# OCT Capillary Depth Measurement in Copper Micro Welding Using Green Lasers
## 3 Results
### Capillary Depth Measurement Using OCT
#### 3.2.3 Aspect Ratio  
In laser welding, the welding process is typically differentiated into two main welding regimes, heat conduction welding and deep penetration welding. For deep penetration in state-of-the-art copper welding processes with infrared laser at \(\sim\)1 \(\upmu\)m, the aspect ratio of the capillary:  
\[R=\frac{s}{d_{f}}, \tag{2}\]  
where \(s\) is the measured capillary depth, and \(d_{f}\) is the focal diameter of the laser, which is typically substantially larger than one [9]. Due to the increased absorptivity of the green wavelength in copper [6], a smooth transition from heat conduction welding to deep penetration welding can be observed, thus allowing for shallow penetration welding. In the following section, the resulting aspect ratios will be presented.  
Using the presented results of the OCT depth measurement in Section 3.2, the resulting aspect ratio can be determined directly. In Figure 9, the aspect ratios are displayed as a function of the welding speed and laser power. The welding speed can be distinguished by color and shape, where red triangles represent a welding speed of \(v=250\)\(\frac{\text{mm}}{\text{s}}\), green diamonds \(v=500\)\(\frac{\text{mm}}{\text{s}}\), and blue circles \(v=750\)\(\frac{\text{mm}}{\text{s}}\). Additionally, both process regimes, heat conduction welding and deep penetration welding, are marked by the background color (heat conduction welding = white; deep penetration welding = grey). The formatting for the process stability follows the formatting in Figure 3 and can be found in Section 3.1.  
For heat conduction welding with low laser powers, the aspect ratio for all welding speeds is \(\sim\)0. With an increase of laser power, an increase of the aspect ratio can be observed. This is not only noticeable when changing from heat conduction welding to deep penetration welding but also during heat conduction welding with high welding speeds. Exemplary, for a welding speed of 750 \(\frac{\text{mm}}{\text{s}}\) and a laser power of 1500 W a depth of \(\sim\)40 \(\upmu\)m was measured, resulting in an aspect ratio of 0.16. This leads to the conclusion that a small melt pit must have already been formed during heat conduction welding.  
For deep penetration welding, aspect ratios as low as 0.35 can be observed for a welding speed of 750 \(\frac{\text{mm}}{\text{s}}\) and a laser power of 1750 W. With welding speeds of 500 \(\frac{\text{mm}}{\text{s}}\) and 250 \(\frac{\text{mm}}{\text{s}}\), the lowest achieved aspect ratio increases to 0.58 and 0.7, respectively. When focusing on the process stability, it can be noticed that, for deep penetration welding with low aspect ratios, no increased spatter formation was observed.  
In conclusion, by using the depth measurement of the OCT System, the aspect ratio of the welding process could be determined directly. It could be shown that, for micro welding of copper with green lasers, aspect ratios less than 1 can be achieved in deep  
Figure 8: (**a**) Percentage difference between measured capillary depth and welding depth as function of the welding depth, white: heat conduction welding, grey: deep penetration welding; (**b**) absolute difference between measured capillary depth and welding depth.  
penetration welding. Additionally, the deep penetration process with low aspect ratio has shown to be more stable towards spatter formation.