The conformal coating process requires watchful administration to ensure successful implementation. Recognizing the unique properties of various coating-types is critical to selecting the kind most applicable to the project and its purposes, while meeting clients’ material and operational specifications. Regardless of the coating material and the substrate, these five fundamental procedures are essential to good conformal films.
Ensuring the Substrate Surface is Free of Contaminants
The presence of organic or inorganic contaminants trapped beneath the liquid/parylene film causes imprecise coating. A clean surface is necessary to assure successful coating application and performance, since contaminants trapped inside parylene coatings lead to structural breakdown, interfering with film adhesion, generating delamination.
Although they’re visually undetectable, ionic contaminants can
- Corrode conformal coatings from within, between substrate and applied film.
- Promote development of dendrites, 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 iso-propyl alcohol.
Visible on the substrate prior to film application, nonionic contaminants like grease, lotion, oil, rosin or silicone attract debris and foreign matter, holding them close to the substrate’s surface. Their presence limits coating-adherence, causing pealing, cracking or other performance dysfunction. Whatever the type, most 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 substrates prior to coating application confirms their surfaces are contaminant-free.
Clear, Consistent Masking Instructions
Insulative qualities of conformal films 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 film coverage; they must remain uncoated to function as designed. Masking safeguards the specified keep-out regions that generate PCBs’ electrical function, preventing conformal film seepage or other migration into restricted assembly areas. Masking materials -- boots, dots, tape and liquid latex materials – applied before coating application assure 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. General guidelines for mask application are useful, and include:
- Gearing masking procedures according to PCB function and topography, the type of coating being used, and the materials comprising the masking appliance.
- For liquid coatings, center mask materials over specified keep-out areas.
- Mask all component edges with smooth seals throughout.
- Make certain masking is tear-free.
- Masking protects keep-out areas from over-dipping or inaccurate spraying of liquid coatings.
- Parylene coating require a separate, significantly more complicated technique.
- Parylene coats all components areas exposed to air through CVD surface permeation.
- More than typical keep-out locations must be effectively masked.
Masking is the only consistently dependable method ensuring conformal coatings do not penetrate keep-out areas.
Experienced Coating Application-Personnel
Skilled, proficient application-personnel enhance the:
- potential for first-application coating success,
- adding quality of the applied film.
For instance, with brushed liquid coatings, experienced applicators increase the likelihood of even-layered films, reducing the need for rework. The same is true for dipped liquid coatings, whose generally higher viscosity/solids’ content can require special handling to assure even-layered film 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/rework similarly benefits from the input of qualified personnel.
Because masking is a temporary process, its removal through de-masking after films dry helps complete the conformal coating process. However, inept de-masking can tear the film, 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, or
- excessive masking that prevents coating from reaching regions on the board requiring coverage.
Operator skill 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/masking-application. Rework will 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/function. Accurate client/provider-generated working-drawings/ specifications define all conformal coating objectives/requirements for the PCB.
The objective is always a clean, durable coating-finish, applied to specification, eliminating costly and time-consuming project-rework. Diamond MT’s trained professionals will work with you through every phase of project- development and implementation to assure your conformal coating is applied correctly and professionally, the first time.
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