Palladium, electroless

Rhoda, R.N. (1952) Electroless palladium plating. Trans. Inst. Met. Finish., 36(3), 82-85. 

Antitamish and preflux Immersion silver Immersion tin Electroless palladium (Pd)... 

Other surface finishes include reflowed tin-lead, electrolytic nickel/electrolytic gold, antitarnish and reflux, electroless palladium, electroless gold, direct immersion gold, and solid solder deposit. 

Electroless palladium can also act as a barrier layer if used in sufficient thickness. Automotive PCBs produced in the 1990s specified a maximum of 0.2 micron of Pd metal with the goal to incorporate the entire Pd layer into the solderjoint. 

There are a number of other surface finishes used in the industry, such as Electroless Nickel Electroless Palladium Immersion Gold (NiPdAu), Immersion Silver, Immersion Gold, Immersion Tin, OSP and Electrolytic Nickel Gold. There are reliability and process trade offs with each surface finish. That is why it is recommended that strain/strain rate characterization and thermal cycling be performed for each set of surface finish before it is selected for the specific end-use conditions in which it will be used. The industry test methods used to evaluate different surface finishes are outlined in detail in the next chapter. 

TABLE 25.2 Composition of a Typical Electroless Palladium Plating Solution... 

Electroless nickel/electroless palladium/immersion gold (ENEPIG)... 

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... 

Solder spread or ability to wet determines the shape of a fillet in a solder joint. Sattiraju et al. investigated the solderability of immersion tin, ENIG, electroless palladium, and OSP surface finishes with Sn-3.4Ag-4.8Bi, Sn-4.0Ag-0.5Cu, Sn-3.5Ag, and Sn-0.7Cu lead-free interconnection alloys both in air and nitrogen using sequential electrochemical reduction analysis (SERA) and wetting balance techniques. It has been reported that Ni/Au finish performed the best and OSP performed the worst in both nitrogen atmosphere and air environment, suggesting a preference for Ni/Au finish. The Sn-0.7Cu alloy was considered superior, whereas Sn-4.0Ag-... 

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. 

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

Catalysts for dielectric surfaces are more complex than the simple salts used on metals. The original catalysts were separate solutions of acidic staimous chloride [7772-99-8J, used to wet the surface and deposit an adherent reducing agent, and acidic palladium chloride [7647-10-17, which was reduced to metallic palladium by the tin. This two-step catalyst system is now essentially obsolete. One-step catalysts consist of a stabilized, pre-reacted solution of the palladium and staimous chlorides. The one-step catalyst is more stable, more active, and more economical than the two-step catalyst (21,23). A separate acceleration or activation solution removes loose palladium and excess tin before the catalyzed part is placed in the electroless bath, prolonging bath life and stability. 

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. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

M.H. Pournaghi-Azar and H. Dastangoo, Electrochemical characteristics of an aluminum electrode modified by a palladium hexacyanoferrate film, synthesized by a simple electroless procedure. J. Electroanal. Chem. 523, 26-33 (2002). 

H. Razmi and A. Azadbakht, Electrochemical characteristics of dopamine oxidation at palladium hexacyanoferrate film, electroless plated on aluminum electrode. Electrochim. Acta 50, 2193 (2005). 

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