1 Technology Overview
Electroless nickel (EN) deposits are classified as a Functional Class of coatings used primarily to enhance the surface performance properties of a variety of substrates. The majority of applications require that the coating provide maximum protection against corrosion and abrasive wear resulting in an extension in the useful life of the component. Because of the unique deposit properties and uniformity of the resultant film, many other engineered applications have emerged within the electronics industry. Methods to improve solderability, enhance thermo magnetic stability and effectively replace Hot Air Solder Leveling (HASL) with electroless nickel/immersion gold on printed circuit boards have been well documented in the literature.
The majority of electroless nickel films used commercially are deposited from solutions formulated with sodium hypophosphite as the reducing agent. This results in nickel films that are alloyed with phosphorous in ranges between 1 to 12 weight percent.
The mechanical properties of electroless nickel phosphorous (ENP) deposits can been further enhanced not only by the co-deposition of inert particles such as Teflon, silicon carbide or boron nitride but also by alloying with a third element, forming a ternary alloy of NiPX, where X can be copper, tungsten, molybdenum or tin depending on the particular formulation.
Electroless nickel boron (NiB) alloys are also well cited in the literature, although is less commercially viable than the nickel phosphorous alloys. The films are generated using either sodium borohydride or dimethylaminoborane as the reducing agent and can range in boron content from 1 to
5 weight percent. Nickel boron films are typically used in the electronics industry where low resistivity coatings are required and also find use in industrial applications when extreme wear and increased coating hardness is specified.
Optimum performance of the mechanical and physical properties of the electroless nickel films will result only if the coating is free of micro defects. It is well documented that deposit phosphorus content plays an important role in determining how well the coating will perform. Equally important is the presence of these micro defects including micro pores, nodules, ductility and intrinsic stress. All can contribute to premature performance failure and do not necessarily correlate to the phosphorous content of the bulk deposit. Interestingly, they appear to be associated more with the deposition mechanism itself, primarily the diffusion characteristics of the plating solution and the related catalytic activity of the growing film during the plating process. Ultimately, this has been found to have the greatest impact on composition, microstructure and performance of the resultant film.
2 The Autocatalytic Deposition Process
The continuous deposition process for chemical reduction of nickel ions in an aqueous solution, utilizing a reducing agent, is termed autocatalytic. The most common reducing agent is sodium hypophosphite and for the purpose of brevity will be referenced throughout this section. The plating process is heterogeneous since the reactants, namely nickel ions and sodium hypophosphite are evenly distributed throughout the plating solution. Since the reaction does not occur in the bulk of the plating solution but rather at the interface of the solution and the catalytic surface, two conditions must be fulfilled for the reaction to occur. First, the interface must be a catalytically active site for the oxidation of hypophosphite to occur, readily adsorbing the hydrogen produced and secondly, the reactants must easily migrate to this site for the reaction to continue and self propagate.
The requisite migration of the dissolved ions to the catalytic surface can occur either by diffusion or solution agitation (convection). Ions are charged species that are constantly moving throughout the plating solution to maintain an even distribution. However, because charged species tend to adhere to stationary solid surfaces and there is depletion of ions at the plating interface a concentration gradient is established and diffusion will take over. This part of the solution is called the diffusion zone and is defined as follows:
- diffusion of reactants to the catalytic surface
- adsorption of the reactants onto the catalytic surface
- reaction on the catalytic surface
- desorption of by-products from the catalytic surface
- diffusion of by-products away from the catalytic surface
An important point to note is that all species do not diffuse linearly to the catalytic surface. Additives such as heavy metal stabilizers, divalent sulfur compounds, heavy metal brighteners, usually present in ppm quantities, diffuse non-linearly to the catalytic surface. Controlling both the concentration and type of additive used in the formulation of the chemistry is a critical component in optimizing the performance of the plated electroless nickel phosphorous film.
3 Key Physical and Mechanical Properties
3.1 The Microstructure and Composition of ENP Films
One of the distinct advantages of the electroless nickel deposition process is the ability to produce an alloy of nickel and phosphorous in varying composition. Depending on the formulation and the operation of the chemistry the film compositions can vary from 2 to 13 weight percent phosphorous. This variation in alloy content has a significant affect on the deposit microstructure and performance characteristics and offers flexibility to well-informed platers and engineers that can take full advantage of these differences.
Electrodeposited nickel has a purity of greater than 99 % and is highly crystalline. On the other hand, electroless nickel deposits that contain more than 10.5 % phosphorous appear to be amorphous, i.e., lacking crystal structure. Electroless nickel deposits with less than 7 weight percent phosphorus have a clear microcrystalline structure (2 to 6 nm grain size) and film properties are distinctly different. Some studies have found that higher phosphorous deposits (above 10.5 weight percent phosphorus) may not be truly amorphous but rather a mixture of microcrystalline and amorphous phases.
The degree of amorphous character can be altered for a given high phosphorous formulation by the addition of additives that affect the growth process of the film. Deposits with a high degree of amorphous composition are free of grain boundaries, which typically act as sites for intergranular corrosion commonly encountered in crystalline deposits.
3.2 Deposit Uniformity
A significant advantage of the electroless nickel process is the ability to produce deposits with uniform thickness on parts with complex geometries and shapes. Since this is a chemical reaction, any catalytic surface exposed to the plating solution will plate uniformly, provided it meets the criteria established a few paragraphs earlier. The current density affects typically associated with electroplating are not a factor, therefore sharp edges, deep recesses and blind holes are readily plated to uniform thickness with electroless nickel chemistry. Many applications for electroless nickel exist today because it is often the only way to plate certain components. The difference in deposit uniformity is illustrated in Figure 1.
Surprisingly, the degree of uniformity can vary on edges, threads, small holes or deep recesses where exchange of fresh solution may be difficult. This can also occur under conditions with excessive bath agitation, especially in the presence of heavy metals. This thickness variation may be controlled by optimizing solution dynamics and/or by controlling the concentration of certain additives formulated into the electroless nickel plating bath.
3.3 Melting Point
Unlike electrolytically deposited nickel, electroless nickel deposits do not have a precise melting point but rather have a melting range. Pure nickel has a melting point of 1455 °C, however as the phosphorous content is increased within the film, the deposit begins to soften at lower temperatures and continues to soften until it eventually melts. The melting range decreases linearly as the phosphorous levels increase. The eutectic or lowest melting point for nickel phosphorous alloys is 880 °C and occurs at a deposit phosphorus content of 11 % b.w.
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Fig. 1: Differences in deposit uniformity of galvanic and electroless plating
