SOLDERING :
The following information will aid you in learning basic soldering skills. It should enable you to solder wires to electrical connectors, splices, and terminal lugs that we have discussed earlier in the chapter. Special skills and schooling are required for the soldering techniques used in printed circuit boards and microminiature component repair.
SOLDERING PROCESS:
Cleanliness is essential for efficient, effective soldering. Solder will not adhere to dirty, greasy, or oxidized surfaces. Heated metals tend to oxidize rapidly. This is the reason the oxides, scale, and dirt must be removed by chemical or mechanical means. Grease or oil films can be removed with a suitable solvent. Connections to be soldered should be cleaned just prior to the actual soldering operation.
Items to be soldered should normally be "tinned" before making a mechanical connection. Tinning is the coating of the material to be soldered with a light coat of solder. When the surface has been properly cleaned, a thin, even coating of flux should be placed over the surface to be tinned. This will prevent oxidation while the part is being heated to soldering temperature. Rosin-core solder is usually preferred in electrical work. However, a separate rosin flux may be used instead. Separate rosin flux is frequently used when wires in cable fabrication are tinned.
TINNING COPPER WIRE AND CABLE :
Wires to be soldered to connectors should be stripped so that when the wire is placed in the barrel, there will be a gap of approximately 1/32 inch between the end of the barrel and the end of the insulation. This is done to prevent burning the insulation during the soldering process and to allow the wire to flex easier at a stress point. Before copper wires are soldered to connectors, the ends exposed by stripping are tinned to hold the strands solidly together. The tinning operation is satisfactory when the ends and sides of the wire strands are fused together with a coat of solder. Do not tin wires that are to be crimped to solderless terminals or splices.
Copper wires are usually tinned by dipping them into flux (view A of figure 1.) and then into a solder bath (pot) (view B of the figure). In the field, copper wires can be tinned with a soldering iron and rosin-core solder. Tin the conductor for about half its exposed length. Tinning or solder on the wire above the barrel causes the wire to be stiff at the point where flexing takes place. This will result in the wire breaking.
Figure 1. - Dip-tinning In a solder pot.
The flux used in tinning copper wire is a mixture of denatured alcohol and freshly ground rosin. This type of flux may be mixed just prior to use. A premixed paste flux may also be used. The solder used for terminal lugs, splices, and connectors is a mixture of 60-percent tin and 40-percent lead. Maintain the temperature of the solder bath (pot) between 450 and 500°F. This keeps the solder in a liquid state. Skim the surface of the solder pot, as necessary, with a metal spoon or blade. This keeps the solder clean and free from oxides, dirt, and so forth.
Dip-tin wires smaller than No. 8 in groups of 8 or 10. Dip-tin wires size No. 8 and larger individually. The procedure for dip-tinning is as follows:
ALTERNATIVE DIP-TINNING PROCEDURE
If an electrically heated solder pot is not available, a small number of wires can be tinned using the following procedure (see figure 2):
Figure 2-. - Alternate dip-tinning method.
Cut off the beveled section of the tip of a discarded soldering iron tip.
Drill a hole (1/4- to 3/8-inch diameter) in the round part of the tip about two-thirds through.
Heat the iron and melt the rosin-core solder into the hole.
Tin the wires by dipping them into the molten solder one at a time.
Keep adding fresh rosin-core solder as the flux burns away.
PROCEDURE FOR TINNING COPPER WIRE WITH A SOLDERING IRON :
In the field, wires smaller than size No. 10 can be tinned with a soldering iron and rosin-core solder as follows (see figure 2.1):
Figure 2-1. - Tinning wire with a soldering iron.
Select a soldering iron with the correct heat capacity for the wire size (see table ). Make sure that the iron is clean and well tinned.
Table. - Approximate Soldering Iron Size for Tinning
Wire Size (AWG) Soldering Iron Size (Heat Capacity)
#20 - #16 65 Watts
#14 & #12 100 Watts
#10 & #8 200 Watts
The tinned surfaces to be joined should be shaped, fitted, and then mechanically joined to make a good mechanical and electrical contact. The parts must be held still. Any motion between the parts while the solder is cooling usually results in a poor solder connection, commonly called a "fractured solder" joint.
SOLDERING TOOLS :
Many types of soldering tools are in use today. Some of the more common types are the soldering iron, soldering gun, resistance soldering set, and pencil iron. The following discussion will provide you with a working knowledge of these tools.
Soldering Irons :
Some common types of hand soldering irons are shown in figure 2-2. All high-quality soldering irons operate in the temperature range of 500 to 600°F. Even the 25-watt midget irons produce this temperature. The important difference in iron sizes is not temperature, but thermal inertia. Thermal inertia is the capacity of the iron to generate and maintain a satisfactory soldering temperature while giving up heat to the joint to be soldered. Although it is not practical to solder large conductors with the 25-watt iron, this iron is quite suitable for replacing a half-watt resistor in an electronic circuit or soldering a miniature connector. One advantage of using a small iron for small work is that it is light and easy to handle and has a small tip that is easily used in close places. Even though its temperature is high enough, a midget iron does not have the thermal inertia to solder large conductors.
Figure 2-2 - Types of hand soldering Irons.
A well-designed iron is self-regulating. The resistance of its element increases with rising temperature. This limits the flow of current. Some common tip shapes of the soldering irons are shown in figure 2-3.
Figure 2-3. - Soldering iron tip shapes.
An iron should be tinned (the application of solder to the tip after the iron is heated) prior to soldering a component in a circuit. After extended use of an iron, the tip tends to become pitted due to oxidation. Pitting indicates the need for retinning. The tip is retinned after first filing the tip until it is smooth (see figure 2-4.
Figure 2-4. - Reconditioning pitted soldering iron tip.
Soldering Gun :
The soldering gun (figure 2-5) has gained great popularity in recent years because it heats and cools rapidly. It is especially well adapted to maintenance and troubleshooting work where only a small part of the technician's time is spent actually soldering.
Figure 2-5. - Soldering gun.
A transformer in the soldering gun supplies approximately 1 volt at high current to a loop of copper, which acts as the soldering tip. It heats to soldering temperature in 3 to 5 seconds. However, it may overheat to the point of incandescence if left on over 30 seconds. This should be avoided because excess heat will burn the insulation off the wiring. The gun is operated by a finger switch. The gun heats only while the switch is pressed.
Since the gun normally operates only for short periods at a time, it is comparatively easy to keep clean and well tinned. Short operating time allows little oxidation to form. Because the tip is made of pure copper, it is likely to pit, due to the dissolving action of the solder.
The gun or iron should always be kept tinned to permit proper heat transfer to the connection to be soldered. Tinning also helps control the heat to prevent solder buildup on the tip. This control reduces the chance of the solder spilling over to nearby components and causing short circuits. Maintaining the proper tinning on the iron or gun, however, may be made easier by tinning with silver solder (a composition of silver, copper, and zinc). The temperature at which the bond is formed between the copper tip and the silver solder is much higher than with lead-tin solder. This tends to decrease the pitting action of the solder on the copper tip.
Overheating small or delicate wiring can easily occur when a soldering gun is used. For most jobs, even the LOW position of the trigger overheats the gun after 10 seconds. With practice, the heat can be controlled by pulsing the gun on and off with its trigger. The HIGH position is used only for fast heating and for soldering heavy connections.
When a soldering iron or gun is used, heating and cooling cycles tend to loosen the nuts or screws that hold the replaceable tips. When the nut on a gun becomes loose, the resistance of the tip connection increases. The temperature of the connection is increased, thus reducing the heat at the tip. Continued loosening may eventually cause an open circuit. Therefore, check and tighten the nut or screw, as needed.
Soldering guns should never be used to solder electronic components, such as resistors, capacitors, and transistors, because the heat generated can destroy the components. They should be used only on terminals, splices, and connectors (not the miniature type).
Resistance Soldering Set :
A time-controlled resistance soldering set (figure 2-6) is now used at many maintenance activities. The set consists of a transformer that supplies 3 or 6 volts at a high current to stainless steel or carbon tips. The transformer is turned ON by a foot switch and OFF by an electronic timer. The timer can be adjusted for as long as 3 seconds soldering time. This set is especially useful for soldering cables to plugs and similar connectors; even the smallest types.
Figure 2-6. - Resistance soldering set
In use, the double-tip probes of the soldering unit are adjusted to straddle the connector cup (connector barrel) to be soldered. One pulse of current heats it for tinning. After the wire is inserted, a second pulse of current solders the connection and completes the job. Since the soldering tips are hot only during the brief period of actual soldering, burning of wire insulation and melting of connector inserts are greatly The greatest difficulty with this device is keeping the probe tips free of rosin and corrosion. A cleaning block is mounted on the transformer case for this purpose. Some technicians prefer fine sandpaper for cleaning the double tips.
Do not use steel wool for cleaning tips. It is dangerous when used around electrical equipment because the strands can fall into the equipment and cause short circuits.
Pencil Iron and Special Tips :
An almost indispensable item is the pencil-type soldering iron with an assortment of tips (figure 2-7). Miniature soldering irons have a wattage rating of less than 40 watts. They are easy to use, and are recommended for soldering small components, such as miniature connectors.
Figure 2-7. - Pencil iron with special tips.
One type of pencil iron is equipped with several different tips that range from one-fourth to one-half inch in size (diameter) and are of various shapes. This feature makes it adaptable to a variety of jobs. Unlike most tips that are held in place by setscrews, these tips have threads and screw into the barrel. This feature provides excellent contact with the heating element, thus improving heat transfer efficiency. "Antiseize" compound is generally applied to the threads of the tip each time a tip is installed into the iron. This allows the tip to be easily removed when another is to be inserted.
A special feature of this iron is the soldering pot that screws in like a tip and holds about a thimbleful of solder. It is useful for tinning the ends of a large number of wires.
The interchangeable tips are of various sizes and shapes for specific uses. Extra tips can be obtained and shaped to serve special purposes. The thread-in units are useful in soldering small items.
Another advantage of the pencil soldering iron is that it can be used as an improvised light source to inspect the completed work. Simply remove the soldering tip and insert a 120-volt, 6-watt, type 6S6 lamp bulb into the socket.
If leads, tabs, or small wires are bent against a board or terminal, slotted tips are provided to simultaneously melt the solder and straighten the leads.
If no suitable tip is available for a particular operation, an improvised tip can be made (see figure 2-8). Wrap a length of bare copper wire around one of the regular tips and bend the wire into the proper shape for the purpose. This method also serves to reduce thermal inertia when a larger iron must be used on small components.
Figure 2.8. - Improvised tip.
SOLDER :
Any discussion of soldering techniques should include an explanation of solder itself. Ordinary soft solder is a fusible alloy consisting chiefly of tin and lead. It is used to join two or more metals at temperatures below their melting point. In addition to tin and lead, soft solders occasionally contain varying amounts of antimony, bismuth, cadmium, or silver. These are added to change the melting point or physical properties of the alloy. Ordinary table salt has to be heated to 1,488°F before it melts. However, when a little water is added, it dissolves easily at room temperature. The action of molten solder on a metal like copper may be compared to the action of water on salt.
The solder bonds the connection by dissolving a small amount of the copper at temperatures quite below its melting point. Thus, the soldering process involves a metal solvent action between the solder and the metal being joined. A solder joint is therefore chemical in nature rather than purely physical. The bond is formed in part by chemical action and part by a physical bond.
The properties of a solder joint are different from those of the original solder. The solder is converted to a new and different alloy through the solvent action. Two metals soldered together behave like one solid metal. It is unlike two metals bolted, wired, or otherwise physically attached. These types of connections are still two pieces of metal. They are not even in direct contact due to an insulating film of oxide on the surfaces of the metals.
Temperature change does not affect the solder alloy. It withstands stress and strains without damaging the joint. An unsoldered connection eventually becomes loosened by small movements caused by temperature variations and by the gradual buildup of oxides on the metal surfaces.
To understand fully the alloy or solvent action on molten solder, look at the tin-lead fusion diagram shown in figure 2-35. This diagram shows that pure lead (point A) melts at 621°F. Point C shows the lowest melting point of the tin and lead alloy. The alloy at point C consists of 63-percent tin (SN63) and 37-percent lead. This is commonly called 63/37 solder. It has a melting point of 361°F. This type of solder, because of its very low melting point, is used in printed circuit boards and microminiature electronic repair. As you can see from the chart, the melting point of the alloy is lowered when tin is added to lead.
Figure 2-9. - Tin-lead fusion diagram.
The solder used to solder wires to electrical connectors, splices, and terminal lugs is a combination of 60-percent tin to 40-percent lead (60/40 solder). The melting point of 60/40 solder is 370°F, as shown at point B of the figure. Type 60/40 solder is less expensive than 63/37 solder and is suitable for all general uses.
FLUX :
As you know, flux is a cleaning agent to remove oxidation during soldering. Heating a metal causes rapid oxidation. Oxidation prevents solder from reacting chemically with a metal. Flux cleans the metal by removing the oxide layer. This operation is shown in figure 2-10. As the iron is moved in the direction shown, the boiling flux floats away the oxide film. The molten solder following the iron then fuses rapidly with the clean surface of the metal.Figure 2-10. - Action of flux.
There are two classes of flux: corrosive and noncorrosive. Zinc chloride, hydrochloric acid, and sal ammoniac are corrosive fluxes. Corrosive flux should NEVER be used in electrical or electronic repair work. Use only rosin fluxes. Any flux remaining in the joint corrodes the connection and creates a defective circuit. Rosin is a noncorrosive flux and is available in paste, liquid, or powder form.
SOLVENTS:
A solvent is used for cleaning and removing contaminants (oil, grease, dirt, and so forth) from the soldered connection. Solvents must be nonconductive and noncorrosive. Solvents must be used in a manner that keeps dissolved flux residue from "contact" surfaces, such as those in switches, potentiometers, or connectors. Ethyl and isopropyl alcohol are acceptable solvents.
These cleaning solvents are highly flammable and may give off toxic vapors. Follow Navy safety precautions and take extreme care when using any flammable solvent.
Soldering Aids :
Some type of heat shunt must be used in all soldering operations that involve heat-sensitive components. A typical heat shunt (figure 2-11) permits soldering the leads of component parts without overheating the part itself. The heat shunt should be attached carefully to prevent damage to the leads, terminals, or component parts. The shunt should be clipped to the lead, between the joint and the part being protected. As the joint is heated, the shunt absorbs the excess heat before it can reach the part and cause damage.
Figure 2-11. - Heat shunt.
A small piece of beeswax may be placed between the protected unit and the heat shunt. When the beeswax begins to melt, the temperature limit has been reached. The heat source should be removed immediately, but the shunt should be left in place.
Removing the shunt too soon permits the heat to flow from the melted solder into the component. The shunt should be allowed to remain in place until it cools to room temperature. A clip-on shunt is preferred because it requires positive action for removal. It does not require that the technician maintain pressure to hold it in place. This leaves both hands free to solder the connection.
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