Conformal coating – coating intended to protect the printed board assemblies from external environmental influences and mechanical influences during operation, greatly increasing the life of printed circuit assemblies. There are basically four main methods for applying this coating: dip, automatic selective, spray, and brush.
What is PCB conformal Coating? Conformal Coatings in Brief
Conformal coatings are applied to printed circuit boards to protect installed electronic components from contaminants such as dust, sand, moisture, salt spray and conductive particles. They provide mechanical and environmental protection that greatly increases the durability of the components and the chain.
Typically, conformal coatings only a few tens of microns thick also provide resistance to external influences, including friction, impact, temperature, and ozone and UV degradation. They improve performance and allow for a higher component density due to the increased dielectric strength between conductors.
Typically, conformal coatings used on printed circuit boards are acrylic or silicone, but can also be epoxy, urethane, parksilene (parylene) and UV curable. They are usually applied by dipping, spraying, or simply watering, and increasingly with the choice of coating and robotic dispenser.
Genuine military specifications for conformal coatings refer to 1966 year. Since then, they have evolved from solvent-based coatings, which take hours to cure, to solvent-free coatings that cure quickly in minutes with gentle heat.
Printed circuit boards are used for mounting electronic components. Modern PCBs use surface mounting technology and typically place parts on both sides. The most common board material is FR4 fiberglass, but these can be phenol or Teflon. Most boards consist of 6 to 8 layers from top to bottom of the following materials:
- Solder resistance
- Copper (conductors / pads, as a ground plane)
- Fiberglass (backing material)
Conformal coating is a protective coating designed to protect the surface of the mounting base and components of the finished electronic assembly from external environmental influences during operation: high temperatures, moisture, pollution, chemical and mechanical influences, as well as to reduce electromigration and increase the mechanical strength of soldered joints. The coating has strong adhesion and conforms to the shape of the finished assembly.
Conformal coatings are made on the basis of acrylic, urethane, parylene, silicone and are applied by dipping, spraying, brushing, as well as on automated selective coating plants.
After application, the coating undergoes a curing operation, carried out under the influence of UV radiation or heat (infrared radiation and convection). Several materials offer a dual cure mechanism – UV and atmospheric moisture, facilitating curing in shady areas. To facilitate visual inspection of the coating’s integrity under UV irradiation, the corresponding materials at the points of formation of the coating glow with blue light.
Quality Control of Conformal Coatings: What do We See?
Quality control is a key aspect of the conformal coating process and is the key to the success of this process. The article discusses the standards for conformal coatings, the meaning of their provisions, the possibilities of new automated technical means for controlling the quality of the deposition of conformal coatings, as well as factors that must be taken into account to ensure reliable control.
A conformal coating is a thin transparent polymer layer applied to the surfaces of PCBs to protect them from external influences. The term “conformal” comes from the Latin conformis, it defines the ability of the coating to repeat the shape of the protected printed circuit assembly. Conformal coating standards
Today, the main international standard for conformal coating used by most companies around the world is IPC-A-610 Acceptability of Electronic Assemblies, the current edition of which (IPC-A-610E) can be ordered at IPC. There are other standards, including company codes of practice, but this article focuses on the A610 to help identify quality control needs for conformal coatings.
The Range of Issues Covered in the IPC-A-610 Standard
Study the IPC-A-610 standard in sections. This will facilitate understanding of both the operator’s needs and the requirements for the conformal coating process itself. The standard consists of three sections: “General Information”, “Coverage Completeness” and “Coating Thickness”.
General Information on Conformal Coatings
IPC-A-610 specifies that, in general, conformal coatings should be transparent and uniform in color and consistency, and evenly coat the printed circuit board with the components mounted on it. The coverage depends on the application method.
PCB Conformal Coating Spray
There is a lot of room for interpretation here, which can lead to problems if misunderstood. Note that any conformal coating technology – be it brushing, selective robotic application with an airless valve or aerosol spraying – has its own characteristics. They all give different levels of finish, which further vary depending on the organization of the technological process, the personality of the operator and the conditions of the working environment.
The terms “uniformity” and “uniformity” used in the text of the standard are of interest. By themselves, they are very ambiguous, but should be understood in the context of the requirements for the completeness and thickness of the coating considered below. Without such context, these terms ultimately do little to clarify.
Further, if the coating is to be transparent, the question arises as to the permissibility of using pigmented coatings. This should be discussed with the client and the effect of the pigment on the performance of the conformal coating should be assessed.
PCB Conformal Coating Process
Most conformal coatings now contain ultraviolet (UV) luminescent additives. This makes it easier to control the quality of the coating. However, some defects are not visible in ultraviolet light, and control in natural (white) light may be required. Some coatings naturally lack UV luminescence, such as many organ silicon coatings. This can make it difficult to control.
It is equally important whether the laminate or photoresist has its own luminescent radiation, comparable in intensity to the radiation of the coating: some conformal coatings are deliberately made non-luminous in ultraviolet light, since under operating conditions the used luminescent additive adversely affects the coating and the printed circuit assembly.
For completeness, the standard sets targets for the quality of the topcoat and different levels of quality – classes 1, 2 and 3. Targets include the following:
- Lack of areas with loss of adhesion;
- No voids or bubbles;
- Lack of desamination, local exfoliation, wrinkles, cracks, ripples, “fisheye” and “orange peel” defects;
- Absence of foreign inclusions;
- No discoloration or loss of transparency;
- Complete curing and homogeneous structure.
Many coating technologies, types of printed circuit boards and materials do not practically achieve all of the above targets. Their systematic achievement will generally be extremely costly, both financially and investment-wise, and in terms of time and effort spent on process control.
Let’s pay attention to such a target as the absence of bubbles. Even if you look at a printed circuit board with the naked eye, it is usually impossible to find a copy that does not have bubbles in one place or another, unless the following conditions are met:
- The technological process of applying a conformal coating is completely controlled;
- The coating material is correctly selected to achieve this result;
- Process conditions are fully optimized;
- Operators have received extensive training on the causes of bubbles and are able to control the process accordingly;
- PCB laminates, assembly processes, components, or conformal coatings are not modified in any way that could cause an undesirable reaction.
Fortunately, achieving these targets, while desirable, is not necessary for most companies – otherwise conformal coating would be an exclusive task for a few experts and an overwhelming task for most. IPC assists in this regard by offering its quality criteria for the specified targets:
- The coating is fully cured and has a uniform structure;
- The coating is applied only to those areas where it is required;
- Adhesion of the coating in the vicinity of the masked areas;
No Bridges between Adjacent Pads or Conductive Surfaces due to the Following Reasons:
- Loss of adhesion,
- Voids or bubbles
Foreign inclusions do not violate the minimum clearance requirements between components, contact pads or conductive surfaces. The coating is thin but reaches the edges of components and devices.
All of this seems reasonable until you try to understand in detail exactly what IPC is proposing to achieve in a conformal coating workflow. You may come to the conclusion that the workflow you are using or requested by your client is not as obvious as it seems at first glance.
First, consider the requirement to coat the edges of components and devices with a thin layer. It is extremely difficult to fulfill this requirement, it is practically impossible when using most standard technological processes of coating. It can be difficult to determine during the normal quality control process whether sharp edges are coated. If the client claims that this is his condition, it must be carefully considered.
Now let’s move on to the requirement that all the listed defects are absent, as well as bridges between adjacent conducting sections. This implies that the operator must examine the gaps between all conductive elements on the printed circuit board with components mounted on it and ensure that there are no defects, such as bubbles, that would violate this quality criterion. Such a task involves not only the highest level of qualifications, but also huge time costs, and in case of large-scale production, also the presence of a whole army of quality control specialists.
Before you come to an agreement with the client or your own design engineer about all the quality criteria, understand in detail what you are agreeing to.
PCB Conformal Coating Thickness Measurement
The last aspect addressed in the IPC-A-610 standard is the thickness of the conformal coating. The table in the standard specifies the allowable dry film thickness ranges for various polymer materials, such as acrylic conformal coatings, in the range 0.03–0.13 mm, or 30–130 µm. This is a wide range for conformal coating if all the processes are set up correctly. It is also easy to go beyond the specified acceptable limits if you do not have an idea of the main problems. The main thing is to understand the principles of the applied conformal coating technology and the capabilities of the material.
For example, if a plant has an automated dipping system, it may be difficult to obtain a dry film of an acrylic or polyurethane solvent-based coating thicker than 30 microns and exclude all defects listed in the quality criteria. The coating tends to be thinner and may not be thick enough to meet the criteria.
Moreover, there is a direct relationship between the amount of bubbles in the dry film coating and the wet film thickness applied in one pass.
It is easy to find out: if too thick layer is applied in one pass, then its near-surface part will harden (dry) before bubbles can emerge from the thickness, and they remain inside. Coating in thin layers is essential to eliminate bubbles. However, the selective coating robot generally operates in a single pass mode. Therefore, a compromise must be found and the coating process must be calibrated to obtain optimum results.
What does the requirement for uniformity of the coating and its uniform application actually mean? Do you mean “uniformity” in the range of 30-130 microns? Do I need to take care to cover the sharp edges from which the coating tends to spread with a thin layer? Finally, as noted in the standard, if coating builds up underneath the device, it is easy to exceed the 130 µm thickness limit in certain areas. Unfortunately, contrary to everyday intuition, more is not always better, and overly thick coatings should be avoided as over-thick coatings tend to crack in the long term.
Technological Processes for Automatic Quality Control of Conformal Coatings
As noted, in order to meet these quality criteria, a thorough inspection of the entire PCB will be required. It is an extremely difficult task due to factors such as eye fatigue, distraction and limited bandwidth. Can you automate quality control of conformal coating?
It is possible, but with some reservations and restrictions.
Consider the automated conformal coating systems on the market. Among them there are systems of a very high technical level with excellent cameras and scanners, excellent software and the highest quality process control. They can provide serial processing of products or be integrated into production lines and, it would seem, bridge the existing technological gap.
Cameras are mounted on three or four axis systems. In each chamber, parallax must be eliminated when inspecting bulk PCBs where hidden areas will exist along the sides of the components. The same parallax distortion occurs in scanner-based systems, and now there are scanning systems on the market that have eliminated parallax.
However, all of these systems have a certain flaw: they can examine every centimeter of the PCB from all kinds of angles and still miss problem areas. But this is not usually the determining factor in the automated quality control of the deposition of conformal coatings. Automated optical inspection (AOI) systems highlight the challenge of meeting IPC quality criteria in standard conformal coating processes. These systems reveal defects within the PCB coating and “see” much more than any operator.
To the user of the system, this may resemble an opened Pandora’s box, since he now has a whole line of printed circuit boards with defects all over the surface. If this is the case and the automated optical inspection system is configured to check the printed circuit boards according to these rules, then after a short time the production line will stop.
Is the Control System to Blame, or is the Conformal Coating Process? What to Blame?
The answer is simple: most technological processes do not provide the level of quality that is required by the criteria of the IPC standard. Automated optical inspection systems clearly identify all defects (as far as mechanical and optical factors allow). Moreover, they see the existing defects more clearly than the naked eye.
It is necessary to implement an iterative process to develop the optimal solution.
- Establish and define what defects (quality criteria) are acceptable.
- Determine what level of control is achievable within the existing and new technological process for applying conformal coatings and what defects can be generated by either process.
- If the system allows the criteria to be met, then all parties will be satisfied. Otherwise, the criteria or workflow should be changed.
Ultimately, you need to be guided by common sense, and then with the right level of knowledge, you can come to the right decision. By developing an optimal quality control process, you will be able to avoid unnecessary costs, disputes and counter-accusations later, when certain problems arise.