CHELATING METALS

Abstract: Chelating a metal such as lead in an aqueous sulfate electrolyte permits the co-deposit of lead together with tin on a substrate including a metallic element. Chelation is effected by forming a complex with a complexing agent. Electrical terminals so plated are not subject to dendrite growth. Chelation is effected by bifunctional complexing agents that can form a 5 or 6 member ring with lead.

Description: DESCRIPTION CHELATING METALS Background This invention relates to electroplating metal elements. In microelectronic devices there are semi-conductor components with integrated circuits formed on a substrate, often of leaded glass. The circuits or chips, as they are often called, have metallic elements, terminals or leads, which leave the integrated circuit for connection to other circuitry. These terminals are conventionally formed as spaced parallel fingers and have in the past been plated with pure tin to a thickness of about 200 to 300 micro inches. The plating has included as a co-deposit a brightner such as Janus Green, or a product 6487-Igepal from Dow Chemicals, or Schlotter Tin, a product available through Learonal Corporation in the United States. These organic brightners provide a uniform flow- ability to the tin and also provide a better brighter cosmetic finish. As such these terminals permit the soldering of very fine wires. Without such brightners the appearance of the terminals will be matt-like and the performance of the terminals in so far as the uniform flowability will be reduced. In practice it has been found with such terminals that whiskers or dendrites gradually grow between the parallel terminals as a result of electrical current passing through these terminals. Over time the dendrites form a short circuit between these terminals with con¬ sequent serious results, especially where the micro¬ electronic device is involved in high technology equipment such as spacecraft, computers, aircraft and the like. As a result of the failure rate with these known components, the United States government has recently OMP introduced higher specifications for the components to be applied in military applications. This requires that the plating no longer includes the organic brightners of the kind mentioned. The specifications, however, do call for a product having sufficient metal flow of the terminals that solderability and corrosion resistance requirements are achieved. Preferably the product should also has the cosmetic brightness characteristics of known products having brighteners. Specifically, the specification calls for the tin plate to be between 200 to 800 micro inches (5.08 to 20.32 micro millimeters) thick. It should also be dense, homogenous, continuous and free of co-deposited organic material. Bright acid tin plate is prohibited. One solution to obtain the improved reflow charac- teristics of such tin plating is to use organic and inorganic fluxes; however, this is generally undesirable since the fluxes normally contain chlorides which attack the leaded glass substrate. Another prepared solution to the above difficulties is the addition of lead to the plating composition. This prevents the growth of whiskers and dendrites. One method to apply a tin-lead composition to the terminals is to dip a tinned terminal after electroplating into a hot solder dip of tin and lead mixture so as to obtain an eutectic coating on the terminals. A disadvan¬ tage with this approach is that the hot dip tends to break the glass substrate as a result of the sudden temperature change. A possible solution is the co-deposit of tfn and lead during electroplating. Where tin-lead plating as a co-deposit is used as an alternative to tin plating the Government specifications require that the lead proportion shall be 2% to 50% by weight and it should be homogenously co-deposited. However, electroplating with tin and lead in a chloride or borate solution is not feasible because chlorides and borates attack leaded glass. Further lead is not soluble in a sulfate plating solution. Thus there is a need for a method to plate a metallic lead or terminal of an integrated circuit having a leaded glass component with a plating composition that avoids dendrite growth, that can be soldered, and is corrosion resistant. Summary The present invention is directed to a method that satisfies this need. According to this method, a metallic element is electroplated with lead and tin simultaneously by forming an aqueous plating solution that comprises water, sulfate ion, tin, and chelated lead. The tin is present in an amount effective for electroplating, and in an amount of at least 0.1 ounce as tin sulfate per gallon of water. The lead is present in an amount of at least about 2 parts by weight per 100 parts by weight tin. The element to be plated is placed in this plating solution, and an electrical current is passed through this solution to deposit lead and tin on the metallic element. By adjusting the amount of tin and lead present in the plating solution, a plating comprising at least 90% by weight tin and at least 2% by weight lead can be formed. The lead is solublized in the sulfate solution by use of a chelating agent that is bifunctional and is capable of forming with lead a five or six member group that is soluble in a sulfate solution. The chelating agent can be an organo-oxyanion compound. The plating solution and the plated substrate pro- duced by the method of the present invention are also novel. The plated substrate can include a plating com¬ prising co-deposited tin and lead, and preferably consists essentially of tin and lead, containing essentially no brighteners. Thus, the present invention can result in an inte¬ grated circuit that has a leaded glass component, where the leads or terminals of the circuit are coated with a plating composition that avoids dendrite growth, that can be soldered, and is corrosion resistant. The organo-oxy-anion chelating agent can be an organic acid of molecular weight less than 250 grams/mole, preferably, a gluconic acid, substituted gluconate or alkylacetonates. Detailed Description A method is provided for the plating of a metallic element such as the parallel terminals from a micro¬ electronic component which includes an integrated circuit with a leaded glass substrate. The terminals are metallic elements, typically made of Kovac. ™ (Alloy 42) which are to be plated such that there is a plating comprising a co-deposit of tin and lead. The metallic element can be made of any electrically conductive metal or alloy. This co-deposit should be in the range of at least 50% tin and no more than about 50% lead, and preferably at least 95% tin and no more than 5% lead. In a preferred embodiment the plating has about 98% by weight tin and about 2% by weight lead. Preferably the coating consists essen¬ tially of tin and lead and contains essentially no bright- eners. As lead is normally insoluble in an aqueous sulfate solution which is constituted by a solution of sulfuric acid as the electrolyte in an electroplating system, the lead is chelated to ensure a sufficiently soluble solution whereby the lead can co-deposit on the metallic element to be plated. In the electroplating system the anode would be constituted by an allotrop of tin which in solution becomes stannic tin and stannous tin, and the metallic element to be plated constitutes the cathode. During electroplating, the tin from the anodes passes to deposit on the cathode eleent. By placing in the electolyte solution a lead-chelate composition, the lead itself be retained sufficiently soluble in the solution so to effectively co-deposit on the cathode. The chelated lead is preferably added to the sulfate solution as a liquid of desired concentration, or as a solid wherein complexing has been effected to a chelating agent, preferably an organo-oxy-anionic chelating agent to render the lead complex soluble in the aqueous sulfate bath. The organo-oxy-anionic chelating agent can be selected from the group of glycines, carboxylic acids and alkylacetonates having a molecular weight of less than 250 grams/mole. The plating solution contains water, sulfuric acid, a wetting agent, tin, and chelated lead. Typically the sulfuric acid is present in amount of 10% by volume based on the volume of the sulfuric acid and water. The wetting agent is used to assure that an even deposit of plating occurs on the metallic element. A suitable wetting element is Triton X-100 available from Rohm & Haas in an amount of about 2 grams per gallon of water. The tin can be provided as stannous sulfate in an amount of at least about 0.1 ounce (weight), and typically from about 2 to about 4 ounces (weight) per gallon of water. The amount of chelated lead used depends upon the composition of the plating desired. For the plating to contain at least about 95% tin and from about 2 to about 5% lead, the plating solution contains chelated lead in an amount sufficient to yield at least 2 parts by weight, and preferably from about 5 to about 10 parts by weight per 100 parts by weight tin. The chelated lead can be prepared according to conventional techniques. In one technique, the chelating agent is dissolved in water in an amount of 10% by weight chelating agent. Then lead nitrate is added to the water with a palladium or platinum charcoal catalyst, the mixture is heated to about 65° C, to yield the chelated lead plus excess chelating agent, which is present in more than stoichiometric quantity. The chelated lead is extracted from this mixture at about 40° C with an ethanol/ methanol mixture, about 95 pbw ethanol and about 5 pbw methanol. The solvent is evaporated to yield a solid lead chelate, which can be directly added to the plating solution, or first dissolved in water. In an exemplary electroplating composition the proportions of the elements are as follows: In an electrolyte being one U.S. gallon (4 liters), the sulfuric acid constitutes 10% by weight, and the water 90% by weight. To this in solution there are added about four ounces (120 grams) of tin sulfate. A lead chelated with diethylglycine or EDTA is added to the solution in an amount of half to one ounce (20 grams) so as to constitute about 16% of the solution. In this lead complex about half is lead metal thereby constituting about 5% of the metal relative to the tin metal content. By applying an electric potential between the cathode and anode with an electrolytic bath so composed, an effective co-deposit of 5% tin and 95% lead is achieved on the metallic element at the cathode. The current applied is in the order of from 1 to 10 amps per square foot of substrate being plated. For high speed plating, 500 amps or higher per square foot can be used. It takes about 10 to about 40 minutes to plate a substrate with a thickness of about 200 micro-inches. The chelating agent is bifunctional, i.e., has at least two chelating groups, either two acid groups, an acid and a base group, or two base groups. Suitable chelating agents for the lead are bifunctional chelating agents capable of forming a five of six member ring with the lead and being capable of solublizing lead in a sulfate solution. Exemplary of the chelating agents that MP have been found successful are gluconic acid, EDTA, diethyl glycine, triethylphosphine, ethylene diamine. Among the materials that are found to be unsuccessful are triethylamine tria ine and acetic acid. Combinations of more than one chelating agent can be used. The complex between lead ions and the complexing agent produces sufficiently soluble in the sulfate solu¬ tion to permit effective electroplating of the lead on the substrate. In tests with a metallic element so-plated, life cycling at a temperature of 200° F for over 48 hours shows a highly utile product which has superior soldering capabilities and an acceptable appearance. Dendrite and whisker growth has not occured. The plated element satisfies MIL Spec 3.5.6.2. • Although the invention has been described with reference to terminals for microelectronic circuitry, the invention also has application to other products which require plating. The relative amounts of metal being co-deposited can be changed. Thus the amount of lead complex agent present can be adjusted between different percentages to provide co-deposits between 50% lead and 50% tin on the other hand. Preferably the chelating agent provides an organo- oxy-anion for oxygen bonding, such as substituted glycines such as dimethylglycine (DMG) , dibutylglycine (DBG). Among suitable acids besides gluconic acid there are acetic acid and ascorbic acid. Suitable alkylacetonates that are commercially available include for example ethylene acetate, diethylene acetate, and the like. Instead of an organo-oxy-anion complexing agent, there can be achieved suitable metal complexing with selected organo-hetero-anionic complexing agents. This means that the metal ions are bonded with the anions of nitrogens, or oxygen, sulfur, or phosphorous. Individ¬ ually such anions would be nitronium, oxonium, sulfoniu , and phosphonium. Nitrogen complexing with lead can be effected, for example, by ethylene diamine. Sulfur complexing can be effected with thio acids. Phosphorous complexing can be with trialkylphos- phides. Many changes with widely differing embodiments can be provided for this invention without departing from the scope thereof. All matter contained in the above de¬ scription shall be interpreted as illustrative and not limiting, the invention being interpreted solely by the scope of the appended claims. • E OMPI

Claims: Claims; 1. A method for electroplating a metallic element with lead and tin simultaneously, the plating comprising at least 2% by weight lead and at least 90% by weight tin, the metallic element being part of a component that includes a leaded glass portion, the method comprising the steps of: (a) forming an aqueous plating solution com¬ prising water, sulfate ion tin in an amount effective for electroplating, and chelated lead in an amount of at least about two parts by weight lead per 100 parts by weight tin, where the lead is chelated with a bifunctional chelating agent capable of forming with lead a five or six member group that is soluble in a sulfate solution; (b) placing a component comprising .a metallic element and a leaded glass portion in the plating solu¬ tion; and (c) passing an electrical current through the solution to deposit lead and tin on the metallic element to form a plating comprising at least about 90% by weight tin and at least about 2% by weight lead. 2. A method for electroplating a metallic element with lead comprising the steps of: (a) forming an aqueous plating solution com¬ prising water, sulfate ion, and chelated lead in an amount effective for electroplating; (b) placing a metallic element in the aqueous plating solution; and (c) - passing an electrical current through the solution to deposit lead on the me¬ tallic element. 3. The method of claim 2 in which the step of forming comprises forming a plating solution comprising tin. 4. The method of claim 2 in which the lead is chelated with a bifunctional chelating agent capable of forming ^ with lead a five or six member group that -is soluble in a sulfate solution. 5. The method of claim 1 or 4 in which the chela¬ ting agent is an organo-oxy-anion compound. 6. The method of claim 5 in which the chelating agent is gluconic acid. 7. The method of claim 5 in which the chelating agent is EDTA. 8. The method of claim 5 in which the chelating agent is diethylglycine. 9. The method of claim 1 or 2 in which the plating solution contains at least one ounce by weight of tin as SnSθ4 per gallon of water. 10. The method of claim 3 in which the plating solution comprises at least two parts by weight lead per 100 parts by weight tin. 11. A composition suitable for electroplating com¬ prising: (a) water; (b) sulfate ion; and (c) chelated lead in an amount sufficient for electroplating a metallic element. 12. The composition of claim 11 including tin in an amount effective for electroplating a metallic element. 13. The composition of claim 11 comprising at least 2 parts by weight lead per 100 parts by weight tin. Cftf OM 14. The composition of claim 11 in which the lead is chelated with a bifunctional chelating agent capable of forming with lead a five or six member group that is soluble in a sulfate solution. 15. The composition of claim 14 in which the chelating agent is an organo-oxy-anion. 16. An electro-plating solution comprising: (a) water; (b) sulfuric acid in an amount of 10% by volume of the volume of water; (c) a wetting agent; (d) at least about 0.1 ounce (weight) of tin sulfate per gallon of water; and (e) chelated lead in an amount of at least about 2 parts by weight lead per 100 parts by weight tin, the lead being chelated with a bifunctional chelating agent capable of forming with lead a five or six member group that is soluble in a sulfate solution. 17. The solution of claim 16 in which the chelating agent is an organo-oxy-anion. 18. The solution of claim 17 in which the chelating agent is gluconic acid. 19. The solution of claim 17 in which the chelating agent is EDTA. 20. The solution of claim 17 in which the chelating agent is diethyl glycine. 21. An article comprising a metal substrate electro¬ plated with a plating comprising codeposited tin and lead, the lead being present in an amount of at least about 2% OM by weight of the plating and the tin being present in an amount of at least about 50% by weight of the plating. 22. The article of claim 21 in which the plating consists essentially of tin and lead. 23. The article of claim 21 or 22 in which the plating comprises at least about 95% by weight tin. 24. The article of claim 21 comprising a leaded glass portion. 25. The article of claim 21 or 22 in which the plating contains essentially no brightener. 26. An article comprising a leaded glass portion and * a metal substrate electroplated with a plating consisting essentially of codeposited tin and lead, the lead being present in an amount of at least about 2% by weight of the plating, the tin being present in an amount of at least about 95% by weight of the plating, the plating containing essentially no brightener. 27. A method of producing an aqueous sulfate solu¬ tion of lead comprising forming a complex of the lead with a selected organo-hetero-anionic complexing agent thereby to produce a sufficiently soluble complex in the sulfate solution so as to permit effective electroplating of the metal. ". 28. A method as claimed in claim 27 wherein the selected organo-hetero-anionic complexing agent is se¬ lected from the group consisting of organo-oxy-anions. 29. A method as claimed in claim 28 wherein the organo-oxy-anion is selected from the group of glycines, carboxylic acids and alkylacetonates. 30. A method as claimed in claim 2-9 wherein the organo-oxy-anion is selected from the group of diethylglycine, gluconic acid, and ethylenediaminete- traacetic acid. 31. A method of producing in an aqueous sulfate solution a complex of a lead ion, said lead ion normally being insoluble in said solution, comprising forming a complex of the lead ion with a selected organo-oxy-anionic complexing agent selected from the group of substituted glycines, carboxylic acids and alkylacetonates thereby to produce a sufficiently soluble complex in the sulfate solution so as to permit effective electroplating of said lead, and tin ions also being present in said sulfate solution. 32. A method of electroplating a substrate with lead from an aqueous solution of lead, the lead ion normally being insoluble in said solution, comprising adding a pre-determined quantity of an organic acid chelating agent having a molecular weight not greater than about 250 grams/mole to the solution to complex with the lead to thereby form a sufficiently soluble complex whereby the lead is adapted to form a plate deposit one substrate in the presence of an electric potential in the sulfate solution. 33. A process for chelating ionic lead in an aqueous sulfate solution comprising 'mixing together a lead com¬ pound with an organic acid chelating agent thereby to effect a soluble complex of the ionic lead and chelating agent in said sulfate solution. 34. A metallic element produced by the method of claim 1, 2, or 3.

Inventors: DRAZIN SHEPARD, VAN ANTWERP WILLIAM PETER, DRAZIN, SHEPARD, VAN ANTWERP, WILLIAM, PETER
Applicants: GSP METALS CHEMICALS CORP, GSP METALS CHEMICALS CORPORATION
Application year: 1983
Main classification: C25D3
IPC codes: C25D3, H05K3