Cyanide copper plating reinvents itself

Cyanide Copper Plating Reinvents Itself Workhorse system finds a niche despite decfin/ng demand/n

key markets. By Ned Mandich, Consulting Electrochemical Engineer, HBM Electrochemical & Engineering Co.

Copper plated on any metal serves as a protective layer, not in terms of corrosion protection, but as a layer that lowers the stress, providing a crystal orientation favorable to the start of subsequent layers. T u e name "copper" comes from the Latin word For example, the first layer deposited onto metal folprum, which m e a n s "from the Island of yprus." Archeological evidence suggests that lows the structure of the basic metal, and then gradually forms its own characteristic structure. The people have been using copper for at least 11,000 resulting stress is very high at that interface. Since years. The first authenticated use of copper plating is copper is very ductile, the stresses induced by a difascribed to Henry Bessemer, better known as a pioferent structure at the basis metal surface are born neer in steel manufacture. In 1831 Bessemer plated by the copper layer, reducing overall stress from the castings of insects, frogs, and plants by immersing subsequent plated deposits. them in copper sulfate solutions Due to ever-increasing enviin a zinc tray. Electroplating with r o n m e n t a l pressure, t h e r e has an external source of electricity been an evident decline in the followed quickly in 1983. J. use of cyanide copper on steel Wright, a Birmingham surgeon, s u b s t r a t e s in the automotive, developed the first cyanide platmotorcycle, and allied indusing solution. tries, with most parts now plated Copper plating from cyanide directly with nickel to the solutions is an important techrequired thickness. However, the nological process. It has been basic demand for cyanide copper successfully operated since the on die-castings and as a strike mid 19th century. It is safe to say bath and stop-off has remained that there is probably no electrounchanged. Despite the fact that plating practitioner who somecyanide copper solutions have time in his career has not The name copper comes from the Latin been successfully operated for e n c o u n t e r e d this workhorse of word cuprum, which means "from the Island of Cyprus." m a n y years, H o r n e r 2 rightfully the plating industry. As early as declared in 1964 that the prac1918, the importance of proper tice of cyanide copper plating is dominated in "conchemistry was recognized and studied in detail. 1 fusion and contradictory opinions." Cyanide copper solutions are used in present-day technology for six principal purposes: 1. As the undercoat deposit for plating copper-nickTHEORETICAL CONSIDERATIONS el-chromium on zinc-base die-castings and steel; There are a number of reasons for the use of more 2. as a strike solution over steel for subsequent expensive cyanide solutions in preference to the plating with acid copper, nickel, or other metals; simpler sulfate bath. When plating from the latter 3. as a stop-off for the selective nitriding and caron steel, zinc die-castings, or any metal above copper burizing of steel parts, by preventing diffusion of in the EMF series, copper will deposit on the base carbon to underlying metal; metal by displacement. This invariably results in 4. for the salvage buildup of specific worn parts; nonadherent deposits. 5. copper from cyanide baths is also oxidized to proIron and zinc do not displace copper when they are duce antique finishes; immersed in the cyanide copper solution because in 6. and for electroforming, magnetic shielding, and the presence of cyanide, copper is displaced upward production of PC boards. Editor's Note: This is the first part of a multi-part article. The second part will appear in the April issue.

March 2005

29

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in the EMF series, i.e. becomes less noble than the base metals. The equilibrium reduction potential of copper in cyanide copper solutions has been report-

°

ed 3 to be about -0.5 to -0.6 V ( H 2 scale), while the standard reduction potential of Cu 2÷ is +0.34 V. This may be due to the reduction of Cu concentration to a very low value indicated by the instability constant of the complex ion involved, or to the deposition potential of the complex ion itself. 4 Although cyanide copper plating solutions have been in use for many years, and a number of articles have been written during this time on their constitution, it must be admitted that the mechanism of copper deposition and the nature and concentration PROGRESSIVE is a recognized leader in the of the various complex ions presdesign and manufacture of pneumatic abrasive ent is still open for conjecture. blast cleaning and shot peening equipment. Expertise and experience makes our line of It is generally agreed that copper equipment benchmarks for industry. We have is present largely as [Cu (CN)3 ]2-, competitive solutions to all your Blast Cleaning and though some [Cu (CN)2]- and [Cu Shot Peening needs. (CN)4 ]3- complexes are also pres4201 Patterson Ave. SE * Grand Rapids, MI * USA 49512-4033 ent. 5-s Excess cyanide lowers the PH 800.968.0871 * 616.957.0871 * FAX 616.957.3484 cathode efficiency and raises the www.ptihome.com polarization, while an increase in Circle 036 on reader information card or go to www.thru.to/webconnect temperature, within limits, has the opposite effect. These facts would indicate that the effect of higher cyanide concentration is to cause a shift to the tetracyano complex, and that of higher temperature, to the dicyano complex. 9 A number 1, 3,10 of investigators have found that an increase of both copper and cyanide, in the ratio that they are present in the tricyano complex ion, increases both anode and cathode efficiency and decreases both anode and cathode polarization. This can be explained only by considering the [Cu (CN) 3] 2- ion as a source of both copper and cyanide, by a conversion to the dicyano form with increasing concentration. It should be emphasized that the values determined for "free cyanide" and "combined cyanide," though important in the production control of cyanide copper plating solutions, have little reference to the real composition of the solution. When one deterULTRASOb mines "free cyanide" by the standard procedure of titrating with

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AgNO 3 with KI an as indicator, all one finds is how much cyanide is less tightly bound in the copper plating solution than in the [Ag (CN2)]- complex ion. The end point of this titration occurs when the solubility product ( _) of AgI is exceeded. Taking K s of AgI as 1.7 x 10-1K~'and the concentration of I as ~0 -2 M/l, the concentration of Ag ÷ at the end point is a p p r o x i m a t e l y 2 x 10 -14 m/1. With dissociation (instability) constant 11 for [Ag (CN)2]- complex, Ki.nst = 4 x 10 .22 and assuming a l m concentration of this ion, the (CN) concentration at the end point of this titration is 10 .4 m/1. Clearly, as Thompson 12 has shown, the value of free cyanide obtained depends on the amount of indicator used. The main constituents of the cyanide copper plating b a t h are copper cyanide (CuCN), and sodium (NaCN) or potassium cyanide (KCN). Since CuCN ("single salt") is not soluble in water, some NaCN or KCN is needed to solubilize CuCN and as a source of"free cyanide." Together, these form a series of soluble complex salts in the following manner13: CuCN + N a C N -~NaCu(CN) 2 NaCu(CN) 2 + NaCN --~ Na2Cu(CN) 3

(1) (2)

Na2[Cu(CN) 3] --~ [Cu(CN)3 ]2- +2 Na + (3) [Cu(CN)3] 2- ~ [Cu(CN)2]- +2 CN(3-a) Although Na2Cu(CN) 3 is the most important complex salt formed from CuCN and NaCN, f u r t h e r complexing may occur to produce small concentrations of Na3Cu(CN)4: Na2Cu(CN) 3 + NaCN ~ Na3Cu(CN) 4

(4)

A very important feature of the [Cu(CN)3]2- complex is its degree of complete dissociation: [Cu(CN)3]2- ~ Cu + + 3CN-

(5)

is extremely small giving for dissociation constant: Kdiss = [Cu =] [CN-]3/[Cu(CN)3 ]2- = 5.6 x 10 .28

(6)

Hence, the effective concentration of free cuprous ions will be very low, and equilibrium potential of Cu in cyanide baths will be much more negative (by up to 1V) t h a n in acid sulfate solutions, and copper will not be displaced by iron or zinc. It is evident that free cyanide content of the b a t h will

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tend to suppress the decomposition of the [Cu(CN)2]- complex by shifting to the left and the decrease of the concentration of free copper will increase cathode polarization. The high polarization in conjunction with the good electrical conductivity and decreased cathode efficiency with increased current density impact the good throwing power of cyanide solutions. In potassium-base solutions, K replaces Na. The n a t u r e of the complexes present in copper cyanide electrolytes has been investigated by m e a s u r i n g the potential of copper in solutions of varying compositions 6,7,14, as well as by infra-red absorption spectroscopy. 15 All these methods confirm the existence of the [Cu(CN)4]3- , [Cu(CN)'3 ]2- and [Cu(CN)2]I- complex ions and show t h a t the proportion in which they are present varies with the free cyanide concentration and the temperature. In the concentration ranges of most cyanide copper plating baths, the [Cu(CN)3 ]2- ion is the most important, though at higher free-cyanide concentrations the [Cu(CN)4]3- ions predominates. Deposition from these kinds of complexes gives rise to the relatively uniform macroscopic distribution and fine-grained deposits.

Table h Composition

Operating Conditions of Cyanide Copper Solutions

& A

B

C R8 T y p e (Rochelle)

Hi-Efficiency

(8.0)

31-G6(4.2-4.3)

90 (t2.0)

--

4t-45 (5.5-6.0)

--

Composition g~ (oz/gal)

Hi Efficiency

Strike

cu-cyanide,

D PR

'

CuCN

......

26 (3.5)

60

cyanide,

Sodium NaCN-total

36 (4.0)

NaCN-free Pota~idm

--

7.5 (I.0) .

.

.

.

.

cyanide, KCN, total

--

4%45 (5.5-&0)

. . . . . . . . . . . . . . . . . . . --

98 (12.5)

_

_

7.5 (1.0)

_

Illp o t ~ l ~ ~ul~

144 (1G.0) .....

cyanide, KCN, free ' Na-carbonate, NafiO~

15 (2.0)

K-earbo'n'atel .... K~CQ,

-

Rochelle salt (KNaC~H,O, H~O)

30 (4.0)

36-45 (4,8-6,0)

15 (2.0)

Na-hydroxide, NaOH

9.0 (t.2)

-

19.0 (2.4)

.......... 54-60 (7.2-8.0)

-

--

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . to p H --

--

K-hydroxide, KOH

-

15-30 (2-4)

15-23 (2-3)

15-30 (2-4)

. . . 130-160

145-170

Operating Co.ditions Temperaturel . oF Tern peratu re

.

.

.

. . . 120-t60

.

.

.

. . . 140-170

.

.

.

49 -71

60-75

54-71

63 -75

*Cathode CD, Adm..~

0,5-15

2-6.5

2.2-6.5

32-11

*Cathode CD, Aft ~

(4.6-14)

(~20-60)

(20-60)

(~30-100)

30-50

60-95

5040

~0-95

°C

*Cathode CE(ave), %

Agitation

Optional

'yes

yes'

Yes

Anodes

Cu

Cu

Cu . . . . . . . .

C~

Anode/cathode ratio

3:1

2: t

2:1

2:1

Bath Voltage, V

2-4

3-4

4-6

Varies

....

T Y P E S OF C Y A N I D E Limiting 2.5 Unlimited 1.3 Unlimited Thickness ~n PLATING SOLUTIONS eelI values. Working CD wiif be much lower. *Varies widely with CD. A cyanide copper solution can be * These a r e H u l l used for rack, continuous strip, Rochelle solutions are used when only an intermediwire, or barrel plating. It can be classified in three catary thickness of copper is needed and where the egories: strike, Rochelle, and high efficiency (HE) baths. Strike solutions are used as a first plating bath. demands for higher plating rates and full brightness are not necessary. The purpose is to deposit a thin, adherent layer of The desired characteristics are achieved by opermetal that can completely cover the active base metal ating at higher temperatures, controlled pH, and such as zinc, aluminum, and steel. Due to the low higher Cu metal content, as well as adding a submetal and high cyanide contents, the strike type baths stantial amount of Rochelle salts or equivalent prohave a low efficiency and, consequently, high covering prietary complexants. The good throwing power is a and throwing power, as well as good adhesion. consequence of the decrease of cathode current effiThe heavier strike must be used when the subseciency (CE) with current density (CE). quent plating operation is from an acid copper bath. March 2005

33

high-efficiency (HE) type used to provide deposits 5microns (0.20 mils) thick and thicker. Bath C is a medium efficiency, simple Rochelle salt (RS) composition that can serve as a strike or as a plating bath. This bath does not make a good strike nor a good plate, but it is a compromise for the lines that are limited to only one cyanide copper bath. Bath D is a formulation recommended for periodic reverse current (PR) plating and usually contains a proprietary brightener(s). Solutions of the type of Bath A are frequently operated at room temperature. "Room temperature" is a loose term to be avoided, or at least needs defining. In Minneapolis in the winter, this can be much too cold. Cu-strikes are more commonly run at 43 ° to 49°C (100 ° to 120°F) except for striking A1 alloys, where it may be about 32°C (-90°F) and at CDs between 0.5 and 1.0 A/din 2 (-5 to 10 A/ft2). CDs may be increased to 1.5 A/din 2 at higher temperatures. Although such solutions are still widely used today, their applications are limited to the strike type plating to provide optimum bond between the base metal and s u b s e q u e n t HE bath. When used prior to p y r o p h o s p h a t e and acid copper plating, thicker deposits are required. The limitation of older formulations is in their slow plating speeds and low maximum deposit thickness. Later developments have brought to the forefront the more concentrated solutions, which may be operated at higher CDs and exhibit better CEs. With the help of proprietary addition agents, moreover, semi- and fully bright deposits may be obtained.

HE, a.k.a. "high-speed" b a t h s were i n t r o d u c e d commercially in 1938. They are characterized by higher m e t a l concentration, and lower free cyanide content. They are operated u n d e r conditions t h a t give h i g h e r cathode and anode CEs, than those from strike or Rochelle type solutions. With proprietary additives, they are used to produce deposits of various degrees of brightness and leveling, with t h i c k n e s s e s r a n g i n g from 7.5-50 (0.3-2.0 mils). The bright and ductile deposits can be produced in routine operations. SOLUTION

COMPOSITION

Four typical compositions and operating conditions of cyanide copper solutions are given, as illustrated in Table I. However, there is lots of room for successful variations in the plating baths as well as the strike baths. There are a few practical points that should be considered. Generally, as the CuCN is increased, the free cyanide can be increased; some platers observe a ratio between the two. Current density ranges listed earlier, are found in Hull cell testing and are not based on actual areas of parts being plated. They are presented for comparative purposes. Even on relatively simple-shaped parts, a plater would be hard pressed to get anywhere near 6 A/din2 (-60 A/ft 2) and more likely 2 to 2.5 A/dm 2 (~20 to 25 A/ft 2 ) on parts. All four type of plating baths normally contain proprietary addition agents. Bath A is a plain cyanide copper strike solution, used mainly to produce deposits 0.25- to 2.5-microns (0.01- to 0.10-mils) thick. Bath B is a high-speed,

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ADDITION

2. Horner, J., Proc. A E S 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, NY; 2000.

AGENTS

Proprietary additives are available for all cyanide copper baths. The main function is to improve one or more properties of the copper deposit. The addition agents to sodium or potassium-based cyanide baths are identical. These p r o p r i e t a r y addition agents include special w e t t i n g agents, brighteners, and Rochelle salt replacements, along with reducing agents for hexavalent chromium. Modern brighteners have replaced the sulfocyanides and the organic compounds that had active sulfur groups. Metallo-organic compounds, with selenium or tellurium with lead, or thallium as a secondary brightener, are commonly used to produce very bright deposits. Other metals have been mentioned, such as antimony and arsenic, but have not found much, if any, commercial use. Small amounts of arsenic in these bright baths have been traced to extreme fine pitting. Other m a t e r i a l s mentioned in the l i t e r a t u r e include amines or their reaction products with active sulfur compounds It should be noted t h a t active sulfur-bearing materials have been replaced with contemporary, improved additives such nitrogen and sulfur heterocyclic compounds, u n s a t u r a t e d alcohols, saccharin, and polyethylene glycol. Only a few of these are in use today. Passa116 presents an extensive listing on additives prior to 1959 as well as the history of cyanide copper plating. The newer ones are given in R e f 17.

N e d Mandich is consulting electrochemical engineer with H B M Electrochemical & Engineering Co. in Lansing, Ill. He can be contacted at (e-mail) [email protected]. ITIf

REFERENCES

1. Mathers, F.C., Trans. Electrochem. Soc., 33:1; 1918. OUALITY •Extensive Testing •Major Company Approvals

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