As a matter of fact, PCBs are widely used in various household, industrial, military, and automotive equipment. In order to ensure the operation of the equipment and prolong the service life, it is crucial to ensure that the circuit boards and components on the board are protected from any danger, as any error in them can affect the device’s performance or even cause the failure of the entire device. PCB coating is a very effective way to protect the PCB, in this article, MOKO Technology will take you to learn more about PCB coating!
The Definition and Types of PCB Coating
PCB coating is a technology that protects PCB from unfavorable factors such as corrosion, moisture, and physical shocks by applying a thin and non-conductive layer on the surface of the circuit board and electronic components. With PCB coating, the possibility of circuit failure of the PCB and electronic components is reduced, and the service life of the final electronic product can also be extended.
In terms of the materials used, PCB coatings can be divided into five types:
- Acrylic –easy to apply and repair, featured with good moisture and abrasion protection, and good mechanical strength. But this kind of coating has poor solvent resistivity and poor temperature resistance.
- Polyurethane –same as acrylic, it provides excellent humidity and abrasion resistance, in addition, it provides better resistance to solvents. The downside, however, is that the coating is difficult to remove.
- Epoxy – it hasexcellent mechanical strength and can offer good protection from abrasion and moisture. But epoxy coatings have poor flexibility and temperature resistance and are hard to repa
- Silicone – this type of coating is available in moisture cure, UV cure, and heat cure, it has excellent temperature and humidity resistance, but it has poor performance in abrasion resistivity and mechanical strength.
- Paralyene – paralyene coatings are available for any surface, which is a perfect option for devices used inaerospace and medical applications, as they can protect PCBs from organic solvents, inorganic reagents, and acids. Their disadvantage is that they are difficult to rework.
Key Considerations When Choosing PCB Coating
Now that we understand the properties of different PCB coatings, then what factors should we consider to choose the most suitable coating for your project? Well, below we have listed some key considerations:
- Cost of product.
- Determination of effectiveness.
- Time for production.
- Space availability.
- Environmental impact.
- Electrical resistivity.
- End-user feasibility.
Different Methods to Apply PCB Coating
This method is suitable for low-volume production runs as it is a time-consuming process. Normally, we use an aerosol can or handheld spray gun to apply the coating, and before spraying, those areas that do not require coating need to be covered. The coating effects would a little different between different batches due to the manual operation.
It refers to an automatic coating process that applies a coating to the specific areas on the circuit boards by using programmed robotic spray nozzles, and there is no need to cover areas that are not to be sprayed. This process is featured with high efficiency and accuracy, suitable for high volume production.
For this method, PCBs would be immersed in the coating solution first and then withdrawn. Many factors would affect the coating effect such as immersion and withdrawal speed, dipping time, etc. There is extensive masking required before the coating process, so it is suitable for those PCBs that require coating for both sides.
A brush is used to apply a coating to specific areas, and it is a method mainly used for repairing and reworking. The process takes much time and requires a lot of labor, the final coating effect is dependent on operator proficiency.
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 involve both basic and advanced testing.
First, basic testing checks the accelerated humidity and electrical performance.
Second, advanced 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
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 for 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.
Volatile Solvent Testing.
EU Solvent Emission Directives.