Coatings Technology Handbook Episode 1 Part 8 ppsx

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.. On the m

Trang 2

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, single-use processes may be practically acceptable even in mass production

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

during these processes Moreover, the hydride theory explains only the reactions with strong

DK4036_book.fm Page 5 Monday, April 25, 2005 12:18 PM

Trang 3

27-6 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 fundamen-tally 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

(27.2) and cathodic reduction of metal ions

(27.3) 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

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

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

polarization resistance Rp is inversely proportional to the process rate i:

(27.4)

(27.5)

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)

Red→Ox ne+

Men++ ne

= +

a c

p( a c)

di i

p=





=0

Trang 4

Electroless Plating 27-7

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 decom-poses 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

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 Stabi-lizers, 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

alcohols, hydroxyacid anions) and compounds having a tertiary amine group and hydroxy groups (hydroxyamines, EDTA, and others) In practice, tartrate, EDTA, and tetraoxypropylethyl ethylenedi-amine (Quadrol) are used most often

In the course of copper plating, along with the main reduction reaction,

(27.6)

Trang 5

27-8 Coatings Technology Handbook, Third Edition

for the deposition of 1 mole of copper During copper plating, much alkali is used including the

(amounts of substances in moles):

(27.7) Various formulations of copper plating solutions, which are totally stable and suitable for long

exploi-tation (e.g., solution B in Table 27.2), have been developed Three types of electroless copper plating

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

Autocatalytic nickel ion reduction by hypophosphite occurs both in acid and in alkaline solutions In a

reduction reaction,

(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,

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

TABLE 27.2 Examples of Electroless Copper Plating Solutions

Components (g/l) and Parameters

Solutions

2–0.03 2–0.05

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

∆OH−=3 ∆ Cu II( )+1 2/ ∆ CH O2

Trang 6

Electroless Plating 27-9

Stabilizing additions for nickel plating solutions are less necessary than for copper solutions; never-theless, 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, dimeth-ylaminoborane (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

(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

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 (<1 µm) Such unstable solutions are more suitable for aerosol spray

More effective electroless silver plating solutions have been developed using cyanide Ag(I) complex and aminoboranes or hydrazine as reducers: at temperatures of 40 to 50°C, the deposition rate is 3 to 4 µm/h, and in the presence of stabilizers, these solutions are quite stable Sufﬁciently stable electroless silver plating solutions may be obtained using metal ions such as Co(II) compounds as reducers

TABLE 27.3 Examples of Electroless Nickel Plating

Solutions Components (g/l) and Parameters

Solutions

Nickel deposition rate µm/h 15 7–15 1.8

Trang 7

27-10 Coatings Technology Handbook, Third Edition

Gold coatings may be deposited employing various reducers: however, the solutions are usually unsta-ble Solutions of sufﬁcient stability have been developed with borohydride or DMAB as reducers using

h, and gold coatings of sufﬁcient purity are obtained

Thin gold coatings may be deposited on plastics by an aerosol spray method: gold complexes with amines are employed with hydrazine as a reducer, and a relatively thick coat (deposition rate as high as 0.4 µm/min) may be obtained

Palladium coatings are easily deposited with hypophosphite as a reducer in alkaline solutions, in which

performed at 40 to 50°C, the deposition rate of the Pd–P (4 to 8 P) coat being in the range of 2 to 5 µm/h Coatings of platinum, ruthenium, and rhodium may be deposited using borohydride or hydrazine as

a reducing agent The process rate in a stable solution is low (0.5 to 2 µm/h)

27.7.5 Deposition of Metal Alloys

About 60 coatings of a different qualitative composition containing two or more metals may be deposited Such metals as copper, iron, zinc, tin, rhenium, tungsten, molybdenum, manganese, thallium, and platinum group metals may be introduced into nickel and cobalt coats, and nickel, cobalt, tin, zinc, cadmium, antimony, bismuth, lead, and gold into copper coats

In the electroless deposition of metal alloys, the same thermodynamic relationships as those of alloy deposition by electroplating techniques are valid; it is clear that it is difﬁcult to introduce into coatings metals that are difﬁcult to reduce, such as chromium and manganese Besides, in the case of chemical reduction, an additional factor — catalytic properties of metals — becomes apparent Great amounts of additional metal may be introduced into a coat of nickel, copper, and so on, only when that metal is catalytic or, at least, inert with respect to oxidation of the reducer The amount of metals–catalysts in the alloy may be as high as 100%, that of catalytically inert metals may be up to 50%, and that of metals–inhibitors may be only 10 to 20% When a less catalytically active metal is introduced, the deposition rate decreased

27.8 Properties of Chemically Deposited Metal Coatings

Only in rare cases are chemically deposited metal coatings so pure, and have so regular a structure, that their properties are the same as those of the corresponding chemically pure substance Very different properties may be exhibited by coatings containing a nonmetallic component — phosphorus or boron The density of coatings is a little lower than that of bulk metal This is related to a rather irregular coatings structure: they contain more defects (pores and inclusions of foreign matter) For example, chemically deposited copper usually has a great number of microscopic voids 20 to 300 Å in diameter, formed by the hydrogen occluded in the coating Ni–P and Ni–B coatings usually have a layered structure, which results from the nonuniform distribution of phosphorus and boron in the coatings

Mechanical properties of the coatings may vary within a wide range depending on the electroless plating conditions, plating solutions composition, and deposition rate

For chemically deposited ﬁnish copper coatings, such as those on printed circuit boards, sufﬁcient resistance and ductility are of great importance Coatings that have a tensile strength of about 40 to 50

ductility, may be as high as 6 to 8% Copper coatings obtained at room temperature are more brittle Highly ductile coatings may be obtained only from solutions containing special additives Ductility

Ni–P and Ni–B coatings are relatively hard; after deposition, their hardness, which depends on the

Trang 8

Electroless Plating 27-11

The ductility of nickel coatings, if their hardness is taken into account, is rather high: their ultimate elongation is less than 2% Such a combination of hardness, wear resistance, and ductility is unique The electrical conductivity of chemically deposited coatings is usually lower than of the respective pure metals Resistivity of thin copper coatings (0.5 to 1.0 µm) deposited at room temperature is 3 to 4

× 1.0–8 Ω⋅m — twice as great as that of pure bulky copper The surface resistance of such coatings is 0.03

to 0.07 Ω/m However, ductile copper coatings that are obtained at temperatures of 50 to 70°C have a resistivity of 2 × 10–8 Ω⋅m, close to that of pure copper

The resistivity of Ni–P and Ni–B coatings depends on the amount of nonmetallic component, and it

× 10–7 Ω⋅m) Heating causes reduction in resistivity

Magnetic properties of the coatings of such ferromagnetic materials as nickel and cobalt may vary within a very wide range With an increase in the amount of phosphorus in nickel coatings, their ferromagnetism decreases, and coatings containing more than 8 mass percent of phosphorus or 6.5 mass percent of boron are nonmagnetic

Coatings of Co–P, Co–B, and cobalt alloys with other metals have highly different magnetic properties These depend on the composition of the coating, their structures, and their thicknesses, and they may

be controlled by changing the composition, pH, and temperature of electroless plating solutions Usually, cobalt coatings exhibit a high coercivity (15 to 80 kA/m); however, soft magnetic coatings (0.1 to 1.0 kA/ m) may be deposited as well

Optical properties of coatings are less varied and do not differ so much from those of pure metals Chemically deposited coatings are usually dull; when special additives are introduced, bright coatings are obtained Since they are not used as ﬁnish decorative coatings, properties of appearance and brightness usually are not essential

Silver and gold coatings are often used as mirrors, but the light-reﬂecting surface is usually the inner surface, which is adjacent to the smooth glass surface Chemically deposited thin gold ﬁlms are employed

as optical ﬁlters; they pass visible light but reﬂect infrared rays and radio waves

Chemically deposited coatings are usually less porous than the respective electroplates; therefore, they provide better protection of the basis metal against corrosion Corrosion resistance of the coatings themselves may be different depending on structure and composition Ni–P and Ni–B coatings are more resistant to corrosion than nickel electroplates; this may be due to their ﬁne crystalline structure

References

1 A Brenner and G Riddell, J Res Natl Bur Stand., 37, 31 (1946).

2 W Goldie, Metallic Coating of Plastics Hatch End, Middlesex, England: Electrochemical

Publica-tions Ltd., Vol 1, 1968; Vol 2, 1969

3 F Pearlstein, “Electroless plating,” in Modern Electroplating, 3rd ed., F A Lowenheim, Ed New

York: Wiley, 1974, p 710

5 R M Lukes, Plating, 51, 969, 1066 (1964).

6 M Paunovic, Plating, 55, 1161 (1968).

7 F M Donahue, Oberfläche-Surface, 13, 301 (1972).

8 A Vakelis and J Jaciauskiene, Elektrokhimiya, 17, 816 (1981).

9 I Ohno and S Haruyama, Surface Technol., 13, 1 (1981).

10 A Vakelis, Elektrokhimiya, 14, 1970 (1978).

11 E B Saubestre, Plating, 59, 563 (1972).

12 K M Gorbunova and A A Nikiforova, Physicochemical Principles of Nickel Plating, Translated

from Russian, 1963, TT 63–11003

Trang 9

27-12 Coatings Technology Handbook, Third Edition

13 G Gawrilov, ChemischelStromläse/Vernickelung Saulgau, Wurt.: Eugen Leuze Verlag, 1974.

14 K M Gorbunova et al., Fiziko-Khimichesklye Osnovy Processa Khimicheskogo Kobaltirovaniya.

Moscow: Nauka, 1974

15 A Molenaar and J J C Coumans, Surface Technol., 16, 265 (1982).

16 Y Okinaka, in Gold Plating Technology, H Reid and W Goldie, Eds Hatch End, Middlesex, England:

Electrochemical Publications, Ltd., 1974, p 82

Trang 10

28 The Electrolizing

Thin, Dense, Chromium Process

28.1 General Definition 28-1 28.2 Applications 28-2

28.3

28.4 Solution 28-5 28.5 Properties 28-5

28.1 General Definition

The Electrolizing process uniformly deposits a dense, high chromium, nonmagnetic alloy on the surface

of the basic metal being treated The alloy used in Electrolizing provides an unusual combination of bearing properties: remarkable wear resistance, an extremely low coefficient of friction, smooth sliding properties, excellent antiseizure characteristics, and beneficial corrosion resistance Electrolized parts perform better and last up to 10 times longer than untreated ones

The solution and application processes are carefully monitored at all Electrolizing facilities The result

is a fine-grained chromium coating that is very hard, thin, and dense and has absolute adhesive qualities The Electrolizing process deposits a 99% chromium coating on the basis metallic surfaces, whereas normal conventional chromium plating processes tend to deposit 82 to 88% chromium in most applications Electrolizing calls for the cleaning and removal of the matrix on the basis metal’s surface by multi-cleaning process, using a modified electrocoating process that causes the chromium metallic elements of the solution to bond to the surface porosity of the basis metal It is during this process that the absolute adhesive characteristics and qualities of Electrolizing are generated The Electrolizing coating will not flake, chip, or peel off the basis metal substrate when conventional ASTM bend tests and impact tests are performed Three basic factors are always present after applying Electrolizing to metal surfaces:

• Added lubricity characteristics

• Excellent corrosion resistance

Michael O’Mary

The Armoloy Corporation

General • Specific

Thickness • Adhesion • Corrosion • Wear Resistance (Surface Hardness) • Lubricity • Conformity • Heat Resistance •

Surface Preparation 28-4

Định dạng
Số trang	15
Dung lượng	395,54 KB