Metal treatment – Compositions – Fluxing
Reexamination Certificate
2000-12-01
2003-07-29
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Compositions
Fluxing
C148S024000, C219S085150
Reexamination Certificate
active
06599372
ABSTRACT:
BACKGROUND
Electronics products, such as computers and communications equipment, employ printed wiring boards (PWB's), or printed circuit boards (PCB's, the two terms used interchangeably throughout). During the assembly of a PCB, components, such as integrated circuits, connectors, dual in-line packages, capacitors and resistors, are soldered to conductive contacts that form part of a circuit-interconnecting pattern on the board. In a solder wave process, components can be mounted on the bottom side of the board with adhesive if they are lead less and on the top of the board via through-holes if the components are leaded wherein leads of a component are passed through a plated through hole in the board. Portions of the leads that protrude through the back or underside of the board are then crimped over and secured to the board with solder that is applied as a molten “solder wave” across the underside of the board. The solder that is thereby applied to the board forms a bond and electrical connection between the surfaces of the component leads and the contacts on the board.
Prior to the actual soldering of such assemblies, a flux is ordinarily applied to the surfaces to be joined to chemically prepare the surfaces to receive the solder. The flux removes and prevents formation of oxides on the surfaces and thereby promotes wetting and continuity of the solder at its interface with the circuit. Consequently, the quality and integrity of electrical and mechanical connections between adjoining surfaces are likewise promoted. Typically, the flux is applied by spray, wave or foam. The fluxed assembly is then pre-heated prior to soldering to volatilize the flux carrier from the surfaces of the PCB.
Soldering fluxes that have been used by the electronics industry in the wave soldering of printed wiring assemblies have traditionally included ingredients that are detrimental to the environment and/or that necessitate the use of post-soldering cleaning agents, which also present undesirable environmental risk. For example, typical resin/rosin-containing fluxes are solvent based (e.g., isopropyl alcohol based), wherein the solvent generally volatilizes in the form of volatile organic compounds (VOC's) that pollute the environment and that may also harm the ozone layer.
As the environmental consequences of electronics production processes have become better understood, and in order to meet increasingly strict government regulations, the art has sought to avoid the above-described problems by proposing water-based fluxes that include acidic rosin. To improve the solubility of the acidic rosin in water, a volatile basic compound, such as ammonia or an amine, is incorporated into the composition.
SUMMARY
The soldering flux described herein can be used for flux coating when soldering electronic assemblies such as printed wiring assemblies. The flux is an aqueous composition that includes a resin.
The flux can be substantially free of volatile organic compounds such as solvents and amines. In addition to the water and resin, the flux can further include activating agent(s) such as organic acids (e.g., mono-, di-, and tricaboxylic acids) and surface-active agent(s) that promote surface wetting. In one embodiment, a non-acidic resin in the form of a pentaerythritol ester of hydrogenated rosin is used. In other embodiments, the resin can be a rosin.
The flux can be applied to a substrate to clean the oxidation from the metallic surfaces (e.g., surfaces of copper, tin, tin-silver, tin-lead, palladium, etc.) and to protect the surfaces from further oxidation. The water can then be evaporated from the flux, followed by application of solder (e.g., via a wave soldering process) to the flux and use of the solder to secure an electronic component to the substrate. The flux residues need not be removed from the board before the board is put into use in an electronics application.
Various embodiments of these soldering fluxes offer numerous advantages over those known in the art. In particular, many advantages can be derived from the use of fluxes that are free of volatile organic compounds.
For example, basic nitrogen compounds, particularly amines (volatile and/or non-volatile at soldering temperatures), that have been used in known flux compositions tend to be flammable, and noxious if volatile and, accordingly, present numerous health and environmental hazards. Further, if the basic compounds that are used are non-volatile, they can react with the acidic resins and metallic ions to produce residues in the form of rosin soaps and metal salts, which are hygroscopic and conductive and, if left on the soldered circuits, can compromise the surface insulation of the PCB assembly. Moreover, in hot and humid environments, these residues absorb moisture, which promotes dendritic growth, which, in turn, causes current leakage. Consequently, an extra step is generally needed after soldering to completely eliminate the residues of the basic compound from the PCB. This post-soldering cleaning is often performed with a cleaning agent (e.g., a chlorofluorocarbon or other organic solvent mixtures), which, itself, presents environmental hazards. Yet another disadvantage of using amines in accordance with known teachings is that these amines can react with an organic acid activator in the flux solution. To offset the loss due to this reaction, relatively large quantities of amines may be needed to provide a stable solution.
Fluxes that are free or substantially free of amines, such as those described herein, can avoid or ameliorate each of these problems. Consequently, flux residues can remain on the board without compromising the electrical reliability of the circuit assemblies, thereby eliminating the need to either wash the flux residues from the board surface or cover the residues with a conformal coating.
Further, fluxes that are also free or substantially free of volatile organic solvents that are typically used in known fluxes also remove or minimize the health and environmental hazards associated with those components. By eliminating the presence of both the amines (volatile or non-volatile) and the volatile organic compounds (solvents) from the flux, as described herein, many of these problems can be entirely or substantially eliminated.
Additionally, the fluxes described herein can provide a high grade of solderability. The term, “solderability,” as used herein, refers to an ability to provide a clean metallic surface on the substrate, as well as on the component leads, so that a high degree of wetting and coalescence of molten solder on the PCB surface can be achieved.
DETAILED DESCRIPTION
The soldering fluxes of this disclosure are aqueous compositions comprising one or more dispersed resins. The fluxes typically also include activating agent(s) and surface-active agent(s) that promote surface wetting. The flux can easily be prepared by the addition of appropriate proportions of resin dispersion, activators and surfactants to water at room temperature along with other property-enhancing additives, and mixing to form an aqueous solution. The end composition is 70 to 97% water by weight. Preferred embodiments include fluxes in which the range of water is 90 to 96% by weight of the flux.
In particular embodiments, the resin is a pentaerythritol ester of hydrogenated rosin, commercially available in the form of an aqueous dispersion, known as PENTALYN H-55WBX resin dispersion (available from Hercules, Inc., Wilmington, Del., USA). Examples of other resins that can be used, and which are also available in the form of aqueous dispersions, include TACOLYN 1065 resin dispersion, TACOLYN 1070 resin dispersion and FORAL 85-55WKX resin dispersion (each of which is also available from Hercules, Inc., Wilmington, Del., USA). The resin solid forms 0.1 to 15%, by weight, of the flux, with a preferred range of 0.1 to 10%, by weight, resin solid in the flux. The preferred range can be formed by adding between about 0.2 to about 18.2%, by weight, resin dispersion (55% solid resin) to the aqueous composition.
Arora Sanyogita
Schneider Alvin F.
Tellefsen Karen A.
Fry's Metals, Inc.
Jenkins Daniel J.
Mintz, Lewin, Cohn, Ferris, Glovsky and Popeo, P.C.
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