# Laser-Induced Period Surface Structures to Improve Solderability of Electrical Solder Pads
## 3 Results  
The first step of our study is the generation of LIPSS on the copper substrate. Referring to our previous studies on LIPSS generation on metals [19; 24], the pitch between two consequent pulses is chosen to be 10 um, i.e., 80% spatial overlap for both the scanning direction and perpendicular thereto. Using a pulse repetition rate of 100 kHz leads to the corresponding scanning speed of 1 m/s. The scanning direction is parallel to the linear polarization direction of the electrical field of the laser beam. A homogenous coverage of LIPSS on the copper surface can be achieved by a laser fluence of 0.8J/cm\({}^{2}\) and a laser pulse duration of 900 fs. The resulting surface modulation is shown in Figure 2. By the bidirectional scan direction of the hatch, jump delays can almost be neglected, and therefore, a high structuring rate of 10 mm\({}^{2}\)/s is achieved. The spatial wavelength of the modulation is 992 nm; i.e., it is slightly lower than the applied laser wavelength of 1030 nm, which was a phenomenon that has been observed before [25]. The orientation of the wavy structures is perpendicular to the polarization of the laser beam. Hence, the structures can be identified as low spatial frequency LIPSS (LSFL) [26]. The mean modulation depth is derived by Figure 2b to be approximately 120 nm. The first investigation of the potential improvement of solderability is performed on fresh polished and cleaned (ultrasonic and aceton) copper substrate. The generation of LIPSS and the solder test are performed as described above. The solder-covered surface was measured for four samples. The comparison of the solder behavior for the reference (fresh polished) and laser treatment (LIPSS) surface is shown in Figure 3.  
The purpose of the initial polishing step is the removal of the natural oxide layer and therefore an improvement in the surface chemical wettability. The comparison in Figure 3 reveals the influence of the topographical modulation generated by the laser treatment. Both surfaces show a very good wettability for leaded and lead free solder. The dashed line marks the initial surface area provided by the stencil aperture of \(33.2\,\mathrm{mm}^{2}\). The lead-free solder enlarges the covered surface after the solder process to \((36.8\pm 1.3)\,\mathrm{mm}^{2}\). On the surface having a laser treatment, the covered surface enlarges to an area of \((38.2\pm 1.1)\,\mathrm{mm}^{2}\); i.e., the presence of the laser laser-induced micro- and nanostructures enables a higher surface wettability with a 4% larger coverage. A similar effect is observed for the application of leaded solder paste. The application of LIPSS on the copper surface increases the covered surface by 5%. The polished surface leads to a surface area of \((39.3\pm 1.4)\,\mathrm{mm}^{2}\), while LIPSS leads to a mean surface area of \((41.3\pm 0.7)\,\mathrm{mm}^{2}\). The evaluation of the electrical resistance reveals no significant influence of the surface preparation. Regardless of the surface condition and solder material, the mean contact resistance of the four solder joints is in a range of 14-17 \(\mathrm{m}\Omega\). Figure 3: (**a**) Solder-covered surface for leaded and lead-free solder paste on fresh polished and LIPSS surface. (**b,c**) Comparison of soldering on reference and LIPSS surface for leaded solder paste. Figure 2: Surface topography after laser structuring using \(0.8\mathrm{J}/\mathrm{cm}^{2}\) and a spatial overlap of 80% using atomic force microscopy. These findings can be explained by the improved surface wetting on a rough surface described in the Wenzel model [27]. The initial wetting behavior of a smooth surface is governed by the relative surface advance introduced by the roughness. According to our previous studies [20], the surface of LSFL can be described by a \(|\sin(x)|\) course, and the relative surface advance, considering the modulation period and depth, is calculated using a numerical approach.