# Laser-Induced Period Surface Structures to Improve Solderability of Electrical Solder Pads
## 1 Introduction  
Electrical connection pads, e.g., metallic solder pads, represent a key part in the packaging of printed circuit boards in electronics [1]. They provide an electrical connection between the electronic circuit and the electronic components. When exposed to ambient atmosphere, the oxidation of such electrical connection pads is a naturally occurring phenomenon [2]. As a result, after transport or storage, electrical connection pads can be covered by a layer of metal oxide, in turn leading to a gradual digression of the solderability of the electrical connection pads [3]. In the worst case, they might be no longer usable and must be disposed with both negative economic and environmental impacts. Therefore, to overcome this deficiency, protective layers are considered. For example, layers of noble metals such as silver or gold are used, covering the electrical connection pads during transportation or storage in order to avoid exposure to ambient atmosphere and hence to prohibit the formation of oxide layers [4]. Nowadays, there are several standards for a protective surface finish of printed circuit boards such as hot air leveling (HAL) [5], electroloss nickel immersion gold (ENIG) [6], or organic solderability preservative (OSP) [7]. However, the additional material required for the protective layer increases the production costs and requires dedicated steps and equipment in the manufacturing process of printed circuit boards. Concurrently, continuous efforts are taken in the electronics industry for improving the solderability of electrical connection pads, since it plays a key role in the durability, performance, and reliability of the resulting electronic devices [8, 9]. Driven by the European restriction of hazardous substances directive (RoHS), the demand of lead-free solder material rose and has led to a high demand of environmentally sound soldering joints using tin-based, lead-free solder; however, this is still a challenge for many applicationsdue to, e.g., a higher fusion temperature [10]. Overall, demand continues for technical improvements regarding the solderability of electrical connection pads and their manufacturing without the usage of an additional surface coating. These improvements shall also be applicable for lead-free solders. A possible method of coating-free surface functionalization is laser-based surface structuring [11]. Especially micro and nanostructures, particularly so-called laser-induced periodic surface structures (LIPSS) generated by ultrashort laser pulses, are highly interesting due to their versatile application [12]. In recent studies, the broad range of surface functionalization based on LIPSS has been demonstrated in fields of structural color [13], renewable energies [14], cell growth [15; 16], tribology [17; 18] and wettability [19], respectively. In particular, the later application is triggered by both the generation of hierarchic micro- and nanostructures and a surface chemical modification [20], enabling hydrophobic and hydrophilic wetting states. [PERSON] et al. [8] already demonstrated the possibility of femtosecond-based surface structuring to improve the solderability of Sn-Ag-Cu solder on copper by the laser-based generation of a pillar-like surface with ablation trenches having a width of 10-40 \(\upmu\)m, a depth of 20 \(\upmu\)m, and a pillar size of 40 \(\upmu\)m. These structural features reveal a decreased wetting angle that implies an improved solderability. However, the introduced method using a large trench depth of 20 \(\upmu\)m is incompatible for state of the art multilayer PCB having top-copper layer thickness of only several \(\upmu\)m. In contrast, the typical modulation depth of laser-induced periodic surface structures is below the applied laser wavelength [21] and therefore appears most suitable for thin solder pads. Against this background, this study addresses the application of laser-induced periodic surface structures on copper and evaluates the potential influence on the solderability.