It is correctly called “conformal coating”, but some also call it protective coating or coating. We work with lacquers that are selectively applied to the assemblies. This is the process we have chosen as an EMS partner. You can find out here why we selectively coat our PCBs and what these coatings can do.
Why we as assemblers also do conformal coating
The reason is simple: We wanted to offer our customers more “from a single source”. On the one hand, our customers then have the same contact person for coating and PCB assembly. On the other hand, outsourcing the coating of entire assembly series has always cost time, both for us and our customers. Since we additionally do this service in-house, we save time and effort. So these are the same reasons why we also offer logistics and complete assembly: It is more flexible, faster and ultimately cheaper for us and our customers.
Why conformal coating?
Coatings can be applied to assemblies in various ways. For example, by painting, spraying and dipping. We prefer conformal coating. A machine pours the lacquer with little pressure onto exactly the points on the board that we specify. Plugs or other elements on the assembly that do not need or cannot tolerate paint can thus be left out. If we were to spray, for example, such precise selections would not be possible. Conformal coating therefore also saves material. As a matter of principle, we use the lacquers of our renowned business partner Lackwerke Peters GmbH & Co. KG.
For the layer thicknesses, we naturally comply with the IPC standards
The layer thicknesses are between 70-110 µ in the standard, we usually manage 80-90 µ, depending on the structure of the PCB. Some components “pull” a little paint through cavities, so we have to apply thicker. Our experienced experts take special features such as these into account as a matter of course. You can find the relevant standards, for example, at our umbrella association. The correct layer thickness should not be underestimated. Too thick layers lead to trapped air or do not cure completely, too thin layers do not protect sufficiently.
“We have an edge sharpness of 1-2 millimetres in conformal coating. But in hazardous areas on assemblies, we can work even more precisely with insulating and filling materials. For example, with plugs or push-buttons.”
Head of Department Conformal Coating
Conformal coating is the more precise choice for high-volume PCB applications
Selective coating is a fast and, above all, accurate method of applying coatings. A robot system with corresponding coating heads paints exactly those areas of the board where the assembled PCB also requires coating. In conformal coating, we use an applicator that moves over the PCB using a programmed coordinate system and dispenses the coating material in selected areas.
Conformal coating is also digitised with us
We programme the flow rates in a computer system, the material viscosity is specified by the paint manufacturer. The system controls the applicator so that it maintains the desired layer thickness everywhere. We have various potting nozzles available for this purpose, with which we can also coat individual components within an assembly in a targeted manner. The only prerequisite for conformal coating is that the PCBs and the assembly are designed for the method. Challenges can come in the form of capillary effects around flat connectors that inadvertently soak up the coating. This is another reason why a professional operator for conformal coating is absolutely necessary.
Most PCB assemblers coat assemblies with a transparent coating that is easy and simple to inspect. In the past, however, it was desirable, especially at trade fairs, for the coating to be black instead of transparent. In this way, the manufacturers prevented industrial espionage from being carried out directly at the stand by taking photographs.
Why assemblies need an individual conformal coating
We use coatings to protect electronic components from environmental factors. And in day-to-day operations, there are quite a few of these environmental factors. An overview of what the coatings we use can do depending on the requirements:
- A high-quality (selective) coating prevents corrosion on the PCB, e.g. due to high humidity, dew water or salty air, for example in offshore applications or on ships.
- A coating insulates. This allows smaller distances between the components: The coating prevents current leakage between closely spaced components.
- The coating minimises the need for complex enclosures that would otherwise have to protect individual components. The lacquer then takes over the task of a casing and protects against dust and mechanical abrasion.
- The right paint is a complete protection of the assembly against chemical attack.
- Mechanical protection against vibration and impact.
- Conformal coating minimises performance degradation due to environmental hazards such as high thermal loads. When used in desert regions, a unit must withstand temperature fluctuations of up to 40 degrees in one day.
- Conformal coating minimises exposure to mould.
- Electromigration – the removal of material by ionic movement in the solid conductor (conductor disruption).
What is a good coating?
The guidelines for evaluating protective coatings can be found in the standards IEC 61086, IEC60464, IPC CC 830, IPC HDBK-830. But in concrete terms: A good coating has minimal impact on component weight – and nowadays is even partly water-based. The lacquers used for coating PCBAs must also be breathable. This allows the moisture trapped in electronic circuit boards to escape, while at the same time maintaining protection against environmental influences. However, because of this, these coatings are often not sealing materials, which means that transmission and degradation occur with prolonged exposure to vapours. The selection of the right coating material is therefore an important decision:
- What is the conformal coating supposed to protect against?
- In which temperature range is the electrical device used?
- What are the physical, electrical and chemical requirements for the coating material itself?
- Are there requirements for electrical, chemical and mechanical compatibility with the parts and substances to be coated? For example, does it have to match the coefficient of expansion of chip components?
The answers to these questions determine the suitability of a particular material, such as acrylic, polyurethane, silicone, epoxy. The choice of conformal coating material is made carefully and in relation to the application method. Incorrect selection can affect the long-term reliability of the PCB and cause processing and cost problems.