# Microstructure of Joint between Stranded Wire and Substrate Weld by Ultrasonic Welding
## 2 Materials and Methods  
The stranded wire consisted of 7 Cu wires with a 70 \(\upmu\)m in diameter. Each wire was coated with an Ag layer 1\(\upmu\)m in thickness. Figure 1 shows the electron back scatter diffraction-inverse pole figure (EBSD-IPF) map of one of the stranded wires cut parallel to the wire direction. The grain size of the Cu ranged from 5 to 10 \(\upmu\)m. The Cu substrate was 100 \(\upmu\)m in thickness and covered by 20 \(\upmu\)m thick Ni and 100 nm thick Au. Figure 2 shows the schematic illustration of the ultrasonic welding machine. The welding machine used in this study was a rotary head type ultrasonic wire bonder (REBO-7 WL, Ultrasonic Engineering Co., LTD., Tokyo, Japan). The wedge tool had a V-groove structure. The depth and the width of the grooves were 140 \(\upmu\)m and 220 \(\upmu\)m, respectively. The length of the groove was 300 \(\upmu\)m. The stranded wire was put on the substrate between the tool and an anvil. Ultrasonic vibration and load were applied to the wire through the wedge tool during the ultrasonic welding. The welding frequency was 110 kHz, and the welding time was fixed at 0.2 s. The loading force and ultrasonic power were changed from 720 to 780 gf and, 110 to 150 a.u., respectively. After the welding, a tensile shear test was carried out for each joint. In the test, the substrate was fixed, and the wire was pulled parallel to the substrate surface with a cross-head speed of 25 mm/min. The joint welded with 750 gf in loading force and 130 a.u. in ultrasonic power showed a maximum strength of 4.8 N and was used for the microstructure observation. Joints welded over 760 gf in loading force and 150 a.u. in ultrasonic power were broken at the boundary of the wire between the original part and the applied ultrasonic part due to the damage produced by the ultrasonic welding. A detailed analysis is beyond the scope of the present paper and will be conducted in future.  
Figure 3a shows the scanning electron microscope (SEM) image of the joint between the coaxial cable and the substrate after ultrasonic welding. The outer cables and covering materials in the coaxial cable were removed and the stranded wire was welded to the substrate. Figure 3b shows the morphology of the joint with maximum strength after the tensile shear test. The welded region remained on the substrate and the sample was broken in the wire.  
As for the microstructure observation, the joint was cut parallel and perpendicular to the wire direction. A cross-section of the joint was polished mechanically and finished by ion-milling (IM4000, Hitachi High-Technologies, Tokyo, Japan), and observed by SEM (SU-5000, Hitachi High-Technologies, Tokyo, Japan). For the transmission electron microscope (TEM) observation, a thin sample was picked up from the cross-section using a focused ion beam (FIB) technique (JIB-4600F, JEOL, Tokyo, Japan) and observed by TEM and scanning transmission electron microscope (STEM) (JEM-2010, JEM-2800, JEOL, Tokyo, Japan).  
Figure 1: EBSD map of one of the stranded wires cut parallel to the wire direction.  
Figure 2: Schematic illustration of the ultrasonic welding machine.