Cyanide Copper Plating Reinvents Itself Understanding the influence of bath parameters helps achiove the mos t from a copper cyanide bath. B y N e d M a n d i c h , C o n s u l t i n g E l e c t r o c h e m i c a l E n g i n e e r , H B M E l e c t r o c h e m i c a l & E n g i n e e r i n g Co.

Edi tor ' s Note: This is the second of a multi-part series. The first installment appeared in the March issue.

C opper cyanide (single salt) provides the source of the metal ion. It is insoluble in water, but soluble in sodium or potassium cyanide solu-

tions to form soluble complexes. It is available as a dry powder and also as a water-soluble "double salt"--Na2Cu(CN) 3 and K2Cu(CN) 3.

Although all of these are commercial ly pure grades, it is recommended that a new bath made up from the salts be purified with act ivated carbon t r ea tmen t before use, especially when mak- ing up a new bath and some- times after making a large addi- tion to an existing bath.

The term "free cyanide" refers to the cyanide (as NaCN or KCN) present in excess of that required to form the Na2Cu (CN) 3 complex salt, or equiva- lent po tass ium salt. The term "total cyanide" denotes all the cyanide present in the solution expressed as sodium or potassi- um cyanide. The free cyanide is normally determined by t i trat- ing the sample of the bath, at or below room tempera ture , with AgNO3, using KI as an indicator. It should be noted that it is important that the tempera ture be con- stant for control purposes.

If "room tempera ture" is used, it has a built-in inaccuracy. It is preferable to specify the tempera- ture as 20 to 250°C (68 to 77°F). The free-cyanide concentration increases with higher temperatures, since lower complexes are formed and free cyanide is, in that way, released. The main functions of free cyanide are to promote anode corrosion , reduce roughness, and provide quality deposits in the lower CD areas. In addition, it has a role as a carrier for Cu ions from anodes to the parts being plated; to maintain the conductivity of the solution; and con-

The name copper comes from the Latin word cuprum, which means "from the Island of Cyprus."

tribute to the formation of sound, fine-grained cop- per deposits.

An increase in the free cyanide concentrat ion causes an increase in cathode polarization, reducing the cathode current densi ty at which burning occurs, although it is claimed that this trend may be reversed in ba ths of high copper concentration. 13 The cyanide re leased during deposit ion (see Equation 3-a on page 31 of the March issue) shifts the distribution of the complexes at the surface to a completely sa tu ra ted s ta te and resul ts in a

decreased local copper deposi- tion rate. 6

The free cyanide/Cu metal ratios in strike and RS baths are somewhat higher compared to HE baths, which account, in part, for the lower cathode CE. The advantage of this is the excess hydrogen given off pro- vides an extra "scrubbing" action on the surface of the part . Cyanide salts have been recog- nized as good cleaners. If the free-cyanide is too low, the anodes will develop a greenish- blue film of insoluble CuCN, which inhibits proper anode dis- solution On the other hand, if the free-cyanide content is exces-

sive, the cathode CE is reduced, the deposit becomes very coarse, burned, and in extreme cases, no depo- sition may take place.

A large excess of free cyanide can simultaneously dissolve the copper layer as soon as it is plated. A compromise is therefore required, with the optimum free-cyanide content depending on the copper con- centration and operating conditions.

Clearly, in order to ensure the satisfactory work- ing of the bath, this optimum concentration of the free cyanide must be maintained, since under condi- tions of low CE, the rate of the hydrogen evolution will be high and this can lead to blistering of the deposit, possibly as the result of hydrogen penetra-

36 www.metalfinishing.com

tion. However, it is more likely the deposits will become "burned" and unusable long before any blis- tering occurs.

Sodium (NaOH) or potassium (KOH) hydroxide is used to provide conductivity and improve quality of the deposits. Besides increasing the electrical con- duct ivi ty of the plat ing solution, the presence of hydroxide helps protect steel that is in contact with the plating solution from corrosion of wetted steel. It also aids in prevent ing excessive cyanide break- down. This is a major and important function, which has both economic and safety implications.

Wi thout any alkali, one can usua l ly smell the cyanide and ammonia coming off the anodes. KOH or N a O H also inf luences the br igh tness of the deposits from HE baths. Additionally, hydroxides can help in saponifying some organic contaminants and affect the codeposition of many metallic ones. Hydroxide also plays a role in the format ion of anode films and anode dissolution. Strike solutions over zinc-based die casting often contain 2 to 4 g/L ( -0 .25 to 0.5 oz/gal) of hydroxide, which is not excessive. The deposi t from the s t r ike solut ion mus t be thick enough and nonporous enough to protect the par t in the copper plating solution.

Copper plat ing solutions have been successfully operated at 15 to 30 g/L (2 to 4 oz/gal) of KOH. It is also important in the plating of zinc-based die cast- ings that the hydroxide be closely controlled; it must be used caut iously and in moderat ion to avoid a t tack of hydroxide on zinc. Larger additions can increase the deposition of any dissolved Zn to the point where the low current density areas may be "brassy" in color.

Although not regarded as essential , carbonates are often added to the freshly prepared bath. Carbonates are readily formed by hydrolysis and oxidation of cyanide, and by reaction of NaOH or KOH with atmospheric carbon dioxide (CO2). The KOH/NaOH solutions are effective CO 2 absorbers and used in the analysis of gases for carbon dioxide based on this efficient reaction.

The rate of bui ldup is directly re la ted to the amount of CO s in the ambient air. In a closed plat- ing facility in the winter time with people, machin- ery, gas, and/or gasoline-powered fork lift trucks, etc., CO 2 reaches significant levels. In an air-agitat- ed cyanide copper bath, the rate of buildup of car- bonates can be substantial.

In barrel plating, without air agitation but with the int imate mixing of room air and plating solu- tion, the increase can be measured in oz/gal/per week. One way that has been used to reduce the absorption from the air is to pipe in outside fresh air

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April 2005 37

with a lower CO 2 content. One plant went further and had a CO 2 (home-made) absorpt ion tower put on the roof. Caustic solution is used to absorb the CO 2 in the o air before the air enters the plat- ........... ing tank. is A third source of rapid 25.4%

carbonate bui ldup is the use of non-Cu auxiliary or intermediate anodes that are sometimes used to plate complex-shaped parts. Unless the concen- tration is excessive, e.g. above 90 to 112.5 g/L (12 to 15 oz/gal), carbonate forms a useful buffer at pH 10.8 to 11.5. In low pH solutions, sodium bicarbon- ate (NaHCO 3) is often regarded as an impor tant constituent, as it reduces anode polarization.

However, this happens only up to a limit, above which the anode efficiency is diminished at an increasing rate. Experimental data 19 on the effect of carbonates on the cathode CE for strike type solu- tions at 1.5 A/dm 2 (14 A/ft 2) is presented in Table II. From Table II, it follows that 37.5 g/L (5.0 oz/gal) of Na2CO 3 may be the optimum concentration.

Clearly, in order to avoid carbonate buildup by oxi-

::'raNe :,: C=thodo c~(,~ , ~ :carm=to ~ = ~ , ' ~ a ~ . o 4o~ ( lo4~

37,5 (5,0) 75 (10.0) 112.5 (15.0)

32.7% 24.95% 24.1%

dation of the cyanide, excessive anode polarization should be avoided. Excessive carbonates in all CuCN baths reduce the bright plat ing range and produce rough and grainy deposits. Carbonates are more difficult to remove by freezing out in potassi- um-based vs. sodium-based baths.

Sodium-potassium tar tarate , commonly called Rochelle salts, potassium bi-tartarate, glycolates, glu- conates, heptonates, tartaric acid, citrates, and similar proprietary complexants have a number of functions in cyanide copper baths (tartaric acid and citrates are rarely used, if at all). They promote anode corrosion, exert some buffering action, contribute to formation of fine-grained deposits, and slightly increase cathode

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CE. The latter is probably due to the formation ofcom- plexes with copper in the cathode film.

A similar mechanism may account for an impor- rant increase in anode CE, resul t ing in the free- cyanide content becoming somewhat less critical. The main benefit of using anode aids is it allows higher anode CDs without polarization. The other important benefit is these compounds are good com- plexing agents, and the effects of metallic contami- nants are reduced.

Rochelle baths can be used for PR plating, but not with as good results as the higher metal baths. An increase in operat ing t empera tu re in RS baths increases anode and cathode CEs at the expense of faster decomposition of free cyanide.

The above five components constitute the princi- pal ingredients in cyanide copper baths. Unders tanding the effects of variations in concen- t rat ions of these should help in get t ing the most from the bath with the fewest problem.

POTASSIgM SALTS The advantage of po tass ium salts is multifold.

The fact tha t one can plate at a higher CD without

burn ing is p robab ly the main reason for us ing more soluble KCN. Also, Cu-metal concentrations can be higher in KCN baths. In practice, however, this is not always impor tan t because equivalent performance is often obtained at lower metal con- centra t ion in the po tass ium bath. Metal concen- t ra t ion can be kept lower for one advantage, an often overlooked benefit. An essential component in commercial cyanide copper plat ing ba ths is the wet t ing agent(s).

These organic compounds may have limited solu- bility in hot, concentrated cyanide copper plating solutions where, as the concentrations of the more soluble materials (Na and K cyanides, hydroxides, carbonates) are increased, the solubility of the less soluble we t t ing agents decreases to the p o i n t where it may become insoluble. At tha t point, it usual ly floats on top of the solution, resembling a "grease." This may be an advantage in using the more soluble KCN.

As the CD in which hydrogen is l iberated is high- er in the po tas s ium-based bath , the increase in cathode CE is part icularly marked in an agitated bath. 2° The disadvantages of the potass ium bath

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April 2005 39

are its higher cost and problems with freezing out the excess carbonates. One kg or I lb. of NaCN and NaOH is equivalent in cyanide and caustic content to 1.16 kg (1.16 lb.) KCN and 1.4 kg (1.4 lb.) KOH, respectively. Mixed sodium-potassium baths are also frequently employed, particularly for bright plating.

REFERENCES 1. Mathers, F.C., Trans. Electrochem. Soc., 33:1; 1918. 2. Horner, J., Proc. AES 51st Annual Technical

Conference, pp. 71-81; 1964. 3. Petrocelli, J.V., Trans. Electrochem. Soc., 77:133; 1940. 4. Senderoff, S., Metal Finishing, 48(7):59; 1950. 5. Graham, A.K. and H.J. Read, Trans. Electrochem. Soc.,

80:344; 1941. 6. Chu, D. and P.S. Fedkiw, J. Electroanal. Chem.,

345:107; 1993. 7. Dudek, D.A. and F.S. Fedkiw, Proc. AES 86th Annual

Technical Conference; 1999. 8. Costa, C.M., J. Rech. CNRS, 64:258; 1963. 9. Thompson, M.R., Trans. Electrochem. Soc., 79:417;

1941. 10. Read, H.J. and A.K. Graham, Trans. Electrochem. Soc.,

74:411; 1938.

11. Britton, H.T.S. and E.N. Dodd, J. Chem. Soc., 135:1940; 1932.

12. Thompson, M.R., Monthly Rev. Electropl. Soc., 18(5):31; 1931.

13. Silman, H., et al., Protective and Decorative Coating

for Metals, p. 307, Finishing Publications, Teddington, U.K.; 1987.

14. Rothbaum, H.P., J. Electrochem. Soc., 104(11):682; 1957.

15. Pennerman, R. and L.R. Jones, Chem. Phys., 24:293; 1956.

16. Passal, F., Plating, 46(6):628; 1959.

17. Dini, J.W., "Electrodeposition of Copper," Chapter 2, pp. 104-118, Part B, in Modern Electroplating 4th

Edition, M. Schlesinger and M. Paunovic, Eds., J. Willey & Sons, N.Y.; 2000.

18. Horner, John D., private communication; Dec. 2003.

Ned Mandich is a consulting electrochemical engi-

neer with HBM Electrochemical & Engineering Co.

in Lansing, Ill. He can be contacted at (e-mail) drne-

nad@ix, netcom.com, mf

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E~y.

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