US 2923671 A
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U ed States Pat COPPER ELECTRODEPOSITION PROCESS AND ANODE FOR USE IN SAME Gerald C. Van Tilburg," Port Chester, N.Y., assignor to American Metal'Climax, Inc., New York, N.Y., a corporation of New York r.
No Drawing. Application'March 19, 1957 Serial N0- 646,985
4 Claims. (11. 204-52 formation and further resulting in the formation of an improved copper deposit on the cathode.
In copper electrodeposition processes commonly re- 1 ferred to as copper electroplating, the problem of roughness of the. cathode electrodeposit i.e. the copper deposit on the cathode is of extreme importance. Although the use of various addition reagents to the bath composition has proven helpful for improving the properties and characteristics of the copper deposit on the cathode, this approach has not proved entirely satisfactory as a solutio of the problem. Y
-A major cause of rough cathodic deposits is considered to be the formation and presence of anode sludge which occurs during the course of the electrodeposition process. The formation of such sludge is thought to be due to a combination of factors including electrochemical action at the face of the anode, and a preferential attack by the electrolyte solutions on the boundaries between the crystals of the copper constituting the structure of the anode. Such attack has the result that loose particles of the anode are detached from the body of the anode, particularly when the crystals are small, and are carried into the electrolyte forming a sludge which settles mostly in the bottom of the tank. During the process, however, some of these loose particles aremechanically carried to the cathode where they produce a deleterious rough plate upon the cathode. Another effect of such intercrystalline attack is an uneven anode dissolution which results in excessive losses of copper in terms of the anode remmant which must then beprematurely discarded.
Several methods have been suggested to overcome the undesirable effects of such sludgingor to reduce the formation of anode sludges Oneproposal involves bagging of the anode whereby the sludge is caught in the bags. Others have proposed the addition of elements to copper anodes made of ordinary that is, oxygen bearing or tough pitch copper whereby chemical films are formed on the surfaces of the anode to inhibit sludge formation.-
certain advantages in some instances, such anodes have generally'been found to possess limited applicationsw'ith respect to the bath compositions suitable for use therewith.
In order to obtain more uniform anode dissolution, there is evidence to indicate that dissolution should occur through the metal crystal rather thanat the crystal boundary; For best results,'it becomesjaccording ly apparent that the consumable copperanode should possess a substantially uniformly dense crystal structure as well as clean crystal boundaries. The need for these requirements are obvious since non-uniform" density'of thefcopper would incur'more rapid dissolution through the less dense portion resulting in unevenoverall -dissolution,of the anode. "Then too," the presence oroxygen "0'1'" oxides and other impurities in tli'e'grain 'boundary'whichf are .usually more soluble than'copper lead to grain boundary corrosion which ultimately causes increased sludgeflformation.
In view of the above, the improved platingjcharacteristics and more efficient utilization of the anode made possible by the more recently increasing use of'high purity, oxygen-free, high conductivity copper anodes in conventional acid type and alkaline cyanide electrodeposition processes are notsurpri'singa'since such anodes are characterized by (1) higher purity, (2) more uniform and increased de nsity and (3) the absence in 'the copper crystal boundaries of oxygen or oxygen in combination witha chemical deoxidant'asis'normally found in anodes/composed of tough' pitch of chemically deoxidize'd copper. i n x In accordance with the' foregoing, it will be seen that a single, pure, dense crystal of copper would be ideal for the purpose of electroplating. Since this is presently impossible commercially, a'feasible approach to the problem consists of making available a'copper anode'which possesses the combination of (1) largercrystal size,-(2-) uniform density of the crystals','and (3) minimum number of crystal boundaries which should be as clean as'pos- 'sible particularly of oxide inclusions. l
I havenow found that by the addition of extremely small quantities of: lithium to oxygen-free high conductivtiy copper, a substantial average enlargement'of the crystal structure is obtained whereby it becomes possible to reduce in anodes 'of equivalent size the overallnumber and extent of. crystal boundaries presented to the sludge-forming preferential attack of electrolyte solutions during. the electrodeposition process; Moreover, I have found that none of the other desirable properties connected with the use of oxygen-free, high conductivity copper anodes in either an acid sulfate or fluoborate as Well as a cyanide copper electroplating process are in an way iadversely affected by such lithium additions;
It is the principal object of this invention to provide a copper electrodeposition process utilizing an oxygen-free, high purity copper anode having an enlarged average crystal structure whereby preferential attack during ele'ctroplating is minimized and the more uniform dissolution of the anode results in reduced sludge formation and improved smoothness of thecopper deposit on the cathode. a
' Another object of thisinvention'is'to provide an'oxygen-free, high purity'copper anode having a relatively enlarged crystal structure which is conducive to improved results in copper electrodeposition processes.
Other specific objects and advantages of the invention will become apparent as this specification proceeds.
For the purpose of thepresent invention, it is essential that the copper used for the fabrication of the anodes be oxygen-free high conductivity copper. By the term oxygen-free copper" is meant a high purity copper which contains no oxygen by .virtue of the process used in production in accordancewith known methods therefor exclusive of the use of chemical deoxidizers such as phosphorus, and the like. Thus, copper which has been produced in a reducing atmosphere such asthe commercially available OFHC brand copper may be used as well as copper prepared in an inert atmosphere or in a vacu- 11m. Such copper is usually of 99.98% or even higher purity and theextremely small amount of impurities which 'im y be present is not objectionable for the stated purpose. Atypical analysis of the composition of OFHC brand copper with respect to the impurity content thereof is illustrated by the following:
. Percent Silver 0.001 Iron 0.001 .Sillflit' 0.002 TLead L 0.0003 Tin p 0.0001 Nickel 0.0008 Antimony Less than 0.0005 Bismuth Less than 0.0001 Zinc Less than 0.0003 Manganese Less than 0.00005 Oxygen None The copper anodes for use in the electrodeposition processes as herein contemplated are made by the addition of extremely :small amounts of lithium to the oxygen- :free high conductivity copper using oxygen-excluding :practices .in accordance with known practices during the :entire process. The amount of added lithium in the form of a master alloy or-as lithium metal may be varied between 0.0001 to 0.001% by weight. Preferably, between 0.00015 to 0.0003% may be used with optimum results being achieved with the use of about 0.0002%.
The lower limit of 0.0001% for the amount of lithium :to-be added to the anode metal'is determined by the observation that any'smaller addition does .not achieve the (desired crystal enlarging effect. While the upper limit is :less critical, it is desirable to keep the addition to the necessary minimum not only for cost considerations but also toensure an anode of maximum copper purity. For this reason the use ofabout 0.0002% as previously indicated is most desirable in that adequate crystal enlargementis achieved without any appreciable impairment of purity, conductivity or density of the oxygen-free high conductivity copper.
Although the reasons for the unexpected grain enlargement achieved in oxygen-free high conductivity copper by the addition of such extremely small amounts of lithium are not clearly understood, it is believed that the highly eflicient scavenging action of lithium serves in some man- .ner to effect a reduction in the nucleation centers present inthe copper. Theeffect is unique, however, since the .efliect associated with lithium addition to ordinary copper even in such small quantities is generally grain refinement as opposed to the actual grain enlargement obtained in the case of oxygen-free copper.
Lithium-treated oxygen-free copper anodes preferably made by the addition of the desired amount of lithium .metal to a copper melt employing conventional oxygenexcluding practices during the melting, lithium addition and casting steps may be used in the usual manner using conventional bath compositions of the acid or alkaline type. By way of specific illustration, a conventional aqueous acid sulfate plating bath consisting essentially ofcupric sulfate and sulfuric acid may be used utilizing, for example, acurrentdensity ofabout 100 amperes per square foot of anode surface. Similarly the new and improved anode may be advantageously used in conventional cyanide plating processes utilizing an electrolyte bath composition, for example, which consists essentially of an aqueous solution of copper cyanide and sodium and/ or potassium hydroxide with anode current densities averaging about 15 amperes per square foot.
Whether or not the advantageous results and improvements are attributable solely to the enlarged grain structure perse of the modified anodes herein described cannot be positively ascertained at this time. Whatever the reasons may be for the superior performance of the lithium-treated oxygen-free copper anodes of the type herein disclosed, the fact remains that, in addition to the adaptability of the anode to both acid and cyanide processes, smooth and fine-textured cathode deposits of substantially pure copper are consistently obtained which are substantially free from nodular growths notwithstanding the thickness of the deposited plate. Furthermore, the reduced sludge formation and minimized cathoderoughness resulting therefrom entirely eliminates the need for bags or diaphragms around the anodes. The more uniform dissolution characteristics imparted to the anode in addition to reducing copper losses by sludge formation also permits a more complete utilization of the anode during the electrodeposition process whereby scrap losses are appreciably less. I
Since certain changes maybe made in the above invention and different embodiments of the invention may be made without departing from the scope hereof, it is intended that all matter contained in the disclosure shall be interpreted as illustrative and notin a limiting sense.
I claim: I
1. In a copper electrodeposition process wherein an electric current is passedvfrom a consumable copper anode through an electrolyteto a cathode to provide a deposit of substantially pure copper on said cathode, the improvement which comprises subjecting to the action of said current an anode consisting of oxygen-free high conductivity copper to which is added from '0.0.001'% to 0.001% by weight of lithium to effect dissolution of improved uniformity of said anode with minimal sludge formation while forming a cathode deposit free of nodular growths.
2. In a copper electrodeposition process wherein an electric current is passed from a consumable copper 'anode through an electrolyte to a cathode whereby a deposit of substantially pure copper is effected on said cathode, the improvement which comprises subjecting to the action of said current an anode consisting of oxygenfree high conductivity copper containing theimpurities normally present therein to which is added from 0.00015% to 0.0003% by weight of lithium toeifect more uniform dissolution of said anode whereby less sludge is formed during'the process.
3. The processofclaim 2 wherein theelectrolyte is an acid electrolyte comprising an aqueous solution of cupric sulfate and sulfuric acid. 7 V
4. The process of claim 2 wherein the electrolyte is otthe alkaline cyanide type.
References Cited-in the file of this patent UNITED STATES PATENTS.
1,812,992 Smith July '7, 1931 2,689,216 Nevers et a1. Sept. 14, 1954 2,690,997 Jernstedt Oct. 5, 1954 2,809,929 OStrow-et-al. Oct. 15, 1957