GB2154250A

GB2154250A - Complexing agent for electroless copper plating

Info

Publication number: GB2154250A
Application number: GB08503104A
Authority: GB
Inventors: Donald A Arcilesi; Oscar E Roberto
Original assignee: OMI International Corp
Current assignee: OMI International Corp
Priority date: 1984-02-17
Filing date: 1985-02-07
Publication date: 1985-09-04
Also published as: CA1248705A; GB8503104D0; JPH0218387B2; DE3504150A1; DE3504150C2; FR2559788B1; JPS60194081A; GB2154250B; FR2559788A1

Abstract

An aqueous alkaline electroless copper plating solution includes a composite complexing agent comprising an amine compound which is ethylenediaminetetraacetic acid and/or N, N, N', N'-tetrakis (2-hydroxy-propyl) ethylenediamine and a hydroxy acid which is gluconic and/or glucoheptonoic acid, or bath soluble and compatible salts and partial salts thereof, in which the hydroxy acid compound is present relative to the amine compound at a mol ratio of from 0.1 to 3:1. The synergistic effect of the use of the controlled combination of complexing agents provides for bath stability and commercially acceptable plating rates of adherent, ductile bright copper deposits over a broad range of temperatures, pH and processing conditions.

Description

SPECIFICATION Electroless copper plating The present invention broadly relates to an improved composition and process for producing electroless or catalytic copper deposits on substrates, and, particularly, on nonconductive substrates such as various plastics which have been subjected to various pretreatments to render the nonconductive substrate receptive to the copper deposit. Electroless copper plating baths of the types heretofore known conventionally comprise an aqueous alkaline solution containing copper ions, a complexing agent for the copper ions to prevent precipitation thereof, a reducing agent for reducing the copper ions to the metallic state, a pH regulator, a stabilizing agent, a rate controller and optionally, but preferably, wetting agents to improve coverage and distribution of the copper deposit.

The increased use of decorative plated trim components on automobiles has provided impetus for further research and development efforts to improve prior art electroless copper plating solutions to reduce their cost and simplify their maintenance and control, to increase their stability and to provide uniform, adherent copper deposits at commercially acceptable deposition rates. The present invention achieves the foregoing benefits and objectives by employing a controlled combination of complexing agents providing a synergistic effect whereby a substantial reduction in the quantity of complexing agent required is achieved providing for significant savings in the cost of make-up and replenishment of such electroless copper baths.Additionally, the present invention enables operation of the bath under commercial conditions at a lower pH level thereby reducing the quantity of the alkaline pH control agent necessary which in turn substantially reduces the degradation reaction of the alkaline agent such as caustic and the reducing agent such as formaldehyde providing for a significant reduction in the consumption of reducing agent. Still a further advantage of the bath of the present invention resides in the use of higher than normal room temperature enabling the bath to be operated at a temperature up to about 1 50 F (66"C) to achieve the desired bath activity for autocatalytically depositing copper.Such higher temperature enables the bath to be cooled to about room temperature during periods of nonuse such as during shutdown over weekends at which temperature the bath is of comparatively low activity enhancing its stability and inhibiting autocatalytic decomposition during periods of nonuse. At such higher operating temperatures, evaporation of the bath normally also occurs providing volumetric space in the plating tank for the addition of replenishing chemicals as is necessary during normal operation due to consumption and drag-out of the constituents. In contrast, electroless copper plating baths of the types heretofore known adapted to operate at room temperature have frequently required the removal of some of the operating solution to provide volumetric space for replenishment which not only waste valuable solution but has also presented a waste disposal problem.The improved composition of the present invention has been found to also enable a substantial reduction, and in some instances a complete elimination, of the need of rate controllers such as cyanide and iodide compounds conventionally employed in prior art electroless copper baths thereby further simplifying control and replenishment of the bath and a cost reduction in the materials required.

The benefits and advantages of the present invention are achieved, in accordance with the composition aspects thereof, by an electroless copper plating bath comprising an aqueous alkaline solution containing copper ions in an amount sufficient to deposit copper autocatalytically, which usually ranges in a concentration of about 0.5 to about 30 grams per litre (girl), a reducing agent present in an amount sufficient to effect a reduction of the copper ions to the metallic state, of which formaldehyde is preferred and can generally be present in an amount of about 0.1 up to about 40 gil, a complexing agent present in an amount to complex the copper ions present in the bath and usually present in a mol ratio of complexing agent to copper ions of from about 1:1 up to about 5:1.The complexing agent comprises a mixture of an amine compound which is ethylenediamine-tetra-acetic acid (EDTA) or N,N,N',N'-tetrakis (2-hydroxypropyl) ethylenediamine (THPEDA) and a hydroxy acid which is gluconic acid or glucoheptonoic acid (or bath soluble and compatible salts and mixtures thereof) wherein the molar ratio of the hydroxy acid amine compound is from about 0.1:1 to about 3:1. The bath can further contain a stabilizing agent, preferably a heterocyclic organic sulphide compound present in an amount up to about 2.5 gull, hydroxyl ions to provide a pH of from about 9 up to about 14, and/or, optionally, a rate controller such as cyanide, iodide and derivatives thereof present in an amount up to about 1 gil and/or a bath soluble and compatible wetting agent which may be present in an amount up to about 10 girl.

In accordance with the process aspects of the present invention, a substrate to be copper plated is immersed in the electroless copper solution maintained at a temperature of about room temperature (70"F or 21"C) up to about 150"F (66"C) under agitation for a period of time sufficient to effect a uniform, dense and adherent copper deposit over the surfaces thereof to the desired thickness. If desired, the substrate incorporating the electroless copper deposit thereover can be subjected to further conventional electroplating operation to apply one or a plurality of overlying electrodeposits thereon to achieve the desired physical characteristics and appearance.

Additional benefits and advantages of the present invention will become apparent upon a reading of the following description of the preferred embodiments taken in conjunction with the accompanying examples.

In accordance with the composition aspects of the present invention, the aqueous alkaline electroless or autocatalytic copper plating solution contains as its essential constituents, copper ions in an amount sufficient to deposit metallic copper on a substrate, a reducing agent for reducing the copper ions to the metallic state and a complexing agent present in an amount sufficient to maintain the copper ions in solution in the alkaline medium, hydroxyl ions to provide an alkaline pH and, optionally, stabilizing agents, rate controllers and/or wetting agents present in amounts sufficient to stabilize the solution and provide appropriate plating activity and good wetting and uniformity of the copper deposit produced.

The copper ions can be introduced into the aqueous alkaline solution in the form of any bath soluble and compatible copper salt in which the associated anion does not have any deleterious effects on the plating characteristics of the process.

Typically, cupric chloride dihydrate and copper sulphate pentahydrate can be used as the source of the copper ions. The concentration of the copper ions can generally range from as low as about 0.5 to as high as about 30 g/l with concentrations of from about 1 to about 5 g/l being preferred. Higher concentrations of copper ions are required when the bath is operated at or about room temperature to provide for a satisfactory rate of copper deposition whereas lower concentrations such as about 0.5 g/l can be employed when the temperature of the bath is at an elevated temperature such as from about 140"F (60"C) to about 1 50 F (66"C).

In addition to the copper ions, the aqueous alkaline solution contains a reducing agent present in an amount sufficient to reduce the cupric ions to the metallic state for deposition on the surface of the substrate being plated. For this purpose, formal- dehyde comprises a preferred material although paraformaldehyde can also be satisfactorily employed. In addition to the foregoing, hypophosphite and hydrazine as well as derivatives thereof have heretofore been used or proposed for use as reducing agents but are generally not as effective as formaldehyde itself. The concentration of the reducing agent generally used will vary in relationship to the copper ion concentration present in the bath and may range from as low as about 0.1 up to about 40 g/l calculated on a weight equivalent basis as formaldehyde with amounts of about 1 to about 5 g/l being preferred.

In addition to the copper ions and reducing agent, the aqueous alkaline solution further contains a controlled mixture of an organic complexing agent to maintain the copper ions in solution which otherwise would precipitate as copper hydroxide in the alkaline medium. The total complexing agent is generally controlled in concentration to provide at least about 1 mol of complexing agent per mol of copper ions present with mol ratios of complexing agent to copper ions as high as about 5:1 being feasible. Preferably the complexing agent is controlled at a mol ratio of about 1.3:1 to about 3.5:1 with a mol ratio of about 1.7:1 being typical.

It has been discovered that a synergistic effect in the complexing characteristics and in the activity and stability of the bath are achieved when the complexing agent comprises a controlled mixture of an amine compound which is ethylenediaminetetraacetic acid (EDTA) or N,N,N',N'-tetrakis (2hydroxypropyl) ethylenediamine (THPEDA) and an organic hydroxy acid which is gluconic acid or glucoheptonoic acid ortheir bath soluble and compatible salts, or a mixture thereof. The mol ratio of the hydroxy acid to amine compound ranges from 0.1 to about 3:1. The particular mol ratio will depend somewhat on the type of amine compound employed in combination with the hydroxy acid compound.For example, when the amine compound comprises THPEDA, the gluconic acid and/or glucoheptonoic acid can be present in a mol ratio relative to the amine compound within a range of about 0.11:1 up to about 2.45:1,with mol ratios of about 0.2:1 to about 1:1 being preferred and with a mol ratio of about 0.26:1 being optimum. On the other hand, when the amine compound comprises EDTA, the gluconic acid and/or glucoheptonoic acid can be present in a mol ratio of about 0.25:1 up to about3:1, preferably at a mol ratio of about0.6:1 to about 2:1 with a mol ratio of about 1.3:1 being optimum.The controlled mixture of complexing agents can conveniently be introduced in the bath in the form of their alkali metal neutralized salts which avoids any significant acidification of the bath and a reduction in its pH in comparison to that which occurs when the acid form of the complexing agents are added. The sodium salts are particularly satisfactory for this purpose.

By employing the specific controlled mixture of the two complexing agents as hereinabove set forth in the proportions indicated, a substantial reduction in the quantity of complexing agent can be achieved, and the two types of complexing agents may individually be present in amounts insufficient by themselves to complex all of the copper ions present.

The electroless copper solution is on the alkaline side and contains hydroxyl ions generally in an amountto provide a pH ranging from about 9 up to about 14, with a pH range of about 10.5 to about 12.5 being preferred. Typically, a pH of about 11.5 can be satisfactorily used: this provides for a commercially satisfactory copper deposition rate while at the same time requiring a lower hydroxyl ion concentration than has been used, which reduces the tendency of reaction and degradation of the formaldehyde reducing agent. This leads to a lower replenishing requirement and further economy in the operation of the process. The pH of the copper electroless solution can be maintained within the foregoing range by the addition of any alkali metal hydroxide of which sodium hydroxide itself constitutes a preferred material.

In addition of the foregoing constituents, it is also contemplated that the electroless copper bath can contain a stabilizing agent of any of the various types known in the art to inhibit spontaneous copper deposition on catalytic particles formed in the bath during the plating operation, which rapidly depletes the solution of copper ions. A variety of compounds have heretofore been used or proposed for this purpose of which 2-mercaptobenzothiazole has been in widespread use.Alternative stabilizing agents heretofore used or proposed for use include 2,5 dimercapto-1 3,4-thiodizole, 8-mercaptopurine, o- phenanthroline, 1 -phenyl-5-mercaptotetrazole, 2,2dipyridyl, 2-(2-pyridyl)-benzimidazole, ben zothiazole-thioetherpolyethyleneglycol, thiazoles, isothiazoles, thiozines, benzotriazole, diazole, imidazole, guanidine, pyrimidine,2,2'-biquinoline, 2,9dimethylphenanthroline and 4,7-diphenyl-1,10phenanthroline. Such stabilizing agents can optionally and preferably be employed in an amount up to about 2.5 g/l with concentrations of about 0.0001 to about 0.5 g/l being usually preferred.

While the novel combination ofcomplexing agents has been found to obviate the necessity of incorporating rate controllers in the bath of the types conventionally required, it is contemplated, under certain conditions, that rate controlling agents such as cyanide, iodide, or derivative compounds thereof can be incorporated in the electroless copper plating bath in amounts usually up to about 1 g/l. Such rate controllers accommodate the stabilizers by retarding the plating rate of the bath. It has also been found that the use of such rate controllers in relatively small proportions improves the lustre and ductility of the copper deposit. When employed, such rate controllers can be used in amounts up to about 1 g/l with amounts of from about 1 ppm up to about 220 ppm being more typical.

Optionally, and preferably, the electroless copper plating bath further contains small controlled amounts of bath soluble and compatible wetting agents to enhance uniformity of coverage of the copper plate on the substrate. Wetting agents of the general types which can be satisfactorily used are those conventionally employed in electroless copper plating baths among which Pluronic P85, available from BASF Corporation, comprising a nonionic block copolymer of ethylene oxide and propylene oxide and Gafac RE 610, available from GAF Corporation, an anionic phosphate ester, are typical examples.

The words 'Pluronic' and 'Gafac' are trade marks.

When employed, such wetting agents can be incorporated in amounts up to about 10 gil while amounts of about 0.1 to about 3 g/l are more typical and preferred.

In accordance with the process aspect of the present invention, an aqueous alkaline electrolyte of the composition as hereinbefore described is formed and is brought (for example heated) to an operating temperature of from about room temperature (60 to 70"F or to 21"C) up to about 160"F (71"C), and preferably from about 80 to about 150"F (27 to 66"C). Substrates to be copper plated are subjected to appropriate cleaning treatments, if necessary, to remove surface contaminants therefrom. In the case of nonconductive substrates such as plastics, for example, a preliminary pre-treatment of the nonconductive substrate is performed.This may include immersion with a tin-palladium complex treating solution to form active sites on the surfaces thereof usually followed by an accelerating treatment whereafter the pretreated plastic is susceptible for autocatalytic copper deposition. As a general rule, the plating rate of the solution can be adjusted to meet any desired commercial situation by varying the concentration of copper ions, reducing agent, temperature, pH and complexing agent concentration to attain the desired result. Increasing copper ion concentration, reducing agent concentration, increasing temperature and pH and reducing complexing agent concentration all contribute toward an increased rate of deposition of copper. Generally speaking, a copper deposition rate of about at least 20 microinches (0.50 microns) in 10 minutes is considered commercially satisfactory.Agitation of the solution also increases plating rate and can be attained by air agitation, cathode rod agitation or other mechanical agitating means.

In order further to illustrate the present invention, the following examples are provided. It will be understood that the examples are provided for illustrative purposes and are not intended to be limiting of the scope of the present invention as herein described and as set forth in the claims.

Example 1 An aqueous alkaline electroless copper solution is prepared by dissolving in water cupric chloride to provide a copper ion concentration of about 2 g/l (0.032 mol), EDTA tetra sodium salt in an amount of about 9 g/l (0.023 mol), sodium glucoheptonate dihydrate in an amount of about 9 gil (0.032 mol), formaldehyde as a reducing agent in an amount to provide a formaldehyde concentration of about 3 g/l, sodium hydroxide to adjust the pH of the solution to about 11.6 and a sulphur containing stabilizing agent such as 2-mercaptobenzothiazole in an amount of about 0.05 to about 10 ppm. The bath is aged corresponding to a turnover of about five to ten times of the copper ion concentration by replenishment of the original copper concentration and the other bath constituents.The aged bath contains a mol ratio of glucoheptonate to EDTA of about 1.4:1 with a total complexor to copper ion mol ratio of about 1.7:1.

The bath at a temperature of about 60"C (140"F) in the presence of air agitation is employed for depositing copper on test panels producing a bright, smooth, uniform pink copper deposit at a plating rate of about 45 microinches (1.14 microns) in 10 minutes. The bath is stable.

Examples 2A to 2E An aged aqueous alkaline electroless copper solution is prepared identical to that as previously described in Example 1 with the exception that no complexing agent is added. To separate portions of the complex-free aged solution, selected amounts and combinations of the EDTA and glucoheptonate complexors are added and the stability of the resulting solution under typical operating conditions is observed. To sample A, 0.032 mol EDTA is added and the bath is observed to be unstable. To a separate sample B, a combination of 0.023 mol EDTA and 0.014 mol glucoheptonate is added providing a mol ratio of glucoheptonate to EDTA of about 0.6:1.

The copper deposit is semi-lustrous and provides a plating rate of about 60 microinches (1.52 microns) in 10 minutes.

To sample C, a mixture of 0.021 mol glucoheptonate and 0.016 mol EDTA is added providing a relative mol ratio of glucoheptonate to EDTA of about 1.3:1.

The resultant electroless copper solution provides a pink copper deposit at deposition rate of about 56 microinches (1.42 microns) in 10 minutes. The bath is stable.

To sample D, a mixture of 0.028 mol glucoheptonate and 0.008 mol EDTA is added providing a mol ratio of glucoheptonate to EDTA of about 3.5:1. The bath is unstable and produces a copper deposition rate at about 34 microinches (0.86 microns) in 10 minutes.

To sample E, 0.035 mol glucoheptonate is added without any EDTA. The bath provides a copper deposition rate of about 16 microinches (0.41 microns) in 10 minutes. The bath is unstable and decomposes.

The foregoing tests indicate that the use of substantially equal molar amounts of EDTA or glucoheptonate by themselves does not provide a stable aqueous alkaline electroless copper solution.

When employing a combination of glucoheptonate and EDTA within the molar ratios as hereinabove specified, namely, from about 0.1:1 to about 3:1 and preferably from about 0.6:1 to about 2:1 mols glucoheptonate to EDTA, satisfactory copper deposits are obtained with relative bath stability.

Example 3 An aged aqueous alkaline electroless copper solution contains about 2 g/l (0.032 mol) copper ions, about 9 g/l (0.023 mol) EDTA, about 5.3 g/l (0.021 mol) gluconate providing a mol ratio of gluconate to EDTA of about 0.9:1 about 3 g/l formaldehyde and sufficient sodium hydroxide to provide a pH of about 11.6. The bath further contains a sulphur stabilizing compound such as 2-mercaptobenzothiazole in an amount up to about 0.25 ppm.

The solution is at a temperature of about 60"C (140"F) and is subject to air agitation. A bright pink copper deposit is obtained on test panel surfaces at a plating rate of about 37 microinches (0.94 microns) in 10 minutes.

Example 4 An aged aqueous alkaline electroless copper solution contains 2 gll (0.032 mol) copper ions, about 8 gll (0.027 mol) THPEDA, about 2 gIl (0.007 mol) sodium glucoheptonate dihydrate to provide a glucoheptonate to THPEDA mol ratio of about 0.26:1, about 3 gll formaldehyde and sufficient sodium hydroxide to provide a pH of about 12.2.

The solution is at a temperature of about 60"C ('140"F) and is subject to air agitation. A bright, smooth, pink copper deposit is obtained on test panel surfaces at a plating rate of 140 microinches (3.56 microns) in 10 minutes.

Example 5 An aged aqueous alkaline electroless copper solution contains about 5 g/l (0.08 mol) copper ions, about 15 g/l (0.05 mol) THPEDA, about 7.5 gll (0.02 mol) EDTA, about 3.75 gll (0.013 mol) sodium glucoheptonate dihydrate providing a mol ratio of glucoheptonate to the two amine compounds present of about 0.18:1, about 5 gll formaldehyde and sodium hydroxide to provide a pH of about 12.2.

The solution is at a temperature of about 60"C (140"F) and is subject to air agitation. A bright, pink copper deposit is obtained on test panel surfaces at a plating rate of about 123 microinches (3.12 microns) in 10 minutes.

While it will be apparent that the prefered embodiments of the invention disclosed are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the claims.

Claims

1. An aqueous alkaline electroless copper plating solution containing copper ions in an amount sufficient to deposit copper, a reducing agent, hydroxyl ions sufficient to provide a pH on the alkaline side, and a complexing agent present in an amount sufficient to complex the copper ions present and comprising a mixture of an amine compound which is ethylenediamine-tetraacetic acid or N,N,N',N'tetrakis (2-hydroxypropyl) ethylenediamine and a hydroxy acid compound which is gluconic acid or glucoheptonoic acid, or bath soluble and compatible salts and partial salts thereof, or a mixture thereof, in which the hydroxy acid compound and the amine compound are present in the mixture at a mol ratio of from 0.1 to 3:1.

2. An aqueous alkaline electroless copper plating solution as claimed in claim 1 in which the hydroxyl ions are present in an amount to provide a pH of from 9 to 14.

3. An aqueous alkaline electroless copper plating solution as claimed in claim 1 in which the hydroxyl ions are present in an amount to provide a pH of from 10.5 to 12.5.

4. An aqueous alkaline electroless copper plating solution as claimed in claim 1,2 or 3, in which the amine compound comprises ethylenediaminetetraacetic acid and the hydroxy acid compound is present in an amount to provide a mol ratio of hydroxy acid compound to amine compound of from 0.25:1 to 3:1.

5. An aqueous alkaline electroless copper plating solution as claimed in claim 1,2 or 3 in which the amine compound comprises ethylenediaminetetraacetic acid and the hydroxy acid compound is present in an amount to provide a mol ratio of hydroxy acid compound to amine compound of from 0.6:1 to 2:1.

6. An aqueous alkaline electroless copper plating solution as claimed in claim 1, 2 or 3 in which the amine compound comprises ethylenediaminetetraacetic acid and the hydroxy acid compound is present in an amount to provide a mol ratio of about 1.3:1.

7. An aqueous alkaline electroless copper plating solution as claimed in claim 1,2 or 3 in which the amine compound comprises N,N,N',N'-tetrakis (2hydroxy-propyl) ethylenediamine and the hydroxy acid compound is present in an amount to provide a mol ratio of hydroxy acid compound to amine compound offrom 0.11:1 to 2.45:1.

8. An aqueous alkaline electroless copper plating solution as claimed in claim 1, 2 or 3, in which the amine compound comprises N,N,N',N-tetrakis (2hydroxypropyl) ethylenediamine and the hydroxy acid compound is present in an amount to provide a mol ratio of hydroxy acid compound to amine compound offrom 0.2:1 to 1:1.

9. An aqueous alkaline electroless copper plating solution as claimed in claim 1,2 or 3, in which the amine compound comprises N,N,N',N'-tetrakis (2hydroxypropyl) ethylenediamine and the hydroxy acid compound is present in an amount to provide a mol ratio of about 0.26:1.

10. An aqueous alkaline electroless copper plating solution as claimed in any one of claims 1 to 9, in which the complexing agent and the copper ions are present to provide a mol ratio of complexing agent to copper ions of from 1:1 to 5:1.

11. An aqueous alkaline electroless copper plating solution as claimed in any one of claims 1 to 10, including a stabilizing agent present in an amount up to about 2.5 g/l.

12. An aqueous alkaline electroless copper plating solution as claimed in any one of claims 1 to 11 further including a rate controller present in an amount up to about 1 g!l.

13. An aqueous alkaline electroless copper plating solution as claimed in any one of claims 1 to 12 including a wetting agent present in an amount up to about 10 g/l.

14. A process for depositing copper plating on a substrate which comprises the steps of immersing the substrate in an aqueous alkaline electroless copper plating solution as claimed in any one of claims 1 to 13, controlling the temperature of said solution within a range of from 60 up to 1 60"F (16 to 71"C), and continuing the immersion of the substrate until a desired thickness of copper has been deposited thereon.

15. A complexing agent for an aqueous alkaline electroless copper plating solution, the agent comprising a mixture of an amine compound which is ethylenediamine-tetraacetic acid or N,N,N',N'tetrakis (2-hydroxypropyl) ethylenediamine and a hydroxy acid compound which is gluconic acid or glucoheptonoic acid, or bath soluble and compatible salts and partial salts thereof, or a mixture thereof, in which the hydroxy acid compound and the amine compound are present in the mixture at a mol ratio of from 0.1 to 3:1.

16. A complexing agent as claimed in claim 15, in which the amine compound comprises ethylenediamine-tetraacetic acid and the hydroxy acid compound is present in an amount to provide a mol ratio of hydroxy acid compound to amine compound of from 0.25:1 to 3:1.

17. A complexing agent as claimed in claim 15, in which the amine compound comprises ethylenediamine-tetraacetic acid and the hydroxy acid compound is present in an amountto provide a mol ratio of hydroxy acid compound to amine compound of from 0.6:1 to 2:1.

18. A complexing agent as claimed in claim 15, in which the amine compound comprises ethylenediamine-tetraacetic acid and the hydroxy acid compound is present in an amount to provide a mol ratio of about 1.3:1.

19. A complexing agent as claimed in claim 15, in which the amine compound comprises N,N,N', N'-tetrakis (2-hydroxypropyl) ethylenediamine and the hydroxy acid compound is present in an amount to provide a mol ratio of hydroxyacid compound to amine compound of from 0.11:1 to 2.45:1.

20. A complexing agent as claimed in claim 15, in which the amine compound comprises N,N,N',N'tetrakis (2-hydroxypropyl) ethylenediamine and the hydroxy acid compound is present in an amount to provide a mol ratio of hydroxy acid compound to amine compound of from 0.2:1 to 1:1.

21. A complexing agent as claimed in claim 15, in which the amine compound comprises N,N,N',N'tetrakis (2-hydroxypropyl) ethylenediamine and the hydroxy acid compound is present in an amount to provide a mol ratio of about 0.26:1.

22. An aqueous alkaline electroless copper plating solution substantially as herein described in any one of Examples 1, 2B, 2C, 3,4 and 5.

23. A process for depositing copper plating on a substrate substantially as herein described in any one of Examples 1, 2B, 2C, 3, 4 and 5.

24. A complexing agent for an aqueous alkaline electroless copper plating solution substantially as herein described in any one of Examples 1, 2B, 2C, 3, 4 and 5.