# Comparison of Tensile and Fatigue Properties of Copper Thin Film Depending on Process Method
## 3 Tensile and Fatigue Test
### Fatigue Test and Fatigue Properties
Therefore, in order to compare the results of the rolled and the electrodeposited copper which are the experimental results of this study, the fatigue life equation for alternating stress was obtained. As a result, fatigue strength coefficients of 910 MPa, 1191 MPa, and 1743 MPa were obtained for each plating type, and the fatigue strength exponents were \(-0.08955\), \(-0.10913\), and \(-0.15072\), respectively. The fatigue properties are different according to the plating method, and it can be seen that the fatigue strength is the highest when the double-plating is performed. However, as a result of comparing both the fatigue properties according to the process method and the plating method, it can be seen that the copper thin film by the rolling process method has the highest fatigue strength. The P-S-N diagram showing the confidence band of fatigue life for each process is shown in Figure 11. When the fatigue life is 50%, the equation of the rolled thin film obtained by regression analysis is \(S_{a}=513(2N_{f})^{-0.1113}\) and the equation of the electrodeposited thin film is \(S_{a}=277(2N_{f})^{-0.0759}\). Figure 10: Stress-Fatigue life curve (S-N curve): Comparison of fatigue properties of Cu thin films depending on process method. In this study, tensile and fatigue tests were performed and analyzed to evaluate an influence of fabrication method on mechanical characteristics of copper thin film which widely used in flexible printed circuit board (FPCB). Tensile test results showed that the elastic modulus was similar for each process, but the 0.2% yield strength and ultimate tensile strength were greater in the rolling process. The elastic modulus of the rolled copper thin films and the electrodeposited copper thin films are 63 and 61 GPa, respectively, which are lower than the bulk value, 117 GPa [15]. In addition, the 0.2% offset yield strength of the rolled copper thin films and the electrodeposited copper thin films are 306 and 233 MPa, respectively, higher than 33 MPa of bulk copper and the ultimate tensile strength of the rolled copper thin films and the electrodeposited copper thin films are 414 and 367 MPa, respectively, about higher than 210 MPa of bulk copper. The tensile test results show that the copper thin films by the rolling process are more brittle than by the electrodeposition process. In addition, fatigue tests were performed with three or more specimens at four levels and S-N curves and P-S-N curves were calculated by regression analysis with the obtained data. The fatigue test results show that the copper thin films by the rolling process are better than by the electrodeposition process in fatigue life in all region. The fatigue strength coefficient of the rolled copper thin films and the electrodeposited copper thin films are 513 and 277 MPa, respectively. In addition, the fatigue strength exponent of the rolled copper thin films and the electrodeposited copper thin films are \(-0.1113\) and \(-0.07590\), respectively. From the experimental results, the mechanical characteristics of the thin film are influenced by the manufacturing method even if they are the same material, and it is important to directly obtain the mechanical properties to secure the reliability of an electric product.