Electroless electrolyte

Electroless Electrolytic Plating. In electroless or autocatalytic plating, no external voltage/current source is required (21). The voltage/current is suppHed by the chemical reduction of an agent at the deposit surface. The reduction reaction must be catalyzed, and often boron or phosphoms is used as the catalyst. Materials that are commonly deposited by electroless plating (qv) are Ni, Cu, Au, Pd, Pt, Ag, Co, and Ni—Fe (permalloy). In order to initiate the electroless deposition process, a catalyst must be present on the surface. A common catalyst for electroless nickel is tin. Often an accelerator is needed to remove the protective coat on the catalysis and start the reaction. 

MOLDING/METALLIZATION. Molded thermoplastic circuit board substrates may be rendered selectively conductive by several additive process techniques including conductive polymeric thick film inks (PTF), and semi and fully additive electroless/electrolytic platings. Of the various chemical process methods developed to produce circuitry on a molded plastic substrate, one method practiced by Pathtek, A Kodak Company, combines both "catalytic" and "non-catalytic" resins in a highly automated commercialized two-shot molding/selective metallization process. 

Electroless, electrolytic plating, and vacuum metallizing are processes used to deposit metal surfaces on plastics materials. However, metal surfaces can also be provided by adhesives or hot-stamp methods. Some finished plastic parts must have shiny metallic surfaces. Besides providing a decorative finish, metal coatings may provide an electrical conducting surface, a wear- and corrosion-resistant surface, or added heat deflection. 

Electroless electrolytic cleaning relies on the difference in electromotive potentials to remove material from one surface and deposit it on another (i.e. displacement plating. Sec. 1.1.2). 

Electroless plating on metal substrates can be improved by addition of pentaerythritol, either to a photosensitive composition of a noble metal salt (99), or with glycerine to nickel plating solutions (100). Both resolution and covering power of the electrolyte are improved. 

The theory and practice of electroless plating parallels that of electrolytic plating. 

The engineering properties of electroless nickel have been summarhed (28). The Ni—P aHoy has good corrosion resistance, lubricity, and especiaHy high hardness. This aHoy can be heat-treated to a hardness equivalent to electrolytic hard chromium [7440-47-3] (Table 2), and the lubricity is also comparable. The wear characteristics ate extremely good, especiaHy with composites of electroless nickel and silicon carbide or fluorochloropolymers. Thus the main appHcations for electroless nickel are in replacement of hard chromium (29,30). 

Chemistry. Successful electroless plating depends on the optimized interaction of five separate complex chemical solutions (1) to clean, roughen, and catalyze the surface before plating. These steps are critical for formation of an adherent continuous electroless coating, and for optimum durabHity after electrolytic plating. 

Numerous variations exist in the electroless plating solutions, processes, and techniques employed both in laboratory and commercial form, to create a great variety of products (39). AH produce a layer of highly conductive copper in specified areas. Modem electroless copper films have a ductiHty and conductivity identical to that of electrolytic copper (40). The three basic classes of copper baths are... 

Fig. 2. Multilayer printed circuit board composite. Constmction is multiple layers of epoxy—glass and foil copper. Foil copper outermost layer and drilled through-holes are sequentially plated with electroless copper, electrolytic copper, electroless nickel, and electroless gold.

Yullj Additive Method. No electrolytic plating step is used ia the fully additive process. The copper circuit is formed directly on the board without a continuous copper film. Heavy-build electroless coppers are used to iacrease the final thickness of the entire circuit. This process is much more difficult to control than the others. Additive processiag is becoming increasingly important ia high aspect ratio, very small diameter through-holes that caimot be easily electrolyticaHy plated. 

Electroless nickel baths are usually preferred to electroless copper, since they tend to be more stable and are less likely to deposit metal on unwanted areas, such as plating racks. Electrolytic copper is then plated before the final application of nickel and chromium, where this is the required finish, as it... 

Resistance to corrosion Most authors who compare resistance to corrosion of electroless nickel with that of electrodeposited nickel conclude that the electroless deposit is the superior material when assessed by salt spray testing, seaside exposure or subjection to nitric acid. Also, resistance to corrosion of electroless nickel is said to increase with increasing phosphorus level. However, unpublished results from International Nickel s Birmingham research laboratory showed that electroless nickel-phosphorus and electrolytic nickel deposits were not significantly different on roof exposure or when compared by polarisation data. 

Electroless nickel engineering deposits Electroless nickel is not usually deposited to thicknesses greater than about 125/xm. Where a greater total thickness is required, an electrolytic nickel undercoat should be used. 

A comprehensive work on the electrodeposition chemistry and characterization of anodically synthesized CdTe thin films has been presented by Ham et al. [98]. In this work, along with the electrolytic anodic synthesis of CdTe by using Cd anodes in alkaline solutions of sodium telluride, an electroless route of anodizing a Cd electrode held at open circuit in the same solution was also introduced. The anodic method was expected to produce CdTe with little contamination from Te on account of the thermodynamic properties of the system the open-circuit potential of Cd anodes in the Te electrolyte lies negative of the Te redox point, so... 

Mital et al. [40] studied the electroless deposition of Ni from DMAB and hypophosphite electrolytes, employing a variety of electrochemical techniques. They concluded that an electrochemical mechanism predominated in the case of the DMAB reductant, whereas reduction by hypophosphite was chemically controlled. The conclusion was based on mixed-potential theory the electrochemical oxidation rate of hypophosphite was found, in the absence of Ni2 + ions, to be significantly less than its oxidation rate at an equivalent potential during the electroless process. These authors do not take into account the possible implication of Ni2+ (or Co2+) ions to the mechanism of electrochemical reactions of hypophosphite. 

The cycling improvement for the Cu-metallized graphite over the pristine graphite was also observed by K. Guo et al. [15] in their study of electroless Cu deposited on graphite cycled in a lithium cell with a 20% PC blend electrolyte. Also, they recorded a rate capability improvement in their Cu graphite material as well. At a current density of 1.4 mA/cm2, the cell achieved about 60% ( 200 mAh/g) of the charge capacity measured at 0.14 mA/cm2, compared to about 30% ( 100 mAh/g) for the non-treated pristine natural graphite cell [15]. 

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