## 2 Materials and Methods  
In this work, three different PDP solutions were tested to study the displacement of electroless deposition of palladium, both as a method to obtain nanometre-sized continuous layers and as a pre-deposition treatment applicable to PCP with the aim of producing high-quality micrometre-sized palladium layers. These three different displacement deposition solutions, from three different authors, were selected as the sole alternatives available in the literature. All the solutions involved in this study were prepared with ultrapure water (18.2 M\(\Omega\), TOC \(<\) 5 ppb) obtained with a Sartorius Arium(r) mini system (Gottingen, Germany). The PDP solutions were prepared using different quantities of PdCl\({}_{2}\) salt 99.9% (Alfa Aesar, Heysham, Lancashire LA3 2 XY, UK) and HCl 37% (VWR AnalaR(r) NORMAPUR(r), Radnor, PA, USA). The PCP solutions were obtained using the same PdCl\({}_{2}\) salt and HCl purchased before for the PDP solutions and NH\({}_{3}\) 28% (VWR AnalaR(r) NORMAPUR(r), Radnor, PA, USA), NaH\({}_{2}\)PO\({}_{2}\)\(\cdot\)H\({}_{2}\)O ([PERSON], Cornaredo, Italy) and NH\({}_{4}\)Cl (Merck Millipore Suprapur(r), Burlington, MA, USA) were added. The composition of PDP solutions studied are reported in Table 1.  
Solutions A, B, and C were developed, respectively, by [PERSON] (1998), [PERSON] (1998) and [PERSON] (1998), and were prepared simply by adding palladium salt in a HCl solution, and then stirred until complete dissolution of the salt at room temperature. The bath developed by [PERSON] and [PERSON] (1998) (Table 2) was selected as the PCP solution used in this work due to its efficiency and its stability over time and work cycles. To obtain the best performances in terms of Pd film quality and bath stability, the PCP solution (presented in Table 2) preparation was meticulously carried out. The Pd solution was prepared by dissolving 0.1 g of PdCl\({}_{2}\) in a flask containing 0.2 mL of 37% HCl. Within the flask, palladium was present in the form of [PdCl\({}_{4}\)]\({}^{2-}\). To the previously prepared solution, 8 mL of 28% NH\({}_{3}\) was added little by little under stirring, forming a light-pink precipitate. The flask was closed and left under stirring for an hour until the solution became transparent. It was then left to mature for a total of 24 h. Transparency indicated the formation of the chelate [Pd(NH\({}_{3}\))\({}_{4}\)]\({}^{2+}\). The contents of the flask were subsequently filtered into a 25 mL flask with a syringe and a glass fibre filter to remove any pollutants that could lead to the deterioration of the solution. In parallel to this, a solution of sodium hypophosphite monohydrate was made by dissolving 0.5 g in 25 mL of ultrapure water. Finally, the two solutions were combined in a 50 mL flask. The copper substrates (pure copper, dimensions 50 mm \(\times\) 10 mm \(\times\) 1 mm) used in the entire work were pre-treated using different steps. Pre-treatment of the copper substrates was an irreplaceable step due to the copper oxide layer that naturally formed over the entire surface, which passivated it, blocking the galvanic exchange reaction between copper atoms and palladium ions, which produced the electroless deposited film. First, the metallic slabs were polished with a polishing machine using a soft polishing brush and polishing wax. Then, to completely remove the organic residues due to the polishing wax, it was necessary to stir the copper plates inside toluene for about 3 min and the rinse with abundant ultrapure water. Each copper plate was masked with the help of Teflon tape to obtain a reproducible deposition area (approximately 1 cm \(\times\) 1 cm on the bottom of the plate) and further degreasing with acetone was performed. In a second step, an electrochemical cathodic degreasing was then carried out using copper as a cathode and a mixed oxide counter electrode inside a commercially available alkaline degreasing solution (DEXO 102, Italifimet, Arezzo, Italy).