Features

Conformal coatings protect electronic circuits from harsh environments via a chemical film that “conforms” to a circuit board’s topography. These coatings protect against moisture, chemicals, contamination, vibration, corrosion, and thermal stress while improving product reliability by reducing failures. The benefits conformal coating can provide often vary in importance by industry. In aerospace electronics there is an emphasis on benefits of protection from corrosive gasses, outgassing, tin whiskers, and radiation. For the purpose of this article, we will focus on liquid coating materials.

Coating Specifications

A good first stop for new conformal coating users is two widely used coating specifications. MIL-I-46058 is the military specification on conformal coating. This document is still widely referenced today, although it has been inactive for new board designs developed within the last 20 years. This doesn’t necessarily stop end users from trying to conform to this standard however, even today.

The more recent specification is IPC-CC-830, which also has a valuable sister handbook (IPC-CC-830-HDBK) documenting best practices. Most manufacturers use one of these specifications as a baseline to guide coating selection and compliance of their application process.

Coating Selection

There are various coating types and application methods. Both the IPC and MIL specifications note coating types that include silicones (SR), acrylics (AR), urethanes (UR), and epoxy (ER). The selection of the proper coating for a specific application is a very personal decision based on the pros and cons of each coating type, and the corresponding protection provided by each chemistry. As an end user you must know your market, the use environment for the end product, anticipated life of the product, and subsequently what protection best positions you to pass testing parameters you, or your customer, have established. Of particular note, MIL-STD-883, MIL-STD-810, and DO-160 all provide testing guidelines relevant to the aerospace industry.

Defining a Process

Figure 1. Coating thickness by chemistry as recommended by IPC-CC-830

Prior to determining the preferred application method for your conformal coating process, it is often critical to evaluate the coverage requirements for your circuit boards. Some components such as connectors, test points, RF shielding, and switches are often not coated. Understanding the required coverage area, keep-out areas, and having an understanding with your customer on how to approach “optional” areas are all important aspects of establishing your coating process.

Optional areas are often non-metal components or bare board regions where it is acceptable to apply conformal coating, though it may not be required. Sometimes a designer or quality department may want to see coating applied on all non-keep-out areas. In many cases there is a trade-off between the desired coated area and aesthetics, with a number of customers seeking a visually appealing process.

Coating Thickness

Figure 2. The Delta 6 is a flexible robotic conformal coating/dispensing system that is ideal for selective coating, potting, bead, and meter-mix dispensing applications.

Conformal coating thickness is tested on bare, flat board surfaces or test coupons. Preferred, cured, conformal coating thickness does vary by chemistry. The IPC (Association Connecting Electronics Industries) standard deems cured films to be between 25-75 μm (0.98-2.95 mil) for acrylic, urethane, and epoxy resins. Silicones are applied roughly twice as thick. Coating thickness per chemistry as recommended by IPC-CC-830 is noted in Figure 1.

Thickness is often also addressed in establishing baseline performance criteria on a formulator’s technical data sheet. These performance characteristics often reflect values determined within the thickness range noted in Figure 1. Lastly, thickness may also be addressed in the most pertinent place of all, the circuit assembly’s engineering drawing.

Increasing the coating thickness in your application does not correlate to an increase in protection of your assembly. Nevertheless, it is not unusual for end users to push the limits on coating thickness. End users can favor thicker films of coating for various reasons. The assembly could be installed in a harsh environment or you could simply be providing the process engineer additional peace of mind. That being said, requiring a thicker than recommended film should not be pursued blindly. A formulator may not guarantee product performance outside of their recommended thickness range. Further, the product itself may have characteristics that are counter to the protection process at high film thickness. Hard coatings, such as acrylics and urethanes, can often crack as cured film builds increase.

In short, there are published standards for appropriate film thickness for your process. Outside of this independent research, end-users often set their own standards that may fall outside of the recommended target range. These instances should always be qualified prior to production and in conference with the formulator. You must also be mindful of what these coverage requirements do to your resulting application method. Sufficient coverage on packages, particularly on corners, sides, or under leads (if required) can be challenging in thin film applications, whereas thick film processes can increase the flow characteristics of a coating application and be more difficult to manage around keep-out areas. The coating type, thickness, and application method require a delicate balance in building a defect-free process.

Application Methods

Manufacturers have a myriad of application methods to choose from in building their coating process. Factors such as budget, board volume, and product mix are typically obvious factors in making this decision. However, quality and taking into account the established coverage area required as dictated by the engineering requirements must be a major factor in this decision-making process.

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