Quality Control of Conformal Coating Application

Quality control is a critical aspect of the conformal coating process and is key to successfully completing this operation. This article discusses the standards for conformal coatings, the meaning of their regulations, the ability of new automated technology means to apply quality control to conformal coatings, and the factors that must be considered to ensure reliable control.

Conformal coatings are thin, transparent polymer layers that are applied to the surfaces of printed circuit assemblies to protect them from external factors. The word "conformal" is derived from the Latin conformis - "similar", "resembling", that is, it determines the ability of the coating to replicate the shape of the protected printed circuit assembly.

Conformal Coating Standards:

Today, the primary international standard used by most companies around the world in the area of conformal coating is IPC-A-610 Standard for Acceptability of Electronic Assemblies, the current version of which (IPC-A-610E) can be ordered from IPC. There are other standards, including company regulations, but this article focuses on A610 to help determine the quality control needs of conformal coating applications.

Scope of issues covered by IPC-A-610

IPC-A-610 should be studied section by section. This will help to understand both the needs of the operator and the requirements of the conformal coating process itself. The standard consists of three sections: General information, Coating Coverage, and Coating Thickness

General information about conformal coatings:

IPC-A-610 states that conformal coatings should generally be clear and uniform in color and consistency, and should uniformly cover the printed circuit board and its components. The extent of coverage depends on the application method.

There is a lot of room for interpretation here, which can lead to problems if misunderstood. It is worth noting that each conformal coating application technology – be it brush application, selective robotic application with an airless valve or aerosol spraying – has its own characteristics. They all produce 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 production environment.

The terms "homogeneity" and "uniformity" used in the text of the standard are of interest. In themselves, they are quite ambiguous, but must be understood in the context of the requirements for the completeness and thickness of the coating discussed below. Without such a context, these terms ultimately clarify little.

Furthermore, if the coating is to be transparent, the question arises as to whether pigmented coatings are acceptable. This should be discussed with the customer and the effect of the pigment on the performance of the conformal coating assessed.

Completeness of coating coverage:

Most conformal coatings now contain luminescent additives that glow under ultraviolet (UV) light. This makes it easier to control the quality of the coating application. However, some defects are not visible in UV light and may require control in natural (white) light. Some coatings do not have sufficient UV luminescence by nature, such as many organosilicon coatings. This can complicate control.

It is equally important whether the laminate or photoresist has its own luminescent emission comparable in intensity to the emission of the coating: some conformal coatings are deliberately made non-luminescent in ultraviolet light, since under operating conditions the luminescent additive used has an adverse effect on the coating and the printed circuit board.

In terms of coverage, the standard sets quality targets for the finish coating and different quality levels – classes 1, 2 and 3. The targets include the following:

• Absence of areas with loss of adhesion;

• Absence of voids or bubbles;

• Absence of dewetting, local peeling, shagreen, wrinkles, cracks, ripples, defects such as “fish eye” and “orange peel”;

• 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 allow achieving all the above-mentioned target indicators in practice. Systematic achievement of them will generally be extremely expensive both in financial and investment terms, and in terms of time and effort spent on process control.

Let's pay attention to such a target indicator as the absence of bubbles. Even if you look at the printed circuit board with the naked eye, it is usually impossible to find a sample that does not have bubbles in one place or another, unless the following conditions are met:

• The conformal coating process is fully controlled;

• The correct coating material is selected to achieve this result;

• Process conditions are fully optimized;

• Operators are extensively trained on the causes of bubbles and are able to control the process accordingly;

• No changes have occurred in the PCB laminate, assembly process, components, or conformal coating that could cause an adverse reaction.

Fortunately, achieving these targets, while desirable, is not necessary for most companies – otherwise, conformal coating would be the exclusive domain of a few experts and an impossible task for many. IPC helps in this regard by offering its own quality criteria for these targets:

• The coating is fully cured and structurally uniform;

• The coating is applied only to areas where it is needed;

• Adhesion of the coating near masked areas;

• No bridging between adjacent pads or conductive surfaces due to:

-- Loss of adhesion,

-- Voids or bubbles,

-- Dewetting,

-- Cracking,

-- Waviness,

-- Fisheyes or sharkskin;

• Foreign inclusions do not violate the minimum insulation gap requirements between components, contact pads or conductive surfaces;

• The coating is thin but still reaches the edges of components and devices.

This all seems reasonable until you take a closer look at what IPC is proposing to achieve with its conformal coating process. You may find that the process you are using or the one your customer is asking for is not as obvious as it first appears.

First, consider the requirement to coat the edges of components and devices with a thin layer. This requirement is extremely difficult, if not impossible, to meet using most standard coating processes. It is quite difficult to determine whether sharp edges are coated during a normal quality control process. If a customer states that this is their requirement, it is important to consider this carefully.

Now let's move on to the requirement for the absence of all of the above defects, as well as bridges between adjacent conductive sections. This means that the operator must examine the gaps between all conductive elements on the printed circuit board with the components mounted on it and ensure that there are no defects, such as bubbles, that would violate this quality criterion. Such a task requires not only the highest level of qualification, but also enormous time expenditures, and in large-scale production, the presence of an entire army of quality control specialists.

Before you agree with the client or your own design engineer on all the quality criteria, understand in detail what exactly you are agreeing to.

Conformal Coating Thickness:

The final area addressed by IPC-A-610 is conformal coating thickness. The standard's table sets acceptable dry film thickness ranges for various polymeric materials, such as acrylic conformal coatings, ranging from 0.03 mm to 0.13 mm, or 30 µm to 130 µm. This is a wide range for a conformal coating application if the process is properly implemented. It is also easy to exceed these limits if you are not aware of the underlying issues. The key is to understand the principles of the conformal coating process being used and the capabilities of the material.

For example, if a facility has an automated dip coating system, it may be difficult to achieve a dry film of solvent-based acrylic or polyurethane coating greater than 30 microns thick and avoid all of the defects listed in the quality criteria. The coating will typically be thinner and may not be thick enough to meet the criteria.

Moreover, there is a direct relationship between the number of bubbles in the dry coating film and the thickness of the wet coating film applied in one pass. This is easy to find out: if you apply too thick a layer in one pass, then its surface part will harden (dry) before the bubbles can float up from the thickness, and they will remain inside. Applying the coating in thin layers is the most important condition for eliminating the occurrence of bubbles. However, the robot for selective coating usually works in a single-pass mode. Therefore, it is necessary to find a compromise and adjust the technological process of coating application in such a way as to obtain optimal results.

What does it actually mean to require a uniform coating and a uniform application? Does it mean “uniform” in the range of 30–130 µm? Do we need to be careful to apply a thin layer to sharp edges where the coating tends to spread? Finally, as noted in the standard, if the coating accumulates under the device, it is easy to exceed the permissible thickness limit of 130 µm in certain areas. Unfortunately, contrary to common sense, more is not always better, and excessively thick coatings should be avoided, as excessively thick coatings tend to crack in the long term.

Technological processes for automatic quality control of conformal coating application:

As noted, to meet the quality criteria outlined above, a thorough inspection of the entire PCB is required. This is an extremely difficult task due to factors such as eye fatigue, distraction, and limited throughput. Can conformal coating quality control be automated?

It is possible, but with some reservations and limitations.

Let's look at the automated conformal coating systems available on the market. They include some very high-tech systems with excellent cameras and scanners, excellent software, and the highest quality of process control. They can handle serial processing of products or be integrated into production lines, and seem to bridge the existing technological gap.

The cameras are mounted on three- or four-axis systems. Each camera must eliminate parallax distortion when inspecting large printed circuit boards where there will be hidden areas along the sides of components. Scanner-based systems suffer from the same parallax distortion, and there are now scanning systems available that eliminate parallax.

However, all of these systems have a drawback: they can examine every inch of the PCB from every angle and still miss problem areas. But that is not usually the determining factor in automated conformal coating quality control. Automated optical inspection (AOI) systems highlight the difficulty of meeting IPC's quality criteria within standard conformal coating processes. These systems show defects within the PCB coating and "see" much more than any operator could.

For the system user, this may seem like a Pandora's box opening, as he now has a whole line of printed circuit boards with defects all over their surface. If this is the case, and the automated optical inspection system is set to inspect the printed circuit boards according to these rules, then after a short time the production line will stop. Is the inspection system to blame, or the conformal coating process? Where should the blame lie?

The answer is simple: most technological processes do not provide the level of quality required by the IPC standard criteria. Automated optical inspection systems clearly identify all defects (as far as mechanical and optical factors allow). Moreover, they see existing defects more clearly than the naked eye.

Conformal Coating Solution：

It is necessary to implement an iterative process for developing an optimal solution.

1. stablish which defects (quality criteria) are acceptable and define them.

2. Determine what level of control is achievable within the existing and new conformal coating process and what defects can be generated by both processes

3. If the system allows the criteria to be met, all parties will be satisfied. Otherwise, the criteria or the process should be changed.

Ultimately, common sense should be used, and then with the right level of knowledge, the right decision can be made. By developing an optimal quality control process, unnecessary costs, disputes and counter-accusations can be avoided later when problems arise.