Hygro-Mechanical Analysis of LCD Panels

Abstract

Liquid crystal displays (LCDs) are getting larger, and the homogeneity of an LCD panel is becoming very important for the quality of the display. Inhomogeneity in an LCD panel can be caused by inhomogeneity of its materials and the defective production process, warpage of the panel due to changes in the temperature and humidity, and so on. In this study, we developed a scheme of hygro-mechanical analysis to reduce the warpage of an LCD. First, we measured the diffusion coefficients and Henry’s law coefficients of the respective components of an LCD using a thermo-gravimetric analyzer (TGA) under controlled humidity. We then measured the coefficients of moisture expansion (CME) of the componenets using a humidity-controlled thermo-mechanical analyzer (TMA). We analyzed the hygro-mechanical deformations of the respective components, a polarizing plate and an LCD panel using the finite element method (FEM) with measured diffusion coefficients, Henry’s law coefficients and the CMEs of the respective components. The analyzed deformations of the respective components corresponded quantitatively with the deformations measured experimentally. However, the analyzed deformation of the polarizing plate did not correspond with the measured deformation perfectly. A polarizing plate is made by sandwiching a polarizer by two sheets of protective film; the effect of the thin boundary layer between the polarizer and its protecting film was ignored in this analysis. The effect of this boundary layer on the diffusion of moisture may have caused the difference between the analysis and the measurement. The expected warpage of the analyzed LCD corresponded qualitatively with the measured warpage. In LCD panels, glass plates and polarizing plates are bonded using pressure-sensitive adhesive. Slippage between the glass plates and the polarizing plates may occur during the deformation of an LCD. We investigated the warpage of LCDs with two types of protecting film and different directions of polarizing plates using the developed technique of FEM analysis.