Transient Stress Distributions in NCF-Bonded COG Packages due to Moisture Diffusion

Abstract

The nonconductive film (NCF) adhesives have potentials to replace the anisotropic conductive film (ACF) adhesive in the chip-on-glass (COG) or other packages in liquid crystal displays (LCDs) due to their feasible features of finer bump pitch and cost reduction. Prior to applying the NCF to the COG packages, reliability issues, such as interfacial delamination and increase of bump contact resistance under thermal and moisture loads, have to be overcome. The objective of this paper is to investigate the effect of moisture ingression on stress field of the COG packages with NCF bonding. The coefficients of moisture diffusion and the hygro-strains of the NCF adhesive have been determined by an innovative approach of measuring the out-of-plane deformation of the bimaterial specimens during moisture absorption at the condition of 30degC/85%RH using Twyman-Green interferometry, associated with a finite element analysis and bimaterial theory. In this study, the results show that the average of the moisture diffusion coefficients and saturated hygro-strains of the NCF under 30degC/85%RH are 1.09 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /s and 1.51 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , respectively. After being validated by the diffusion theory, the moisture- transient finite element analysis reveals that the moisture absorption of the COG package under 30degC/85%RH is nearly saturated after 1000 h. It is also found that there are large tensile stresses at pad corner and mediate ones across bump joint, and these tensile stresses gradually increase with moisture absorption and up to the maximum at the state of moisture saturation. Through the parametric study by the validated finite element model, the results show that the tensile bump-joint and interfacial stresses increase nonlinearly with the NCF elastic modulus and linearly with the saturated moisture-induced strain (SMIS) of the NCFs. Furthermore, the COG package with polyimide bumps is found to have lower moisture-induced tensile stresses than that with gold bumps. It is further shown that the tensile stresses from the longitudinal-section and transverse-section models are similar only with a few percent difference in values. It can be concluded that the stress states of the NCF-bonded COG packages induced by transient moisture ingression have been fully addressed through experimental, theoretical and numerical approaches in this study.