# Comparison of Tensile and Fatigue Properties of Copper Thin Film Depending on Process Method
## 1 Introduction  
Modern electronic devices are required to be thin, lightweight, and functionally sophisticated. Therefore, flexible printed circuit boards(FPCBs) can be used as alternatives to rigid printed circuit board (PCB) in certain applications because of its excellent flexibility, twistability, and light weight. FPCB facilitates a space-efficient system design, reduces size and weight, improves the internal appearance of a system, and offers low-cost installation. Over the years, FPCBs were widely used in numerous electronic devices such as smartphones and LCDs [1, 2]. Therefore, FPCB has become one of the important parts for securing the reliability of electronic products.  
FPCB is constructed from polyester or polyimide film and copper film. In particular, one of the roles of the FPCB is to make electricity flow well. FPCB breaks down due to heat generated by joule heating and mechanical fatigue loads at the joints. The reliability of electronic device using FPCB is strongly dependent on the fatigue characteristics of materials used such as copper.  
It is well known that tensile and/or fatigue properties of such thin films are not the same as those of bulk materials and that they could strongly depend on the size [3, 4, 5, 6, 7]. In general, manufacturing methods are known to affect the mechanical properties of materials, especially for thin films [8, 9]. Thus, it is essential to perform mechanical (tensile and fatigue) tests to obtain relevant mechanical properties [10, 11]. Therefore, authors reported the tensile and fatigue characteristics of the film thecopper film coated by Sn used in electronic devices such as LCD. The copper film was fabricated by electro-plating on polyimide tape and then pure Sn was coated by electroless plating to prevent oxidization of copper film. Finally, by a diffusion process, the film was treated by heating. Some smooth specimens for the tests were fabricated by an etching process and to make a free standing specimen, the polyimide tape was etched. The tensile tests for the film were performed at a displacement rate of 100 \(\upmu\)m/min. The high cycle fatigue tests were performed with 20 Hz frequency, in ambient environment and two level mean stresses. The tester used in the test was closed-loop feedback controlled.  
In this study, the tensile and high cycle fatigue tests for copper thin film, especially used in FPCB were performed using the test machine developed by authors. The copper films were produced by the rolling process and the electrodeposited process. The copper film was attached on UV film for handing. Smooth specimens for the tests were fabricated by an etching process and to make a free standing specimen, the UV film was peeled off with acetone. The tensile tests for the film were performed at a displacement rate of 50 \(\upmu\)m/min. The high cycle fatigue tests were performed with 3 Hz frequency in ambient environment at R = 0.1. The tester used in the test was closed-loop feedback controlled. To analyze the influence of the fabrication, the test results were compared and, especially with the test results of copper thin films fabricated by completely different processes published by the authors [12].