Patent Application
20050268991
This invention relates to methods and compositions for inhibiting corrosion of tin-based surfaces, and to inhibiting corrosion and enhancing solderability of such surfaces.
Tin-based coatings are frequently applied to surfaces of copper- and nickel-based workpieces such as surfaces of electrical connectors, engineering, functional, and decorative devices in order to prevent the copper or nickel-based surface from oxidizing or tarnishing and/or to enhance solderability. Under conditions such as elevated temperatures in air or in other oxidizing atmospheres, tin-coated surfaces of electronic package leads and electrical connectors have a tendency to form oxide films during periods of shipment and storage between manufacture and assembly into electronic devices. The oxide coats, typically only about 50-200 Angstroms (Å) in thickness, discolor the surface of the tin-coated surface and impart a yellowish color which many consumers consider unacceptable. Furthermore, the oxide may degrade the contact resistance of a coated electrical terminal. A tarnish-free surface has lower electrical contact resistance and better solderability than an oxide coated surface.
S. Chen, et al., in U.S. Pat. No. 6,136,460 disclose approaches to reducing the oxidation of tin which involve inclusion of elements that have a more negative free energy of oxide formation than tin into the tin matrix. Such elements include potassium, sodium, calcium, chromium, manganese, magnesium, aluminum, vanadium, zinc, indium and phosphorus amongst several others. Such inclusion may be accomplished by electrolytic deposition of the element onto the tin surface, immersion of the surface in melts of the element, or immersion in salt solutions of the element followed by high temperature reflow techniques. The high temperature exposure renders the process unacceptable for certain temperature-sensitive applications, and increases processing time, cost, and equipment requirements.
H. E. Fuchs, et al., in U.S. Pat. No. 5,853,797 disclose a method and solution for providing corrosion protection of coated electrical contact surfaces which involve exposure of such surfaces to a solution containing phosphonates, lubricants and various volatile organic solvents. Evaporation of such solvents for disposal is fraught with environmental concerns such as handling, hazard to workers, and disposal of waste into streams.
The above approaches suffer from inherent disadvantages such as high processing temperatures, expense, and difficult solvents. Therefore, there is a need for a low temperature, environmentally friendly, and inexpensive method to provide an anti-tarnish, oxidation resistant, agent onto tin-based surfaces in order to provide protection against the yellowing and oxidation typically encountered by electrical terminals and connectors during storage and shipment. The present invention provides a solution to that need.
Among the objects of the invention, therefore, is to provide a method and compositions for imparting corrosion resistance and enhancing solderability of a tin-based surface.
Briefly, therefore, the invention is directed to a method for enhancing corrosion resistance of a tin-based surface of a workpiece comprising contacting the tin-based surface with a composition comprising a phosphonic acid compound and water to form a phosphorus-based film over the tin-based coating thereby inhibiting corrosion of the tin-based surface.
The invention is also directed to various compositions for enhancing corrosion resistance of a tin-based surface on a workpiece comprising between about 0.01 and about 10% w/v of a phosphonic acid compound and water; and such compositions alternatively also containing a concentration up to about 30vol. % of an organic solvent.
Other objects and features of the invention are disclosed.
Corresponding reference characters indicate corresponding parts throughout the drawings.
In one aspect the invention is directed to a method for enhancing corrosion resistance of a tin-based surface of a workpiece. For purposes of illustration, one such workpiece is an electronic component such as an electronic lead of an encapsulated electronic package, or is an electrical connector; but the invention is applicable to any tin-based surface whether part of an electronic device, engineering, functional, decorative, or otherwise. With regard to tin-based surfaces for electronic devices, the method enhances corrosion resistance and also preserves solderability of tin-based surfaces during storage prior to a soldering operation involving reflow of a portion of the tin-based surface.
In accordance with the invention, the tin-based surface is immersed or otherwise contacted with a composition comprising a phosphonic acid compound and water to form a phosphorus-based film over the tin-based surface. This film inhibits corrosion of the tin-based surface. The water is preferably deionized. The phosphonic acid compound has the formula:
where R is hydrocarbyl or substituted hydrocarbyl and the H ions can be replaced by sodium or potassium to produce a phosphonate salt. In one embodiment, R is a long-chain linear or branched substituent and the H ions can be replaced by sodium or potassium to produce a phosphonate salt. In a preferred embodiment, the phosphonic acid compound is a phosphonic acid of the general formula CH3(CH2)nP(O)(OH)2 wherein n is in the range 5 to 17. In one currently preferred embodiment, the composition contains octyl phosphonic acid as the phosphonic acid compound. It is believed that the PO3−2 moiety facilitates attachment of the compound to the substrate surface.
Unless otherwise indicated, the “substituted hydrocarbyl” moieties described herein are hydrocarbyl moieties which are substituted with at least one atom other than carbon, including moieties in which a carbon chain atom is substituted with a hetero atom such as nitrogen, oxygen, silicon, phosphorous, boron, sulfur, or a halogen atom. The hydrocarbyl moieties may be substituted with one or more of the following substituents: halogen, heterocyclo, alkoxy, alkenoxy, alkynoxy, aryloxy, hydroxy, protected hydroxy, hydroxycarbonyl, keto, acyl, acyloxy, nitro, amino, amido, nitro, phosphono, cyano, thiol, ketals, acetals, esters and ethers.
In general, the range of concentration of the phosphonic acid compound is about 0.01 to 10 weight/volume percent (wt./vol. %)(1 wt./vol. %=10 grams in 1 liter). Concentrations of less than 0.01 percent are not effective in corrosion protection, while concentrations above 10 weight percent tend to result in a material with too high a viscosity to be useful for most applications.
Among the various embodiments of the invention are methods which employ compositions comprising the phosphonic acid compound, water, and other optional components as might be desired under certain circumstances. There are other embodiments in which the compositions comprise the phosphonic acid compound, organic solvent, water, and other optional components as might be desired under certain circumstances. And there are other embodiments which consist essentially of these components, i.e., they contain no additional components which materially affect the basic properties of the invention. In the embodiments where components are specifically excluded, such exclusion is critical to achieving certain of the additional properties of those compositions, such as simplicity, low cost, predictability, reduction of risk of interactivity, and stability.
In one embodiment of the invention, organic solvents are employed to assist dissolving the phosphonic acid compound, but only to only a limited extent. In particular, the compositions of this embodiment contain no more than about 30% organic solvent by volume (vol. %), and in some embodiments, less than about 5% organic solvent by volume. The organic solvent may be an alcohol, and in one preferred embodiment, the organic solvent is ethanol. In this embodiment the foregoing concerns are substantially reduced. Advantageously, by keeping the concentration of such organic solvents at a low level, concerns such as handling, hazard to workers, disposal of waste into streams, flash point, and expense are substantially reduced.
In an alternative embodiment, the composition is essentially free of organic solvents. This embodiment contains the phosphonic acid compound and water. Advantageously, by eliminating organic solvents concerns with such solvents such as handling, hazard to workers, and disposal of waste into streams, and expense are eliminated.
The invention is also directed to concentrates containing much less water, which are to be diluted prior to use.
In performing the method of the invention, tin-based surfaces to be treated are exposed to the composition of the invention by immersion, cascading, spraying, or brushing. The exposure time in one embodiment is between about 1 sec and about 60 sec. The temperature of the composition is between about 20 C and about 45 C. The treated surfaces are normally not rinsed. In some cases, the surface may be rinsed to increase the brightness of the surface. However, this rinse can reduce the effectiveness of the film because it partially removes it. The composition is allowed to air dry on the surface to leave a thin phosphorus-based film estimated to be between about 5 Å and about 10000 Å thick.
After drying, the components are stored and/or shipped to a subsequent manufacturing operation where they are incorporated into an electronic device by a metals joining technique, i.e., soldering. Exposure of the treated surfaces to temperatures above about 230 C are specifically avoided between deposition of the film and such time as an actual manufacturing operation. In the manufacturing operation the tin surface is reflowed at a temperature above about 230 C as part of an assembly operation involving connection of the surfaces to a substrate of a device. Accordingly, the first exposure of the phosphorus film-bearing tin-based surface to a temperature above about 230 C is a joining operation.
In one aspect, the method of the invention involves a lead 13 (
An electronic package of the type treated in accordance with one embodiment of the invention is manufactured in part from a lead frame 30 shown in
In another aspect, this invention encompasses an electronic connector as shown in
These figures are schematic and the various respective layers are not drawn to scale.
Further details of the invention are given in the following examples.
Copper sheets having a tin-based surface thereon were immersed in three distinct compositions of the invention in a balance of deionized water for about 10 seconds:
These sheets, and a copper sheet with a tin-based surface thereon with no phosphorus-based composition treatment according to the invention, were exposed to steam aging under the conditions of 85 C and 85% relative humidity (RH). The samples were thereafter observed for discoloration (yellowish color) as follows, where X is the number of days until first observation of discoloration was made:
These results illustrate that with the method and compositions of the invention A, AA, and AAA, there was no noticeable discoloration even after two months.
Photographs were taken of the as-plated copper sheet without the treatment of the invention, and of the copper sheet receiving the treatment of the invention with composition A by immersion for about 10 seconds. Photographs taken after 23 days of 85 C/85% RH steam aging are presented in
Photographs were taken of the as-plated copper sheet without the treatment of the invention, and of the copper sheet receiving the treatment of the invention with composition A by immersion for 10 seconds. Photographs taken after 106 days of 85 C/85% RH steam aging are presented in
Tin-plated samples having undergone steam aging for 18 hours under the conditions of 85 C and 85% relative humidity were subjected to solderability wetting balance tests (Joint Industry Standard J-STD-002). The wetting balance test conditions were: Sn63Pb37 solder, 235 C, R-type non-activated flux. The results of five tests conducted on as-plated samples not receiving the treatment of the invention are presented in
The results of five tests conducted on samples receiving the treatment of the invention (1.6 wt/vol % octylphosphonic acid in water and 0.2% ethanol) are presented in
The present invention is not limited to the above embodiments and can be variously modified. The above description of preferred embodiments is intended only to acquaint others skilled in the art with the invention, its principles and its practical application so that others skilled in the art may adapt and apply the invention in its numerous forms, as may be best suited to the requirements of a particular use.
With reference to the use of the word(s) “comprise” or “comprises” or “comprising” in this entire specification (including the claims below), it is noted that unless the context requires otherwise, those words are used on the basis and clear understanding that they are to be interpreted inclusively, rather than exclusively, and that it is intended each of those words to be so interpreted in construing this entire specification.
