Electroless Deposition Of Palladium Alloys

Abstract

A plating solution for providing an electroless deposit of palladium alloys wherein palladium predominates, with a minor amount of nickel, cobalt, or zinc. The electroless palladium alloys contain up to about 6% nickel, or 10% cobalt, or 36% zinc, each with phosphorus. Preferred bath compositions comprise 29.6 g/l NiSO.sub.4.sup.. 6H.sub.2 O, or 29.6 g/l CoSO.sub.4.sup.. 6H.sub.2 O, or 36.0 g/l ZnSO.sub.4.sup.. 8H.sub.2 O, with: PdCl.sub.2 --2 g/l Hcl (38%)--4 ml/l Nh.sub.4 oh (28%)--160 ml/l Nh.sub.4 cl--27 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O--10 g/l

Parent Case Text

This application is a continuation-in-part of our copending application, Ser. No. 74,038, filed Sept. 21, 1970, now abandoned, for "Electroless Deposition of Palladium and Palladium Alloys", and assigned to the same assignee hereof.

Description

The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

The present invention relates to the electroless deposition of palladium alloys and improved bath compositions therefor.

Electroless deposition (autocatalytic chemical reduction) of palladium has been accomplished through the use of hydrazine-based baths which, however, have a short shelf life and deposition rates decrease rapidly after initial use but prior to significant depletion. These deficiences have been substantially minimized by our electroless plating bath which provides binary alloys of Pd-P and ternary alloys of Pd-P and an additional metal such as nickel, cobalt, or zinc.

Electrolessly deposited palladium alloys are most useful metals having diverse and important commercial and military applications. They may be used on electrical contacts for communications systems and to provide reliable and substantially noise-free transmission in voice circuits. Electroless palladium alloy deposits should find broad application to electronic components such as connectors, terminals, slip rings and electrical brushes where low contact resistance and high wear resistance is required. Electroless palladium alloy deposits may be applied directly to non-conductive substrates for use in catalytic chemical processing and for fuel cell electrodes. Electroless deposition of the palladium alloys will provide certain advantages over electroplated palladium. In general an electroless palladium alloy deposit

A. IS MORE UNIFORM RESULTING IN A MINIMUM AMOUNT OF METAL REQUIRED FOR A GIVEN APPLICATION.

B. YIELDS LESS PORUS DEPOSITS AND HENCE THINNER DEPOSITS MAY BE FOR A GIVEN APPLICATION.

C. IS MORE VERSATILE, PROVIDING A VARIETY OF CONTROLLABLE PROPERTIES SUCH AS HARDNESS, WEAR RESISTANCE, AND CATALYTIC ACTIVITY CHANGE WITH ALLOY COMPOSITION.

Accordingly, it is an object of this invention to provide an improved bath composition for permitting the electroless deposition of high quality palladium alloy deposits.

Another object of the invention is to provide improved bath compositions as above discussed having a shelf life of at least seven months and yet with deposition rates that are relatively constant so long as bath replenishment is accomplished.

Still another object of the invention is to provide improved bath compositions which yield palladium phosphorus alloys or having codeposited therewith, nickel or cobalt, or zinc.

The exact nature of this invention as well as other objects and advantages thereof will be apparent from consideration of the following description and drawings wherein:

FIG. 1 graphically represents effect of temperature on electroless palladium phosphorus alloy deposition rates, and

FIG. 2 is a graphical representation showing effects of depletion and replenishment on electroless deposition rates.

In accordance with the above objects, we have discovered that excellent electroless deposits of palladium alloys may be obtained through the use of our invention. When palladium ions and hypophosphite ions are present in solution, homogeneous chemical reduction occurs such that palladium metal is formed throughout the solution as well as on the desired surface. The nuclei of metal thus formed in the solution provide a large surface area for catalytic oxidation of hypophosphite and reduction of palladium ions thereby resulting in rapid and wasteful depletion of the reacting chemicals. A stable hypophosphite-based electroless palladium plating solution was developed wherein the acidified palladium chloride stock solution utilized for solution preparation consisted of 20 g/l PdCl.sub.2 -- 40 ml/l HCl (38%). All solutions hereinafter described will thus be understood to contain 2 ml/l HCl (38%) for each g/l PdCl.sub.2 present. The appropriate quantity of acidified PdCl.sub.2 solution was added to the NH.sub.4 OH solution, allowed to stand 20 hours and filtered before addition of other test solution constituents.

Test solutions for electroless palladium phosphorus alloy deposition were prepared and heated in a constant temperature bath. An activated tantalum panel of 10 cm.sup.2 area was immersed into 200 ml of test solution. The deposit weight was found by difference after stripping the deposit for 10 minutes in aqua regia. Activation of the panel comprised immersing it in 5 g/l of SnCl.sub.2 -5 ml/l HCl solution for 2 minutes; rinse; then in 0.2 g/l PdCl.sub.2 -1 ml/l HCl solution for 2 more minutes; and rinse.

Test electroless palladium alloy plating solutions were prepared of 1 g/l PdCl.sub.2, 160 ml/l NH.sub.4 OH (28%) and varying concentrations of hypophosphite. Increasing NaH.sub.2 PO.sub.2.sup.. H.sub.2 O resulted in increased deposition rate at 40.degree.C to a maximum at about 20 g/l. At 60.degree.C, the solutions containing 20 or 40 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O were unstable and rapid solution decomposition occurred before deposition tests could be conducted. 10 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O yielded optimum results.

Solutions were prepared with 0.5, 1.0, 2.0 and 4.0 g/l PdCl.sub.2, 160 m/l NH.sub.4 OH (28%) and 10 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O. The deposition rates from the solutions at 40.degree.C increased with increasing palladium content but there was relatively little benefit from concentrations above 2.0 g/l PdCl.sub.2.

Additional deposition rate determinations were made using solutions containing 2.0 g/l PdCl.sub.2, 10 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O and various concentrations of ammonium hydroxide. The solution containing 40 ml/l NH.sub.4 OH (28 percent) was unstable at 60.degree.C. There was otherwise little effect on deposition rate or stability of solutions containing 80 to 320 ml/l NH.sub.4 OH.

The effect of NH.sub.4 Cl additions on electroless palladium phosphorus alloy deposition rates was determined from solutions containing 2 g/l PdCl.sub.2, 160 m/l NH.sub.4 OH (28%) and 10 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O. Increasing concentration of NH.sub.4 Cl resulted in marked decrease in deposition rates presumably by more effectively complexing the palladium. The stability of the electroless solutions on extended use were improved with increasing NH.sub.4 Cl concentration. It is thus considered very desirable to include NH.sub.4 Cl in the electroless palladium phosphorus alloy plating solution.

Bath compositions are presented in the following Tables for providing electroless deposits of palladium-phosphorus alloy or palladium-phosphorus alloys additionally containing nickel, cobalt or zinc:

TABLE I

Bath composition For Electroless Deposition Of Pd-P

Alloy Preferred Compound Concentration Effective Range PdCl.sub.2 2 g/l 0.5 to 4 g/l HCl (38%) 4 ml/l 1.0 to 8 ml/l NH.sub.4 OH (28%) 160 m/l 80 to 320 ml/l NH.sub.4 Cl 27 g/l 10 to 54 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O 10 g/l 5 to 20 g/l

TABLE II

Bath Composition for Electroless Deposition of Pd-Ni-P Alloy Compound Preferred Concentration Effective Range PdCl.sub.2 2 g/l 0.5 to 4 g/l HCl (38%) 4 ml/l 1.0 to 8 ml/l NH.sub.4 OH (28%) 160 ml/l 80 to 320 ml/l NH.sub.4 Cl 27 g/l 50 to 54 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O 10 g/l 5 to 20 g/l NiSO.sub.4.sup.. 6H.sub.2 O 29.6 g/l 1 to 40 g/l

TABLE III

Bath Composition for Electroless Deposition of Pd-Co-P Alloy Compound Preferred Concentration Effective Range PdCl.sub.2 2 g/l 0.5 to 4 g/l HCl (38%) 4 ml/l 1.0 to 8 ml/l NH.sub.4 OH (28%) 160 ml/l 80 to 320 ml/l NH.sub.4 Cl 27 g/l 10 to 54 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O 10 g/l 5 to 20 g/l CoSO.sub.4.sup.. 6H.sub.2 O 29.6 g/l 1 to 40 g/l

TABLE IV

Bath Composition For Electroless Deposition of Pd-Zn-P Alloy Compound Preferred Concentration Effective Range PdCl.sub.2 2 g/l 0.5 to 4 g/l HCl (38%) 4 ml/l 1.0 to 8 ml/l NH.sub.4 OH (28%) 160 ml/l 80 to 320 ml/l NH.sub.4 Cl 27 g/l 10 to 54 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O 10 g/l 5 to 20 g/l ZnSO.sub.4.sup.. 8H.sub.2 O 36.0 g/l 1 to 40 g/l

The preferred pH of our preferred concentration baths is about 9.8 .+-. 0.2.

The bath of preferred composition (Table I) for providing electroless palladium-phosphorus alloy deposition was studied for effect of temperature on deposition rate and stability. The results are shown in FIG. 1. The deposition rate increased with increasing temperature. However, at 80.degree.C, solution decomposition ensued before the deposition test was completed and at 90.degree.C deposition tests could not be conducted because of premature decomposition. The solution may be used at 70.degree. or lower. However, deposition at 50.degree. to 60.degree.C is advisable with deposition rates of over 2.5 .mu.m/hr. (0.1 mil/hr) (3 mg/cm.sup.2 /hr).

The electroless palladium phosphorus alloy plating solution was prepared and filtered, and left in a glass stoppered bottle at room conditions (22.degree. to 28.degree.C) for seven months without evidence of solution decomposition and without reduced effectiveness of decomposition. Poor shelf life was characteristic of prior art electroless palladium plating solutions. However, during use of the palladium plating solution, entrance of particulate matter may affect solution stability by providing nuclei for catalytic reduction of palladium. Filtration into a new container is advisable if gassing is evident in the solution or on the container bottom indicating presence of catalytic nuclei.

A number of consecutive one hour deposition tests on 10 cm.sup.2 activated tantalum were conducted using a single 200 ml solution at 40.degree.C. The deposition rates were determined as shown in FIG. 2. The deposition rate decreased as the solution became depleted. After 19 consecutive one hour deposition tests, the deposition rate decreased from about 1.7 to 0.3 mg/cm.sup.2 and over 90 percent of the palladium originally present in the solution was plated out. When the palladium content was restored to the original value by addition as the amine complex in NH.sub.4 OH, the deposition rate was restored to almost the original value even without hypophosphite replenishment. Prior art palladium plating baths were drastically reduced in plating rate after very short usage and replenishment procedures were ineffective.

Hydrogen gas is evolved on the surface during electroless palladium alloy deposition as it is during electroless deposition of other metals when hypophosphite reducing agent is utilized. Of course, hydrogen gas formation represents inefficient use of reducing agent. The efficiency of hypophosphite utilization was determined by analysis of hypophosphite consumed during electroless deposition of 59.4 mg palladium phosphorus alloy at 40.degree.C. Approximately 188 mg NaH.sub.2 PO.sub.2.sup.. H.sub.2 O was consumed. This represents a utilization of efficiency of approximately 31 percent.

A 150 .mu.m electroless palladium phosphorus alloy deposit was produced by immersion in the plating solution at 40.degree.C for an extended period. The deposit was cross-sectioned and tested for microhardness which was found to be approximately 165 kg/mm.sup.2 on the Vickers scale. Microscopic examination of the deposit cross section revealed a crack pattern probably owing to stresses in the deposit.

After alkaline cleaning and immersion in 10 percent (volume) sulfuric acid at 25.degree.C, copper, brass and gold specimens were immersed into the electroless palladium phosphorus alloy plating solution at 55.degree.C. Electroless deposit coverage was achieved on copper after about 3 minutes, on brass after about 1.5 minutes and on gold after about 20 seconds. Immersion of the metal for 30 seconds in 0.1 g/l PdCl.sub.2 --0.5 ml/l HCl (38% ) at 25.degree.C, and rinsing prior to immersion in the electroless palladium phosphorus alloy plating solution, resulted in deposit coverage on all of the metals within about 20 seconds. Electroless palladium phosphorus alloy was also spontaneously deposited on steel or electroless nickel plated steel shortly after immersion in the plating solution.

The tests described above were repeated except that the hypophosphite reducing agent was omitted from the plating solution. No visible deposits were produced on any of the metal surfaces. It is thus evident that the aforementioned deposits were produced by a truly electroless mechanism rather than by electrochemical displacement.

Glass or plastics, activated by stannous chloride and palladium chloride immersions are readily coated with palladium phosphorus alloy from the electroless plating solution.

Production of electroless palladium phosphorus alloys with nickel, cobalt or zinc are presented below.

The possibility of producing the ternary electroless palladium alloys was determined by addition of metal salts to the electroless palladium phosphorous alloy plating solution. Deposits were produced on activated tantalum over a period of 5 hours at 60.degree.C. The deposits were analyzed for constituent elements by wet chemical procedures. The results are shown in Table V below:

TABLE V

ELECTROLESS PALLADIUM ALLOY DEPOSITION

Metal Salt Addition to Deposit Composition, % by Weight Preferred Bath Salt Conc, g/l Metal Phosphorus Palladium NONE 1.52 Balance NiSO.sub.4.sup.. 6H.sub.2 O 29.6 (Nickel) 2.68 Balance 5.99 CoSo.sub.4.sup.. 6H.sub.2 O 29.6 (Cobalt) 2.77 Balance 9.82 Na.sub.2 WO.sub.4.sup.. 2H.sub.2 O 37.2 (Tungsten) 2.69 Balance 0.00 ZnSO.sub.4.sup.. 8H.sub.2 O 36.0 (Zinc) 1.24 61.43 35.99 KReO.sub.4 3.3 (Rhenium) 2.56 Balance Trace

Both nickel and cobalt are capable of independent electroless deposition and were employed in the electroless palladium solutions at 10 times the molar concentration of palladium salt, yet less than 10% cobalt or 6 % nickel was codeposited with palladium. The preferential deposition of palladium is thus evident. However, when zinc sulfate was added to the electroless palladium bath, about 36% zinc was produced in deposits. The palladium content was directly analyzed to be 61.43 percent indicating that the zinc is present in elemental form since the metallic elements account for virtually the entire deposit. The induced chemical reduction of zinc with palladium is significant since the high electronegativity of zinc would be expected to preclude the possibility of alloy formation with palladium. Palladium alloys with tungsten or rhenium were not produced from the electroless palladium solution to which tungstate or perrhenate was added.

It is recognized that by reducing the palladium ion concentration in our alloy plating solution, an increase of the concentration of alloying element in the deposit would be expected.

We wish it to be understood that we do not desire to be limited to the exact details shown and described, for obvious modifications will occur to a person skilled in the art.

Claims

1. A bath composition for the electroless deposition of alloys of palladium phosphorous wherein palladium predominates, said alloy including a minor amount of a member of the group consisting of nickel, cobalt, or zinc, said bath comprising:

0.5 to 4 g/l PdCl.sub.2,

1.0 to 8 ml/l 38% HCl,

80 to 320 ml/l 28% NH.sub.4 OH,

10 to 54 g/l NH.sub.4 Cl, and

5 to 20 g/l NaH.sub.2 PO.sub.2.sup.. H.sub.2 O,

said bath composition including a member of the group consisting of NiSO.sub.4.sup.. 6H.sub.2 O, CoSO.sub.4.sup.. 6H.sub.2 O, and ZnSO.sub.4.sup.. 8H.sub.2 O for forming an alloy of Pd-Ni-P, Pd-Co-P, and Pd-Zn-P respectively.

2. The composition as described in claim 1 wherein said PdCl.sub.2 is present in an amount of 2 g/l, said HCl is present in an amount of 4 ml/l, said NH.sub.4 OH is present in an amount of 160 ml/l, said NH.sub.4 Cl is present in an amount of 27 g/l, and said NaH.sub.2 PO.sub.2.sup.. H.sub.2 O is present in an amount of 10 g/l.

3. The bath composition as described in claim 1 wherein said NiSO.sub.4.sup.. 6H.sub.2 O is present in an amount between about 1 to 40 g/l.

4. The bath composition as described in claim 1 wherein said CoSO.sub.4.sup.. 6H.sub.2 O is present in an amount between about 1 to 40 g/l.

5. The bath composition as described in claim 1 wherein said ZnSO.sub.4.sup.. 8H.sub.2 O is present in an amount between about 1 to 40 g/l.

6. The bath composition as described in claim 1 wherein said NiSO.sub.4.sup.. 6H.sub.2 O is present in an amount of 29.6 g/l.

7. The bath composition as described in claim 1 wherein said CoSO.sub.4.sup.. 6H.sub.2 O is present in an amount of 29.6 g/l.

8. The bath composition as described in claim 1 wherein said ZnSO.sub.4.sup.. 8H.sub.2 O is present in an amount of 36.0 g/l.