5 Keys to the Conformal Coating Process

The conformal coating process requires careful administration to ensure successful implementation. Recognizing the unique properties of various conformal coatings is critical in selecting the most applicable coating to a project while meeting clients’ material and operational specifications. Regardless of the coating material and the component, these five fundamental procedures are essential in producing a good conformal coating.

Ensuring the Component Surface is Free of Contaminants

The presence of organic or inorganic contaminants trapped beneath the liquid or Parylene coating causes imprecise coating. A clean surface is necessary to assure successful coating application and performance. Contaminants trapped inside Parylene coatings lead to structural breakdown, interfering with coating adhesion and generating delamination.

Although they’re visually undetectable, ionic contaminants can

• Corrode conformal coatings from within, between component and applied coating
• Promote development of dendrites, which are small metal outgrowths from a circuit component that further stresses the coating
• Short-circuit the assembly

Most ionic contaminants can be removed by cleansing with purified water or isopropyl alcohol.

Visible on the component prior to coating application, non-ionic contaminants like grease, lotion, oil, rosin or silicone attract debris and foreign matter, holding them close to the component’s surface. Their presence limits coating adherence, causing pealing, cracking or other performance dysfunction. Most non-ionic contaminants can be removed with solvents and surfactants.

Debris caught under coating will corrode assemblies, reducing their overall lifespan, while destabilizing performance prior to breakdown. Thorough cleaning of all components prior to coating application confirms their surfaces are contaminant-free.

Clear, Consistent Masking Instructions

Insulative qualities of conformal coatings interfere with the performance of assemblies’ electrical components. Since PCBs typically have areas that need to remain uncoated, most conformal coating projects require some degree of masking. Performance of components like connectors, switches and ground points are negatively impacted by coating coverage. They must remain uncoated to function as designed. Masking safeguards the specified keep-out regions that generate PCBs’ electrical function, preventing conformal coating seepage or other migration into restricted assembly areas. Masking materials, such as boots, dots, tape and liquid latex materials, applied before coating application assure that the conformal materials do not obstruct the function of a PCB’s electric components.

Client drawings and specifications are the basis of clear, consistent masking instructions. The objective is simple, prevent coating from getting into coating free areas. Gearing masking procedures according to PCB function and topography, the type of coating being used and the materials comprising the masking appliance.

Key masking instructions for liquid coatings include:

• Centering mask materials over specified keep-out areas
• Masking all component edges with smooth seals throughout
• Ensuring that masking is tear-free

Parylene coating requires a separate, significantly more complicated technique. Parylene coats all component areas exposed to air through vapor deposition surface permeation. Therefore, it is crucial that keep-out locations are effectively masked.

Masking is the only consistently dependable method that ensures conformal coatings do not penetrate keep-out areas.

Experienced Coating Application Personnel

Skilled, proficient application personnel enhance the potential for first-application coating success and add quality to the applied coating. For instance, experienced applicators of brush coatings increase the likelihood of even-layered coatings, reducing the need for rework. The same is true for dipped liquid coatings, whose generally higher viscosity and solids’ content can require special handling to assure even-layered coating application. Experienced applicators offer a higher rate of success for selective coating by machine. Explicit operator skill and care is also beneficial for appropriate materials’ selections during masking processes, ensuring they are correctly implemented. Any repair or rework similarly benefits from the input of qualified personnel.

Clean De-masking

Because masking is a temporary process, its removal through de-masking helps complete the conformal coating process. However, inept de-masking can tear the coating, causing a requirement for touchup. Badly implemented de-masking can be the fault of an unskilled operator or the result of poor masking earlier in the conformal coating process. Even the cleanest de-masking job cannot nullify:

• Leakage of coating into keep-out areas due to poor masking
• Excessive masking that prevents coating from reaching regions on the board requiring coverage

Correctly de-masking the component will significantly diminish any undue damage to the coating. Clean de-masking will also ease the conformal coating process to its final stages but cannot overcome problems with coating or masking application. Rework could be necessary despite clean de-masking.

Explicitly Defining Coating Objectives

Explicit detailing of coating materials and thickness, as well as areas to be coated and those to be masked, are essential to generating a successful conformal coating process. Also important is recognition of coating standards for the PCB type and function. Accurate client and provider generated drawings and specifications define all conformal coating objectives and requirements for the PCB.