ELECTROPLATED AUGMENTATIVE REPLACEMENT PROCESSED CONDUCTORS AND MANUFACTURE THEREOF

Abstract: Electrical conductors (Figures 2 and 4) having enhanced properties (such as increased electrical and thermal conductivity and improved surface characteristics for wire bonding and edge connector insertion in an edge connector) are prepared by applying a mixture (101) of a metallic powder and a polymer on a substrate (100), curing the polymer, effecting an augmentative replacement reaction to replace some of the metallic powder with a more noble metal to form a contiguous layer of a conducting metal (102) on the substrate and thereafter electroplating an additional metal layer (103, 104) on the contiguous layer. Principal use is in manufacture of printed circuits.

Description: ELECTROPLATED AUGMENTATIVE REPLACEMENT PROCESSED CONDUCTORS AND MANUFACTURE THEREOF RELATED APPLICATIONS ; This application is related to co-pending applications Serial Nos. 220,331, 220,341, 220,342, 220,343, 220,244 and 220,937, all filed December 29, 1980, and Serial No. 220,332 filed March 11 ,. 1981. BACKGROUND OF THE INVENTION Many types of electronic apparatus are known in which the various electrical components are interconnected by conductors. The intercon¬ necting conductors are fabricated in a wide variety of processes such as, for example, thick- film fired conductor systems, polymer conductors and printed circuit boards. In thick-film fired conductors, a mixture of a conducting metal powder, a ceramic or glass binder and an appropriate vehicle is screen print¬ ed on a substrate. The conductor pattern on the substrate is then fired at a relatively high temperature, typically between 650° and 900°C. As the temperature rises to the firing temperature, the vehicle is volatilized, leaving the metal and binder behind. At the firing temperature, sinter¬ ng of the metal takes place to a greater or lesser extent with the binder providing adhesion between the metal film formed and the substrate. Thick-film fired conductors have classi¬ cally employed precious metals such as gold, silver, platinum and palladium. Recently these noble metals have soared in cost, and new conduc¬ tor systems using copper, nickel and aluminum are being made commercially available. The cost of the precious metal systems is prohibitive where a low cost conductor system is desired. The newer metal systems are not s gnificantly cheaper because of the special chemistry which is required to prevent oxidation of the metal during the fir¬ ing process. Moreover, these systems are very difficult to solder using the conventional tin/ lead solder and the high firing temperatures required during fabrication preclude the use of low cost substrate materials. Some of the nickel systems can be fired on a soda-lime glass at temperatures just below the melting point of the glass but the resulting conductivity of the conductor is relatively low. The term "polymer conductor" is actually a misnomer since the polymer is not actually a conductor. Instead, the polymer is heavily loaded with a conducting metal and screened onto a sub¬ strate. The advantage of this system is that the polymer can be cured either catalytical ly or ther¬ mally at temperatures which range from room temperature to about 125°C, As a result of this so-called "cold processing", it is possible to use very inexpensive substrates such as films of Mylar D (polyethylene terephthal ate). The mechanism by which conductivity is achieved is supplied entirely by contact between individual metallic particles. It has been found that the only metals which can be loaded into the polymer and give acceptable conductivity are the precious metals such as gold and silver. All of the other stan¬ dard conducting metals oxidize over a period of time, reducing the conductivity between the parti¬ cles. Silver has been the predominant choice in polymer conductor systems but the silver systems are generally not solderable because the silver is leached by the lead-tin solder. When the price of silver is about $10-11 per ounce, these conductor systems are competitive with other systems if used on very low-cost substrates such as thin Mylar films. However, when the price of silver is higher, the systems are not competitive with printed circuit boards. The techniques used to prepare printed circuit boards can be divided nto additive, se i- additive and subtractive technologies. In the semi -add ti ve and subtractive techniques, the starting point is a substrate, which can vary widely from phenolics to glass-filled expoxies, on which a copper foil is bonded. In the additive preparatory system, the copper foil is very th n, usually on the order of about 200 icroinches. A resist is patterned such that the copper is expos¬ ed only where the conductors are desired and the board is then electroplated to form copper conduc¬ tors of about 1 mil in thickness. The plating resist is stripped and the copper is etched. In areas where the conductor is not desired, the copper is only about 200 microinches thick so that etching quickly removes this copper while leaving a 1 mil thick conductor. In the subtractive pro¬ cess, the starting thickness of the copper foil is usually between 1 and 2 mils. An etch resist is deposited wherever the conductors are desired, the board is etched and the resist is then removed. The resist prevents etching where the conductors are desired leaving conductor runs. Both the semi -add ti ve and subtractive printed circuit board procedures require the application of a copper foil over the entire sub¬ strate, deposition and removal of a resist, etch¬ ing of the printed circuit board, drilling holes for component insertion, and in one case, the additional step of electroplating. In the additive technique^ conductors are formed by printing a sensiti zer pattern in which the sensitizer contains palladium or a metal which is subsequently replaced by palladium after a sensitizing dip. The substrate is then placed in a catalytictype plating bath to form an electro- less copper or nickel layer in the same area as the sensitizer pattern. This reaction can be O P allowed to continue until a sufficient thickness of conductor is established. However, this reac¬ tion is generally so slow that only a thin layer of conductor is built up in this fashion and the conductor must be subsequently electroplated. The very thin layer of electroless copper initially formed by the catalytic reaction is not capable of sustaining normal plating currents until a sub¬ stantial additional layer of copper has been built up. If too high a current is applied, a condition generally known as burning results. The most significant drawback of the printed circuit board technology is that a sub¬ stantial number of processing steps are necessary and this requires a large amount of associated equipment. In addition, the choice of substrate materials is limited to one of those available for circuit board materials. The number of processing steps and equipment results in relatively high processing costs and the limitation of the substrate material eliminates the opportunity to use a decorative or structural member which may be required in the apparatus as a substrate. In United States Patent Application Serial No. 220,342, filed December 29, 1980, and owned by the assignee of the present invention, which is entirely incorporated herein by refer¬ ence, the formation of an electrical conductor by an augmentative replacement reaction technique is described. The desired conductive design is applied to the substrate with an ink composition which contains a finely divided metal powder, a curable polymer and a solvent. The curable poly¬ mer is at least partially cured and then the resulting ink composition pattern is contacted with a metal salt solution in which the metal cation is more noble than the metal of the finely divided powder and the anion forms a salt with the metal of the salt and the powder which is soluble in the solution. This system is simple to effect, each processing step is relatively fast and the waste materials generated are generally environ¬ mentally safe and do not require special disposal processing. The system can be applied to a multi¬ plicity of low cost substrate materials such as soda-lime glass, plastic and even paper. The augmentative replacement process is basically a surface process and only metal at the surface of the polymer-containing ink enters into the reaction. As a result, the final deposits have a thickness on the order of about 100 to 400 microinches. In many systems, it is desirable to have higher conductivities than can be provided by a 400 microinch thick layer of copper and this is particularly true in power control systems where the high currents can cause substantial heating of thin conductor runs. In other systems, it is desirable to have a relatively thick metallic member as a heat spreading device so that heat can flow away from a power dissipating device and sub¬ sequently flow into the substrate and this also requires a thicker metal layer than is formed by the augmentative replacement process. The finely divided metal powder used in the augmentative replacement process generally has a particle size in the range o 5-50 microns and this results in a conductor with a relatively bumpy surface. While this is not a problem in most applications, there are applications, such as wire bonding, which cannot tolerate such surface irregularities. For example, in one wire bonding technique, it is desirable to ul trasonical ly scrub an aluminum wire into a very smooth mating metal¬ lic surface of, for example, copper, gold or nickel. The adhesion is attained by the inter- metallic attraction between the aluminum and the mating surface. It is also possible that the thin augmen¬ tative replacement conductors will experience pro¬ blems in applications where they are subjected to a high degree of abrasion. Such conditions are encountered, for example, when the conductors are used to form the fingers of an edge connector or when the conductors are used to form part of a switch or slide mechanism for a potentiometer. Accordingly, it is the object of this invention to provide an enhanced low-cost conduc¬ tor system and method of manufacture which is simple to effect, in which each processing step is relatively fast, which permits batch handling as opposed to single piece handling and in which the waste materials generated are generally environ¬ mentally safe and do not require special disposal techniques. It is also the object of this inven¬ tion to provide such conductors with enhanced electrical and/or thermal conductiv ty. Another object of the invention is to provide a conductor of enhanced surface smoothness and/or which exhibits an improved abrasion resis¬ tance. -g JR≤ OMP These and other objects of the invention will become apparent to those skilled in this art from the following detailed description in which: Figure 1 is a cross-section of a conduc¬ t ve path to be enhanced by the present invention; Figure 2 is a cross-section of the con¬ ductor path of Figure 1 which has been enhanced by the present invention; Figure 3 is a cross-section of a second conductive path having an irregular or bumpy sur¬ face to be enhanced by the present invention; and Figure 4 is a cross-section of the con¬ ductor path of Figure 3 after enhancement by the present invention. SUMMARY OF THE INVENTION Printed circuits in desired conductive designs are prepared by: applying the desired design on a substrate with an ink composition which contains a finely divided metal powder, a polymer and a solvent, contacting the resulting ink composition pattern with a metal salt solution in which the metal cation is more noble than the salt solution in which the metal cation is more noble than the metal of the finely divided powder so as to form a contiguous metallic layer on the ink; -n- and thereafter electroplating additional metal on the contiguous layer. DESCRIPTION OF THE INVENTION The process of the present invention, in its broadest form, involves the establ shment of the desired conductive pattern on a substrate by means of a metal -contai ni ng cured polymer which is subjected to an augmentation replacement reaction followed by an electroplating step. The process is particularly adapted for the use of screen printing techniques to establish the conductor patterns on. the substrates although the invention is not so limited. Other types of printing and application techniques can be used including, without limitation, pad flexographic printing, stencil, rotogravure and offset printing. The substrates on which the conductive patterns are formed are not restricted and any insulator to which the metal ink can be adhered is employable. Thus, the usual printed circuit sub¬ strates can be used as well as glass-filled poly¬ esters, phenolic boards, polystyrene and the like. Of particular interest as substrates for use in the present invention are glass and steel which are covered with an insulator such as porce¬ lain or epoxy. The latter materials are often used as structural or decorative elements in many constructions and applying electronic elements directly to them provides advantages with respect to ease of fabrication, essential structural members and cost. The ink composition used in the present invention is a combination of a finely di ided metal powder with a polymer whose viscosity and flow characteristics can be controlled by the incorporation of a solvent therein. The metal can be any metal: which is a stable in the polymer; which can be obtained in finely divided form; and which is situated above that metal, used in the augmentation replacement reaction, in the activity series of the metals. The metal powder generally has a particle size of less than about 50 microns, preferably 3 to about 25 microns and most preferably about 15-25 microns. When the ink is deposited by screen printing, the metal particles must be of a size to pass through the screen, i.e., if a 325 mesh screen is being used, the metal particles should be -325 mesh. The polymer employed In the ink 1s any material (curable or otherwise) or mixture thereof which exhibits a degree of adhesion to the sub¬ strate being employed and to the finely divided metal powder which is dispersed therein. Typical polymers which can be employed include the homo- polymers and copolymers of ethylenical ly unsatu- rated aliphatic, alicyclic and aromatic hydro¬ carbons such as polyethylene, polypropylene, poly- butene, ethylene propylene copolymers, copolymers of ethylene or propylene with other olefins, poly- butadiene, polyisoprene, polystyrene and polymers of pe ' ntene, hexene, heptene, bicyclo-(2, 2, 1)2- heptane, methyl styrene and the like. Other poly¬ mers which can be used Include polylndene, poly¬ mers of acrylate esters, and polymers of ethacrylate esters, acrylate and methacrylate resins such as ethyl acrylate, n-butyl methacry¬ late, isobutyl methacrylate, ethyl methacrylate and methyl methacrylate; alkyd resins; cellulose derivatives such as cellulose acetate, cellulose acetate butyrate, cellulose nitrate, ethyl cellu¬ lose, hydroxyethyl cellulose, methyl cellulose, and sodium carboxy ethyl cellulose; epoxy resins; hydrocarbon resins from petroleum; isobutylene resins; isocyanate resins (polyurethanes) ; mela- OMFI mi ne resins such as mel ami ne-formaldehyde and mel ami ne-urea-f ormal dehyde ; oleo-resins; polyamide polymers such as polyamides and polyamide-epoxy polyesters; polyester resins such as the unsatu¬ rated polyesters of dibasic acids and dihydroxy compounds; polyester elastomer and resorcinol resins such as resorci nol -formaldehyde, resor¬ cinol -furfural , resorcinolphenol -formal dehyde and resorcinol -urea ; rubbers such as natural rubber, reclaimed rubber, chlorinated rubber, butadiene styrene rubber, and butyl rubber, neoprene rubber, poly sul fide, vinyl acetate and vinyl alcohol -acetate copolymers, polyvinyl alcohol, poly vinyl chloride, polyvinyl pyrollidone and polyvi nylidene chloride, polycarbonates, graft copolymers of polymers of unsaturated hydrocarbons and of unsaturated monomers such as graft copoly¬ mers of polybutadiene, styrene and acryloni trile, commonly called ABS resins, polyamides and the like. The polymers and inks of the present invention can contain various other materials such as fillers, e.g. glass fiber, glass powder, glass beads, asbestos, mineral fillers, wood flour and other vegetable fillers, dyes, pigments, waxes, stabilizers, lubricants, curing catalysts such as peroxides, photosensitizers and amines, polymeri¬ zation inhibitors and the like. It is preferred, but not essential, to employ a polymer which exhibits a substantial degree of volumetric shrinkage upon curing. The amounts of the finely divided metal and polymer are adjusted such that the metal constitutes about 60-80% by volume of the mixture after curing. Preferably, the metal is about 70% by volume. It is desired to have a significant amount of the metal particles forming part of the surface of the cured ink to facilitate the sub¬ sequent augmentation replacement reaction. A solvent is used in the ink formulation in order to adjust the viscosity and flow charac¬ teristics for the type of printing desired. In general, the solvent should be employed in an amount sufficient that the ink has a viscosity of 100-200,000 cps at room temperature and preferably about 50,000-150,000 cps. Suitable solvents or diluents can be aliphatic or aromatic and usually contain up to about 30 carbon atoms. They include the hydrocarbons, ethers and thioethers, carbonyl compounds such as esters and ketoπes, nitrogen -^JSI OM containing compounds such as amides, amines, πitriles and nitro compounds, alcohols, phenols, mercaptans and halogen containing compounds. Examples include alcohols such as methanol, ethaπol , propanol, benzyl alcohol, cyclohexanol , ethylene glycol, glycerol and the like, aromatic materials such as benzene, toluene, xylene, ethyl benzene, naphthalene, tetralin and the like, ethers such as methyl ether, ethyl ether, propyl ether, methyl t-butyl ether, and the like, alkanes such as methane, ethane, propane and the like, dimethyl sulf oxide, butyl formate, methyl acetate, ethyl acetate, formamide, dimethyl formamide, acetamide, acetone, nitrobenzene, onochloro- benzene, acetophenone, tetrahydrof uran, chloro¬ form, carbon tetrachloride, trichloroethylene, ethylbromide, phenol, mercaptophenol , and the like. Additionally, reactive solvents or diluents such as triallyl isocyanurate can be used if desired. It is preferred to employ a solvent which is relatively non-volatile at room tempera¬ ture so that the viscosity and flow of the ink is appropriate during application to the substrate and highly volatile at the curing temperature of the polymer or at other temperatures above the OMPI application temperature. The carbitol series of solvents and particularly butyl carbitol (diethy- lene glycol monobutyl ether) has been found to be particularly appropirate. The ink is applied to the substrate to achieve the desired conductor patterns thereon. For example, standard printed circuit application technology can be employed. Any temperature which will not cause premature curing of the ink and at which the viscosity and flow characteristics of the ink are appropriate to the application tech¬ nique used can be employed. It is preferred, but not necessary, to permit at least a portion of the solvent to evaporate after application of the ink to the substrate and before curing. The act of evaporation exposes additional metal powder and increases the ratio of metal powder to polymer so as to achieve a balance between sufficient metal to provide a base for the conductive film to be formed thereon and too little polymer to act as a binder to hold the metal particles. Preferably, the drying is effected for 0.1-1 hour, more pre¬ ferably about 0.25-0.5 hour, at a temperature of about 70-150°C, most preferably about 110-130°C. In the next step in the instant process, the ink polymer is cured or polymerized by the most convenient method. If an autocatalyst has been added, the polymer will cure by itself with no additional initiation. In the case of ultra¬ violet light initiators, the substrates carrying the conductor patterns can be passed under a high intensity ultraviolet source which causes the initiators to begin the curing reaction. It is presently preferred to employ a thermal curing system which is activated by exposure to tempera¬ tures of about 140-200°C, preferably about 150-180 β C, for a time of 0.1-1 hour, preferably 0.15-0.5 hour. As a result of this step, a closely compacted metal powder bound to the sub¬ strate by the cured polymer is achieved. Because of the high percentage of metal and shrinkage of the polymer chosen, the conductive pattern thus obtained may have some conductivity due to physical contact between the metal particles. In the preferred embodiment of this invention that conductivity is on the order of about 30 Kohm per square for a one mil thick deposit. The resis¬ tance will be highly variable and increase sub¬ stantially if the system is subjected to oxidizing conditions for any period of time, since an oxide builds up between particles and reduces conduc- ti vi ty. In some instances, it may be desirable to only partially cure the polymer. For example, occasions arise where it is desirable to mount components by inserting the leads thereof in the polymer ink. In such instances, it may be desir¬ able to partially cure the polymer or only gel the polymer in situations where the polymer employed is gelable, so as to provide an adhesive for the lead wire. The ink composition pattern is subjected to an augmentation replacement reaction in which some of the metal of the powder is replaced by a metal further down in the activity series, i.e., which is more noble. This step takes advantage of the known chemical behavior of metals that any metal will displace any succeeding, less active, metal from a water solution of one of its salts. It has been found that several hundred microinches of conductor material can be built up from a solu¬ tion in a period of 5 minutes. The augmentation reaction reagent is a solution, preferably inorganic and most preferably OMPI y Sfc ' , WI_,P_ ** OU aqueous, of a metal salt. The cation of the metal salt is any more noble or electropositive metal than the metal of the finely divided powder, i.e., lies below the powder metal in the activity series, and which is electrically conductive. Any anion can be used which is relatively inert, i.e., does not deleteriously affect the process and which forms soluble salts with both the cation metal and the powder metal. Typical salts include copper nitrate, copper acetate, copper fluor- oborate, potassium gold cyanide, nickel sulfate, nickel chloride, nickel sulfa ate, potassium ' silver cyanide, silver chloride and the like. The presently preferred metal salt is copper sulfate. The concentration of the metal salt in the solu¬ tion can range from 0.1 molar to saturation but is preferably about 0.5-2.0 molar. Below about 0.5 molar, deposition rates are inordinately slow and there is no improvement in rate at molarities above 2.0. Most preferably, the metal salt is present at a concentration of about 1 molar. Any inert solvent can be used. When copper sulfate is used as the aug¬ mentation metal, a copper layer is formed with new unoxidized copper which can be readily soldered. If further enhancement is desired or if soldering of the circuits is to be delayed for a substantial period of time, the conductor pattern formed can be dipped in a tin plating solution so that the tin will replace some of the copper. Tin and copper are very close in the activity series and the normal replacement reaction would cause copper to be plated out on the tin. However, by adding appropriate complexing ions, the tin will replace the copper. The tin plated copper thus formed is very readily soldered and can be left for periods of a month or more and good soldering can still be achieved. Suitable tin plating solutions are commercially available for plating on copper such as, for example, Coppertech Electroless Tin Plat¬ ing Solution ST-210. The augmentation reaction can be carried out at any suitable temperature although elevated temperatures are generally pre¬ ferred in order to increase reaction rate. Thus, any temperature from ambient up to about 200°C can be employed although the temperature is preferably about 45 β -60 β C. Generally the augmentation reac¬ tion is completed in about 0.1-1 hour or more, preferably about 5 minutes. Figure 1 is a cross-section of a sub¬ strate carrying a conductive pathway in accordance with the procedure which has been described thus far. Substrate 100 carries a layer of cured ink 101 over a portion of its surface. The conductive layer of metal achieved by the autmentation replacement reaction is shown as 102. It is often desirable to use zinc as the powder metal because of its very low cost and because zinc reacts readily with simple copper salt solutions. Unfortunately, the zinc reacts too vigorously resulting in a very porous and spongy copper film. Further, in some fabrication systems using iron powder, there is a moisture susceptibility problem because the iron has a tendency to rust. These problems can be avoided to a great extent by using a mixture of powdered metal s. A preferred powder metal mixture contains about 35-45% zinc, preferably about 40%, and 65-55% nickel, preferably about 60%, by weight. This combination exhibits a high degree of conduc¬ tivity before the augmentative replacement reac¬ tion step and has certain advantages in reducing the rate of reaction with zinc because an enlarged OMPI electrically conductive surface area is presented to the metal salt solution while a relatively smaller proportion of that area is the quite reac¬ tive zinc powder. As a result, a high quality conductive coating is formed which has a ve ry high degree of adhesion even though the reaction involves zinc and copper which are quite displaced from one another in the activity series of the metals. The pressence of the nickel reduces this vigorous reaction. The resulting conductor system also has the advantage of being stable in the presence of high degrees of moisture. When iron is used as the reactive metal, it tends to rust and form an unsightly deposit on the surface of the conductor and in areas immediately adjacent to the conductor on the substrate. In extreme cases, the resistance between closely spaced conductors can actually be reduced. In the nickel zinc system, the zinc does not rust and ve ry little corrosion product is formed even in ve ry high moisture environments. It further has been found that by raising temperatures at which the augmen¬ tative replacement is conducted to about 65°C and by adding a small amount of nitric acid to the copper sulfate solution, improved coating can be achieved which is primarily manifested by reduced resistivity of the conductor. It is believed that the nitric acid probably acts to clean the passi- vated surface of the nickel, allowing it to enter into the replacement reaction rather than merely be coated. In the next step of the present inven¬ tion, a metallic layer is electroplated on the augmentative replacement layer. The augmentative replacement layer is sufficiently conductive so that it can be used as an electrode in the elec¬ troplating procedure simply by attaching a suit¬ able electrical lead to the layer. The electro¬ plating procedures and plating baths which can be used are well known and are described, for example, in Metal Finishing Guidebook and Direc¬ tory which is published annually by Metals and Plastic Publications, Inc. Virtually any plating solution which is known to plate directly to the metal of the augmentative replacement layer can be used. For example, when the layer is of copper, then silver, gold, rhodium, nickel, chromium and the like can be electroplated thereon. The plat¬ ing baths generally contain a salt of the metal to be plated together with various additives and/or brighteners as may be desired. Typical addition agents include gelatin, glue, phenol sulfonic acid, glycine, peptone, pyrogallol, starch, urea, gum arabic, hydroqui none, thiourea, acetyl cyana- mide, di ethylanil i ne, tannin and resorcinol. Typical Class I nickel brighteners include benzene disulfonic acid, benzene trisulfonic acid, naph¬ thalene disulfonic acid, naphthalene trisulfonic acid, benzene sulfonamides and sulfonimides in¬ cluding saccharin. Typical Class II nickel brigh¬ teners include formaldehyde, butynediol, pyrimi- dines, pyrazoles and imidazoles, ethylene cyanohy- drin and the like. Other brighteners which can be used include piperonal, coumarin, furfural, dex¬ trin, milk, sugar, molasses, and the like. Some of the electroplating baths are cap¬ able of being used for the augmentative replace¬ ment reaction step. A typical example is a plat¬ ing solution containing 5/8 M CuSO^.δH O, 0.5 M sulfuric acid and 0.25 M nitric acid. Such plating solutions have the advantage that the electroplated layer can be realized over the augmentative replacement layer without removing the substrate from the solution. One of the preferred electropl ting baths is a fluoroborate copper plating solution, several of which are commercially available. The fluor¬ oborate bath is used where high depositing rates are desired, since this solution can provide adequate coatings at currents up to 1 amp per square inch and, therefore, can deposit a 1 mil coating in less than three minutes. Conventional processing would require initial plating at much lower currents in order to prevent damage to the conductor due to the heating effect of the high currents, but the conductivity of the augmentative replacement layer is sufficiently high to al-low plating to commence immediately at the maximum current. This is particularly advantageous to increase productivity because the augmentative replacement process can be effected in a batch mode, since no electrical connection is necessary and there is no requirement to maintain a specific distance from the plating anode. The electro¬ plating step can be dramatically reduced in time since the full plating current can be applied immediately. Figure 2 shows the conductor of. Figure 1 in which an electroplated copper layer 103 has been established in accordance with the present i nvention. The augmentative replacement process uses a metallic powder where the particles have a size of about 5-50 microns. The augmentative replace¬ ment layer which is established follows the con¬ tour of the ink and as a result a conductor with a relatively bumpy surface is realized. This is illustrated in Figure 3. In those applications where the bumpy or irregular surface may cause problems, such as wire bonding applications, suit¬ able leveling agents are added to the electro¬ plating bath so that the electroplated layer achieved has a re! ati vely smooth surface. Level - SL$ i ng is a property of electroplating processes in which the surface of the deposited metal becomes smoother as the deposit thickness is increased. In the case of nickel, the most effective leveling agents are the Class II brighteners and therefore the semi-bright and bright nickel plating baths are capable of providing level metal surfaces. Suitable leveling agents for other metals are known in the art. The resulting smooth surface is OMP especially desirable when it is required- to directly bond wires to conductors for the purpose of connecting integrated circuit chips, transis¬ tors, diodes or the like. Figure 4 shows the conductor of Figure 3 in which the electroplating process has been carried out with a plating solution containing a leveling agent so that the resulting electroplated layer 104 has a relatively smooth surface. It should be noted that the electro¬ plating step permits a greater degree of flexi¬ bility than would ordinarily be available in the augmentative replacement process. For example, in the embodiment using a nickel -zinc powdered metal mixture described above, the nickel provides good adhesion to the subsequently deposited augmenta¬ tive replacement layer but as the amount of nickel is increased, the conductivity which can be realized is reduced. The electroplating permits superior adhesion to be achieved by using metal powder mixtures which contain a larger weight per¬ centage of nickel than would otherwise be accept¬ able. As one example, between 80 and 90% nickel and between 20 and 10% zinc would give sufficient conductivity for electroplating and would provide more nickel for increased adhesion. Another favorable trade-off is realized when the amount of polymer in the ink is slightly increased since this gives an improvement in the total adhesion to the substrate because there is more polymer avail¬ able for adhering to the surface thereof and the conductivity, which would be adversely affected in the augmentative replacement process alone, is enhanced by the electroplating step. In order to further illustrate the present invention, various non-limiting examples are set forth below. Throughout this specifica¬ tion and claims, all parts and percentages are by weight and all temperatures are in degrees Celsius unless otherwise indicated. EXAMPLE 1 A conductor pattern ink was prepared by combining 80 grams of -325 mesh nickel, 20 grams of -325 mesh zinc, 10 grams of a commercially available epoxy resin containing a thermal curing agent and 3 grams of ethylene glycol ethyl ether acetate as solvent. The ink was printed on a glass substrate through a 200 mesh stainless steel screen to provide a conductor pattern on the sub¬ strate. After drying for 15 minutes at a tempera- ture of 100°C. in a cross-flow oven, the epoxy resin was cured at a temperature of 180°C. for 45 mi nutes. The resulting patterned substrates were immersed into a solution which contained one molar copper sulfate,- 0.5 molar sulfuric acid and 0.25 molar nitric acid, which was maintained at a temperature of 54°C. After two minutes, conductivity of the patterns was measured and found to be 25 milliohms per square. The substrate was rinsed in running water and placed in a room temperature cupric fluor- oboric plating solution and plated at a current density of 1 ampere per square inch for three minutes. Thereafter, the substrate was removed, rinsed and dried. The conductivity was again measured and found to be 0.6 milliohms per square which indi¬ cated that a copper layer of greater than 1 mil in thickness has been built up in the three minutes of electroplating. There were no signs of burning in the conductors and a highly adherent dense cop¬ per layer had been formed on the substrate. EXAMPLE 2 A cured polymer conductive pattern on a glass substrate was prepared as described in Example 1. Copper anodes were placed in a copper plating solution which contained 1 molar copper sulfate, 0.5 molar sulfuric acid and 0.25 molar nitric acid. The patterned substrates were dipped into the plating solution and electroplating was commenced immediately at the rate of 200 milliamps per square inch for a period of 20 minutes. The resulting product was found to be a fine grained copper layer having a conductivity of 0.6 milliohm per square which indicates that a copper layer of greater than 1 mil thickness had been achieved using the same bath for both the augmentative replacement and the electroplating processes. EXAMPLE 3 Several samples having an augmentative replacement layer thereon were prepared by the procedure described in Example 1. Some of these samples were then placed in a bright copper plat¬ ing solution which was commercially available and a plating current density of 200 milliamps per square inch was established. The resulting copper had a ve ry bright appearance. Attempts were then made to bond an alumi¬ num wire to the conductive pathways of the samples which had been electroplated and those which had not been. A 1.25 mil aluminum wire containing 1% silicon was used with a K&E ultrasonic aluminum wire bonder, having settings at power equal to 0.5, time equal to 0.5 and weight equal to 25 grams. Bonds could not be made to the conductive pathways which had not been electroplated, but bonds, with pull test results from 6 to 10 grams, were obtained on the samples which had been elec¬ troplated. Various changes and modifications can be made in the process and products of this invention without departing from the spirit and scope there¬ of. The various embodiments which have been set forth herein were for the purpose of illustrating the invention but were not intended to limit it.

Claims: WHAT IS CLAIMED IS: 1. An electrical conductor adhered to a substrate in which said conductor comprises a polymer containing a finely divided first metal and a second metal therein, a contiguous layer of said second metal on said polymer wherein said second metal is below the first metal in the activity series of metals, and a metallic electro¬ plated layer on said contiguous layer. 2. The electrical conductor of Claim 1, wherein said finely divided first metal has a particle size of less than about 50 microns and the amount of metals in said polymer is about 60-80% by volume. 3. The electrical conductor of Claim 2, wherein said finely divided first metal has a particle size of about 3-25 microns and said poly¬ mer comprises a thermally curable polymeric mate¬ rial which exhibits volumetric shrinkage upon cur¬ ing. 4. The electrical conductor of Claim 1, in which said finely divided fi st metal comprises i ro n . 5. The electricial conductor of Claim 1, wherein said finely divided first metal com¬ prises nickel and zinc. 6. The electrical conductor of Claim 5, wherein said zinc comprises about 10-20% of said first metal by weight and said nickel comprises about 80-90% of said first metal by weight prior to formation of said contiguous layer of said second metal on said polymer. 7. The electrical conductor of Claim 1, wherein said second metal comprises copper. 8. The electrical conductor of Claim 1, wherein said metallic electroplated layer com¬ prises copper. 9. The electrical conductor of Claim 1, wherein said metallic electroplated layer co - pri ses nickel . 10. The electrical conductor of Claim 1, wherein said metallic electroplated layer is leve¬ led. 11. A method of forming an electrical conductor in desired areas of a substrate which comprises applying a desired design to said sub¬ strate with an ink composition comprising finely divided metal powder and a polymer at least par¬ tially curing or drying said polymer; contacting the resulting ink composition pattern with a metal salt solution in which the metal cation is more noble than the finely divided metal and the anion of said metal salt forms soluble salts with both said cation and said finely divided metal, so as to form a contiguous metal layer on said polymer; and electroplating a metallic layer on said con¬ tiguous layer. 12. The method of Claim 11, wherein said finely divided metal has a particle size of less than about 50 microns and said ink composition has a viscosity of about 100-200,000 centipoises at room temperature. , . OMPI 13. The method of Claim 12, wherein said finely divided metal has a particle size of about 3-25 microns, said polymer undergoes volumetric shrinkage upon curing and said ink composition has a viscosity of 50,000-150,000 centepoises at room temperature. 14. The method of Claim 13, wherein said finely divided metal has a particle size of about 15-25 microns and said polymer is adapted to ther¬ mally cure at a temperature of about 140-200°C. 15. The method of Claim 11, wherein said finely divided metal comprises about 60-80% by volume of the mixture of finely divided metal and with said metal salt solution. 16. The method of Claim 11, in which said metal salt solution comprises an inorganiz solution containing 0.1 molar to saturation of said metal salt. OM 17. The method of Claim 16, wherein said metal salt solution comprises an aqueous solution containing about 0.5-2 molar metal salt. 18. The method of Claim 11, wherein said finely divided metal powder comprises iron. 19. The method of Claim 11, wherein said finely divided metal powder comprises zinc and nickel . 20. The method of Claim 19, wherein said finely divided metal comprises about 10-20% by weight of zinc and about 80-90% by weight of nickel . 21. The method of Claim 11, wherein said metallic layer is electroplated from an electro¬ plating solution containing a leveling agent. 22. The method of Claim 11, wherein said electroplated metallic layer comprises copper. _OM 23. The method of Claim 11, wherein said electroplated metallic layer comprises nickel. 24. The method of Claim 11, wherein said metallic layer is electrop ated from said metal salt solution in the electroplating step.

Inventors: AUERBACH ABRAHAM, EICHELBERGER CHARLES WILLIAM, WOJNAROWSKI ROBERT JOHN, AUERBACH, ABRAHAM, EICHELBERGER, CHARLES, WILLIAM, WOJNAROWSKI, ROBERT, JOHN
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1982
Main classification: H05K1
IPC codes: H05K1, H05K3