Electroless palladium-plating bath

The ideal electroless solution deposits metal only on an immersed article, never as a film on the sides of the tank or as a fine powder. Room temperature electroless nickel baths closely approach this ideal electroless copper plating is beginning to approach this stabiHty when carefully controUed. Any metal that can be electroplated can theoretically also be deposited by electroless plating. Only a few metals, ie, nickel, copper, gold, palladium, and silver, are used on any significant commercial scale. 

Photoproduced palladium nanoparticles have an average size ca. 3.3 nm that only slightly exceeds the critical size ensuring their stability in the nickel electroless plating bath. As a result, only 30 % of Pd centers of the latent image exhibit dissolution during Ni deposition, while others initiate the Ni plating... 

The electroless nickel/electroless palladium/immersion gold (ENEPIG) finish is simply an ENIG finish into which an intermediate layer of palladium (0.1 to 0.5 pm thick) is deposited between the electroless nickel and immersion gold coatings. Palladium is deposited from an autocatalytic plating bath. Palladium is a noble metal applied to protect oxidizable Ni. Palladium (m.p. 1552°C) is not fusible but rather dissolves in the molten solder in a manner similar to gold, and... 

The plating bath consists of a palladium ion source [Pd(NH3)4Cl2], a complexant (ethylene-di-amine tefra acetic acid (EDTA)) and a pH controller (ammonia). Deposition is carried out at basic pH (10-12) and at a controlled temperature ranging between 40 and 60°C. In more detail, the electroless technique is based on the following redox reactions, which occur simultaneously in the solution (Bottino et al., 2006) ... 

Table 3.4 Typical composition of palladium electroless plating baths reported in literature...

Acceleration modifies the surface layer of palladium nuclei, and stannous and stannic hydrous oxides and oxychlorides. Any acid or alkaline solution in which excess tin is appreciably soluble and catalytic palladium nuclei become exposed may be used. The activation or acceleration step is needed to remove excess tin from the catalyzed surface, which would inhibit electroless plating. This step also exposes the active palladium sites and removes loose palladium that can destabilize the bath. Accelerators can be any acidic or alkaline solution that solubilizes excess tin. 

Thorough cleaning and the avoidance of any extraneous debris are essential in any membrane fabrication scheme. After EP, a composite membrane can be soaked in hot water or dilute ammonia to help remove any impurities trapped in the porous support that may be detrimental to the palladium film during high temperature operation [72, 73]. However, traces of impurities from the EP bath such as chlorine, sodium, and carbon, inevitably become incorporated into the metal film. Membrane defects can be a consequence of preparation conditions. Fabrication in a dean-room environment has resulted in increased permselectivity [140]. Porous stainless steel (PSS) must be cleaned and pickled before electrodeposition or else activated for electroless plating. 

The Electroless plating deposition (ELP) technique is based upon the controlled auto-catalyzed decomposition or reduction of meta-stable metallic salt complexes on target surfaces [75]. In the case of palladium, usually, the substrate should be pre-seeded with palladium nuclei in an activation solution in order to reduce the induction period of the autocatalytic plating reaction. For some applications, this technique provides strong benefits such as uniformity of deposits on complex shapes and hardness. Palladium and some of its alloys are among the few metals that can be deposited in this way [11]. However, this method presents some drawbacks such as difficult thickness control, costly losses of palladium in the bath, not guaranteed purity of the deposit and so on [75]. 

Electroless plating and deposition Spent electroless copper bath Waste rinse water Acids, palladium, complexed metals, chelating agents, formaldehyde... 

Plastics are very good electrical insulators, and this hampers galvanic metallization in MID technology. The electrons required for the purpose cannot be provided by an external electrical source. Chemically reductive metallization baths, also known as electroless baths, are used instead. The electrons are provided by a component of the bath, namely the reductant. A bath of this nature consists of an aqueous metal salt solution, a reductant, and various additives such as chelating agents and stabilizers. In technical parlance, chemical baths of this nature are termed thermodynamically unstable and kinetically inhibited. The art of designing the bath is to make sure that metal deposition starts only at catalytically active areas (the tracks lasered by LDS or the places activated by selective palladium activation) and does not take place where no plating is deposited. This is accomplished by special stabilizers in the bath and by injection of finely distributed air [30]. 

The electroless plating process is normally carried out in a traditional bath consisting of a palladium ion source (PdCl2,Pd(NH3)4Cl2, Pd(NH3)4(N03)2),... 

Electroless plating was used by Shu et al. (1993) to deposit simultaneously Pd-Ag membrane on porous stainless steel substrate. This work investigated the co-deposition behaviour of Pd and Ag on porous stainless steel by means of electroless plating. Although Pd and Ag can be deposited independently to a considerable thickness, simultaneous deposition of palladium and silver was passivated by the preferential deposition of Ag in an electroless bath containing EDTA as a complexing agent. After effective... 

Hydrazine is the most common and suitable reducing agent for electroless plating of palladium. A typical composition of a hydrazine-based bath is shown in Table 3.4. 

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