What is Conformal coating? it is a common question in the mind of PCBA professionals. Conformal coating is a thin polymeric film coating on a Printed Circuit Board to protect the electronic components mounted on boards from environmental extremes.
Normally the thickness of polymer film is 25-75µm thick (50µm typical). The purpose of conformal coating is to protect the electronic circuitry against dust, moisture, salt, chemical, and varying temperatures.
What Is Conformal Coating?
Coated circuit boards are protected by thin polymeric films that conform to the board’s contours. Electronic circuitry is typically protected with this coating at a thickness of 25-250 *m[1] (micrometers) against moisture, dust, chemicals, and extreme temperatures.
When we manufacture the electronic product, its performance and reliability are up to standard. In addition, this product must withstand extreme environmental conditions, such as dust, moisture, chemicals, etc. Corrosion of electrical components can occur, leading to failures and reliability issues. This can damage the product’s performance and life expectancy, resulting in dissatisfaction from the customer.
- The conductor spacing of a PCB can be reduced due to the insulation properties.
- It can help eliminate the need for complex, sophisticated enclosures.
- Completely protect the assembly from chemical and corrosive attacks.
- PCB assemblies should be protected from environmental stress.
- Ensure that environmental hazards do not degrade performance
- Improve the performance and reliability of products exposed to environmental conditions
How to Decide Which Coating is Best for Your PCB
The choice of PCB conformal coating material depends on the application and functionality of the electronic product. The most important decision a process engineer must make is this one. Our selection criteria include what we want to protect (e.g. moisture, chemicals), the temperature at which the device will operate, and the physical, chemical, or electrical requirements of the coating material.
According to the application for your PCBs, you will need a different kind of conformal coating. You can have difficulty deciding which coating you should use, but you can uncover the best coating options when working with an electronics contract manufacturer. According to the above factors, one can choose between acrylic, polyurethane, silicone, epoxy, etc.
An ideal PCB protective coating possesses excellent electrical properties with low moisture penetration, is adhering, and has excellent electrical properties. In addition to these qualities, conformal coatings require other qualities as well. Apart From that Coatings with conformal properties should also provide corrosion resistance, in addition to protective properties like salt, chemical, UV, and heat resistance.
Which industry requires Conformal Coatings?
The concept of conformal coatings on PCBs was developed through military research. Because conformal coatings are one of the latest developments in the electronics sector, they are mainly used in industries such as aerospace, automotive, military, and marine that rely on accuracy and reliability.
In aerospace & defense and automotive industries, PCBs with conformal coatings are highly preferred. According to a recent scientific study, astronautics and defense industries use conformal coatings on space shuttle controls and systems, aircraft communications and controls, and satellite systems. Following is a list of companies where conformal coatings are recommended for accuracy and product reliability.
- Aerospace industries
- Industrial Controls
- Automotive industries
- Medical equipment
- Telecoms industries
- Deference equipment supply industry
What are the Methods of conformal Coatings?
An appropriate method of conformally coating printed circuit boards can be selected based on your volume and product requirements. A fluorescent dye is used in conformal coatings of modern electronics for inspection aids. These are some of the most common methods of coating. The following are common application methods mainly used for the conformal coatings process
Brush coating method- An application technique for simple conformal coatings on circuit boards is the coating using a brush. After reworking and repairing circuit boards, this application method is mainly used in small production runs.
Spray coating – Spray application conformally coated can be further divided into two types based on their application. In low volume sparing, the spray gun is used manually, and in large volume sparing the spray gun is used by a machine as the PCB is moved by a conveyor.
Conformal coating dipping – The process of conformal coating involves coating both sides of the circuit board, then dipping the circuit board into coating solutions.
Selective coating by machine – Using programmed automated robots, this coating is sprayed on a very specific area of the circuit board.
How do I measure conformally coated thickness?
A conformal coating should be 50–210 *m thick (0.0020–0.0083 in) according to IPC standards. There are several methods of measuring the thickness of the conformal coating on printed circuit boards, but they fall into two main categories: wet film and dry film.
Wet film conformal coating thickness measurement: – This measurement is applicable when the coating is still wet. For thickness measurement, wet film gauges are used. While the coating is still wet, quality control is ensured using the wet film method. A coating applied in excess can be cost-prohibitive. Wet film measurements are also useful for conformal coatings that require destructive testing or for those that are overcoated. In wet conformal coatings, wet film gauges are applied. The teeth indicate the thickness of the coating. From the measurement, one can calculate the dry film thickness.
Dry film conformal coating thickness measurement: – The fastest method of measuring the dry film thickness is using eddy currents. This system works by placing a head-on PCB conformal coatings surface.
Additionally, Test coupons can be used to measure coating thicknesses and are a reliable method for archiving them. Test coupons are coated simultaneously with circuit boards so that an accurate record of the coating thickness will be created.
How do I do conformally coated PCBA testing?
You can perform the adhesion test to test reliability as described in the IPC-TM-650, section 2.4.1. Every batch of coatings is subjected to a frequent inspection.
An adhesive tape is applied, the varnish is scarified, and finally, the tape is removed. The tape must not have any coating on it to pass the test.
What Are The Most Common conformally coated Defects?
There are several types of Conformal Coating defects, below is the list of the most common ones.
Capillary Flow – This PCB conformal coating defect is also known as scavenging, where the low viscosity, high surface tension, or excess coating amounts migrate from one area of the circuit board to another, causing the uneven coating.
Cracking – Cracking is often observed in layers of the coating due to thicker coatings, thermal resistance, or rapid curing.
De-Wetting – In this defect the conformally coated coating is not applied evenly on printed circuit boards due to uneven application, improper mixing of the parts material, buildup on surfaces, etc.
Circuit boards suffering from this defect are unable to protect themselves from the coating layer lifting away. In most cases, these defects are caused by a quick cure, a coating on contaminated surfaces, or a coating that doesn’t adhere to a previous coating.
orange peel – appearance is caused by the conformally coated layer on the PCB layer looking very dull. In this case, painting on a too hot surface and the local environment being too dry when activity is performed are the main contributing factors.
Bubbles, Pinholes, and Foam – when Bubbles are trapped inside the conformally coated layer, they form Bubbles, and when the Bubbles burst, they form Pinholes. Most of these defects occur when the solvent has a high viscosity, the coating is too thick, and it is not applied properly.
Methods Of Drying Or Curing conformally coated?
There are a lot of drying and Curing methods of Conformal Coatings of printed circuit boards. Few of them are listed as per below
Heat Cure / Evaporation Cure Coating – Solvent-based coatings cured when exposed to a certain temperature by evaporation of the solvent.Suitable for most applications.
UV (Ultraviolet) Cure – Ultraviolet light-curing becoming more popular due to faster curing speed for large volumes in industrial requirements such as automotive and consumer electronics.
Moisture (Humidity) Curing:- Silicone and Urethane resins get cured by the Moisture (Humidity) Curing method.As the resin and forms a polymer when exposed to Moisture.
What are Conformally coated PCB removal methods?
Sometimes need component replacement due to damage for repairing the printed circuit board. To do this activity require the removal of a conformal coated layer. According to IPC below methods are recommended to remove the conformal coating in PCB without any quality compromise.
- Thermal or Burn‐through method
- Microblasting method
- Peeling method
- Solvent Removal
- Grinding or Scraping method
Where to buy conformal coating?
Where to buy conformal coating is a very important question for small project owners because it was normally used in bulk for industrial usage.
You can buy the conformal coating online from Amazon portal
Conclusion – Wrapping it up
It is important to consider the application and the functional requirements of the electronics when choosing a coating. In addition to protecting the board, it is crucial to do so with the appropriate materials. Choosing a coating type should be based on the environmental conditions as well as rework or repair requirements.
A conformal coating protects electronic components from the elements they come into contact with. There are many factors that contribute to corrosion, including moisture, salt, chemicals, temperature changes, and mechanical abrasion. Conformal coatings can minimize corrosion on boards.They are also being used to reduce whiskers[3] and keep current from leaking between closely positioned components.
Electronic boards are protected against contamination while being able to release trapped moisture within the coating. The coatings aren’t sealants and will be degraded by prolonged exposure to vapors. Conformal coatings typically fall into four categories: acrylic, urethane, silicone, and varnish. The following functions can be performed by each despite their specific physical and chemical properties:
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