A2. workpiece heating methods A2a. with soldering iron (INS) – Soldering irons are solid copper hand tools, nickel or iron plated, heated electrically or by immersion in a gas flame. Contact between the iron and the base materials conducts heat to the latter so that solder in contact with them will melt and flow into the joint. (Proper technique involves application of heat to the base materials rather than to the solder. The base material must be above the melting point of the solder if the solder is to flow properly into the joint and wet the surfaces.) Soldering irons are useful for prototypes and limited production, and light work such as wiring connections and printed circuit board touch up or repair. Large irons are used for sheet metal work.

A2b. with gas torch - An oxygas torch can be used to heat the workpieces to be joined. One common application is the joining of copper tubing and fittings. The tubing and fittings are heated by torch and the solder then melts and flows into the joint spaces by capillary attraction.

A2c. oven or furnace heating – Several methods are used: circulated air, infrared lamps, electric elements. An inert or reducing atmosphere may be introduced into the oven to facilitate the operation. Conveyorized pass-through ovens can be used for high production applications. Infrared oven heating is widely used for reflow soldering of electronic printed circuit boards and is covered in section 13C6a.

A2d. selective infrared heating – Infrared light can be focused onto very small areas, so the heated area can be limited to the area of the joint. It is not necessary to place the assembly in an oven.

A2e. vapor-phase heating - is a method used to “reflow” solder paste deposits on electronic printed circuit boards. See section 13C6b for a description of the process.

A2f. Resistance heating (RS) - uses the electrical resistance of the workpieces themselves to provide the heat required for soldering. Metal or carbon electrodes, carrying a low-voltage, high amperage current, are attached or clamped to the workpieces or brought in contact with them. The workpieces are usually preassembled with a solder preform and flux or solder paste prior to heating. One common method is to have one part of the assembly clamped to a grounded fixture. The other electrode then can be manually brought in contact with the other part. The heating operation is usually quick. The manually held electrode is lifted from the workpiece by the operator when the parts become hot and the solder starts to flow. Other systems use clamped parts and a manual, timed, or automatic switch. Shut-off can be made automatic with a circuit that senses the drop in electrical resistance when the melting solder wets the parts and improves the conductivity between them. Resistance heating is suitable for higher production levels and is particularly useable when other methods of heating are less workable due to inaccessibility of the joint or the need to avoid overheating other parts of the assembly. If the dimensions of the parts to be joined and their resulting electrical resistance vary somewhat, the process will be more difficult to control. In such situations, the process is limited to less critical applications.

A2g. laser heating - provides very rapid, extremely localized heating. Its principal soldering use is in the reflow soldering of electronic printed circuit boards. The method is described in section 13C6c.

A2h. induction heating - generates heat in the joint from its electrical resistance to induced eddy currents. Eddy currents are induced in the joint by high-frequency alternating current in a coil adjacent to or surrounding the joint. The amount of heat generated depends on the amount of electrical power in the coil, the magnetic properties of the workpiece material and how well the coil and workpiece are electromagnetically “coupled”. The advantages of induction heating are its speed and the fact that the heat is localized in the joint area. The joint is preassembled prior to induction heating, and solder paste or preforms and flux are included in the assembly. Fixturing, either external or by self-fixturing parts, is required to hold the joint in the desired arrangement and the induction coil must be fabricated to fit the shape of the joint. A certain amount of development may be needed to insure good coupling between the coil and the joint to insure proper energy transfer. Because of the cost of these factors, moderate or high levels of production are required for economic operation. However the operation, once established provides very good repeatability and low unit costs. Robotic and other automatic operation can be developed if quantities are sufficiently large to justify the investment required. Typical soldering applications are: fittings for tubing and hose, container seals, and complex machine components where the strength requirements are modest enough to allow use of solder as a filler metal. Induction heating is more commonly used as a heating means for brazing than for soldering. Heating for localized heat treating is another application. Fig. 7A2h shows a typical arrangement of induction coil and workpieces.

Fig. 7A2h Arrangement for induction heating to melt a solder preform in the joining of two pieces of metal tubing.

A2i. hot gas - Hot gas soldering is a technique used in the electronics industry for repair operations. It uses a stream of hot gas to provide localized heating of joints that require rework. See section 13C7.

A3. fluxes - All soldering, brazing and welding requires a clean metal-to-metal contact in order to produce a satisfactory joint. Oxides and other coatings on metal surfaces prevent this contact. Oxidation and other surface tarnishing are very likely at the elevated temperatures required in these metal joining operations. Fluxes are used to prevent and remove such surface coatings and to shield the metal surfaces so tarnishing does not reform. Fluxes also have another function; they provide greater fluidity (spreading and surface wetting ) of the filler metal used. Fluxes may be in gaseous, liquid, or solid form, but solids, pastes, and liquids are the common forms.

Welding fluxes are included in the electrode coating or core of flux-bearing welding wire and include calcium carbonate, fluorspar, dolomite, and sodium silicate that produce shielding gas or shielding slag as well as fluxing action. Welding processes may use inert gases - usually argon – to shield the molten metal from formation of oxides and other unwanted compounds.

In brazing, combinations of borax, borates, boric acid, fluorides, chlorides, and fluoborates are used as fluxes. The most active solderingfluxes include inorganic acids and inorganic chlorides used in structural soldering applications, and hydrogen and hydrogen-chloride gases used in transistor manufacture 6 . Less active solderingfluxes are organic acids and halogens. Rosin, a derivative of pine sap, is a natural mild flux. It is inactive at normal temperatures but provides fluxing action when heated to above its melting temperature. Rosin is used in electronic soldering applications, though less extensively than previously. Almost all commercial fluxes are mixtures of several materials to provide the desired properties. Solvents, viscosity modifiers, combinations of active ingredients, surfactants, and other additives may be included. These ingredients are processed to a blended mixture in mixing equipment of the type described in 11G with the choice of mixing equipment dependent on whether the materials are solid or liquid and, if liquid, the viscosity involved. (See 13C1 for flux application in electronics manufacture.)