The invention relates to autocatalytic electroless depositing compositions, processes for use of the same, autocatalytic alloy deposits formed therefrom, substrates coated with said electroless alloys and optionally overcoated with an electroless metal and to a means for transforming a hypophosphite solution that is not autocatalytic to one that is autocatalytic.
2. Description of the Prior Art
Electroless metal deposition refers to the plating of a metal from solution over a suitable surface by chemical action in the absence of an external electric current. Processes and compositions useful therefor are known in the prior art and described in numerous publications.
There are several methods by which metal may be deposited over a substrate without the use of an external source of electricity. One method, known as immersion or displacement plating, depends upon a galvanic displacement between a metal in solution and a metal comprising a substrate to be plated. For example, tin is plated over copper from solution by the tin displacing the copper on the surface of the substrate plated. Plating continues for as long as copper is available to be displaced by the tin. Once the copper is completely coated by the tin, and copper is no longer available for displacement, plating stops or slows to the point where there is only an infinitessimal increase in deposit thickness with time. Deposits having a thickness of from about 1 to 100 millionths or more of an inch are obtainable by displacement plating, depending upon the porosity of the deposit and the displacement potential between the metal of the substrate and the metal to be displaced.
Plating by chemical reduction is another electroless method for depositing metal over a substrate. Plating takes place by the action of a reducing agent on dissolved metal in the presence of a catalytic surface. The catalytic surface may be a metal substrate or a catalyzed non-metallic substrate. Plating continues for so long as catalytic surface is present to initiate or cause the plating reaction to continue.
Solutions for plating by chemical reduction are either autocatalytic or they are not autocatalytic. Those solutions that are not autocatalytic provide a metal plate which is not catalytic to continued plating. Therefore, once the initial catalytic surface is covered by the metal plate, the plating reaction stops or slows to the point where there is only an infinitessimal increase in deposit thickness with time. Metal plated from a solution that is not autocatalytic is often referred to as a "flash plate" and typically, the thickness of such deposits vary between about 1 and 30 millionths of an inch.
Autocatalytic plating solutions are solutions that deposit a metal plate which itself is catalytic to continued deposition. Consequently, theoretically, metal may be deposited continuously so long as the composition of the plating solution is maintained and deposits of considerable thickness are obtainable. Autocatalytic plating solutions typically plate at a rate of from about 30 to 500 millionths of an inch per hour and the desired deposit thickness is dependent upon the use of the metal plate.
The invention described below is an autocatalytic plating solution for alloys of copper, nickel and/or cobalt and phosphorus. The discussion which follows will therefore be directed to autocatalytic plating solutions unless otherwise stated. Such solutions will be referred to generically as electroless plating solutions as in common in the art.
The two metals most commonly plated by chemical reduction from electroless solutions are nickel-phosphorus alloys and copper. Each has advantages and disadvantages which limit its utility to specific applications. Electroless nickel is most frequently used for metal finishing and plating on plastics where economy is a concern, while electroless copper is most frequently used in the electronics industry, such as in the manufacture of printed circuit boards and semiconductors, and in those automotive plating on plastics applications where electroless nickel is not satisfactory.
Electroless processes for plating nickel phosphorus alloys and compositions useful therefor are described in numerous patents, for example, U.S. Pat. Nos. 2,690,401; 2,690,402; 2,762,723; 2,935,425; 2,929,742; 3,062,666; 3,338,726; 4,042,382 and 4,167,416, all included herein by reference.
Known autocatalytic electroless solutions for plating nickel-phosphorus alloys generally comprise a reaction mixture consisting of at least four ingredients dissolved in a solvent, typically water. They are (1) a source of nickel ions, (2) a hypophosphite compound as a reducing agent, (3) an acid or hydroxide pH adjustor to provide required pH and (4) a complexing agent for metal ions sufficient to prevent their precipitation from solution. Many electroless nickel solutions are described in the above-noted patents. Hypophosphite as the reducing agent is the source of phosphorus in the alloy. Electroless nickel-boron alloys can be formed using a boron compound as a reducing agent such as dimethyl amine borane but most plating solutions use hypophosphite as the reducing agent because it is substantially less expensive and easier to use than the boron compounds.
Autocatalytic electroless copper plating solutions and processes for their use are also well known in the prior art and described in numerous patents including U.S. Pat. Nos. 3,046,159; 3,310,430; 4,118,234; 4,143,186; 4,167,601; and 4,171,225, all incorporated herein by reference. Known electroless copper deposition solutions also generally comprise at least four ingredients dissolved in a solvent, typically water. They are (1) a source of copper ions, (2) a reducing agent such as formaldehyde or a boron compound such as a borohydride or an amine borane, (3) a pH adjustor to provide required pH and (4) a complexing agent for the copper ions sufficient to prevent their precipitation from solution. Many electroless copper formulations are described in the above-noted patents. The most frequently used copper plating solutions use formaldehyde as the reducing agent.
As noted above, the properties of electroless nickel solutions and the deposits plated therefrom make electroless nickel useful for plating on plastic applications where the ease of operation is important.
Electroless nickel solutions using hypophosphite as the reducing agent are easier to use than copper plating solutions for many reasons. The solutions are usually more stable than electroless copper solutions and consequently, have s substantially greater tolerance for contamination as a consequence of drag-in during the conventional plating sequence. Therefore, such solutions are not likely to undergo spontaneous decomposition resulting in loss of the plating solution. In addition, hypophosphite does not undergo undesirable side reactions as does formaldehyde in alkaline electroless copper solutions. Therefore, there is minimal need for expensive equipment to monitor the concentration of components in solution as is frequently required for electroless copper plating solutions. Moreover, hypophosphite is not considered toxic. Consequently, nickel is frequently preferred for use as a base for subsequent electroplating.
Nickel as a conductive coating for electroplating has advantages in addition to low cost. Nickel deposited from electroless nickel solution is not readily burned off by current passing through it during electroplating and the nickel deposit is not readily etched by electroplating solutions. Consequently, nickel is suitable for many plating on plastic operations. Also, electroless nickel has less tendency to plate onto plastisol coated racks, thereby making it especially desirable for straight through processing.
There are disadvantages to the use of nickel as a base metal for electroplating that limits its use for many applications. Nickel, though an electrical conductor, is a poor conductor for electrolytic overcoating. Consequently, large and/or irregularly shaped parts are not readily electroplated over nickel as a base metal and spread of the electroplated deposit from solution is slow, often causing substandard adhesion of the electroplated metal. Nickel deposits, upon standing, tend to passivate, thereby causing plating problems, such as poor adhesion. Further, because nickel is superior to copper with respect to etching, this may be a disadvantage because plated parts that are rejected can not be easily reworked. Thermal cycling is also a problem with nickel as a base metal over plastic because corrosion penetration undercuts adhesion to the plastic. Hence, when temperature extremes are encountered, particularly in automotive parts, delamination of metal from the plastic frequently occurs. This is a serious drawback to the use of nickel for applications where exposure to outside weather extremes is a requirement. Finally, most electroless nickel solutions that are used for plating on plastics contain ammonia, thereby creating waste treatment problems.
It is known that copper has some properties which make it superior to nickel as a base for electroplating for many plating on plastics operations. However, the majority of the electroless copper plating solutions used commercially contain formaldehyde as a reducing agent thereby causing many difficulties in operation and performance. Formaldehyde in alkaline plating solutions is relatively unstable and consequently more difficult to control than hypophosphite in nickel solution. A major reason for this is the volatility of formaldehyde coupled with the known Cannizarro side reaction that takes place in the alkaline bath which combine to consume formaldehyde not consumed by the primary plating reaction. The result is a constantly changing, frequently unpredictable, formaldehyde concentration in solution that required careful monitoring, frequently with expensive monitoring equipment. In addition, the degradation of formaldehyde often is accompanied by a loss of hydroxide. Failure to monitor the concentration of components in a copper plating solution may result in spontaneous decomposition of the solution and deposits of varying quality. Finally, formaldehyde is toxic and highly volatile and its fumes are very irritating. Consequently, substantially greater care is now needed in the use and waste treatment of formaldehyde containing solutions.
Notwithstanding the problems associated with copper plating solutions using formaldehyde as the reducing agent, such solutions are used in significant quantity for plating on plastic operations in the automotive industry because of the superior performance of copper as a base metal for subsequent electroplating. The reasons are better conductivity of copper relative to nickel which permits plating of large and/or irregularly shaped parts, and the ability of parts plated with copper, then subsequently electroplated, to withstand thermal cycling.
The above discussion has been limited to the plating on plastics industry. In the electronics industry, nickel-phosphorus alloy, because of relatively poor conductivity, has found little or no application, notwithstanding the advantages associated with the use of nickel plating solutions. Instead, copper is the metal used for the formation of conductors over various substrates. Virtually all copper used in the electronics industry is plated from formaldehyde based solutions.
Based upon the above, electroless alloy solutions of the two metals would appear to be desirable and attempts to plate such alloys are described in the prior art.
One electroless solution used for plating alloys of nickel and copper over plastics is disclosed in U.S. Pat. Nos. 3,547,784 and 3,692,502, both incorporated herein by reference. The solution disclosed in each of these patents is essentially a formaldehyde copper plating solution to which a nickel salt is added in a weight ratio of approximatey 4 parts copper to 1 part nickel. Since the solution utilizes formaldehyde as the reducing agent, it is subject to the same disadvantages as the conventional formaldehyde copper baths described above.
Another patent disclosing alloy solutions of copper and nickel is U.S. Pat. No. 3,935,013. This patent teaches that alloys of from 9 to 98 mole percent copper and from 2 to 91 mole percent nickel can be obtained by mixing salts of nickel and copper together in a solution containing a reducing agent, complexing agent and a pH adjustor. The reducing agent is disclosed in Column 2, line 62 to Column 3, line 5 as any reducing agent which reduces both copper and nickel ions. Borane reducing agents are disclosed in the working examples but others, including hypophosphite, are disclosed in the above-cited portion of the patent. However, it is believed that if hypophosphite were substituted for the amine-boranes of the examples, the solutions would not be autocatalytic, particularly those solutions having a copper content in excess of several percent by weight copper.
In U.S. Pat. No. 3,832,168, hypophosphite plating solutions are disclosed for deposition of nickel and copper alloys. Though it is not explicitly disclosed in the patent, the patentees teach that codeposited copper in excess of 25 percent by weight was not obtainable. No solutions are taught having a copper content of at least 5 percent by weight.
An effort to obtain an alloy of copper and nickel from a hypophosphite plating solution is reported by Aoki, Takano and Ishibashi in the Journal of The Metal Finishing Society, Japan, Volume 30, No. 3, 1979. The authors report that they were able to obtain deposits containing up to 69 percent by weight copper from a plating solution containing 0.085 moles of nickel sulfate and 0.015 moles of copper sulfate using hypophosphite as a reducing agent when the pH of the solution was adjusted to about 10 with sodium hydroxide (though it should be noted that there is some inconsistency in the publication as from time to time, the 0.015 moles of copper sulfate is written as 0.15 moles). The authors appear to have been unsuccessful in obtaining a higher concentration of codeposited copper because of an inability to obtain an operative plating solution containing more than 20 mole percent copper, the higher concentrations of copper apparently acting to depress or inhibit plating. Moreover, to obtain deposition, the authors found it necessary to induce plating electrolytically. It is believed that the deposits obtained from these solutions were not autocatalytic and deposition was extremely slow.
Another publication relating to the formation of copper-nickel-phosphorus alloys is East German published patent application Ser. No. 109,669, bearing a publication date of Nov. 12, 1974. This publication discloses electroless solutions of nickel sulfate, copper sulfate, citric acid, sodium hypophosphite and in one example, sodium tetraborate. The author suggests that the concentration of citric acid relative to the concentration of the copper and nickel salt is important and that increasing the concentration of citric acid requires an increase in the concentration of the nickel salt relative to the copper salt. This is illustrated in Examples 1 and 2 where a doubling of the citric acid concentration necessitated an increase in the ratio of the nickel salt relative to the copper salt of from 50 to 100 percent in order to obtain a homogeneous deposit. It is believed that the publication fails to disclose any autocatalytic formulation having a copper salt concentration at least equal to the nickel salt concentration. Sodium tetraborate is disclosed as an optional ingredient in several of the examples. Its function is not stated, but it is believed that it acts as a buffer.
A recent patent directed to efforts to plate copper from solutions using a hypophosphite compound as a reducing agent is U.S. Pat. No. 4,209,331. The patent teaches that plating is achieved by optimizing pH for any given complexing agent. In several examples within the patent, nickel salt is added to the plating solution to determine if it promotes deposition, but it is concluded by the patentee that the nickel provides little or no benefit. A careful review of the patent establishes that only thin deposits are obtained before plating stops or slows to an infinitessimally small plating rate. Consequently, the plating solutions of this patent are believed to be non-autocatalytic electroless solutions.
A more recent patent, assigned to the same assignee as the aforesaid patent, is U.S. Pat. No. 4,265,943, incorporated herein by reference. This patent is directed to copper plating solutions using hypophosphite as a reducing agent. The patent teaches that maintaining the pH of the plating solution at a pH preferably within the range of 11 to 14 while adding a minor amount of nickel or cobalt ions permits plating of copper at a substantially constant rate. In all of the examples withint the patent, the plating solutions have a pH in excess of 11 and a second metal ion concentration less than 10 mole percent.