Thermally dissipated soldering flux and method of use

Abstract: A no clean thermally dissipated soldering flux includes camphor as a flux base, an organic activator and an organic diluent. The camphor provides a tacky yet fluid medium which is thermally dissipated during the flux operation and leaves no undesirable residue which would require a postcleaning step for removal.

Description: The present invention relates to soldering fluxes and their use in processes for soldering pieces, particularly metallic pieces, together. The flux compositions of the invention are particularly useful in the field of microelectronics, such as in the soldering of integrated circuit modules and integrated circuit boards. A common task in the manufacture of microelectronic components involves the manufacture of single chip or multi-chip modules having input/output pins which are inserted into a substrate. The input/output pins provide the needed electrical connections to the integrated circuit chip or chips which are subsequently connected to the substrate or carrier. In other manufacturing processes, a chip is soldered directly to a printed circuit board. With either process, solder flux compositions have typically been applied to the pins in order to connect the component to the selected substrate, for instance, the printed circuit board. Flux compositions are employed to remove oxides from the pins and to prevent the pins from oxidizing when subjected to elevated temperatures for soldering, thereby serving to maintain the electrical conductivity of the pins. Once the solder is applied, any flux composition or residue remaining on the pins and substrates must be removed to provide as clean a substrate as possible. This has meant that an extra step of flux removal was necessary in the manufacturing process. The soldering operation, in general, and flux removal, in particular, is increasingly difficult when applied to microelectronics. The pieces to be joined are extremely small, making cleaning, tinning, postcleaning and inspection difficult. In some cases, to avoid over heating, only the lead portion of the parts to be joined can be heated during the soldering operation. Cleaning and postcleaning are difficult due to the small size of the component, their large numbers, and the potential damage to the electronics by the cleaning solutions used, if any. Another problem source results from the fact that many of the known soldering fluxes are corrosive. In the environment of microelectronics, corrosion from any residual flux can ruin an extremely costly device. Many of the organic water soluble fluxes presently available contain corrosive materials such as halides. A flux composition which contains free halogen can result in conversion to hydroacids and corresponding halide ions by hydrolysis at the soldering temperature. Hydroacids can further react with organic materials present in the flux to free halide ions. Accordingly, if the flux residue is not entirely removed, it will lead to corrosion of the parts soldered. Because of these problems, so-called "non-activated" rosin fluxes have been used in the microelectronic environment. This has not generally provided an acceptable solution, however, since the pure rosin alone is limited in oxide removal capability and can require rework to produce an acceptable product. To improve the pure rosin flux oxide removal capability, a number of "activated" or "mildly activated" rosin fluxes have been developed. These products have several shortcomings, including the necessity of a cleaning step to ensure the removal of corrosive agents left behind after the soldering operation. For instance, it was often necessary to employ a hot water rinse in combination with neutralization or a mild hydrochloric acid solution in combination with a hot water rinse and neutralization or to use specialized water-based detergents. These cleaning steps were extremely difficult during the assembly of chips to integrated circuit board where the low stand-off height of the chip to the substrate typically 5.08 X 10⁻⁵ metres (0.002 inches) to 1.02 X 10⁻⁴ metres (0.004 inches) made it extremely difficult to clean underneath the chip with an aqueous or non-solvent process. Other commercially available low residue fluxes have proved to be too thin, running out from under the chip during the manufacturing operation and failing to hold it in place. SU198104 describes a solder flux containing a polyester resin and an organic solvent and includes 4-20% of camphor and 1-2% of dibutylphthalat which are claimed to improve the elasticity of the flux film. According to the invention a thermally dissipated solder flux comprises about 10% to 20% by weight of camphor as a flux base; about 1% to 5% by weight of an organic activator; and the remainder consisting of one or more organic diluents. Preferably the invention provides a thermally dissipated no clean soldering flux adapted for use in assembling chips to integrated circuit boards where the stand off height of the chips to the boards is on the order of about 5.08 x 10₋₅ metres (0.002 inches) to 1.02 x 10₋₄ (0.004 inches) the flux comprising: from about 10 to about 20% by weight of camphor as a flux base; from about 1 to 5% by weight of an organic activator; the remainder being substantially at least one organic diluent and wherein the flux is characterized by the absence of halogen and rosin components and is thermally dissipated, in use, leaving no visible residue which would require a post cleaning step. The invention also provides a method for fluxing the surface of a portion on a piece, comprising the steps of: furnishing a piece to be fluxed; contacting the portion to be fluxed with a solution comprising of a mixture of 10% to 20% by weight of camphor as a flux base, 1% to 5% by weight of an organic activator and the remainder consisting of one or more organic diluents; and heating the contacted portion to a temperature at which the organic activator reacts with oxides present on the surface and the solution evaporates. Preferably the invention provides a method for fluxing the surface of a portion on a piece, comprising the step of: furnishing a piece to be fluxed; contacting the portion to be fluxed with a solution comprising a mixture of 10 to 20% by weight camphor as a flux base, from 1 to 5% adipic acid as an organic activator and the remainder consisting of one or more organic diluents; and heating the contacted portion to a temperature at which the organic activator reacts with oxides present on the surface and the solution evaporates. The invention further provides a method for joining portions of two pieces, comprising the steps of: furnishing the two pieces to be joined; contacting the portions to be joined to a solution comprising 10% to 20% by weight of camphor as a flux base, 1% to 5% by weight of an aliphatic dicarboxylic acid activator and the remainder consisting of one or more alcohol diluents; applying a metallic solder to at least one of the portions to be joined to tin the portion at a temperature above the melting temperature of the solder; and contacting the two portions to be joined at a temperature above the melting point of the metallic solder and then cooling the contacted portions to a temperature below the melting point of the metallic solder, thereby joining the contacted portions together, the method being carried out without any postcleaning of the joined portions. Preferably the invention provides a method for joining portions of two pieces, comprising the steps of: furnishing the two pieces to be joined; contacting the portions to be joined to a solution consisting essentially of 10% to 20% by weight camphor as a flux base, 1% to 5% by weight adipic acid as an acid activator and the remainder being an alcohol diluent; applying a metallic solder to at least one of the portions to be joined to tin the portion at a temperature above the melting temperature of the solder; and contacting the two portions to be joined at a temperature above the melting point of the metallic solder and then cooling the contacted portions to a temperature below the melting point of the metallic solder, thereby joining the contacted portions together, the method being carried out without any postcleaning of the joined portions. In order that the invention may be fully understood preferred embodiments thereof will now be described. In accordance with a preferred embodiment of the invention the soldering flux comprises (1) camphor as a flux base; (2) an organic solvent or diluent; and (3) an organic activator of the type which reacts with oxides present on the surface to be fluxed. The camphor used in the soldering flux of the invention is a known material and is commercially available from a number of sources. It occurs naturally in the camphor tree, laurus camphora and can be synthesized from vinyl chloride and cyclopentadiene. The partial synthesis from pinene is also an important source. It is a translucent mass with a fragrant, penetrating odour and has a slightly bitter, cooling taste. It sublimes appreciably at room temperature and atmospheric pressure, has a molecular weight of approximately 152.24, a melting point of approximately 175°C plus or minus 2°C. It is commercially available from, for instance, Aldrich Chemical Co. Inc., of Milwaukee, Wisconsin, Sigma Chemical Co., of St. Louis, Missouri, and Eastman kodak Co. of Rochester, New York, as "dl-camphor", preferably, in the 96% pure form. The camphor component is employed in the compositions of the present invention in amounts of about 10 to about 20%, most preferably about 14-15%, by weight of the total flux composition. The compositions of the present invention also contain an organic activator which is effective to remove oxides present on the surface to be fluxed and which improves wetting of the substrate. Preferably, the organic activator is ephemeral in nature, completely dissipating when heated in the range of about 200°C and above. The known organic activators include, for example, aliphatic dicarboxylic acids having about 2 to 6 carbon atoms, i.e., oxalic acid, malonic acid, succinic acid, glutaric acid and adipic acid. Other organic activators are referenced in the include, for instance, ethylene diamine tetraacetic acid and/or diethylene triamine pentaacetic acid and/or sodium salts thereof. A particularly preferred organic activator for the present purpose is adipic acid HOCO (CH₂)₄COOH. Adipic acid is a white crystalline solid having a molecular weight of approximately 146.14, and a melting point of 153°C. It is found naturally in beet juice and can be prepared by oxidizing cyclohexanol with concentrated nitric acid. It is commercially available from Aldrich Chemical Co., Inc., Sigma Chemical Co. and Eastman Kodak Co. The organic activator component of the soldering fluxes of the invention is present in the range from 1 to 5% by weight and, preferably about 2.0 to 2.5% by weight of the total flux composition. The remainder of the soldering fluxes of the invention is substantially at least one organic solvent or diluent which will evaporate during the reflow operation. A variety of evaporative organic solvents are known, the preferred diluent being an aliphatic alcohol having about 1 to 4 carbon atoms, i.e., methyl alcohol, ethyl alcohol, isopropyl alcohol, n-propyl alcohol, isobutyl alcohol and n-butyl alcohol. Other organic solvents which are known to be operative include tetrahydrofuran, acetone, ethyl acetate, ethyl formate, hexane, methylene chloride, benzene, toluene, cyclohexanone, and mixtures thereof. However, the aliphatic alcohols are preferred because of the absence of objectionable odours, adverse reaction with the other ingredients and because of the lack of environmental and health concerns. The organic diluent component of the soldering fluxes of the invention is present in the range from about 60 to 90% by weight, preferably about 80 to 90% by weight, most preferably about 83% by weight. The following non-limiting examples are presented in order to further illustrate embodiments of the present invention: EXAMPLE I A soldering flux in accordance with an embodiment of the invention was prepared by mixing 60 ccs of isopropyl alcohol, 1.0 x 1⁻² kg (10 grams) of camphor (96% pure) and approximately 2.0 X 10⁻³kg (2 grams) of adipic acid (2.5% by weight). This can be conveniently accomplished by dissolving the camphor in the isopropyl alcohol and adding the organic acid. In order to test the effectiveness of the soldering flux so prepared, a number of pull tests were conducted in which the force in quanta of 0.45 kilograms (pounds) required to dislocate a chip pin from a substrate was measured using an INSTRON Pull Tester (Instron is a trade mark of Instron Limited). The results for the compositions of the invention are shown in Table I as compared to the results obtained from two commercially available fluxes. Table II shows the result of visual card inspections in which pins soldered using the fluxes of the invention are compared to pins soldered using the same two commercially available fluxes. The soldering fluxes of the invention have improved pull strength and are substantially free of dimples or craters. Note that a number of the Type II test chips were off registration due to the excessively thin nature of the solder flux. EXAMPLE II A soldering flux in accordance with another emodiment of the invention was prepared by mixing 60 cm³ of isopropyl alcohol; 10 grams of camphor (96% pure) and approximately 2 grams (2.5% by weight) of succinic acid. The test results obtained are similar to those obtained in Example I above. An advantage of the invention is that the soldering flux is thermally dissipated during use and leaves no visible residue which would require a postcleaning operation. Another advantage is that the soldering flux exhibits improved wettability producing the activity needed to effectively remove oxides on the surface to be fluxed. The camphor component provides a tacky yet fluid medium which holds a precisely aligned chip in place during reflow and which requires no cleaning. The composition exhibits substantially no corrosivity even if left on the card for a period of time before or after the soldering operation. By-products of the flux as herein described are inert and leave no undesirable residue. The compositions can be formulated without the use of chemicals which are environmental and/or health hazards. A thermally dissipating soldering flux has been described which comprises camphor as a flux base to provide an appropriately tacky yet fluid medium for the flux, a liquid solvent or diluent, such as isopropyl alcohol, and an organic activator composition, such as an aliphatic dicarboxylic acid. By combining these ingredients, a liquid solder flux is produced which remains tacky after application, holding a precisely aligned chip in position for reflow, and which is subsequently thermally dissipated leaving no visible residue which would require cleaning. A method has been descrived in which the aforesaid thermally dissipating soldering flux is used for fluxing the surface of a portion on a piece, such as the pin of an integrated circuit chip and/or the circuit pad on the substrate. The portion to be fluxed is contacted with the aforesaid soldering flux consisting essentially of a mixture of camphor as a flux base, an organic activator and an organic diluent. The contacted portion is then heated to a temperature at which the organic activator reacts with oxides present on the surface of the portion of the piece and the solution evaporates leaving no residue. In joining the chip to a substrate, such as a printed circuit board, the pin and a selected area on the substrate are fluxed using the composition of the invention. A metallic solder is then applied to at least one of the chip and substrate to be joined at a temperature above the melting point of the solder. The chip and substrate are contacted at a temperature above the melting point of the metallic solder and then cooled to a temperature below the melting point of the solder, thereby joining the contacted portions together, without any postcleaning of the joined components. A soldering flux has been described which thermally dissipates after solder reflow so that substantially no visible residue is left on the printed circuit substrate which would be visible with conventional inspection techniques such as light microscopy, or visual inspection at low powers of magnification. The soldering flux is a "no cleaning" necessary flux which results in manufacturing cost savings and which is environmentally better, requiring no chemical reclamation or waste treatment. A "soldering" was described, said flux having a novel flux base component which provides improved wetting properties and which provides a flux which remains tacky after application, holding a precisely aligned chip in proper position. A thermally dissipated soldering flux having the required activity to remove oxides present on the substrate to thereby promote adequate bonding was also described.

Claims: A thermally dissipated soldering flux comprising: 10% to 20% by weight of camphor as a flux base; 1% to 5% by weight of an organic activator; and the remainder consisting of one or more organic diluents. A thermally dissipated soldering flux as claimed in claim 1, wherein said organic activator is an aliphatic dicarboxylic acid having 2 to 6 carbon atoms. A thermally dissipated soldering flux as claimed in claim 2, wherein said organic activator is selected from the group consisting of: oxalic acid, malonic acid, succinic acid, glutaric acid and adipic acid. A thermally dissipated soldering flux as claimed in any of claims 1, 2 or 3 wherein said organic diluent is an aliphatic alcohol having 1 to 4 carbon atoms. A thermally dissipated soldering flux as claimed in claim 4, wherein said organic diluent is selected from the group consisting of: methyl alcohol, ethyl alcohol isopropyl alcohol, n-propyl alcohol, isobutyl alcohol and n-butyl alcohol. A method for fluxing the surface of a portion on a piece, comprising the steps of: furnishing a piece to be fluxed; contacting the portion to be fluxed with a solution comprising: 10% to 20% by weight of camphor as a flux base; 1% to 5% by weight of an organic activator; and the remainder consisting of one or more organic diluents; and heating the contacted portion to a temperature at which the organic activator reacts with oxides present on the surface and the solution evaporates. A method as claimed in claim 6, wherein said organic activator is an aliphatic dicarboxylic acid having 2 to 6 carbon atoms. A method as claimed in any of claims 6 or 7, wherein said organic activator is selected from the group consisting of: oxalic acid, malonic acid, succinic acid, glutaric acid and adipic acid. A method as claimed in any of claims 6, 7 or 8, wherein the organic diluent is an aliphatic alcohol having 1 to 4 carbon atoms. A method for joining portions of two pieces, comprising the steps of: furnishing the two pieces to be joined; contacting the portions to be joined to a solution comprising: 10% to 20% by weight of camphor as a flux base; 1% to 5% by weight of an aliphatic dicarboxylic acid activator; and the remainder consisting of one or more alcohol diluents; applying a metallic solder to at least one of the portions to be joined to tin the portion at a temperature above the melting temperature of the solder; and contacting the two portions to be joined at a temperature above the melting point of the metallic solder and then cooling the contacted portions to a temperature below the melting point of the metallic solder, thereby joining the contacted portions together, the method being carried out without any postcleaning of the joined portions. A method as claimed in claim 10, wherein at least one of the pieces is an electronic component. A method as claimed in any of claims 10 or 11, wherein one of said pieces is an electronic chip and one of said pieces is a printed circuit board.

Inventors: MACE EVERITT WILLIAM, SICKLER JANET, SRINIVASAN VARADARAJAN, MACE, EVERITT WILLIAM, SICKLER, JANET, SRINIVASAN, VARADARAJAN
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1990
Main classification: B23K35
IPC codes: B23K101, B23K35