polymer film, can be used in conjunction with assemblies mounted on assembly surfaces to protect assembly leads, solder joints, exposed wiring, and other metal points on the PCB surface from corrosion, dust, or chemicals due to various operating or environmental conditions.

The conformal coating can be as thin as 25 microns and “conforms” to the shape and component layout of the circuit board. As mentioned above, the reason for applying a conformal coating to the surface (top and bottom) of a PCB is to protect the PCB from adverse external environmental conditions, thereby increasing the service life of the PCB and associated electronic equipment.

Like high temperatures found in industrial, factory, and high-power electronics, these PCBS with conformal coatings can withstand extreme temperatures. Similarly, electronic equipment installed in areas/areas close to the sea or ocean may be affected by high humidity, such as navigation electronic naval equipment may be subject to corrosion/erosion which can lead to metal oxidation. Similarly, in microbiology laboratories and the medical industry, sensitive electronic equipment can be exposed to toxic chemicals, acidic and alkaline solvents that can accidentally spill onto a PCB, but the PCB’s “conformation coating” will protect the PCB and components from fatal injury.

How to apply conformal coating?

In fact, the method of applying “conformal paint” in the right way is so important that careful consideration must be given to how conformal paint is applied. It is as important to choose suitable conformal coating materials.

The main factors determining the proper application of conformal coatings are:

1- Thickness of paint

2- The level of coverage achieved

3- The degree of adhesion of paint to wood panels and their components.

There are five methods for applying conformal coatings:

1- Paint by hand with a brush

2- Aerosol coating

3- Atomizing spray gun coating

4- Automatic dip coating

5- Automatic selection of coating

Conformal coating curing/drying method:

Conformal coatings themselves can be classified according to the drying and curing methods used after conformal coatings are completed. These methods are:

1- Heat/heat curing: Conformal coating is dried at high temperature. Drying rate is much faster than normal room temperature drying/curing.

2- Condensation curing: The conformal coating of PCB is dried at ambient temperature, moisture in the atmosphere slows down the curing or drying process.

3- ULTRAVIOLET (UV) curing: Here PCB with conformal coating is exposed to UV radiation. The uv energy determines the curing speed of PCB conformal coating

4- Oxidation curing: In this method, PCB conformers are exposed to an open air environment with large amounts of atmospheric oxygen, which will aid in the drying/curing of solvent-based conformers

5- Catalytic curing: This is the process of curing conformal coating in which two materials are fused together, one of which is conformal coating. Once the coating is fused with other catalyst materials, the curing process cannot be stopped until it is complete.

Classification of conformal coatings:

There are five main conformal coatings used: acrylic resin, epoxy resin, silicone, polyurethane (PU) and polyparaxylene coating.

L Acrylic resin (AAR) :

Acrylics are ideal for (low-cost and high-volume) common electronics because AAR is inexpensive and can be easily applied to PCB surfaces by brush, dip, and manual or automatic spraying, reducing turnaround time and producing cost-effective products.

Benefits:

1 – low cost

2- Easy for manual or automatic robot applications

3- Easy to rework

4- Excellent moisture protection

5- Good surface elasticity, can withstand static voltage discharge, and does not react with the atmosphere, therefore helps cure through solvent evaporation

Disadvantages:

1- Due to the use of atmospheric curing/drying methods for this material, proper ventilation systems need to be ensured

2- Low viscosity maintenance

3- Low wear resistance and chemical resistance

L Epoxy conformal coating (ER) :

Conformal coatings based on epoxy resins can be completed by hand brush, spray or dip coating. Spraying is recommended for larger volumes and for smaller volume or prototype PCBS.

Benefits:

1- High moisture resistance and good dielectric resistance

2- Excellent chemical resistance, abrasion resistance, moisture resistance and high temperatures up to 150 O C

Disadvantages:

1-epoxy conformal paint is very hard and rigid and can damage PCB and its components if attempted peeling or removal. Remove the coating by using a hazardous solvent

2- Poor low temperature performance

3- High curing shrinkage

4- They are hard to redo

L Silicone resin (OSR) conformal coating:

The softest of the above two types of conformal coatings is silicone resin (OSR) conformal coatings. They are widely used in LED lamp PCBS without reducing light intensity or color change. Ideal for PCB installation at high humidity and exposure to air. Suitable for PCB with high operating temperature and high power

Benefits:

1- Good chemical resistance, moisture resistance, salt spray and high temperatures up to 200 O C

2- Good flexibility makes it resistant to vibration stress on PCB from external environment.

3- Suitable for PCB outdoor applications with high humidity

The bad:

1- Not wear resistant due to rubber properties

2- Can be reworked, but not easily, requiring special solvents, long soaking times and stirring like a brush or ultrasonic bath

3- Low mechanical strength, weak adhesion to PCB substrate

L Polyurethane (PU) conformal coating:

Suitable for PCB applications in automotive, industrial, instrumentation and telecommunications. In aerospace, in particular, fuel vapors constantly collide with the main body of electronic equipment and thus penetrate into the interior and affect the PCB board

Benefits:

1- High resistance to moisture, chemicals (acid and alkali) and wear

Disadvantages:

1- After a long time of complete curing process, it tends to turn yellow at high temperatures due to its high VOC content

2- Like silicon, it is not easy to remove completely

L polyparaxylene conformal coating:

This type of coating is suitable for avionics, microelectronics, sensors, high-frequency circuits, and densely populated PCB-based components. It is applied by means of vapor deposition.

Benefits:

1- Excellent dielectric strength

2- High resistance to moisture, solvents, extreme temperatures and acid corrosion

3- Can be applied evenly with very thin paint.

Disadvantages:

1- Disassembly/rework is very difficult

2- High cost is the biggest drawback.