Electroless Plating Book

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27 Electroless Plating 27.1 Introduction ......................................................................27-1 27.2 Plating Systems ..................................................................27-2 27.3 Electroless Plating Solutions.............................................27-3 Deposition Rate • Solution Life • Reducing Agent Efficiency Factor • Solution Sensitivity to Activation

27.4 27.5 27.6 27.7

Practical Applications........................................................27-4 Mechanisms of Autocatalytic Metal Ion Reduction .......27-5 Stability of Plating Solutions............................................27-7 Electroless Plating..............................................................27-7 Copper Deposition • Nickel Plating • Cobalt, Iron, and Tin Plating • Deposition of Precious Metals • Deposition of Metal Alloys

A. Vakelis Lithuanian Academy of Sciences

27.8 Properties of Chemically Deposited Metal Coatings....27-10 References ...................................................................................27-11

27.1 Introduction In electroless plating, metallic coatings are formed as a result of a chemical reaction between the reducing agent present in the solution and metal ions. The metallic phase that appears in such reactions may be obtained either in the bulk of the solution or as a precipitate in the form of a film on a solid surface. Localization of the chemical process on a particular surface requires that the surface must serve as a catalyst. If the catalyst is a reduction product (metal) itself, autocatalysis is ensured, and in this case, it is possible to deposit a coating, in principle, of unlimited thickness. Such autocatalytic reactions constitute the essence of practical processes of electroless plating. For this reason, these plating processes are sometimes called autocatalytic. Electroless plating may include metal plating techniques in which the metal is obtained as a result of the decomposition reaction of a particular compound; for example, aluminum coatings are deposited during decomposition of complex aluminum hydrides in organic solvents. However, such methods are rare, and their practical significance is not great. In a wider sense, electroless plating also includes other metal deposition processes from solutions in which an external electrical current is not used, such as immersion, and contact plating methods in which another more negative (active) metal is used as a reducing agent. However, such methods have a limited application; they are not suitable for metallization of dielectric materials, and the reactions taking place are not catalytic. Therefore, they usually are not classified as electroless plating. Electroless plating now is widely used in modifying the surface of various materials, such as nonconductors, semiconductors, and metals. Among the methods of applying metallic coatings, it is exceeded in volume only by electroplating techniques, and it is almost equal to vacuum metallization. Electroless plating methods have some advantages over similar electrochemical methods. These are as follows:

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1. Coatings may be deposited on electrically nonconductive materials (on almost any surface that is stable in electroless plating solutions). 2. Coatings have more uniform thickness, irrespective of the shape of the product to be plated. 3. Deposition is simple — it is enough to immerse the (pretreated) product in the electroless plating solution. 4. It is possible to obtain coatings that have unique mechanical, magnetic, and chemical properties. Application of electroless plating, in comparison with electroplating techniques, is limited by two factors: (a) it is more expensive because the reducing agent costs more than an equivalent amount of electricity, and (b) it is less intensive because the metal deposition rate is limited by metal ion reduction in the bulk of the solution.

27.2 Plating Systems To ensure chemical reduction of metal ions in a solution, the solution must contain a sufficiently strong and active reducing agent; that is, it must have a sufficiently negative redox potential. The more easily the metal ions are reduced, the greater is the number of available reducing agents. Because only autocatalytic reduction reactions may be used successfully for deposition of coatings, the number of electroless plating Me-Red (metal-reducing agent) systems suitable for practice is not great (see Table 27.1). Currently known electroless plating methods may be used to deposit 12 different metals, including metals belonging to the groups of iron, copper, and platinum (the well-known catalysts of various reactions) as well as tin and lead (only one solution has been published for deposition of the latter). Although deposition of chromium and cadmium coatings is described in the patent literature, autocatalytic reduction is not realized in these cases. Coatings are deposited on some metals by immersion plating only. In some widely used processes, the deposition of metal is accompanied by precipitation of the reducing agent decomposition products — phosphorus and boron — and so, the respective alloys are obtained. It is not difficult to deposit two or more metals at a time; electroless plating methods are known for deposition of more than 50 alloys of different qualitative composition, mostly based on nickel, cobalt, and copper. The majority of reducing agents used in electroless plating are hydrogen compounds, in which H is linked to phosphorus, nitrogen, and carbon. It is in the reactions of these compounds that significant catalytic effects are possible because in the absence of catalysts, these reactions proceed slowly. The most effective autocatalysis is obtained when the strongest reducer — hypophosphite — is used. In the absence of catalysts, the reducer is inert and does not react even with the strong oxidants; only a TABLE 27.1 Coatings Obtained by Electroless Plating Reducing Agent H2 PO−2

Metal Ni Co Fe Cu Ag Au Pd Rh Ru Pt Sn Pb

N 2H 4

CH2O

Ni–P Co–P

Ni Co

Co

Cu

Cu Ag Au Pd Rh

Pd–P

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Cu Ag Au Pd

BH−4

RBH3

Ni–B Co–B Fe–B Cu Ag Au Pd–B

Ni–B Co–B Cu Ag Au Pd–B

Me ions

Cu Ag

Others

Ag Au Rh

Ru Pt

Pt

Pt Sn

Pb

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Electroless Plating

few catalysts are suitable for it (e.g., Ni, Co, and Pd), but they provide for a catalytic process of the highest rate without reduction in the bulk of a solution. Other reducers are more versatile, for example, by using borohydride, we may deposit coatings of almost all the metals mentioned. The reducing capacity of hydrogen compounds increases with an increase in pH of a solution. For this reason, the majority of electroless plating solutions are alkaline. Such simple reducing agents as metal ions of variable valences (Fe2+, Cr2+, and Ti3+) usually are not suitable for deposition of coatings, because noncatalytic reduction occurs rather easily. Recently, conditions have been established for autocatalytic deposition of tin and silver coatings using as reducing agents such metal complexes as Sn(OH)2− and Co(NH 3 )2− 4 6 . Depositions of some metals (Ag, Au, Cu) by chemical reduction techniques was known as long ago as the 19th century, but it became popular after Brenner found (in 1945) a very efficient electroless nickel plating process using hypophosphite.1 It was then that the term “electroless plating” was coined.

27.3 Electroless Plating Solutions The electroless plating solutions used in practice, in addition to the basic components (the salt of the metal to be deposited and a reducing agent), contain other substances as well. Usually, these are as follows: 1. Ligands, which form soluble complexes with metal ions, are necessary for alkaline solutions. Also, the use of stable complexes sometimes enhances the autocatalytic effect. 2. Substances controlling and maintaining a certain pH value of the solution are used: buffer additions are especially important, because in the course of metal reduction, hydrogen ions are formed. 3. Stabilizers that decelerate reduction reaction in the bulk of a solution and, hence, enhance autocatalysis can be used. Sometimes, agents such as brighteners are also added to the solution. The basic technological parameters of electroless plating solutions are discussed in Sections 27.3.1 through 27.3.4.

27.3.1 Deposition Rate Deposition rate usually is expressed in micrometers per hour (µm/h; or mil/h, µin./h, mg/cm2h). In the course of deposition, if the concentrations of reacting substances are not maintained at a constant level, this rate decreases. The values given in the literature are often averages, reflecting only the initial period. Such average rates depend on the ratio of the surface to be plated to the solution volume (dm2/l). The dependence of the deposition rate (v) on the concentration of reacting substances for a general case is rather complicated. It is often described by empirical equations, for example: v = k[Men+ ]a [Red]b [H + ]c [ L]d

(27.1)

where k is the rate constant (a constant value for a system of the given type), and [L] is the concentration of a free ligand (not bound with metal ions in a complex). The exponents a and b are usually smaller than unity, which c is a negative value (in alkaline solutions OH-ion concentration is used, and in such a case, the exponent is often positive, O < c < 1). Exponent d is usually close to zero; when the ligand is substituted, however, the deposition rate may change substantially. With constant concentrations of other solution components, the deposition rate decreases when the stability of a metal complex increases (when the concentration of free metal ions is lowered); however, this relationship for a general case is not rigorous. The electroless deposition rate of most metals under suitable conditions is about 2 to 5 µm/h, and only electroless nickel plating rate may be as high as 20 µm/h (this corresponds to an electroplating process at current densities of 200 A/m2).

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27.3.2 Solution Life Solution life represents the maximum duration of solution usefulness. The beginning of metal ion reduction in the bulk of a solution may terminate its exploitation. In most modern electroless plating solutions, however, the reduction in the bulk usually does not occur under normal operating conditions, and the solution life is limited by the accumulation of reaction products or impurities. Thus, it is better to characterize the life of a solution not by time, which depends on the intensity of exploitation, but rather by the maximum amount of metal deposited from a volume unit of the solution (g/l or µm/l) or by turnover number showing how many times the initial amount of metal in the solution may be deposited in the form of a coating. This number may be as big as 10 to 20. After removal of undesirable substances accumulated in the solution, it may be used longer, just like electrolytes for electroplating. After protracted exploitation of solutions, a certain amount of sediment may appear, as the bulk reaction may proceed on a limited scale even in fully stable solution.

27.3.3 Reducing Agent Efficiency Factor The amount of reducing agent (in moles or grams) that is consumed for deposition of a mole or gram of coating is indicated by the reducing agent efficiency factors. The required amount (according to the reduction reaction) of a reducing agent, which is equal, for example, to 2 moles for 1 mole of metal (nickel ion reduction by hyposphosphite or copper ion reduction by formaldehyde) is exceeded in real electroless plating processes as a result of the side reactions taking place.

27.3.4 Solution Sensitivity to Activation The minimum amount of catalyst that must be present on the dielectric surface to initiate a reduction reaction is shown by the solution sensitivity to activation. This parameter is related to solution stability. The lower the stability of a solution, the easier is the initiation of a reaction, even on surfaces with low catalytic activity. A high sensitivity of a solution to activation is not always desirable because metal from such solutions may be deposited even on surfaces that had not been activated; in such cases, selective plating becomes impossible. When palladium compounds are used for activation, there should be no less than 0.01 and 0.03 to 0.05 µg of lead per square centimeter of a dielectric surface for nickel and copper plating, respectively. When silver is used as an activator, which is suitable only for some electroless copper plating solutions, it is necessary to have about 0.4 µg of silver per square centimeter.

27.4 Practical Applications2–4 The applications of chemically deposited coatings may be divided into two groups. For decorative metallization of plastics, a thin (0.3 to 1.0 µm) layer of metal is chemically deposited on a dielectric surface, and its thickness is then increased by electroplating techniques. In this case, the properties of chemically deposited coatings and the nature of the metal are not of great significance; it is important only to ensure compactness and sufficient electrical conductivity of such coatings for subsequent electroplating and for providing the required adhesion of the metal layer. The metal for the chemically deposited underlayer is selected for process convenience and cost. For this purpose, nickel and copper coatings are used. Nickel is more convenient, since electroless nickel plating solutions are more stable and their compositions simpler than those of similar electroless copper plating solutions. The adhesion of a coating to the nonconducting surface is essentially determined by the state of the surface, while the nature of the metal (at least for nickel and copper) usually has only a slight effect on adhesion. Copper coatings might be preferred because of their higher electrical conductivity. A copper underlayer is almost always used in the production of printed circuit boards. Chemically deposited finished coatings, on the other hand, are thicker, and their use is determined by their mechanical, electrical, and magnetic properties. The most popular are nickel (Ni–P and Ni–B) coatings deposited on metal products. Copper coatings 20 to 30 µm thick, deposited on plastics, exhibit

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Electroless Plating

27-5

good electrical conductivity and ductility and, therefore, are used in the production of printed circuit boards by additive processes. The entire circuit pattern is obtained by electroless techniques. Coatings of cobalt and its alloys may be used to take advantage of their specific magnetic properties; silver and gold coatings are used because of their good electrical conductivity, optical properties, and inertness. Electroless plating may be performed by using the plating solution once (until a greater part of any component in the solution is consumed and the reaction rate has sharply decreased) or by replenishing the substances that have been consumed in the course of plating. Long-term exploitation of solutions reduces the amount of plating wastes and ensures a higher labor productivity, but at the same time, it imposes more stringent requirements on plating solutions: they must be stable, and their parameters should not vary significantly with time. Besides, special equipment is required for monitoring and controlling the composition of such solutions. For this reason, long-term exploitation of solutions is applied only in large-scale production processes. Single-use solutions are more versatile, but they are less economical and less efficient. A single-use method may be applied rather efficiently, however, when the solution has a simple composition and the basic components (first of all, metal ions) are fully consumed in the plating process, while the remaining components (such as ligands) are inexpensive and do not pollute the environment. In this case, singleuse processes may be practically acceptable even in mass production. An extreme case of single use of plating solutions is aerosol spray plating,5 in which droplets of two solutions begin sprayed by a special gun collide on, or close to, the surface being plated. One solution usually contains metal ions, while the other contains the reducing agent. Metal ion reduction in this case should be rapid enough to permit a greater part of the metal to precipitate on the surface before the solution film runs off it. This method is practical for deposition of such easily reducible metals as silver and gold, though such aerosol solutions are known for deposition of copper and nickel as well. The aerosol spray method is highly suitable for deposition of thin coatings on large, flat surfaces: this process is similar to spray painting. Since the components of electroless plating solutions, first of all metal ions, may be toxic and pollute the environment, techniques have been developed for recovery of metals from spent plating solutions and rinse water. Other valuable solution components, such as ligands (EDTA, tartrate), may also be recovered. Electroless plating usually does not require sophisticated equipment. The tank for keeping plating solutions must exhibit sufficient chemical inertness, and its lining should not catalyze deposition of metals. Such tanks are usually made of chemically stable plastics; metal tanks may be used as well — they can be made of stainless steel or titanium. To prevent possible deposition of metals on the walls, a sufficiently positive potential is applied to them using a special current source (anodic protection). Parts for plating may be mounted on racks; small parts may be placed in barrels immersed in the plating bath. Heating and filtration of solutions are carried out in the same way as in electroplating processes. Special automatic devices have been developed for monitoring and controlling the composition of plating solutions.

27.5 Mechanisms of Autocatalytic Metal Ion Reduction Autocatalytic metal ion reduction processes are highly complex: they contain many stages, and their mechanism is not understood in detail. At present, it is possible to give an accurate description only of the basic stages of the catalytic process. Localization of the reduction reaction on the metal–catalyst surface (the cause of catalysis) is usually attributed to the requirement for a catalytic surface for one or more stages of the process to proceed. In accordance with one of the earlier explanations, only on a catalytic surface is an active intermediate product obtained, which then reduces metal ions. First, atomic hydrogen and, later, a negative hydrogen ionhydride were considered to be such products. A reaction scheme with an intermediate hydride gives a good explanation of the relationships observed in nickel and copper plating processes.5 However, there is no direct proof that hydride ions are really formed during these processes. Moreover, the hydride theory explains only the reactions with strong hydrogencontaining reducers, which really may be H– donors.

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Coatings Technology Handbook, Third Edition

A more versatile explanation of the causes of catalysis in these processes is based on electrochemical reactions. It is suggested that reducing agents are anodically oxidized on the catalyst surface and the electrons obtained are transferred to metal ions, which are cathodically reduced. The catalytic process comprises two simultaneous and mutually compensating electrochemical reactions. In this explanation of the catalytic process, electrons are the active intermediate product. However, electrons are fundamentally different from the conversational intermediate products of reactions. They may be easily transferred along the catalyst without transfer of the mass, and for this reason, the catalyst reaction, contrary to all other possible mechanisms (which are conventionally called “chemical mechanisms”), occurs not as a result of direct contact between the reactants, or the reactants, or the reactant and an intermediate substance, but because of the exchange of “anonymous” electrons via metal. On the metal surface, when anodic oxidation of the reducer Red → Ox + ne

(27.2)

Men + + ne

(27.3)

and cathodic reduction of metal ions

proceed simultaneously, a steady state in the catalytic system of electroless plating is obtained, in which the rates of both electrochemical reactions are equal, while the metal catalyst acquires a mixed potential Em. The magnitude of this potential is between the equilibrium potentials Ec of the reducer and of the metal. The specific value Em depends on the kinetic parameters of these two electrochemical reactions. Electrochemical studies of catalytic metal deposition reactions have shown that the electrochemical mechanism is realized practically in all the systems of electroless plating.4,6,7 At the same time, it has become clear that the process is often not so simple. It appears that anodic and cathodic reactions occurring simultaneously often do not remain kinetically independent but affect each other. For example, copper ion reduction increases along with anodic oxidation of formaldehyde.8 The cathodic reduction of nickel ions and the anodic oxidation of hypophosphite in electroless nickel plating solutions are faster than in the case in which these electrochemical reactions occur separately. This interaction of electrochemical reactions probably is related to the changes in the state of the metal–catalyst surface. Electrochemical reactions may also hinder each other: for example, in reducing silver ions by hydrazine from cyanide solutions, their rate is lower than is separate Ag–Ag(1) and redox systems. The electrochemical nature of most of the autocatalytic processes discussed enables us to apply electrochemical methods to their investigation. But, they must be applied to the entire system of electroless plating, without separating the anodic and cathodic processes in space. One suitable method is based on the measurement of polarization resistance. It can provide information on the mechanism of the process and may be used for measuring the metal deposition rate (both in laboratory and in industry).9 The polarization resistance Rp is inversely proportional to the process rate i: ba bc Rp (ba + bc )

(27.4)

 dE  Rp =    di  i =0

(27.5)

i=

where ba and bc are Tafel equation coefficients (b ≈ 1/αnf ), α is the transfer coefficient, n is the number of electrons taking part in the reaction for one molecule of reactant, and f = F/RT (F = Faraday number).

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Electroless Plating

Autocatalytic metal reduction reactions also may not proceed in an electrochemical manner. Two courses of such reactions have been shown: (a) an intermediate metal hydride is formed, which decomposes to meta and hydrogen (reduction of copper ions by borohydride); and (b) the metal complex is hydrolyzed, resulting in precipitation of metal oxide on the surface, which then is reduced to metal by the reducer present in the solution (reduction of silver ions by tartrate).

27.6 Stability of Plating Solutions Electroless plating solutions containing metal ions and reducing agents are thermodynamically unstable systems. Metal ion reduction must proceed in the bulk of the solution. The difference in the rate of metal ion reduction on the required surface (controlled catalytic reaction) and that of a reduction reaction in the bulk of a solution shows the effect of catalysis, and it determines, to a substantial degree, the practical usefulness of plating solutions. In an ideal case, the reaction in the bulk of a solution should not occur at all. Formation of metal in the bulk of a solution is hindered by energy barriers: the activation barrier of homogenous reactions between metal ions and reducer and the barrier of the formation of a new phase (metal). The magnitude of the second barrier may be evaluated on the basis of thermodynamic principles.10 It was established empirically that the stability of plating solutions decreases with an increase in the concentration of reactants and temperature, with a decrease in the stability of metal ion complexes, and with the presence of solid foreign particles in the solution. Besides, it was found that stability decreases as the catalytic process rate and load increase. This may be attributed to the transfer of intermediate catalytic reaction products from the catalytic surface to the solution, where they may initiate a reduction reaction. To enhance the stability of solutions, it is recommended that lower concentration solutions and more stable metal complexes be used and that solid particles in the solution be removed by filtration. The most effective solution stabilization method is the introduction of special addition agents — that is, stabilizers.4,11 Stabilizers, the number of which is very great, may be divided into two large groups: (a) catalytic poisons, such as S(II), Se(II) compounds, cyanides, heterocyclic compounds with nitrogen and sulfur, and some metal ions, and (b) oxidizers. It is assumed that stabilizers hinder the growth of fine metal particles, close to critical ones, by absorbing on them (catalytic poison) or passivating them (oxidizers). Modern electroless plating solutions always contain stabilizers. Their concentration may be within the range of 1 to 100 mg/l. Stabilizers, by hindering deposition of metal on fine particles, usually slow the rate of the catalytic process on the surface being plated. This process may stop completely at a sufficiently high concentration of the stabilizers. In some cases, however, small amounts of stabilizers increase the deposition rate.

27.7 Electroless Plating 27.7.1 Copper Deposition Though copper coatings may be deposited using various reducers, only formaldehyde copper plating solutions are of practical importance. Autocatalytic reduction of copper ions by formaldehyde proceeds at room temperature in alkaline solutions (pH = 11–14); here, copper ions must be bound into a complex. Suitable Cu2+ ligands for electroless copper plating solutions are polyhydroxy compounds (polyhydroxy alcohols, hydroxyacid anions) and compounds having a tertiary amine group and hydroxy groups (hydroxyamines, EDTA, and others). In practice, tartrate, EDTA, and tetraoxypropylethyl ethylenediamine (Quadrol) are used most often. In the course of copper plating, along with the main reduction reaction, Cu 2 + 2CH 2 O + 40 H − → Cu + 2 HCOO − + H 2 + 2 H 2 O

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(27.6)

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Coatings Technology Handbook, Third Edition

TABLE 27.2 Examples of Electroless Copper Plating Solutions Components (g/l) and Parameters CuSO4 ⋅ 5H2O K-Na tartrate Na2EDTA NaOH Formaldehyde (40%) ml/l Additivesa pH Temperature, °C Deposition rate µm/h

Solutions A

B

C

7 25

15

15

4.5 25 2 12.2–12.5 20 0.4–0.5

30 10 20 1–0.005 2–0.03 12.7 20 2

45 10 10 1–0.03 2–0.05 12.6 70 3

Solution A: NiCl2 ⋅ 6H2O; solution B: sodium diethyldithiocarbamate, K4Fe(CN)6; solution C: 2.2′-dipyridyl, polyethylene glycol (MW = 600). a

formaldehyde is consumed in the Cannizzaro reaction, and a total of 3 to 6 moles of CH2O is consumed for the deposition of 1 mole of copper. During copper plating, much alkali is used including the Cannizzaro reaction. Consumption of OH– may be determined according to the following equation (amounts of substances in moles): ∆OH − = 3 ∆ Cu( II) + 1 / 2 ∆ CH 2 O

(27.7)

Various formulations of copper plating solutions, which are totally stable and suitable for long exploitation (e.g., solution B in Table 27.2), have been developed. Three types of electroless copper plating solution have been distinguished in the literature: (a) low deposition rate solutions (0.5 to 1.0 µm/h), suitable for deposition of a copper underlayer; (b) solutions giving deposition rates of 4 to 5 µm/h (i.e., exhibiting a higher autocatalytic effect); and (c) solutions for deposition of highly ductile and strong copper coats (e.g., solution C in Table 27.2). All these solutions, essentially, have the same composition: they differ mostly by their additives. Besides, highly ductile coatings, which are used in the production of printed circuit boards by additives processes, are obtained at higher temperatures (>40°C) and at a relatively low copper deposition rate.

27.7.2 Nickel Plating Electroless nickel plating, in which hypophosphite is used as a reducer, is the most popular process.12,13 Autocatalytic nickel ion reduction by hypophosphite occurs both in acid and in alkaline solutions. In a stable solution with a high coating quality, the deposition rate may be as high as 20 to 25 µm/h. This requires, however, a relatively high temperature, about 90°C. Because hydrogen ions are formed in the reduction reaction, Ni 2+ + 2 H 2 PO 2− + 2 H 2 O → Ni + 2 H 2 PO −3 + H 2 + 2 H +

(27.8)

a high buffering capacity of the solution is necessary to ensure a steady-state process. For this reason, acetate, citrate, propionate, glycolate, lactate, or aminoacetate is added to the solutions; these substances, along with buffering, may form complexes with nickel ions. Binding Ni2+ ions into a complex is required in alkaline solutions (here, besides citrate and aminoacetate, ammonia and pyrophosphate may be added); moreover, such binding is desirable in acid solutions, because free nickel ions form a compound with the reaction product (i.e., phosphate), which precipitates and hinders further use of the solution.

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TABLE 27.3 Examples of Electroless Nickel Plating Solutions Components (g/l) and Parameters NiCl2⋅6H2O NaH2PO2⋅H2O Sodium acetate NH4Cl NH4OH (25%), ml/l Glycine NaNO2 pH Temperature, °C Nickel deposition rate µm/h

Solutions A

B

C

30 10 8

30 20

25 30–40 30 30–35

20 5 90 15

6 80–90 7–15

0.02–0.1 9 30 1.8

Stabilizing additions for nickel plating solutions are less necessary than for copper solutions; nevertheless, they are added to ensure the stability of long-lived solutions. Phosphorus is always present in the coatings when reduction is performed by hypophosphite. Its amount (in the range of 2 to 15 mass percent) depends on pH, buffering capacity, ligands, and other parameters of electroless solutions. Borohydride and its derivatives may also be used as reducers for electroless nickel plating solutions. While temperatures of 60 to 90°C are required for the reduction of nickel ions by borohydride, dimethylaminoborane (DMAB) enables the deposition of Ni–B coatings with a small amount of boron (0.5 to 1.0 mass percent) at temperatures in the range of 30 to 40°C. Neutral and alkaline solutions may be used, and their compositions are similar to those of hypophosphite solutions (Table 27.3).

27.7.3 Cobalt, Iron, and Tin Plating Deposition of cobalt is similar to that of nickel — the same reducers (hypophosphite, borohydride, and its derivatives) are used, and reduction relationships are similar.14 Reduction of cobalt is more difficult, however, and cobalt deposition rates are lower than those of nickel; it should be noted that it is difficult to deposit cobalt from acid solutions. The Co–P and Co–B coatings obtained are of particular interest due to their magnetic properties. Electroless iron plating is more difficult, and only one sufficiently effective iron plating solution is known, in which Fe ions form a complex with tartrate and NaBH4 is used as a reducer. Fe–B coatings (about 6% B) are obtained in an alkaline solution (pH 12) at a temperature of 40°C and a deposition rate of about 2 µm/h. It is rather difficult to realize an autocatalytic tin deposition process. A sufficiently effective tin deposition method is based on the tin (II) disproportionation reaction in an alkaline medium.15 In 1 to 5M NaOH solutions at 80 to 90°C, it is possible to obtain a deposition rate of a few micrometers per hour.

27.7.4 Deposition of Precious Metals Electroless silver plating is the oldest electroless metallization process; its present performance however, lags behind nickel or copper plating.1 Unstable single-use ammonia silver plating solutions (with glucose, tartrate, formaldehyde, etc., as reducers) are usually employed. The thickness of coatings from such solutions is not great (