# Comparison of Tensile and Fatigue Properties of Copper Thin Film Depending on Process Method
## 3 Tensile and Fatigue Test  
### Tensile Test and Mechanical Properties  
The mechanical properties of the thin films, such as modulus of elasticity, yield strength and tensile strength, can be obtained from the stress-strain curve. To compare the mechanical properties of the copper thin film according to process method, we performed a repetitive test five times or more under the same conditions on each of the rolled copper and the electrodeposited copper specimens. In this test, the tensile rate was fixed at 50 \(\upmu\)m/min (0.84 \(\upmu\)m/s). The thin film specimen has limitations in using attached sensors such as extensometers. Therefore, in this paper, we use a capacitance displacement sensor for measuring the displacement during the tensile test. To obtain the tensile properties accurately, it has to be measured the displacement of the interesting range in the specimen (5000 \(\upmu\)m in Figure 1a). However, the capacitance displacement sensor can measure only the length of reduced section in the specimen (13,500 \(\upmu\)m in Figure 1a). Therefore, we use the finite element analysis method to calculate the ratio of reduced section and an interesting range as shown in Figure 7. In addition, the deformation of interesting range is yield by multiplying the measurement data and the ratio. Figure 8 shows the results of applying finite element analysis. The result of ratio is 0.635 [14]. Figure 6: The fracture images of specimen when alignment is correct. Figure 7: The ratio for reduced section and interesting range at the specimen. The stress-strain curve for copper thin film is obtained from load and displacement, as shown in Figure 9. For the load, the data obtained from the load cell are used, and the strain is calculated using the data obtained from the displacement sensor. To analyze the mechanical properties of thin film copper according to the process method, we compare the typical mechanical properties such as elastic modulus, 0.2% offset yield strength and ultimate tensile strength. The unloading test is essential for obtaining the elastic modulus more accurately. Therefore, the unloading test is applied to the electrodeposited copper around the strain 0.015, as shown in Figure 9b. However, the unloading test cannot be applied on the rolled copper because it had a brittle fracture. The mechanical properties of the rolled and electrodeposited copper are shown in Table 1, which are experimental results obtained from the stress-strain curve. As a result, the elastic modulus of the rolled and electrodeposited copper is 63 GPa and 61 GPa. 0.2% yield strength is 306 MPa, 233 MPa, and ultimate tensile strength is 414 MPa, 367 MPa. The elastic modulus is similar depending on the process method, but the 0.2% yield strength and ultimate tensile strength are different depending on the process method. In particular, it can be seen that the electrodeposited copper has a lower yield strength than the rolled copper. The mechanical properties of the copper thin films obtained in this study were compared with those of the copper thin films tested in [12]. To compare the mechanical properties of copper thin films according to the fabrication method, the specimens in [12] were fabricated with pre-plating (type A), the other with post-plating (type B), and finally double-plated (type C). A copper film fabricated pre-plating (type A) was deposited by electro-plating the whole pattern with SR (solder resist) printing. In addition, the SR film was cured to protect the pattern. A post-plating (type B) was first fabricated the SR coating and after was cured. A double-plated (type C) was fabricated by electro-plating on polyimide and pure Sn was coated before and SR film was printed and cured. The specimens were dog-bone type and length of specimen was 8000 \(\upmu\)m, width was 2000 \(\upmu\)m, and thickness was 15.26 \(\upmu\)m.