Originally published in the IPC Proceedings, the article “Effectiveness of Conformal Coat to Prevent Corrosion of Terminals“ was published online by circuit insight (http://www.circuitinsight.com/programs/54223.html). Author Michael Osterman is affiliated with the Center for Advanced Life Cycle Engineering, University of Maryland (College Park, MD).
As the title suggests, the article empirically investigates different conformal coatings’ success limiting or eliminating incursion of creep corrosion on component terminals with surfaces of nickel-palladium-gold plate, during operation. Terminals of printed circuit boards (PCBs) may permanently deform under the influence of mechanical stresses, because of long-term exposure to high levels of heat and stress; development of deformities increases as temperatures near the effected materials’ melting point[s]. Conformal coatings are applied to PCBs during manufacture for many reasons (insulation, moisture control, etc.), including protection from the development of creep corrosion.
Osterman conducted experiments testing the corrosion-resistant power of selected conformal coating materials subjected to corrosive influences.
Liquid (wet) coatings.
Four types of liquid coating were tested – 2 kinds of acrylic resin (AR1, AR2), in addition to resins of silicon (SR) and urethane (UR). Wet coatings are so-designated because of their deposition methods, as liquid materials, via brushing, dipping (immersion) or spraying the coating substance onto the substrate; these methods provide strictly surface coverage of substrate materials.
In this study, machine spraying was used for AR1, AR2, and SR; UR was coated by manual spraying. These processes yielded non-uniform coating thickness for all coating materials, across PCB topography (board surface in comparison to terminal topside). Uneven coats are a major source of both coating and component failure. Osterman’s measurements showed disparities of coating thicknesses between board surface and terminal topside.
Coating Application Board Coating Terminal Topside
Material Method Thickness (micrometers) Thickness
AR1 Machine spray 20.13 (0.31) 5.14 (1.61)
SR Machine spray 66.45 (2.05) 11.41 (3.39)
UR Hand spray 25.74 (2.71) 7.96 (1.57)
AR2 Machine spray 33.39 (1.32) 37.24 (2.05)
Registering uneven coating, these measurements suggested a tendency toward coating failure and corrosion development. All liquid coating types failed to completely cover terminal edges, leaving regions exposed. Corrosion occurred at these regions of limited coverage, when exposed to chlorine and sulfur gases.
Osterman provided photographs to accompany the text, demonstrating this tendency. He also provided high-magnification images (X-ray energy dispersive spectroscopy [EDS]), demonstrating the build-up of corrosive substances on terminal surfaces, despite covering by liquid conformal films. Encrustations of copper sulfide (Cu2S) and copper chloride (CuCl2) were the dominant corrosive substances detected. The worst corrosion was observed on the silicone-coated specimens, but none of the liquid coatings protected terminals from corrosive impact.
Unlike the liquid resins, parylene uses a chemical vapor deposition (CVD) method, which transforms solid parylene dimer into a gas, under heat in a vacuum. The parylene penetrates deep into the surface of the substrate, creating a truly conformal, pinhole-free, micron-thin coating film. with high dielectric and insulative qualities, generally superior to liquid films for most purposes.
In this study, Parylene C was used for terminal coverage. CVD enabled coverage of the entire board/terminal configuration, coating terminal edges and other surface regions missed by the liquid materials. It registered minimal variance of coating thickness, regardless of the PCB’s surface topography; thickness measurements matched closely, at 17.99 (0.44) across the PCB flat surfaces and 18.09 (2.71) at terminal topside. Corrosion was detected only where the parylene coating had been damaged by human handling; the coating itself repelled adhesion of corrosive substances.
Atomic Layer Deposition (ALD).
Similar to parylene, ALD methods use a vacuum deposition format. The deposited coats of both substances are micron-thin, adaptable for a wide range of microelectricalmechanical (MEMS)/nano-tech purposes. In this study, the similarity ended there. With ALD, binary reaction sequences requiring two surface reactions deposit a binary-compound film, sequentially, with atomic-level control on targeted surfaces Deposited ALD-Cap O5TA200(ALD), an A12O3 conformal film, was precisely uniform, measuring 0.1 micrometers on both board and terminal surfaces. Unfortunately, these coating did not repel corrosive development, despite completely covering all terminal edges. They registered the second worst protection performance in the study.
Parylene exhibited corrosion only in areas where the coating had been previously damaged, by handling. The liquid conformal coatings – 2 types of acrylic (AR1, AR2), silicon and urethane – and the atomic layer deposition coating were unable to prevent corrosion on the tested nickel-palladium-gold-finished copper terminations. Although ALD’s entirely uniform coating-performance surpassed parylene for coating-uniformity, corrosion occurred despite vapor deposition application methods. Apparently, ALD’s ultra-thin (0.01) nano-meter layers were insufficient to prevent the development of substantial corrosive structures identified by EDS analysis as containing copper, sulphur and chlorine.
In this study, neither liquid coatings nor ALD adequately covered edge/corner areas of the components, leaving them susceptible to admission of corrosive elements on the exposed terminal areas. Silicone registered the worst corrosive build-up, followed by ALD. However, liquid coating protection may improve if thicker films of the coating materials are applied. The same was not suggested for ALD. Parylene covered terminals completely and provided reliable protection against corrosion.
The study was well-organized and written. Osterman’s provision of graphic data, photographic- and EDS-imagery added to its verifiability as empirically-generated scientific evidence. References cited accurately supported the study.
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