PALLADIUM ELECTROPLATING PROCESS

Abstract: A palladium electroplating process in which the source of palladium is an organic ammine or ammonia complexed palladium hydroxide. Particularly useful is palladium diammine hydroxine. Palladium electroplating processes using such sources of palladium yield excellent plated films of palladium even at high plating rates without the danger of incorporating hydrogen from the plating process.

Description: PALLADIUM ELECTROPLATING PROCESS Technical Field The invention is a palladium electroplating bath with a unique source of palladium. Background of the Invention Noble metals, particularly gold, have been used extensively in the electronics industry both for high quality electric contacts and for high quality electrical conducting paths. The high price of gold and the frequent variations in the price of gold has made it desirable to consider other noble metals as a substitute for gold. Palladium appears to be a suitable alternative to gold in many applications in the electronics industry. Palladium has two main economic advantages over gold. First, its cost per unit weight is much less than gold and second its density is much less than gold. Thus, substitution of palladium for gold in electrical components would lead to considerable cost savings. A particular disadvantage of palladium in the past has been the difficulty in electrodepositing the palladium to produce a metal film with suitable physical properties. Because of the economic advantages of palladium, it is highly desirable to obtain a palladium electroplating process in which palladium can be electroplated at rates of commercial interest and reliably will yield palladium deposits with suitable properties for use in the electronics industry. Summary of the Invention The invention is a palladium or palladium alloy electroplating process in which the source of palladium in the aqueous bath is a palladium hydroxide complex. Organic amines with up to five carbon atoms are very useful. Ammonia is preferred as the complexing agent. Most preferred as the source of palladium is the dimeric compound di-u-hydroxo-bis-[cis-diaminepalladium (II)] dihydroxide. Such a palladium electroplating process yields excellent palladium deposits even when plating takes place at extremely high rates, A large variety of complexing agents can be used provided they complex with palladium in the palladium hydroxide complex. Typical complexing agents are ammonia, organic amines (monoamines and polyamines) with up to 30 carbon atoms. Oxylates are also useful as complexing agents. Brief Description of the Drawing FIG. 1 shows a voltammogram on coordinates of current versus potential of a palladium diammine hydroxide in aqueous solution; and FIG. 2 shows a typical electroplating apparatus useful in the practice of the invention. Detailed Description The invention involves the use of certain palladium hydroxide complexes as the source of palladium in palladium or palladium alloy electroplating processes. A typical complexed palladium hyroxide is palladium diammine dihydroxide believed to have the structural formula shown below: with the chemical name di-y-hydroxo-bis-[cis-diammine palladium (II)] dihydroxide with nominal formula [P ( H3) 2 (OH) ] 2 (OH) 2 . The invention is based on the observation that certain palladium hydroxide complexes have unusually good properties as a source of palladium in palladium electroplating procedures. A great variety of complexing agents may be used including ammonia, organic amines (including organic polyamines), etc. The principal advantage of the palladium hydroxide complexes involves their electrochemical properties. It is found that the reduction potential of these complexes in water is much greater (lesser on a negative scale) than usually found for palladium species in aqueous solution so that the reduction potential for palladium is far removed from the reduction potential for water to form hydrogen. Indeed, the palladium reduction potential for the palladium diammine hydroxide is so far removed from the hydrogen potential that under the usual plating conditions hydrogen will actually oxidize at the potential at which palladium metal electroplates from a palladium hydroxide complex solution. This is illustrated in the voltammogram shown in FIG. 1. In such an experiment the potential on a cathode is varied and the current measured. The cathode is immersed in a plating solution which contains [Pd(NH 3 ) 2 (OH)] 2 (OH) 2 and phosphate ion. The concentration of the two species were 6.2 mM in terms of palladium metal and 1.0 M for the phosphate ion. The pH was 11.8 (measured at 25 degrees C) and the measurements were carried out at about 70 degrees C. The curve labeled 1 in FIG. 1 resulted from decreasing the potential (voltage measured on the SCE scale) from 0.0 in the negative direction. Initially, little current is passed until at about -0.6 volts (point 2) at which potential, current is drawn into the cathode. This represents the potentials at which palladium electroplates (is reduced) at the cathode. Further reduction in potential leads to evolution of hydrogen (reduction of hydrogen ions to hydrogen at point 3). As can be seen, the hydrogen reduction potential (about -0.92) is well separated from the palladium reduction potential (about -0.58). On increasing the potential from about -1.2 volts, a dip occurs at about -0.65 (labeled 4 in FIG. 1). This is the oxidation potential for hydrogen and occurs at a more negative potential than the electroplating potential for palladium. This insures the absence of hydrogen in the electroplated palladium. There are additional advantages to the use of the palladium amine hydroxide complex as a source of palladium in palladium electroplating processes. For example, the complex has high solubility in water (about 350 gms/liter in terms of palladium metal at room temperature) so that high concentrations may be maintained without the danger of precipitates forming during the plating procedure. Also, the plating operation can be carried out at a high solution pH without the danger of precipitating insoluble palladium compounds such as palladium oxide. This is particularly advantageous because high pH is often preferred in palladium electroplating procedures. Other palladium bath chemistries often result in precipitation of palladium compounds such as palladium oxide in highly alkaline solutions. A large variety of palladium hydroxide complexes may be used in the practice of the inventions. Generally, any complexing agent may be used provided it yields a complex like that described above as palladium diammine hydroxide. In other words, other complexing agents can be substituted for ammonia in the palladium diammine dihydroxide including organic amines (monoamines and polyamines) with up to 30 carbon atoms. Typical organic monoamines may be primary, secondary or tertiary amines with linear or branched chains with substituted or unsubstituted groups. Both alkyl and aryl groups may be present. Typical substituents aside from hydrogen are halide atoms, oxygen, nitrogen, sulfur, arsenic and phosphorous atoms, etc. Generally, relatively simple complexing substances are preferred with up to 10 carbon atoms since these are relatively easily available and have reasonable solubility. Other complexing agents such as oxylates might also be useful in the practice of the invention. The various palladium hydroxide complexes are usually made by adding the particular complexing agent to a solution of palladium diammine dihydroxide. A particular advantage of various complexes is that the electrochemical properties of the bath can be changed to fit a particular plating problem. For example, the potential for electroplating palladium can be increased or decreased by suitable addition of complexing agent. Alloy plating may also be carried out using the present procedure. Typical elements are silver, copper, nickel, cobalt, iron, gold, chromium, manganese, ruthenium, platinum and iridium. Particularly useful are copper, nickel and silver. Preferred are alloys comprising at least 10 mole percent palladium, remainder copper, silver and/or nickel. Other useful alloys are 60 mole percent palladium, remainder silver, copper and/or nickel, etc. The palladium-silver alloys are particularly useful, especially for electrical contact surfaces. A particular advantage in the palladium hydroxide complex chemistry is the limitation in the amount and variety of ions in the bath. Some counter ions (anions and cations) are often introduced as for example to adjust the pH of the bath or buffer agents to maintain the pH of the bath. Also, initial synthesis of the palladium hydroxide complex might involve introduction of limited amounts of counter ions. Generally, such counter ions should be stable (chemically and electrochemically) and in particular not subject to oxidation or reduction under conditions- of the electroplating process. In addition, the anion should not interfere with the plating process by either chemical attack on the surface being plated or on the metal complex system. Typical anions are halides, nitrate, sulfate and phosphates. Also, certain ions, including those set forth above, may be used as supporting electrolyte to increase conductivity of the electroplating bath. The cation used for the supporting electrolyte may be any soluble ion which does not interfere with the electroplating process. Alkali-metal ions (Na, K, Li) are particularly preferred because of solubility and stability. It is pointed out that in most instances, particularly where high-speed plating is contemplated, the concentration of palladium complex would be sufficiently high to provide high conductivity of the electroplating solution. Thus a supporting electrolyte is not usually necessary. Also, for many purposes, a bath free of anions other than hydroxide ions is highly advantageous. It is generally contemplated that the palladium hydroxide complex is either made up in place in the bath or added either in solid form or concentrated solution. The palladium a ine hydroxide complex is made in place by adding ammonia to a solution of Pd(NH3) 2 Cl 2 (or similar starting material) and replacing the chloride ion with hydroxide ion either by hyroxide ion exchange or chemical means. The plating process may be carried out under a large variety of conditions. Generally, an alkaline pH is preferred, with pH from 7.5 to 14.0 preferred and 11.0 to 13.5 most preferred. High pH generally permits more rapid plating without evolving hydrogen and less chance of substrate degradation (especially with copper). For certain palladium alloys, lower pH values might be preferred. For example, for palladium-nickel, pH values from 8 to 11 are preferred. The bath temperature may vary over large limits, typically from the freezing point to the boiling point of the electroplating bath. Often, the preferred plating temperature range depends on bath composition and concentration, plating cell design, pH and plating rate. Preferred temperatures for typical conditions are from room temperature to about 80 degrees C with 45 to 65 degrees C most preferred. It should be understood that for some purposes the convenience of ambiant temperatures might offset any small advantage to elevated temperatures. Concentration ranges may vary over large limits, generally from about 0.001 molar to saturation (about 3.5 molar), with from 0.05 to 2.5 molar typical. High concentrations are preferred because it permits higher plating rates but loss of palladium from drag out is more severe at higher palladium concentrations. Preferred concentration ranges often depend on plating speed. For example, for high-speed plating [above 5.38A/sq. dm (50 ASF)] preferred molar concentration range varies from 0.1 to 1.0 molar ? for low-speed plating, the preferred range is* from 0.05 to 0.2 molar. The plating process may be carried out with or without a buffer system. A buffer system is often preferred because it maintains constant pH and adds to the conductivity of the bath. Typical buffer systems are the phosphate system, borate system, citrates, ammonia, etc. Preferred is the HP0 - 4 2/P0- 4 3 system often made by adding an alkali-metal hydroxide (KOH, NaOH, etc.) to an aqueous solution of the hydrogen phosphate ion. Generally, the concentration of buffer varies from about 0.1 molar to 2 molar (about 0.5 ^ 0.2 molar preferred) and the mole ratio of hydrogen phosphate to phosphate varies from 5/1 to 1/5 (with equal mole amounts within _+ 50 percent preferred) . These mole ratios often depend on the particular pH desired for the plating bath. The phosphate system is preferred because it is stable under the conditions of the plating process and tends to stabilize the palladium complex without the use of excess ammonia. Excess ammonia is often to be avoided because of possible chemical attack on the surface to be plated (e.g., copper surfaces). Various surfaces may be plated using the disclosed process. Usually, the plating would be carried out on a metal surface or alloy surface, but any conducting surface would appear sufficient. Also, electrolessly plated surfaces may be useful. Typical metal and alloy surfaces are copper, nickel, gold, platinum, palladium (as, for example, a surface electrolessly plated with palladium and then electroplated with palladium in accordance with the invention). Various alloy surfaces may also be used such as copper-nickel-tin alloys and berylium- copper alloys. The amount of complexing agent in the solution presents special problems in the practice of the invention. Excess complexing agent (beyond that needed to complex the palladium present) is generally preferred because it stabilizes the palladium complex, insures continued solubility of the palladium, etc. For some complexing agents (e.g., NH 3 ), the presence of free complexing agent in the solution should, in some cases, be minimized because it might chemically attack the surface being plated. For example, with the plating of a copper surface, free ammonia might react with the copper surface and free ammonia in the bath should be minimized. Often, the palladium complex can be stabilized in other ways including the addition of phosphate buffer. Where excess complexing agent is used, the mole concentration range is from 2 to 18 times the mole concentration of palladium ion. Most preferred is about 6 times the mole concentration of palladium. A number of experiments are carried out to demonstrate the efficiency of the disclosed process. The bath is made up using palladium diammine hydroxide as the source of palladium. The concentration of palladium is 0.1 molar. Phosphate buffer is used with one molar K 2 HP0 4 and sufficient KOH to make the pH about 11.5. The temperature is maintained between 50 and 65 degrees C and the experiments carried out in an electroplating cell with high bath agitation. Current passes through anode, electroplating bath and cathode. The electrical energy is supplied by a conventional power supply. The current density is 21.53A/sq. dm (200 ASF). Typical thicknesses in these experiments are 30 to 170 microniches. The deposit is crack free as determined by a scanning electron micrograph at 10,000 magnification. Both adherence and ductility are excellent. Similar results are obtained using 0.1 molar and 2.0 molar. Plating rate is often determined by the thickness desired after a predetermined period of plating. For example, in a strip line plating apparatus (see, for example, U.S. Patent No. 4,153,523 issued to D. E. Koontz and D. R. Turner on May 8, 1979 and U.S. Patent No. 4,230,538 issued to D. R. Turner on October 28, 1980) the strip line being plated is exposed to the plating solution for a set period of time (depending on the speed the strip, is moving down the line and the length of the plating cell) and the plating rate is adjusted to give the desired thickness in this period of time. Experiments are carried out at other plating speeds, for example 1.08A/sq. dm (10 ASF) and 69.97A/sq. dm (650 ASF) with excellent results. Other complexing agents are also used in the practice of the invention. Usually, these complexing agents are added to the bath so as to displace ammonia. The following complexing agents are useful: ethylamine, ethylenediamine, diethylenetriamine, 1, 6 hexadiamine, 1, 4 butadiamine, 1 , 4 diaminobutane, 1 , 3 diaminopropane and N,N dimethyl -1, 3 - propane diamine. Each complexing agent yields excellent results even at high plating rates. Excellent results are also obtained with palladium alloy plating. Generally, the alloying element is introduced by replacement of part of the palladium in the palladium hydroxide complex. Use of a different complex for the alloying element is also useful. The apparatus described in the above-cited patents are particularly advantageous for carrying out the process. They permit good control of the bath conditions, the rate of plating and permit rapid palladium plating. The palladium plating process is highly advantageous for plating electrical contact pins for electrical connectors such as described in the above references. FIG. 2 shows apparatus W. use f u l i- n fcne practice of the invention. The surface to be plated 11 is made the cathode in the electrolytic process. The anode 12 is conveniently made of platinized titanium or may be made of various other materials such as oxides of platinum group metals, binder metal oxides, etc. Both anode and cathode are at least partially immersed in the electroplating bath 13 containing source of palladium such as palladium diammine hydroxide. A container 14 is used to hold the palladium plating solution and the anode 12 and cathode 11 are electrically connected to source of electrical energy 15. An ammeter 16 and voltmeter 17 are used to monitor current and voltage. The voltage and current are controlled inside the source of electrical energy 15.

Claims: Claims 1. A process for electroplating a metallic substance on a surface, said metallic substance comprising palladium, comprising the step of passing current through a cathode, an electroplating bath and an anode with cathode potential great enough to electroplate palladium, said electroplating bath having conductivity greater than 10 ♦ mho-cm and pH greater than 7, characterized in that the electroplating bath comprises palladium hydroxide complex. 2. The process according to claim 1 CHARACTERIZED IN THAT the complexing agent is an organic amine with up to 30 carbon atoms. 3. The process according to claim 2, CHARACTERIZED IN THAT the complexing agent has up to 10 carbon atoms. 4. The process according to claim 1, CHARACTERIZED IN THAT the complexing agent is ammonia. 5. The process according to claim 4 CHARACTERIZED IN THAT the palladium hydroxide complex is palladium diammine dihydroxide. 6. The method according to claim 1, CHARACTERIZED IN THAT said complex is the same as results from reacting palladium diammine dihydroxide with a complexing agent, said complexing agent comprising at least one compound selected from an organic ammine with up to 30 carbon atoms. 7. The process according to claim 5 or 6, CHARACTERIZED IN THAT the palladium hydroxide complex is di-μ- hydroxo-bis-[cis-diammine palladium (II)]. 8. The process according to claim 1, CHARACTERIZED IN THAT the electroplating bath comprises excess complexing agents. 9. The process according to claim 8 CHARACTERIZED IN THAT • the mole concentration of complexing agent is from 2 to 18 times the mole concentration of palladium ion. 10. The process of claim 1 the metallic substance consists essentially of at least 10 mole percent palladium, remainder being at least one of silver, copper, and nickel. 11. The process according to claim 1 , CHARACTERIZED IN THAT the metallic substance consists essentially of palladium. 12. The process according claim 1, CHARACTERIZED IN THAT the pH varies from 7.5 to 14.0, preferably from 11.0 to 13.5. 13. The process according to claim 1, CHARACTERIZED IN THAT the electroplating bath comprises a buffer comprising at least one of hydrogen phosphate ion and phosphate ion. 14. The process according to claim 7, CHARACTERIZED IN THAT the buffer concentration varies from 0.2 to 2 molar and the ratio of hydrogen phosphate to phosphate ion is from 5/1 to 1/5. 15. The process according to claim 1, CHARACTERIZED IN THAT the palladium concentration is from 0.001 molar to saturation, preferably from 0.05 to 2.5 molar. 16. The process according to claim 1, CHARACTERIZED IN THAT the palladium in the electroplating bath is replenished by the addition of a source of palladium. 17. The process according to claim 16, CHARACTERIZED IN THAT the source of palladium is palladium diammine 5 dihydroxide. 18. The process according to claim 1, CHARACTERIZED IN THAT the plating current density is up to 50 ASF. 19. The process according to claim 1, 0 CHARACTERIZED IN THAT the plating current density is between 50 and 1000 ASF. 20. The process according to claim 1, CHARACTERIZED IN THAT 5 the electroplating process is carried out at a temperature between room temperature 80 degrees C, preferably betweeen 45 and 65 degrees C. 0 5 0 5

Inventors: ABYS ANTHONY JOSEPH, ABYS, ANTHONY, JOSEPH
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE & TELEGRAPH COMPANY
Application year: 1985
Main classification: C25D3
IPC codes: C25D3