PCB Coating: Conformal Coating

As a matter of fact, the electronics industry is one of the most rapidly growing industries due to its unlimited applications. Moreover, all kinds of domestic, industrial, military and automotive devices are using PCB because it ensures protection from various dangers. Also, the reduction in performance or complete failure can occur in case of the absence of the protection of equipment. In this article, MOKO Technology will guide you to learn PCB coating!

PCB Coating Standards

In the world of engineering, there are certain standards for every application. Similarly, in conformal coating, there are a series of standards that require its usage under certain conditions like the military, automobile, domestic use, etc. Most commonly the conformal coatings qualify to either MIL-I-46058C or IPC-CC-830B specification which relates closely to MIL-I-46058C.

In addition to these UL94V0 and UL746E specifications, there are other standards that involve flammability assessment of permanent coating and electrical properties assessment in the conformal coating by underwriters laboratories. However, the standards that are common in circuit board insulation are as follows;

  • MIL-I-46058C: A common conformal coating standard in the industry, also known as Military insulating compound. It requires testing from any MIL authorized laboratories and is still used even after the deactivation since 1998 for new designs. This test requires a standard Qualified product list (QPL).
  • Def Stan 59/47: A similar standard to 46058C used for coating high-end devices for military use but the UK Ministry of Defense must approve them first.
  • IEC 61086: A standard based on self-certification by the supplier with similar requirements to 46058c. International Electrotechnical Commission governs it.
  • IPC-CC-830B: Actively used and continuously updated standard similar to 46058C, introduced when 46058C remains inactive. Material standardized for 46058C that follows these specifications. No testing is available as no QPL is maintained.
  • UL94V0: Relates to the conformal coating property of self-extinguishing on an FR4 substrate. V0 is the highest achievable category with V1 and V2 as its successors.
  • UL746E: A series of testing for dielectric breakdown evaluations along with UL94V0.
  • UL approved material does not require any testing.
  • Environmental Testing.
  • Volatile Solvent Testing.
  • EU Solvent Emission Directives.

Conformal Coating vs. Potting (Encapsulation)

In order to provide protection to any circuit, there are two methods of PCB; Conformal coating and Potting

Conformal coating is a thin layer of protection; a non-conductive dielectric applies on the coated-to-be surface (known as substrate). Not to mention that it allows PCB insulation coating without adding any significant weight or thickness to the substrate.

Potting or encapsulation is a process of adding protection to the substrate in which they place the substrate inside a shell and adds liquid to the shell. The liquid in the shell covers the substrate. When it cools down, it hardens and becomes a part of the final product. Subsequently. it adds weight and thickness. Furthermore, both potting and conformal coating is effective if necessary conditions and environment are available.

However, the main aim of using the conformal coating or potting is to provide;

  • Shielding of the circuit board and an increase in the electrical performance of the equipment
  • Prevention from failure in the harsh environment and vibrations along with protection from any other potential damage

The consideration that helps you choosing conformal coating PCB

In the field of engineering, many parameters decide the design and the type of PCB coating for the equipment. The conformal coating is very thin and provides insulation to the circuit in complex devices like cell phones, laptops, etc. Moreover, this protection is extensive because it requires the following of strict weight and thickness restriction.

The parameters of the process of conformal coating include;

  • Cost of product.
  • Determination of effectiveness.
  • Time for production.
  • Space availability.
  • Environmental impact.
  • Electrical resistivity.
  • Temperature.
  • End-user feasibility.

It should be noted that there are additionally five types of coatings for protection in conformal PCB coating, but Parylene is a gold standard among them.

Conformal PCB Coating Testing

Testing is an important phase for checking the performance range and limitations of circuit insulated coating under various environmental conditions to obtain desired characteristics. Depending on the application, the conformal coating can be different.

An ideal coating has a feature of excessive adhesion, good electrical properties and physical characteristics with low humidity.

These tests for the testing of coating involves both basic and advanced testing.

First, basic testing checks the accelerated humidity and electrical performance.

Second, advance testing checks the salt mist, rapid environmental change, and the temperature limit.

These tests include the following categories;

  • Surface insulation resistance test: 1010 ohms before the test, above 108 ohms after the test
  • Flexibility: no crack or delamination shall take place on the cure
  • Adhesion: tested by using a cross-hatch technique
  • Environmental testing and cycling
  • Humidity test
  • Salt Mist Test
  • Corrosive Gas test
  • Electrical Testing
  • Water Immersion
  • Thermal Cycling / Shock
  • UV Test

Step-by-Step Design for conformal coating PCB

In doing conformal coating, the ease and method of application play an important role in choosing the type of coating.

In comparison to the required protection, these parameters come second after the protection. While doing conformal coating this PCB coating method considers the following points;

  • The production not only needs preparation for coating and speed but also handling of the board after coating.
  • Design requirements of the board: connector, solvents and other problems affecting the decision.
  • The requirement of equipment: checking for the required equipment for the coating process.
  • Process before coating: Masking and taping
  • Quality Standard: the high scale of repeatability and reliability for mission-critical devices will tend to go for an automated process. The coating can use following methods;

○ Manual Spraying

○ Automated Spraying

○ Selective Coating

○ Dipping

○ Brushing

  • Thickness Measurement: Use of thin layers makes it possible to avoid heat entrapment, weight addition, and other factors. Normal thickness ranges anywhere between 1 to 5 Mils (25 to 127 Microns). Measurement of thickness can use the following;
  • Wet film thickness gauge.
  • Micrometer
  • Eddy current probes
  • Ultrasonic thickness gauge
  • Curing: check for any errors in the coating process
  • Conformal Coating Removal: for any rework or to remove any damaged parts from the circuit. Method and material are equally used based on coating resins
  • Certification

 

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