INTRODUCTION TO THE ELECTROPLATING PROCESS
1.3 THE ELECTROPLATING STAGE
1.33.1 INORGANIC COMPOUNDS IN THE ELECTROPLATING SOLUTION
The various electrolytes present in the plating bath are used to increase conductivity of the plating solution and improve anode corrosion.69 Conductivity is the ability of a solution to carry an electric current.75 For example the addition of sulfuric acid to copper sulfate and tin sulfate baths, the addition of sodium hydroxide to cyanide baths, the addition of nickel chloride to nickel baths, the addition of carbonate to silver and gold baths are all used to increase the electrical conductivity of the baths. In chromium plating the chromic acid (C1O3) concentration of 400 g/L gives the best conductivity.63 Compounds such as carbonates in cyanide copper baths and chloride in nickel baths are used to obtain a proper dissolution of the anodes.
Some electrolytes are also used to buffer the plating solution to give a good quality deposit. A buffer is a solution that resists a change in pH when small amounts of acid or alkali are added or when dilution occurs.76 For example boric acid buffers nickel plating solutions (see Table 1.3) by controlling the pH in the cathode film.77
H3BO3 (aq)+ H20(1) - H2B03"(aq) + H30+(aq) Equation 1.17 Equation 1.17 shows a simplified version of the buffer action of boric acid at the concentration found in a nickel plating solution. It can be seen that the addition of excess acid in the solution (H30+) shifts the equilibrium to the left side of the reaction to consume the W^f. Any OH' produced from the electrolysis of water at the cathode (see Equation 1.7) would cause the equilibrium shift to the right side to replace the H30+ which is removed by OH". For either situation, the equilibrium constant of the buffer solution remains the same. Therefore, the pH remains practically constant to within the operating range.
In some plating solutions, for instance nickel, controlling the pH of a plating bath is essential for achieving good deposits and efficient use of materials and electricity. In nickel plating, if the pH of the bath is higher than the optimum recommended value, the deposit will be rough, burned, dull and brittle due to precipitation of metallic contaminants and increased consumption of brightener components.58,77 On the other hand, low pH causes evolution of hydrogen which consequently results in a decrease in cathode efficiency.58 This in turn leads to an accumulation of hydroxide ions in the cathode region and consequent precipitation of basic salts that may get included in the electrodeposits, thereby altering the deposit properties.
Therefore, the pH of a bath must be regulated without a significant change in the bath composition. This can be done automatically by the buffer within certain limits or by chemical additions made by the operators. To increase the pH of acidic baths, for example, nickel baths, nickel carbonate or nickel hydroxide can be used. To decrease the pH of chloride or sulfate baths of nickel and acid zinc, dilute acids such as hydrochloric or sulfuric acid may be employed. The pH of alkaline baths containing hydroxides can be increased by adding sodium or ammonium hydroxide. Generally, highly acidic baths such as chromic acid, acid copper sulfate, fluroborate formulation, or alkaline baths such as a stannate tin bath cannot use pH
C O
measurements to monitor and control the composition of the plating solution.
Complexing agents are compounds or ions that combine with metallic ions in solution to form complex ions.43 The most common complexing agents used in the electroplating industry are cyanides, hydroxide and the sulfamate ion. Complexing agents have been widely used in plating baths to make the deposition potential more negative when it is necessary to prevent a spontaneous chemical reaction between the cathode and the plating ion e.g. plating of copper onto iron as shown in Equation 1.18.69
Cu2+(aq) + Fe(s) - Cu(s) + Fe2+(aq) Equation 1.18 The E° values for the reduction of Cu2+ and Fe2+ are 0.34 V and -0.44 V respectively (see Equation 1.18). The addition of a complexing agent makes the potential of the Cu /Cu couple more negative than that for the Fe2+/Fe couple thus Equation 1.18 is no longer thermodynamically favoured.69
Complexing agents have also been used to improve the throwing power. This is affected by the electrical conductivity of the solution, the degree to which the cathode polarizes with an increase in current density and the relationship between cathode current efficiency and cathode current density.40' 58 The steeper the slope of cathode polarisation and the greater the conductivity of the solution, the more uniform will be the current diffusion at the cathode. The evenness of metal distribution will be higher, the greater the decrease of cathode current efficiency with an increase in current density.
Complexing agents have been used to enhance the solubility of slightly soluble salts added to the plating bath during top-up. For instance, silver cyanide and copper(I) cyanide are slightly soluble in water. However, these dissolve readily in sodium or potassium cyanide solutions to form highly soluble metal cyanide complexes (see Equation 1.19).
AgCN(s) + KCN (aq) * K+(aq) + [Ag(CN)2]" (aq) Equation 1.19 Complexing agents also facilitate the dissolution of the anode and in so doing prevent passivation and loss of current efficiency during the oxidation of the anode.69
1.3.3.2 ORGANIC COMPOUNDS IN THE ELECTROPLATING SOLUTION
Additives are often high molecular weight organic compounds or colloids added in relatively low concentration, usually from 10"4 to 10"2 mol/L, to the electroplating bath in order to modify the structure and properties of the cathode deposit.78 They are classified into four major categories: brighteners, levellers (dendrites and roughness inhibitors), structural modifiers e.g.
stress relievers, and wetting agents.69'78 Some additives belong to more than one category because they perform more than one function.63 The only structural modifiers which will be discussed in this section are stress relievers which also serve as brighteners.
Brighteners usually cause the workpiece to have a bright and shiny look compared to the matte or dull deposits obtained from baths without such additives. This means the surface must be even (not rough) so that a high proportion of the light reflected from them is not scattered.
Brighteners usually cause the formation of an even and fine-grained deposit by modifying the deposition processes during plating.
Brighteners are used extensively in nickel plating baths. There are two types of nickel brighteners: class I (primary or maintenance brighteners) and class II brighteners (secondary or carrier brighteners).63
A) Sodium naphthalene trisulfonate S03Na
Na03S- S03Na
B) AIM sulfonic acid
CH2 = CH —CH2—S03H