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

Abstract

This paper addresses the performance of various electronic conformal coatings specifically for the protection of the intemal 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 steadystate 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 elcvated 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. Steady-state damp heat: temperature of 40 12°C and relative humidity of 93 +2, -3%RH Cyclic damp heat: temperatures varying cyclically from 25 +3C to 55 *2OC on a 24 hour period, with constant relative humidity of 93 +3%RH. A number of samples were withdrawn at weekly intervals and subject to shear strength measurements. Two sets of additional tests were also conducted on unaged samples of the sealant materials: water absorption, and chloride ion content. In particular, moisture in the presence of ionic contaminants such as chlorides has been reported [I], [2] as potentially problematic for the indium tin oxide traces (ITO) within the LCD module as they are oxidized to chloride while IT0 reduces. Experimental Methods In this section, the entire testing procedure used in this investigation is described starting with the methods used for sample preparation and ageing of the sealant materials. Additionally techniques to assess the mechanical performance and chemical stability of the aged sealants are also explained along with thc procedures for determining their chloride ion content and water absorption ability.