The soldering process is the joining of metals by causing a lower-melting-point metal to wet or alloy with the joint surfaces and then solidify in place. Solders are used to establish reliable electrical connections, make a liquid-or gas-tight joint and hold parts together physically. The soldering process is a highly effective manufacturing process because soldered joints can sustain relatively heavy loads for extended periods of time and provide reliable electrical connections.

Under usual favorable conditions, solders wet the joint surfaces well enough to be drawn into fine crevices or capillaries by surface tension. In order for the surfaces to accept solder readily, the surfaces and the solder must be free from oxide or other obstructing films. When necessary, parts are cleaned chemically or by abrasion.

A material having a high rate of thermal conductivity is used to create the form of the soldering tip. Iron electroplating is disposed on the form of the soldering tip to reduce the erosion of the form. The problem with the electroplating is that it has a characteristically rough, grainy surface and areas having a high current density receive excessive deposits thereon.

To improve the performance of the soldering tip, the iron electroplate on the working surface is customarily"tinned."To function properly, the soldering tip must be kept tinned at all times. Preventing the"detinning''of the soldering tip is not always possible. When using a soldering tool, the tip is wiped frequently to remove excess solder, flux and residues. Because of the rough texture of the iron electroplate, the tin can be wiped off the peaks of the grainy electroplating. The exposed iron will oxidize quickly and will not wet again unless corrosive flux is used.

Accordingly, it is desirable to have a soldering tip that retains its tinning and prolongs its service life.

It is also desirable to have a soldering tip whose edges and corners do not have excess deposit thereon.

It is further desirable to reduce the surface area of the electroplating to increase the effectiveness of wetting by flux and solder.

SUMMARY OF THE INVENTION It is an object of the present invention to have a soldering tip that retains its tinning for a longer period of time.

It is another object of the present invention to have a soldering tip with a prolonged service life.

It is yet another object of the present invention to create smooth edges and corners that will not inhibit free flow of flux and solder thereon.

It is yet another object of the present invention to reduce the surface area of the electroplating to minimize the area to be wetted by solder or flux.

The present invention addresses the problems associated with prior art techniques. A polished soldering tip is created by making a form from copper or a copper alloy which has a high rate of thermal conductivity. The working surfaces of the soldering tip are overlaid with electroplated iron of at least. 002 inch thickness. The iron electroplating will resist rapid erosion by the tin in molten tin-bearing solders. The iron electroplate is polished until a roughness average. Ra. of the surface texture falls below 8 microinches. In the preferred embodiment of the present invention, the roughness average of the surface texture should fall in the range of less than 8 microinches.

The desired finish can be obtained using various processes, including, polishing, buffing, mass finishing, electropolishing and adding leveling agents to the plating bath. The polishing is usually done is several stages to produce successive surface textures with decreasing levels of roughness. The polishing is initiated by using a relatively coarse abrasive to reduce the texture roughness. This step is repeated using progressively finer abrasives until a smooth finish is obtained. The soldering tip can also be buffed until the working surface is reduced to a near mirror finish. Once the desired finish is realized, the iron electroplate is cleaned, fluxed and tinned.

DESCRIPTION OF THE DRAWINGS Other objects and avantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings, in which: FIG. 1 is a side view of an exemplary soldering tip having a chisel point with cross- hatches illustrating the working area: FIG. 2 is an end view of the chisel point soldering tip shown in FIG. 1 with cross-hatches illustrating the working area: FIG. 3 is a side view of an exemplary soldering tip having a cone point with cross-hatches illustrating the working area : FIG. 4 is an end view of the cone point soldering tip shown in FIG. 3 with cross-hatches illustrating the working area:

FIG. 5 is a side view of an exemplary soldering tip having beveled point with cross- hatches illustrating the working area : FIG. 6 is an end view of the beveled point soldering tip shown in FIG. 5 with cross- hatches illustrating the working area; FIG. 7 is a graphical representation of the roughness average, Ra. of an exemplary surface profile ; and FIG. 8 shows a list of manufacturing processes and the corresponding roughness average that can be achieved by using each of the processes.

DETAILED DESCRIPTION OF THE INVENTION Various soldering tips intended for use in modem electrically or gas-heated soldering tools are shown in FIGS. 1 through 6. As can be seen in FIGS. 1. 3 and 5. the point of the soldering tip can have various forms, including chisel, semi-chisel, bevel, cone. and truncated cone shapes. There are two main considerations when choosing a proper tip shape. First. the tip shape must be able to access the solder joint. Second, it is desirable to maximize the working surface of the soldering tip to increase the heat transfer to the solder joint members and reduce the total time necessary to make a quality solder joint. The working surface is defined as the portion of the soldering tip that is in close proximity to the joint to be soldered and makes contact with the area to be soldered.

FIG. 1 depicts an exemplary soldering tip 10 of the present invention having a chisel point 12. The working surface 14 is indicated by cross-hatched lines. Figure 2 shows the end view 16 of the chisel point soldering tip 10. Cross-hatched lines indicate the working surface 18 at the end of tip 10.

FIG. 3 depicts an alternative embodiment of the present invention wherein soldering tip 20 has a cone point 22. The working surface 24 is indicated by cross-hatched lines. FIG. 4 shows the end view 26 of the cone point soldering tip 20. Cross-hatched lines indicate the working surface 28 at the end of tip 20.

FIG. 5 shows yet another embodiment of the present invention. Soldering tip 30 has a beveled point 32. The working surface 34 is indicated by cross-hatched lines. FIG. 6 shows the end view 36 of the beveled point soldering tip 30. Cross-hatched lines indicate the working surface 38.

The soldering tip is customarily manufactured from copper or a copper alloy having a high rate of thermal conductivity. Copper is chosen because excellent heat transmission is required to conduct heat to the joint to be soldered. The problem with copper, however. is that when it comes into contact with the tin in molten solder, it erodes rapidly. To prevent such erosion, the working surfaces of the soldering tip are overlaid with electroplated iron. The electroplate will resist rapid erosion by the tin in molten tin-lead solders. Iron can be wetted quite easily with tin solders but dissolves much less rapidly than copper.

It should be noted that it is possible to manufacture the form of the soldering tip from other materials having a high rate of heat transmission, such as solid iron. It is also possible to use other materials, such as iron alloys, for electroplating over copper substrates. However. pure iron over a copper substrate is the preferred combination. Also, the most commonly used solders are alloys of tin and lead that melt below the melting point of tin. These are the preferred material for the solders of the present invention. However, antimony, bismuth, cadmium, silver, arsenic. and other materials can be added to improve strength, wetting qualities, grain size, or to produce alloys having desired melting ranges.

The thickness of the iron electroplating applied to the copper form should be at least . 002 inches. Usually, the electroplating thickness ranges between. 005 and. 020 inches. In some cases, the electroplate thickness can be as much as. 100 inches. Iron plating in thickness usually employed in soldering tip manufacturing has a grainy surface and can be expected to have a surface finish with a surface roughness, Ra, in the range of 50 to 124 microinches. As a result of the differing current densities of electroplating, the areas with high current density, such as edges and corners. build up excessive deposits. It is common to utilize abrasives or files to smooth over the working surface and eliminate the excess deposits in the high current density areas.

To improve the performance of the soldering tip, the iron electroplate on the working surface is customarily"tinned."Tinning can be accomplished by electroplating with tin or tin- lead alloy or by fluxing the surface and subsequently dipping the soldering tip in molten tin or tin bearing solder. It is important that the working surfaces of the soldering tip be kept tinned in order for it to function properly. When an iron plated soldering tip is tinned, a very thin layer of an intermetallic compound is formed of iron and tin. The presence of this layer is essential if the tip is to be kept tinned while being used with mild fluxes.

The present invention discloses improving the performance of a soldering tip by polishing the iron-plated surface to a very smooth finish prior to the tinning step. The performance of the polished soldering tip is far superior to an unpolished equivalent. First, the soldering tip retains its tinning better if the iron plate surface is polished. In practice, the working surfaces of the hot soldering tip are wiped frequently to remove excess solder, flux and residues from the working area. A variety of materials can be used for wiping such as sponges, metal turnings, cloth, brushes or paper. If the iron-tin compound is wiped off the peaks of the grainy iron electroplate during the wiping procedure the exposed iron will oxidize quickly and will not wet again unless corrosive flux is used. This condition is usually known as"dewetting"or"detinning."The smoother the iron surface. the less likely that the iron-tin compound will be destroyed.

A further avantage of a smooth surface on the working area of a soldering tip is that a smooth surface has less surface area than a rougher surface, and consequently there is less area to be wetted by solder and flux. Accordingly, there is a better chance of the working area being completely covered by solder and flux. This feature is of increasing importance as the soldering

industry continues a trend toward using smaller diameters of solder wire and lesser quantities of flux.

Another benefit of a smooth surface is that when creating a smooth finish, angular edges are radiused which makes it easier for flux and solder to flow over such edges.

A smooth surface is achieved basically by reducing the surface-texture irregularities. The standard measure of surface-texture irregularities is the roughness average Ra. As shown in FIG. 7. roughness average Ra is the arithmetic average of the absolute values of the measured profile height deviations taken within the sampling length, L, and measured from the graphical centerline. Roughness average Ra is also known as arithmetic average (AA) and centerline average (CLA).

In this description. roughness average, Ra, is expressed in microinches. Surface texture can be described using other evaluation parameters such as root-mean-square deviation of the profile (Rq), ten-point height of irregularities (Rz), maximum height of the profile (Ry) and total peak-to-valley height (Rt). However, in characterizing the surface of a soldering tip, Ra is the most meaningful and is well used in the metal working industries.

To achieve the desired performance, the electroplated surface of the soldering tip should be polished until the roughness average is below 8. The performance improves further if the roughness average is subsequently reduced to below 5. In the preferred embodiment of the invention, the working surface is polished until the roughness average is in the range of less than 8 microinches.

There are many methods that can be used to smooth metal surfaces to achieve the desired roughness average. These methods include polishing, buffing, mass finishing, electropolishing and the addition of leveling agents to the plating bath. FIG. 8 discloses a list of manufacturing processes and the corresponding roughness average that can be achieved by using each of the processes.

The desired finish can be obtained by polishing the working surfaces of the solder tip with abrasives to produce stages of decreasing roughness. After the iron electroplating, the solder tip will have a rough surface. Through the use of a relatively coarse abrasive the roughness of the surface can be reduced. At this point the surface will be smoother than the initial rough texture but it will not be a very smooth finish and it will contain lines that would need to be smoothed out. The smoothing process can be repeated using progressively finer abrasives. Alternatively. using a cloth buff. the working surface can be reduced to a very smooth finish. The buffing can be performed on a fabric wheel with such compounds as tripoli, red rouge, lime bar or other polishing compounds used to produce a high luster on metals.

The finish can also be obtained through mass finishing by tumbling or rolling of parts in rotating barrels or tubs. Alternatively, mass finishing can be accomplished by the agitation of <BR> <BR> <BR> parts in shaker containers, vibratory tubs or other comparable equipment. This process usually involves treatment with successive steps using abrasives of increasing fineness.

These techniques may also be used in combination to achieve the desired results.

Regardless of the process used, the finish obtained by the processes described above is far smoother than the finish produced by sanding or filing. In the preferred embodiment, the surface is smoothed using mass finishing techniques. Initially, a relatively coarse abrasive, referred to as a"cut-down"medium is used. This abrasive is usually in a ceramic. plastic or similar matrix.

This step is repeated several times using progressively finer abrasives. When the surface finish obtains a roughness average in the range of 20-12 microinches. the tips are further contacted with a less aggressive medium. usually in a plastic or other relatively soft matrix until the desired roughness average is achieved.

While the invention has been described in conjunction with the specific embodiments thereof, it is to be evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.