偏光板元件濕熱變形對液晶面板漏光現象之影響

Abstract

本研究透過彎柄儀 (bending beam) 整體平均應力量測及結合3D結構性應力模擬，探討偏光板元件在高溫高濕可靠度測試下，其組成材料性質、溫度及濕度變化對液晶面板漏光現象之影響。吾人發現控制入射光偏振狀態之聚乙烯醇 (PVA) 薄膜在濕熱條件變化下，所產生收縮應力及壓感黏合劑 (PSA) 之應力釋放能力為控制偏光板x-y平面內應力分佈及整體結構曲率之關鍵因素。在三種不同楊氏係數之壓感黏合劑中，若使用較高楊氏係數之壓感黏合劑 (PSA-h) 時 (E>0.25 MPa)，整個偏光板產生最大曲率變化,但在x-y平面內最小位移量，導致非均向性應力分布且集中於面板邊緣。相反地，使用較低楊氏係數之壓感黏合劑 (PSA-s) 時 (E<0.04 MPa),卻是整個偏光板產生最小曲率變化，而在x-y平面內產生最大位移量，造成等向性應力分佈。此外，3D模擬分析顯示面板漏光根本原因在於應力集中於三醋酸纖維素 (TAC) 薄膜所導致光行進時產生相位遲延 (phase retardation)。針對PSA-h,模擬結果顯示主應力值差異值 ( ) 大於1.125 MPa之分佈區域，主要集中於面板之周圍且對光學之影響性相對較小，因而形成側漏型面板漏光之形貌。相對之下，針對PSA-s其主應力值差異值 ( ) 大於1.125 MPa之分佈區域明顯地由面板側邊拓展至面板中心，因而形成漏斗型，甚至是全面性漏光。 針對濕氣影響之實驗分析，吾人發現聚乙烯醇薄膜及壓感黏合劑其高分子吸濕特性所導致應力、應變之變化量高於溫度改變對其之影響。這應歸咎於高分子在吸水後受到濕氣塑化之影響導致自由體積相對變大因而提升黏合劑應力釋放之能力。簡言之，使用較高楊氏係數之壓感黏合劑或降低三醋酸纖維素之熱機械性質均為有效之方式達到改善面板漏光之現象。

In this study, we combine the stress measurement using bending beam technique with static thermal-dependent and transient time-dependent three-dimensional (3D) finite element analysis (FEA) analysis to examine the light leakage variation of the thin-film transistor liquid-crystal display (TFT-LCD) panel. The objective is to understand the effects of material properties of key components, temperature, and humidity under hygrothermal reliability test on the light leakage phenomena. The shrinkage stress in stretched poly(vinyl alcohol) (PVA) film and stress relaxation ability of pressure-sensitive adhesives (PSA) layers are found to be the key factors determining the stress distribution and out-of-plane displacement of a polarizer stack. For hard-type PSA, its polarizer stack generates the highest bending curvature with maximum out-of-plane displacement but minimum in-plane displacement, leading to anisotropic stress distribution with high stress around the edges. On the other hand, polarizer stack with soft-type PSA yields the maximum in-plane displacement but the minimum out-of-plane displacement, resulting in isotropic stress distribution. 3D FEA shows that a strong correlation exists between light leakage and retardation difference induced by stress on triacetyl cellulose (TAC) films. For hard-type PSA, the area of higher principal stress difference is relatively small and localized on the edges of the panel, thus indicating low dimensional variations. In contrast, funnel-type light leakage and greater polarizer shrinkage are found for soft-type PSA. Moreover, the magnitude of hygroscopic stress in the simulated analysis is found to be significantly higher than that of thermal stress. This can be attributed to moisture’s plasticizing effect on the hydrophilic polymers such as PVA and PSA layers, leading to enhanced stress relaxation and degradation of the display image quality. An increase in Young’s modulus of PSA brings in lower relaxation ability and better resistance to shrinkage in polarizer stack, making it an effective solution to minimize the light leakage. The other effective solution is to develop a TAC film with lower Young’s modulus and/or lower coefficient of thermal expansion (CTE).