# Laser-Induced Period Surface Structures to Improve Solderability of Electrical Solder Pads
## 2 Materials and Methods  
Evaluation of the solderability of different surfaces can be performed by different methods, e.g., by the measurement of the 3-point contact line wetting angle between the solder and the substrate. Good wetting is defined as having a wetting angle of substantially less than 90\({}^{\circ}\), whereas an angle larger than 90\({}^{\circ}\) implies bad wetting. With increasing wettability, the contact angle decreases to rather small values, which makes it difficult to evaluate such surfaces by the contact angle. Yet another method is the characterization of a material's tendency to spread the solder after application. The solder spread, also termed slump, can be used as a qualitative and quantitative evaluation of the solderability [22]. Slump stability describes the solder paste's ability to maintain its structure and shape after printing. Surfaces having a bad solderability will be dewetted during the solder process and will lead to insufficient substrate contact. In addition, the slump of the solder paste should be minimized, as slump creates the risk of forming solder bridges between neighboring pads, creating a short circuit [23]. The general evaluation of the solderability is depicted in Figure 1 and can be divided into 4 major steps: laser structuring, solder past application, solder process, and evaluation. Figure 1: Evaluation of modified solderability. (**a**) Laser structuring of the copper surface, (**b**) solder paste application via screen printing, (**c**) solder process on hotplate, and (**d**) slump analyzation. The laser structuring was performed on commercial available copper sheets with a thickness of 1.5 mm. For a defined initial surface, the pieces were ground with a multi-directional polishing machine (LS3V, REMET, Riale, Italy) with several abrasive strengths (grit size 800-2500) and afterwards polished with suspensions down to a grain size of 1 um to achieve a reference surface with a roughness \(R_{a}\) of 0.03 um (measured by laser-scanning microscope VH-X, Keyence, Osaka, Japan). For laser surface processing, we use a micro-machining station (WSMH, Optec, Frameries, Belgium) equipped with a high-power ultrashort pulsed laser (Amphos200, AMPHOS, Herzogenrath, Germany) having a variable pulse duration between 900 fs and 10 ps (FWHM) and a repetition rate of up to 40 MHz. The maximum average power is 200 W with a maximum pulse energy of 1 mJ. For the presented structuring approach, the fundamental emission wavelength of 1030 nm is used. The energy of the laser is adjusted by an external attenuator based on a rotating wave plate and a polarizer. Using an adjustable beam expander telescope, the raw beam diameter is adjusted to a diameter of 5.5 mm (1/e2) (LaserCam-HR, Coherent, Santa Clara, CA, USA). A galvo scanner (Excelliscan 14, Scanlab, Puchheim, Germany) is used in combination with a telecentric lens (f = 163 mm) to focus the beam onto the sample with a calculated spot diameter of 50 um (1/e2). All given values of the pulse energy and the resulting laser fluence are derived by the laser power measured directly behind the processing optic. To evaluate the laser-based surface modulation, the topography was measured using an atomic force microscope (Dimension Icon, Bruker, Billerica, MA, USA) in PeakForce Tapping Mode. Via manual stencil printing, the solder paste was applied on the relevant surface. The 0.2 mm thick stencil creates 4 circular shapes of solder paste having a diameter of 6.5 mm. We applied leaded (Sn63/Pb36/Ag2) and lead-free (Sn96.5/AG3/Cu0.5) solder paste with a solder powder size of type 3. The solder process was performed via a flat hot plate and a contact time of 20 s. The temperature was set to 250 \({}^{\circ}\)C for both leaded and lead-free solder paste. After the solidification of the solder paste, the measurement of the solder-covered area was performed using an optical 3D profilometer (VR 3200, Keyence, Japan).