Printed circuit boards (PCBs) interconnect microchips and similar electronic components etched or laminated onto flat fiberglass/plastic panels. A variety of conformal coatings – liquid resins of acrylic. epoxy, silicone or urethane and chemically inert parylene -- assure proper insulation for PCBs. Such environmental conditions as contamination from debris, humidity within the unit, solvent incursion or the impact of human handling are minimized by conformal protection, reducing performance degradation while prolonging assembly life.
Materials and Methods
The material-specific performance properties of conformal coatings differ according to material type. Although they can be applied by similar methods – brush, dip, spray – all four liquid coating materials respond differently to various performance criteria and conditions. Liquid application methods include:
· brushing wet coating material directly onto the PCB,
· immersing the assembly in a liquid coating solution (dipping), and
· spraying coating onto the PCB.
Chemical vapor deposition (CVD) applied parylene has a range of operational properties all its own. Regardless of the coating material used, successful application of conformal films is supported by recognizing their distinct operational characteristics.
Quality Assurance (QA) Certifications
To assure reliable, long-lasting film application, clients seeking conformal coating services are advised to select providers who understand not only the differences between the various coating types, but also all aspects of coating application and processing. Accredited professional organizations from both the public and private sector have devised benchmarks outlining appropriate quality management systems (QMS) for the industry. Both the International Standards organization (ISO) and the National Aeronautics and Space Administration (NASA) create professional standards and specifications for conformal coatings, guiding quality assurance (QA) of material and operational conditions.
· ISO is an independent, international/non-governmental organization providing documented world-class specifications for products, services and systems.
· NASA, an independent agency of the U.S. federal executive branch responsible for aeronautics/aerospace research, creates operational standards to ensure process/product quality, safety and efficiency.
Accredited QA-certifications are awarded conformal coating firms that repeatedly demonstrate well-implemented QMS for their processes/products, planning production and service-provision to regularly meet customer requirements. ISO and NASA certs represent verifiable records of industry-standard compliance, performance and risk management
QA-certifications are conferred upon firms meeting industry-best standards/specifications. Achieving and maintaining ISO/NASA certification is necessary for any firm wanting to establish professional QA-reliability. Certification-acquisition signifies a firm’s commitment to providing customers superior conformal coatings, ensuring optimal PCB-function, performance reliability and operational-life.
ISO consensus-based standards’ development procedures involve experts from organizations in over 170 nations, nominated by their national standards bodies. Companies implementing ISO 9001:2015 guidelines commonly report benefits like:
· Enhanced operational efficiency.
· Greater customer satisfaction.
· Lower production costs.
· Better quality processes/products.
The objective of 9001:2015 is enhancing customer satisfaction through the efficient implementation of QA/QMS processes, including those supporting ongoing systemic improvement and appropriate conformance with customer or regulatory/statutory obligations.
NASA’s various projects -- moon/planetary landings, SkyLab space-exploration, the Space Shuttle -- require exceptional QMS to assure safe project implementation, maintenance and completion, using mission-hardware/critical ground-support technology.
A Workmanship Standard for Polymeric Application on Electrical Assemblies, NASA 8739.1 sets practice/documentation requirements for reliable conformal coating of PCBs and related electronic assemblies; included are fabrication/inspection procedures for specialized coating processes for NASA-generated assignments. These may be tailored to the program applications to obtain the most cost-effective, best quality product.
Basic Standards for Coating Application and Rework
According to ISO/NASA standards, conformally coated electronics require thorough inspection for product suitability. Prior to film application, PCBs must be:
· Cleaned/demoisturized 8 hours or less prior to coating-application.
· Only oven-bake or vacuum-bake processes are acceptable for PCB demoisturization, which
· must commence at the temperature/time-duration specified by the product's engineering document.
These quality processes assure the PCB is sufficiently dry to safely accept conformal film application.
Whether liquid methods or parylene CVD are used, QMS processes eliminate excessive filtering and material dripping/oozing during application, supporting intended component coverage. Elimination of material-bubbles in the coated-surface is preferred. However, micro-bubbles that do not bridge component surfaces, expose bare conductors, or exceed 0.76mm (0.03 inch) in any dimension are acceptable. Scratches or other imperfections in the coating surface are acceptable ONLY when they do not expose conductive area.
NASA 8739.1 specifies precise film thicknesses for each coating material, measured in millimeters (inches):
· Parylene -- 0.013 – 0.051 (0.0005 to 0.002).
· Silicone -- 0.051 – 0.203 (0.002 to 0.008).
· Acrylic, urethane, epoxy -- 0.025 – 0.127 (0.001 to 0.005).
These levels of coating support reliable, safe performance of circuits and components.
ISO/NASA QA standards are essential to quality conformal coating performance, with guidelines that:
· reduce the incidence of product/process defects,
· develop/maintain best-practice product design/manufacture, and
· highest-quality conformal film protection,
· minimizing repair/rework.
Clients seeking coating services should pursue providers in possession of accredited QA certification awarded for:
· commitment to using designs/materials/processes
· that support quality performance,
· streamlined failure-cause analyses and
· ongoing product/process evolution.
Recognizing the necessity of matching conformal materials and coating processes with standardized industry-best specifications, Diamond MT maintains up-to-date, audited accreditations. Our primary objective is providing continual product quality, sustaining clients’ expectations, as well as the industry’s material/operational performance requirements.
To learn more about selecting a conformal coating service provider, download our whitepaper now: