# Laser-Induced Period Surface Structures to Improve Solderability of Electrical Solder Pads
## 3 Results
\[RSA=\frac{\int_{0}^{\Lambda}\sqrt{1+\left(\frac{d_{\pi}\cos\left(\frac{\pi \Lambda}{\Lambda}\right)}{\Lambda}\right)^{2}}dx}{\Lambda} \tag{1}\]  
The measured modulation period \(\Lambda\) of 992 nm and the LSFL depth \(d\) of 120 nm lead to a relative surface advance \(RSA\) of 4%. The original Wenzel model uses this roughness factor to determine the static contact angle at the 3-point contact line that is in our case inherent with the increased surface coverage of the solder. Our results show that this obvious small relative surface advance leads to a noticeable improved solderability. Furthermore, the adaption of laser and strategy parameters, e.g., increasing laser fluence, can lead to larger LIPSS modulation depth and the occurrence of additional hierarchical microstructures. For brass, our group achieved a relative surface advance of up to 34% [20]. One possible application scenario is to activate soldering pads with laser structuring directly before the soldering process. The initial materials here would be printed circuit boards, which have already been in storage for a certain period and therefore would exhibit a certain oxide layer on the solder pads surface, thus inhibiting a reliable solder process. In order to investigate the possibility of spatial-selective solder pad activation, the polished copper substrate was stored for 14 days in an air atmosphere at room temperature. The results of this investigations are shown in Figure 4. The oxidation of the reactive copper reduces the wettability of the surface. For solder containing lead, the wetted area corresponds roughly to the size of the stencil but is reduced by approximately 14% as compared to the polished surface, as shown in Figure 3.  
For lead-free solder, partial dewetting of the surface takes place after the expiry of storage. As shown in Figure 4b, the solder material slumps from the circular template geometry and thus clearly shows the problem with aged substrate materials. The solder covered area reduces to a value of \((22.8\pm 6.1)\) mm\({}^{2}\). By applying laser-induced periodic surface structures, the aged surface can be modified in such a way that the increased wettability of freshly polished and additionally laser-structured surfaces is restored for both leaded and lead-free solder. Due to the nat  
Figure 4: (**a**) Solder-covered surface for leaded and lead-free solder paste on 14-day-aged LIPSS and 14-day-aged LIPSS surface. (**b,c**) Comparison of soldering on reference and LIPSS surface for lead-free solder paste. ural oxide layer on copper under normal environmental conditions being only 10-20 nm thick [28], LIPSS can reveal the underlying copper material. Figure 4a depicts the increase of the covered area in comparison to the aged surface. The laser-based activation leads to a covered surface of \((38.8\pm 0.5)\) mm\({}^{2}\) for leaded and \((40.4\pm 1.4)\) mm\({}^{2}\) for lead-free solder. It is worthwhile to stress that even after two weeks of storage after the laser application, again under ambient conditions, further soldering tests showed that the increased wetting still persists. For leaded solder, the covered surface is \((34.2\pm 1.7)\) mm\({}^{2}\), and for lead-free solder, it is \((32.0\pm 2.8)\) mm\({}^{2}\). Although the wettability is strongly influenced by surface aging, the measurement of the electrical resistance shows no significant deterioration of the electrical properties of the solder joints. For all evaluated surface conditions from Figure 4a, the contact resistance is in the range of 13-19 m\(\Omega\). Thus, we conclude that laser activation by introducing LSFL onto the solder pad using a femtosecond laser poses an effective possibility to improve solderability and storage possibilities, especially for lead-free solder. In turn, this provides an option to foster lead-free solder pads with respect to RoHS guidelines.