All about the Conformal Coating Process

Acrylic, epoxy, silicone and urethane resins are applied by wet methods, including brush, dip or spray, and use either manual or robotic processes. These liquid coatings need to be cured before they can be used.

Parylene is applied through a chemical vapor deposition (CVD) process that transforms the raw material for Parylene, dimer, into a gas. The gaseous material permeates all surfaces of the substrate, resulting in a uniform, pinhole-free protective covering. Parylene does not require curing.

Conformal coatings safeguard PCBs, medical devices and other applications from environmental factors that could lead to component dysfunction or failure. Among the most common of these conditions are:

• Chemical incursion
• Contamination (particulate or otherwise)
• Humidity/moisture
• Temperature extremes

Basic Conformal Coating Processes

Obtaining coating specifications is the initial step in the conformal coating process. This should include all pertinent information, including such factors as identifying:

• Required coating thickness
• Keep-out areas of the component that should not be conformally coated
• Materials that best meet the coating requirements
• Acceptance criteria

Process development depends upon whether wet or CVD materials are used. Liquid coating processes need explicit instructions concerning whether brush, dip or spray methods will be used with further details on tooling – brush/nozzle-types, pallets, etc., machine program requirements and the curing procedure. Non-cured Parylene CVD processes also need to be enacted according to customer specifications.

Inspecting the components to be coated is essential to detect the presence of a wide range of contaminants – chemicals, dust, moisture, etc. — that can interrupt the unfettered flow of energy through the circuit, leading to degraded performance accuracy. Cleanliness testing helps detect such contaminants; if required, cleaning is performed prior to coating.

Insulative conformal films can interfere with the function of electrical components; thus, items like connectors, test points and similar keep-out areas require masking. Prominent variables of the masking process include the coating type, the PCB’s surface geometry, the component to be masked and overall production volume. Once completed, the masking inspection verifies compliance with the client’s masking drawing. Masking processes enacted before coating ensure the conformal materials do not invade designated keep-out areas.

Liquid Application Methods

The most common liquid application methods are:

• Brushing: Implemented on a piece-by-piece basis. It is used less frequently because quality is dependent on the manual skill of the operator; uniform coating thickness and bubbling are difficult to control. It is a slow process employed mostly for low-volume coating applications and repair.
• Dipping: Immersing PCBs in a coating solution is fast. It is the most commonly used method for most high-volume applications and requires conscientious preparation to ensure the coating does not penetrate improperly masked keep-out areas. Variables include rates of immersion/withdrawal and viscosity of the coating material.
• Spraying: This is a good choice for medium- and higher-volume applications. Spraying generates reliable conformal films; substrate cleanliness is essential as is superior film adhesion. The objective is to obtain a combination of solvent dilution, nozzle pressure and spray pattern to meet assignment objectives. Spray coating is readily automated.

Robotic technology can selectively coat circuit boards using the processes described above, generally faster and more accurately than manual application. Robotics generate advantages like varied flow rates/vacuum pressure for dipping, and better controlled viscosity while spraying.

Liquid coatings require curing and drying following application, to ensure correct film consistency and thickness. Drying must be implemented in the appropriate environment This is dependent on the specific parameters of the coating application and type of coating used; in some cases, curing can last longer than 30 days at room temperature.

Parylene CVD

Raw, powdered Parylene dimer is heated and the solid-state Parylene is transformed into vapor at the molecular level, sublimating each gaseous molecule and dividing each into a monomer. Parylene vapor attaches to the substrate one molecule at a time. The monomer gas reaches the final deposition phase in the cold trap where temperatures are cooled to levels between -90º and -120ºC, removing any residual Parylene materials from the substrate. The coated assembly requires no curing or solvent catalysts.

De-masking

The masking material is removed as soon as possible after the dried coating is firm enough but still sufficiently soft to prevent tearing the film during removal. Following de-masking, contacts and other regions of the PCB are exposed and ready to perform their electrical functions.

If masking is implemented poorly, the conformal coating can seep under the tape because of bleeding/leaking during film application, necessitating de-masking.

Touch-up

Failure to stop coating penetration into keep-out areas can result in costly, time-consuming repairs, including fixing the leak, removing the conformal coating from the assembly, replacing the improperly masked component or scrapping the entire assembly. Conditions like delamination or tin-whiskers can also necessitate touch-up. The process is job-specific, depending on the type of coating material, its position on the PCB and the board’s components.

Chemical solvents do the least damage to PCBs and are effective for liquid coatings. Most chemical methods do not work well for chemically inert Parylene films; only tetrahydrofuran (THF) consistently removes Parylene from substrates. Abrasion, laser, mechanical, plasmatic and thermal removal methods are more successful for Parylene films; they also work for liquid coatings in many cases.

Touch-up generally entails spot removal from specific regions of the PCB. In most cases, the selected coating can be reapplied. When completed, a thorough, pre-shipment inspection is enacted for quality assurance.