Papers by Author: Ki Jeong Seo

Abstract: Flexible printed circuit board (PCB), which is used for folder and slide type cellular phones, consists of flexible copper clad laminate (FCCL) and cover layer. Through it an electric current is applied to liquid crystal display (LCD) from the main board of cellular phone. In thin Cu foils of flexible PCB fatigue cracks due to repeated bending motion generate and propagate, and they cause a short circuit. Fatigue behavior of thin Cu foils being used for flexible PCB must be evaluated and confirmed to resolve this problem. It is based on findings by several researches that the mechanical properties of thin film materials differ from those of their bulk counterparts. Thin film properties have been investigated over the last years; however fatigue behavior of thin films has not yet been studied as thoroughly as monotonic behavior. In this study fatigue properties of thin Cu foils for the application in flexible PCB are obtained. Fatigue testing was performed for two kinds of Cu foils that were made by rolling and electrochemical procedures respectively. Differences of fabrications in fatigue behavior of thin foils were distinguished. Especially for rolled Cu foils, effects of rolling directions in fatigue properties were evaluated.

Abstract: Nanoimprint lithography is a promising technology to produce sub-100 nm scale features on silicon chips. One of key issues in the nanoimprint lithography is how to make uniform contact between the stamp and the substrate on a large area. In this study a rubber membrane unit under substrate is introduced to resolve this problem. Two layers of membrane were designed to consider air flow in the middle of resist on a silicon wafer. The geometry design for accomplishing uniform contact was carried out using finite element analysis. The material modeling of hyperelastic properties of rubber is characterized by the Mooney-Rivlin strain energy functions. Material constants in the strain energy functions are able to be determined via the curve fitting of experimental stress-strain data. Simple tension and equi-biaxial tests were performed to determine the material constants. To evaluate the effects of a rubber membrane unit, nanoimprint lithography process with it was executed. We could confirm that a distinct improvement of uniform contact was shown and air flow problem was solved during the process.