An investigation of sealant materials for display modules subjected to temperature and humidity conditions

This paper addresses the performance of various electronic conformal coatings specifically for the protection of the internal components of liquid crystal display (LCD) modules from moisture. The sealant materials after curing on a glass substrate were aged according to standard cyclic and steady-state damp heat tests. At various stages during the tests the mechanical performance of the materials was assessed. In addition these data were complemented with further measurements of the chemical stability and tendency of the materials to absorb moisture. Overall, from the tests it can be inferred that cyclic damp heat is much more stressful on the materials than steady state. Selection of an appropriate sealant for LCD modules demands a balance of mechanical durability, residual chloride ion content, tendency to absorb moisture, and the volatility of the material at elevated temperatures. The silicone elastomer was found to have the highest shear strength and a low tendency to absorb moisture, but it demonstrated appreciable chloride ion content and is not repairable. Thermoplastic polyurethane resins appear to be more suitable because, despite lower strength and higher water absorption, they have negligible chloride ion content and are repairable. Finally, it was concluded from the strength measurements that cyclic damp heat is more severe than steady-state conditions as an aging test.