Coating defects

Common Conformal Coating Defects & Their Solutions Capillary FlowCapillary flow, also known as scavenging, occurs when a conformal coating migrates from one area of a PCBA to another, leaving behind an inconsistent, uneven finish on the surface. Common causes for capillary flow include: The application of too much coatingConformal coating with low viscosity or high surface tensionLow surface energy of component being coatedTo avoid capillary flow from occurring: Ensure that the PCBA is entirely clean before coating.Use solvent-based coatings instead of water-based, and decrease the use of thinners to increase viscosity.Before coating, heat the board to decrease dry time. Cracking and RipplesCracking and ripples can compromise a conformal coating film, exposing a substrate or PCBA to potential contaminants. Common causes for cracks and ripples include: Coating that is too thickThermal shockCoating that has been cured too quicklyTo avoid cracks and ripples: Always apply the coating at an appropriate thickness.Allow for extra drying time at room temperature.Consider selecting a coating with a broader effective temperature range.DewettingDe-wetting is when a conformal coating will not evenly coat the surface to which it is being applied. Many things cause dewetting, including: An uneven coating applicationAn improper mixture of two-part materialsResidue on the coating surfaceVariations in Surface Tension and Surface EnergyTo avoid dewetting, thoroughly clean the material being coated before application. DelaminationDelamination occurs when a coating has lifted away from the substrate. This hinders protection because the area below the coating is left exposed. Delamination has many causes, including: Too thick of a coatingCoating not adhering to the previous coating layerContamination on the coating surface prevented a good bond to the surfaceTo prevent delamination: Reduce the thickness of the coating.Thoroughly clean the substrate before coating.Apply a “primer material” that can bond with the substrate and material used for conformal coating.Orange PeelOrange peel is when a coating appears dull, resembling the skin of an orange. It is caused by conditions including: A substrate that is too hotImproper coating mixture or solvent ratioA local environment that is too dry during applicationA second coat has been applied before the first coat is dryTo avoid orange peel: Apply coating at the specified thickness.Reduce the viscosity of the coating. Air bubbles, Pinholes, and FoamBubbles occur when pockets of air get trapped under a conformal coating layer. Foam is an extreme form of bubbling, and sometimes bubbles burst through the conformal coating, forming a pinhole. Bubbles, pinholes, and foam happen when: A coating is too viscousThe coating is too thickApplication equipment has not been setup correctlyTo avoid air bubbles, pinholes, and foam: Use a lower viscosity conformal coatingEnsure the coating is applied at the recommended thickness.When applying the coating, do so with thin coats. Why This Doesn’t Happen at HZOAt HZO, we use a conformal coating application method called chemical vapor deposition (CVD) that circumvents these conformal coating issues. Using CVD, we produce uniform, pinhole-free conformal coatings every time.