Conformal Coating Curing with Thermal Cure, Moisture Cure, Ultraviolet Cure
Conformal coatings are applied to printed circuit assemblies to provide protection against chemicals, dust, moisture, and extreme temperatures that, if uncoated, could result in a failure of the electronics. When electronics must withstand harsh environments, most circuit board assemblers coat electronic assemblies with a layer of transparent conformal coating rather than potting. Recently, conformal coatings have been used to reduce or eliminate the potential for tin whiskers. Conformal coatings come in a variety of material options including Silicones, Acrylics, Epoxies and Urethanes. These different coating materials have very different curing requirements.
Thermal (Heat) Curing: Solvent based coatings cure or dry as solvents evaporate and off-gas from the liquid coating, leaving behind the solids that comprise the protective coating. To accellerate the curing or drying process thermal curing ovens are often used. Thermal curing can also be employed for silicone based coatings as well as a secondary cure mechanism for some UV Curable Coatings.
Ultraviolet (UV) Curing: Some adhesives and conformal coatings, such as those produced by Dymax or Humiseal, cure upon exposure to Ultraviolet (UV) energy and/or Visible Light with wavelengths in the range of ~254-460nm. UV curable coatings are 100% solids and contain no solvents. One problem assosiated with UV curable coatings is shadowing caused by components on the printed circuit assembly. In recent years, coating manufacturers have addressed this issue by offering formulations that offer a secondary curing mechanism. The secondary curing mechanism is typically moisture cure or thermal cure.
Moisture (Humidity) Curing: Silicone coatings offer rapid curing when temperature and humidity are controlled. Typical silicone coating curing conditions are 60 to 80°C at 10 to 15% relative humidity. Since relative humidity falls as temperature rises it is often required to enhance the moisture content in the airstream.