## 1 Introduction
Chemical plating is the most used electroless deposition technique [15] (PCP, palladium chemical plating, from now on) because it is more efficient than displacement plating baths allowing to obtain palladium deposition rates of over 2.5 \(\upmu\)m/h at a deposition temperature of 50-60 \({}^{\circ}\)C [16]. The deposit is formed exclusively on catalytically active substrates, thanks to the reduction of metal ions in the solution by a reducing agent present in the bath. Chemical deposition is defined as a process in which one has the formation of a metal film through a controlled chemical reaction, catalysed by the same deposited metal [17]. This technique is widely used to deposit metallic films on non-metallic surfaces, e.g., glass and ceramics, through a pre-treatment to make catalytically activates the surfaces [18; 19]. Unlike chemical deposition processes, displacement plating does not include an external reducing agent, it ends as soon as the noblest metal, palladium in this case, completely covers the substrate. For this reason, this technique can produce only very thin films, which is an advantage for catalytic applications but an issue for decorative applications. Chemical plating has the advantage of selectively depositing on the conductive area of a substrate, a useful feature in various areas, such as the preparation of printed circuits [20]. Electroless deposition can be found in the literature in a large number of metal/substrate combinations [21; 22]; for example, metallization of semiconductor surfaces [23; 24], deposition of Ag-Pd alloys [25; 26], deposition of Pt on Ti surface [27], deposition of noble metals on Ge [28], Cu film formation on metal nitrides [29], Cu and Ni deposition on Al surfaces [30; 31; 32; 33], metallization of carbon nanotubes [34], and deposition of Ag, Cu and Au surfaces [35; 36]. The particular case of Pd on Cu can be found a limited number of reports [15; 20; 37], where authors did not study the homogeneity of the deposits over the entire substrate area nor their thickness or their adhesion and stability over time. Moreover, the combination of different electroless techniques to develop a complete process with tailorable Pd thicknesses has never been studied, to the best of our knowledge. In particular, [PERSON] [15] studied the influence of temperature and pH on the deposition rate of Pd-P alloys, analysing their crystalline structures. [PERSON] [20] studied both the aging of the plating solution and the thickness of the obtained Pd layer, but due to the date of publishing (1961) the thickness data obtained could be revised using more modern techniques. [PERSON] [37] studied electroless deposition of Pt-Pd alloys as catalysts for HER (hydrogen evolution reactions) reporting the composition and the surface morphology of the deposited alloys, but without paying attention to the adhesion of the obtained structures. In this work we attempted to combine two different palladium electroless processes to deposit palladium films with tailored thicknesses on copper. To achieve this aim, selected PDP baths were tested to obtain thin palladium films. Low-thickness palladium films have the dual role of being applied as an affordable catalytic surface in the energy and automotive sectors or by protecting copper substrates from subsequent PCP baths based on ammonia, which to our knowledge are the best-performing palladium chemical plating baths available in the literature [16; 20]. Ammonia-based PCP baths we used contained hypophosphite as a reducing agent for the formation of the palladium coating. This type of formulation was the best alternative to hydrazine-based PCP baths in terms of performance, although the palladium film could incorporate phosphorus forming Pd-P alloys [13; 15]. Hydrazine represents the first type of reducing agent studied for the chemical deposition of different metals, such as nickel, copper [38; 39; 40], and also palladium [41].