Metal treatment – Compositions – Fluxing
Reexamination Certificate
2001-04-12
2003-02-25
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Compositions
Fluxing
Reexamination Certificate
active
06524398
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to soldering flux compositions useful in soldering applications for electronic assemblies such as printed wiring boards.
BACKGROUND OF THE INVENTION
The manufacture of printed circuit boards (PCBs), also referred to as printed wiring boards (PWBs), is typically divided into two processing categories, fabrication and assembly. Towards the end of the PCB fabrication process, after plating and etching, most of the PCB is covered with a solder mask. The solder mask is used to protect masked areas of the PCB from the attachment of solder. Accordingly, areas of the PCB that will later be soldered, such as the pads and holes, are not covered with the mask.
In a first step of the assembly process, both through-hole and surface mount components, such as integrated circuits, connectors, dual in-line packages, capacitors and resistors, are oriented with the PCB. For example, a component with leads may be mounted on the PCB by placing its leads through holes in the board. Surface mount components can be attached to the board's bottom side (i.e., the surface to be soldered) using adhesive. The components are then ready to be soldered to the PCB to ensure reliable electrical continuity between the components and PCB circuitry. The component leads and terminations or pads can be soldered via a wave solder process.
The wave solder process includes the steps of fluxing, preheating and soldering. In the fluxing step, a flux is used to prepare the surfaces to be soldered. Such preparation is generally needed because the PCB and the components can become contaminated as a consequence of having been stored in a non-clean-room environment before the soldering process. In addition, oxides may have formed on the leads, terminations and/or pads. In addition to reacting with or removing contaminants and oxides, the flux can perform other functions, such as protecting the surfaces from re-oxidation and reducing the interfacial surface tension between the solder and the substrate to enhance wetting.
Typically, a spray, foam or wave fluxing process is used to apply the flux onto the PCB and component surfaces to be soldered. The fluxing procedure is followed by a preheating step to evaporate the solvent carriers in the flux, such as alcohols or water, and to begin heating the surfaces to be joined. The preheat step is followed by a wave solder process in which the PCB, with components mounted thereon, is passed over a wave of molten solder. The solder wave is pumped through a nozzle; and the wave then contacts and deposits solder on the surface to be joined. The deposited solder then serves to bond and electrically connect the leads and terminations of the components with the contacts on the PCB.
Many of the existing low-solids, no-clean soldering fluxes cause an excessive number of solder balls to be left on the PCB surface. These fluxes also cause solder bridging because their surface tensions are too high. In addition, due to their weak activity levels, the fluxes are unable to thoroughly remove the tarnish and oxides from the surfaces to be joined. Solder balls are unwanted balls of solder occurring randomly or non-randomly on the solder mask and/or between the leads of the components on the board; the solder balls can bridge a gap between two conductors resulting in an electrical short. Solder bridges are connections of unwanted solder that can form a short circuit between two traces or leads that were not designed to be connected. Solder bridges and/or solder balls may cause electrical failure of the board. Excessive solder balls and bridging also require costly solder touch-up operations for their removal. Even tiny solder balls, often referred to as micro-solder balls, because they are only visible with magnification (e.g., 10 times magnification), can result in electrical shorting of very-closely-spaced board lines and pads as well as component leads and terminations. Another problem with existing fluxes is that they can leave visible residues on the surfaces of the PCB, which in addition to being unsightly, can cause false rejects with in-circuit pin testing.
Thus, a need exists for a soldering flux composition that effectively prepares the surfaces to be joined and that reduces the number of solder balls and solder bridges by reducing the surface tension between the PCB surfaces, component leads/terminations and molten solder without increasing the amount of visible residue.
SUMMARY
Described herein are fluxes offering reduced-micro-solder-balling, low-residue, low-solids, and no-clean capability. The fluxes include a solvent, an activator in the solvent, a cationic surfactant and a nonionic surfactant. The fluxes are particularly useful for coating a PCB prior to application of solder.
In one embodiment, the solvent is an alcohol, such as isopropyl alcohol. The cationic surfactant can be a quaternary ammonium fluoroalkyl surfactant. The nonionic surfactant can be a nonylphenoxypolyethoxyethanol surfactant. The activator can be a combination of a dicarboxylic acid and a nonionic brominated compound. A printed circuit board coated with a flux described herein includes a substrate on which conductive pathways and conductive contacts, typically formed of metal, are printed and electrically coupled. The flux is coated on the conductive contacts; then solder is applied onto the flux and the board. The solder provides electrical coupling between the contacts and components fixed to the PCB.
Fluxes described herein offer a variety of advantages. The fluxes can effectively remove metal oxides from the PCB surfaces to be soldered so as to promote solder wetting. The fluxes can also substantially lower the interfacial surface tension between the PCB surfaces and a molten solder alloy, thereby promoting drainage of excess solder from the board surface and consequently reducing solder-ball and solder-bridge formation. Moreover, the fluxes can reduce the amount of visible flux residue formed during the wave soldering process.
DETAILED DESCRIPTION
The soldering flux composition incorporates one or more cationic surfactants, one or more nonionic surfactants and activators dissolved in volatile solvent, such as isopropyl alcohol, ethyl alcohol, de-ionized water or mixtures thereof. Suitable concentration ranges (by weight percent) for these components in the flux composition are as follows: 50-98% solvent, 0.2-10% activator, 0.01-1.0% cationic surfactant, and 0.05-10% nonionic surfactant. In particular embodiments, the concentration range (by weight percent) for each of the above-mentioned components is as follows: 75-98% solvent, 0.2-5.0% activator, 0.05-0.5% cationic surfactant, and 0.10-2.0% nonionic surfactant. One or more high-boiling-point additives can also be incorporated into the flux composition, e.g., at a concentration of 0.2-25% by weight.
A cationic quaternary ammonium fluoroalkyl surfactant, such as FLUORAD FC-135 surfactant (manufactured by 3M Co. of St. Paul, Minn.), SURFLON S-121 surfactant (manufactured by Seimi Chemical Co., Japan), or Neos FTERGENT 300 surfactant (manufactured by Neos, Japan), is used to substantially reduce flux residues as well as the surface tension of the board surface and molten solder alloy. Consequently, the volume of random and non-random solder balls on the board surfaces is also reduced. Ammonium or amine fluoroalkyl surfactants that include an aromatic sulfone functional group (such as Neos FTERGENT 300 surfactant, which is a cationic quaternary ammonium fluoroalkyl compound with an aromatic sulfone functional group) have been found to be particularly effective.
Nonionic surfactants are added to further lower the surface tension of the composition and improve the high-temperature survivability of the flux to further reduce solder balling and bridging. The nonionic surfactant can resist decomposition on a boiling solder pot at a temperature of about 500° F. (260° C.). Suitable nonionic surfactants include, but are not limited to, nonylphenoxypolyethoxyethanols, Octylphenoxypolyethoxyethanols, alcohol etho
Arora Sanyogita
Mo Bin
Fry's Metals, Inc.
Jenkins Daniel J.
Mintz, Levin, Cohn, Ferris & Popeo, P.C.
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