Electroless copper plating of resin-coated optical fibres

Surface

Technology,

ELECTROLESS FIBRES

23 (1984)

COPPER

333

333

- 340

PLATING

OF RESIN-COATED

OPTICAL

I3. CHATTERJEE Engelhard

Industries,

(Received

April

Davis Road,

Chessington,

Surrey

KT9

1TD

(et.

Britain)

16, 1984)

Summary The pristine surface of optical fibres precoated with a silicone resin was preserved by hermetically sealing the fibres by means of electroless copper plating. An optimum plating rate of 0.5 pm min-’ at 22 “C and pH 13 provided a high adhesive strength of 7 MPa for the deposit. Measurement of the mechanical properties of the plated fibres was carried out both in air and in water by bend tests; the results showed an improvement in the fibre strengths in both environments compared with those of unplated fibres.

1. Introduction Optical glass fibres are well known to degrade from their inherently high strength pristine condition very quickly after fabrication [l]+ This is due to the propagation of surface microcracks under the action of chemical corrosion by surface moisture and mechanical abrasion, leading to a substantial reduction in the tensile strength (static fatigue), It is therefore essential that the individual fibres are given proper protection so that any subsequent handling does not lead to fibre breakage. Although the glass fibres are inevitably damaged to some extent even during the various stages of fibre drawing, undoubtedly most degradation occurs during subsequent processes. It is therefore important to apply so me protective covering to the fibre surface as soon as possible after fibre draw ing and certainly before the fibre has come into contact with any other surface. Although many materials could be used for protection, plastics have been considered to be suitable in view of their availability and ease of application [2]. Workers at British Telecom have successfully developed a novel technique of coating the fibres with a silicone resin (Sylgard-182 of Dow Corning) which protects the surface with a considerable improvement in strength [ 31. However, any protection with a resin has been reported to deteriorate with time [4] as a result of chemical attack of the glass surface by environmental contaminants such as moisture and sodium ions which can penetrate the resin coatings [ 53. It was found to be necessary to protect the Sylgard-coated optical fibres with a metallic abrasion-resistant coating which 0376-4583/84/$3.00

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would provide a hermetic seal to prevent the ingress of air and moisture and consequently would improve the fibre strength against static fatigue, Several attempts to metallize the resin-coated fibres with low melting metals, such as aluminium, zinc, indium and tin, failed to preserve the pristine surface. The origin of our interest in the present project lies in our long association with British Telecom [6], Initially, application of physical vapour deposition methods was considered to be feasible but was estimated to be quite costly. An electroless plating method has been investigated which involves an appreciably lower cost and less maintenance than any other method. An electroless process to plate copper 5 - 10 pm thick onto optical fibres coated with Sylgard-182 resin 60 pm thick is described in t-his paper. The wet method involved in electroless plating may make the optical fibres susceptible to corrosion via diffusion of water through the resin coating. However, this possibility was minimized by the short time involved at nearambient temperature, 2. Basic considerations The economic implications of fibre-optical communication systems are significant. Detailed consideration must therefore be given to any treatment process such as electroless plating for fibre optics to render the method successful in its eventual application, The present choice of electroless copper plating is primarily due to the low cost involved and the low intrinsic stresses produced in the film compared with those obtained by using solutions of nickel, cobalt, gold, silver etc. In electroless copper plating, the cleaned substrate undergoes (a) etching, (b) sensitization, (c) activation and (d) dipping in a bath made up of a source of copper ions, a complexing agent for copper, an alkali hydroxide and a reducing agent. A stabilizer such as malathion [7] or poly(ethylene glycol) [8], a wetting agent such as sodium lauryl sulphate [9] and other addition agents [lo] are invariably present in any conventional electroless bath, Any chemical or metallurgical process involves a number of parameters which affect the quality of the product. Meek [ll] noticed the effect of the time of sensitization and activation on the purity of the deposited copper films. Data on the relationship between the time of plating and the thickness of the electroless film are important in controlling the thickness of the films. All electroless baths are highly unstable in that they decompose long before all the copper is plated on the substrate, thus causing heavy losses of copper. In fact, the ratio of the amount of copper plated to that of the reducing agent is an important factor for satisfactory electroless plating. It is therefore imperative to develop a process which can overcome all the above-mentioned shortcomings. 3. Selection

of materials

3.1. Etchmt In the electroless because of satisfactory

plating of plastics good adhesion is developed mainly mechanical keying at the metal-plastic interface.

335

Such an effect can be produced by an etching treatment before plating [ 121 as a result of the hydrophobic nature of smooth plastic surfaces. A satisfactory method of etching the silicone resin to render it hydrophilic was established by comparing the structures of the resin etched in alkali (5 - 10 M NaOH) acid) solutions. A stronger reactivity of the and in acid (5% - 10% chromic alkaline etch was evident; it was estimated that about 2 - 3 pm of plastic was etched away after an etching time of 10 - 15 min. The optical fibre coated with resin 60 pm thick would not therefore be exposed by such an etching treatment. 3.2. Sensitizer By far the most commonly used sensitizers are those based on stannous However, Cohen et al. [13] using SnClz as compounds, particularly SnQ. the sensitizer observed that electroless films contained chlorine, oxygen, tin and lead impurities. Such contamination can leave a precipitate of oxychloride on the substrate by oxidation and hydrolysis. AnalaR grade reagents should be used with deionized water. Goldie [ 141 mentioned no variation in his results for the peel strength and the rate of deposition of copper when SnClz was used. 3.3. Activator Among the activators used, sometimes referred to as reduction catalysts, are members of the platinum group, particularly palladium and to a lesser extent platinum and also gold and silver. An acidified solution of palladium chloride was found to be the best activator as it has a maximum catalytic activity for deposition of copper from copper complex solutions under alkaline conditions. Melka [ 151 employed silver nitrate as an activator without any specific advantages.

3.4. Source of copper ions Cupric sulphate, cupric chloride and cupric nitrate are the most common water-soluble copper compounds. Of these, cupric chloride and cupric nitrate solutions undergo hydrolysis in water, thereby precipitating oxy compounds. Moreover, they are not available with purities of more than 98%, whereas cupric sulphate (CuS04* 5H,O) could be obtained with a purity of 99.5%.

3.5, Complexing

agent

Sodium potassium tartrate (Rochelle salt) is the most versatile complexing agent used in electroless copper plating. Complexing agents less frequently tried are citrate, ethylenediaminetetraacetate, triethanolamine and ethylene glycol. The chemical chosen should form a stable complex with a dissociation constant less than that of copper sulphate at the pH of copper deposition. A high rate of deposition leads to soft and coarse-grained films. The lower the dissociation constant, the lower is the rate of copper deposition, resulting in hard and fine-grained films. From such considerations, a copper tartrate complex [ Cu(OH),( tar)] 4- with a dissociation constant of 1.4 X 10ml'at

336

pH > 11 [ 161, compared with a dissociation constant copper citrate complex [ 171 at the same pH, was preferred

of 5 X 10 -20 for for the deposition.

a

3.6. Reducing agent Hydrazine, sodium hypophosphite, glycerol and potassium borohydride were considered as reducing agents from the standpoint of their redox potentials for the reduction of CL?+ ions to copper metal. These are effective only in the pH range 1 - 6 while, at pH values above 10, formaldehyde is unparalleled in its reducing power and hence was used in the present work. 3.7. Alkali metal hydroxide The pH of the copper tion of copper begins at procedure, NaOH was used.

3.8. Additional

tartrate complex is only 5.0 whereas the deposipH 11 or above, Following the conventional

agents

It was decided not to introduce any other chemicals into the bath except those mentioned above, as these could accentuate the problems of process control.

4. Experimental

details

All solutions were prepared from AnalaR grade chemicals and deionized water. In the present work, the effects of the concentrations of the Rochelle salt, the NaOH and the formaldehyde solutions and the number of alternate sensitization and activation treatments of the Sylgard-coated glass fibres on the growth of copper films and their adhesive strengths were investigated. The optimum compositions and conditions for pretreatment and electroless plating are presented in Tables 1 and 2. A thorough rinsing in warm water was carried out at the end of each step. The amount of Rochelle salt used as the complexing agent corresponded to an equimolar ratio of CuS04 to

TABLE 1 Pretreatment

conditions

for electroless

Treatment

Chemical

1 Etching 2 Sensitizing

NaOH SnCl2 2H20 Concentrated acid PdClz Concentrated acid

3 Activating

plating

compound

l

hydrochloric

hydrochloric

at ambient

temperature

Concentration WI

Treatment (min)

10 0.31 1.19

10 - 15 5 - 10

8.4 x 1w3 0.12

3-5

time

337 TABLE

2

Optimum

composition

Chemical

compound

of an electroless

1 CuS04*5Hz0 2 Rochelle salt 3 NaOH 4 Formaldehyde ---._ The pretreated 0,5 E_tmmin?

fibres

are processed

copper

bath

Function

Concentration

Source of metal ions Complexing agent To increase pH Reducing agent

0.16 0.22 0.55 1.51

at 22 “C at pH 13 for 5 - IO min with a plating

(M)

rate of

Rochelle salt as suggested by Carlo [ 181. The electroless solution (Table 2) can be stabilized by considering the pH dependence of the reducing action of formaldehyde [ 12,191. It was found that a solution which starts to decompose during usage can be stabilized during an idle period by lowering the pH to a value below 10 at which the reduction potential of formaldehyde is too low to exert any reducing action. Since the strength of the silicone resin decreases with increasing temperature, all work was carried out close to ambient temperature. Optical microscopy (Fig. 1) was performed by sputter coating the substrate with platinum or gold (about 6 a thick). The presence of occasional dust and/or debris in the microstructures seemed to be unavoidable. The adhesive strength of the copper film on Sylgard resin was evaluated by the peel test using Sellotape and “pull-off” tests. A commercial adherence tester supplied by Quad Group of Santa Barbara, CA, was used for the pulloff tests. The method involved epoxy resin bonding of a stud of known surface area onto the film and pulling under a known load. The stress at which either the epoxy or the film failed was automatically registered. The instrument was capable of providing stress values of up to 69 MPa. 5. Results

and discussion

The study of the influence of the number of sensitization and activation sequences carried out on the substrate showed that a repeated treatment was not essential to cover the substrate completely with the palladium catalyst for the subsequent deposition of copper. On sensitization, dark spots of tinenriched areas developed (Fig. l(c)) compared with the unetched surface (Fig. l(b)). There was no change in the microstructure on subseauent activation with palladium chloride. This is expected since activation dnly involves an exchange redox reaction between Snzt and Pd*+ ions. There was no noticeable increase in the density of the spots on prolonging the treatment. Since the present electroless process provides a pore-free uniform deposit (Fig. l(d)), it is presumed that the catalytic sites, although sparse, are active enough to generate sufficient nucleation for total coverage of the resin

338

(C> Fig. 1. Photomicrographs of Sylgard-coated optical fibres: (a), (b) as received; (c) etched and sensitized; (d) plated with electroless copper. (Magnifications: (a) 67.5x; (b) 375x; (c) 375x;(d) 1500x.)

substrate, It has been reported [ZO] that the high catalytic activity is associated with large colloidal particles and may not require a complete coverage of the surface. Furthermore, it is well known that the use of an autocatalytic electroless bath should ensure complete coverage of the surface once the nucleation of copper has started on the catalytic sites. The dependence of the rate of formation of the electroless coating on the concentration of formaldehyde revealed some interesting facts, A combination of copper ions and formaldehyde in a molar ratio ranging from I:2 to 1~3 was required for satisfactory plating. The range of molar ratio depends

339

on the plating conditions, namely the temperature, the volume, the concentration of the bath and the size of the object to be plated. The present fibre composite consisted of two components with a cylindrical glass core (125 pm in diameter) coated with silicone resin (60 pm thick). A molar ratio of copper ions to formaldehyde greater than 1:2 results in an extremely low rate of copper deposition, and a molar ratio smaller than 1:3 causes decomposition of the bath. The present findings thus suggest that the stability of the bath decreases with increasing concentration of formaldehyde, which can be explained on the basis of the mechanism proposed earlier [Zl], namely Cu*+ + ZHCHO + 4OH-

+

Cu + 2HCOO- + 2H20 + H2

The reaction is a redox reaction comprising the anodic oxidation of formaldehyde and the cathodic reduction of cupric ions at the copper surface [22,23]. The copper surface serves as both the anode and the cathode at which the anodic and cathodic processes proceed at an equal rate. The adhesive bonding between the Sylgard resin and the copper film was found to be satisfactory from the Sellotape pulling test. The strength of adhesion was measured by means of the pull-off test to be as high as 7 MPa, which is comparable with a typical value for an electrodeposit. The strengths of the Sylgard-coated optical fibres plated with electroless copper were measured using a bending technique as detailed elsewhere [24], The fibre was bent through 180” between two grooved stainless steel plates. The plates were attached to a motorized micrometer cross-head and brought together with a uniform velocity (37.5 m s- I). Measurements were carried out on both plated and unplated optical fibres exposed to air and water. The severe deformation caused by the 180” bend test is expected to produce cracks in the plated metal, resulting in a degradation of the initial fibre strength on subsequent exposure to an aggressive environment. However, from the Weibull plots of cumulative per cent failure against fibre strength (per cent elongation) it was estimated that the strengths of the copperplated fibres in both air and water were increased, e.g. by 1.7% in air and 4% in water, compared with the results for the unplated fibres. These results demonstrate the effectiveness of copper plating to inhibit the flow of air or water, and thereby their effectiveness in improving the initial strengths of the Sylgard-coated fibres by providing a satisfactory hermetic sealing. There is no doubt that some detailed assessment of the plating quality on a longterm basis as well as static fatigue testing will eventually be required. HOWever, the present work has at least highlighted for the first time the potential application of the electroless plating method for hermetic sealing of resincoated optical fibres. Acknowledgment The author

wishes

to thank

British

Telecom,

Ipswich,

for their

support.

340

References 1 F. M. Ernsberger, Glass Ind., 47 (1966) 422. 2 R. Hasegawa, M. Kobayashi and H. Tadokoro, PoEym. J., 3 (1972) 591. 3 P. W. France, P. L. Dunn and M. H. Reeve, Fiber Integr. Opt., 2 (1979) 267. 4 D. A. Pinnow, G. D. Robertson and 3. A. Wysocki, Appl. Phys. Lett., 34 (1979) 17. 5 D. A. Pinnow, G. D. Robertson, G. R. Blair and J. A. Wysocki, I’roc. Top. Meet. on Optical Fiber Communication, March 6 - 8, 1979, Optical Society of‘ America, Washington, DC, 1979, p_ 16. 6 K. J. Bealcs and C. R. Day, Phys. Chem. Glasses, 22 (1980) 5. 7 M. N. Gilano, Ger. Patent 2,331,950, 1975. 8 A. Molenaar, J. Meerakker and J. Boven, Plating (East Orange, i2rJ), 61 (1974) 649. 9 L. L. Roger and R. S. Vincent, S. Afr. Patent 6&X,387, 1969. 10 H. Hirohata, Kinzoku Hyomen &‘jutsu, 21 (1970) 142. 11 R, L. Meek,J. Electrochem. Sot., 122 (1975) 1478. 12 R. Weiner, Electroplating of Plastics, Finishing Publications, Hampton Hill, Middx., 1977. 13 R. Cohen, J. D’Amico and K. J. West, J. Electrochem. Sot., 118 (1971) 2042. 14 W. Goldie, Plating (East Orange, NJ), 51 (1964) 1069. 15 3. P. Mclka, Plating (East Orange, NJ), 58 (1971) 1045. 16 1;. M&es, J. Chem. Sot., 71 (1949) 3269. 17 L. Meites, J. Chem. Sot., 72 (1950) 180. 18 F. Carlo, Ann. Chim. Appl., 38 (1948) 84. 19 B. D, Barker, Surf. Technol., 12 (1981) 77. 20 T. Osaka and H. Takematsu, J. Electrochem. SOC., 127 (1980) 1021. 21 R. M. Lukes, Plating (East Orange, NJ), 51 (1964) 1066. 22 M. Saito, J. 1Met. Finish. Sot. Jpn., 2 7 (1966) 14. 23 M. Paunovic, Plating (East Orange, NJ), 55 (1968) 1161. 24 p. W. France, M. J. Paradine, M. H, Reeve and G. R. Newns, J. Mater. Sci., 15 (1980) 825,