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Soldering is a process in which two or more workpieces are joined by melting solder and placing a binder (solder or molten solder) into a joint, with the binder having a lower melting point than the adjacent metal. We can use lead free solder for this process. Unlike welding, brazing does not involve melting the workpieces. In brazing, the workpiece metal does not melt either, but the binder melts at a higher temperature than in brazing. In the past, almost all solders contained lead, but environmental and health issues have increasingly mandated the use of lead-free alloys for electronic and plumbing purposes. How choose best soldering tools? What is flux core solder, hot soldering iron? What are soldering wires and reflow soldering? We write about it bellow!

Where is soldering process used? 

Soldering is used in plumbing, electronics and metalwork, from flashing to jewelry and musical instruments. Soldering irons can make:

  • Soldering provides fairly permanent but reversible connections between copper pipes in plumbing, as well as connections in sheet metal objects such as food cans, roof flashing, rain gutters and car radiators. 
  • Jewelry parts, machine tools and some refrigeration and plumbing components are often assembled and repaired by silver soldering at higher temperatures. Small mechanical parts are also often soldered. 
  • Soldering is also used to join lead and copper foil in stained glass windows. Electronic soldering connects electrical wiring to devices, and electronic components to circuit boards. 
  • Electronic connections can be soldered manually with a soldering iron. Automated methods, such as wave soldering or the use of ovens, can make multiple connections on a complex circuit board in a single operation, significantly reducing the cost of manufacturing electronic devices. 
  • Musical instruments, especially brass and woodwind instruments, use a combination of soldering in their assembly. Brass bodies are often soldered together.
Solderability and types of soldering

The solderability of a substrate is a measure of the ease with which a solder joint can be made with that material. Some metals are easier to solder than others. Copper, zinc, brass, silver and gold are among the simpler ones. Iron, mild steel and nickel are other difficulties, due to thin but strong oxide layers. Stainless steel and some aluminum alloys are even more difficult to solder. It is possible to braze titanium, magnesium, iron, some high-carbon steels, ceramics and graphite, but this requires a process similar to joining carbides: they are first coated with a suitable metal element that induces interfacial bonding.

Laser soldering

Laser soldering is a technique that involves using a 30-50 W laser to melt and solder an electrical connector. Laser diode chips based on semiconductor junctions are used for this purpose. Suzanne Jenniches patented laser soldering in 1980. Wavelengths typically range from 808 nm to 980 nm. The beam is delivered via an optical fiber to a workpiece with a fiber diameter of 800 µm or less. Since the beam coming out of the end of the fiber propagates rapidly, lenses are used to create the right size spot on the workpiece at the right working distance. A wire feeder is used to feed solder. Both lead-tin and silver-tin materials can be soldered. Process recipes will vary depending on the composition of the alloy. When soldering 44-pin chip carriers to a board using solder preforms, power levels were on the order of 10 watts and soldering time was about 1 second. Low power levels can lead to incomplete wetting and the formation of voids, which can weaken the joint.

Induction soldering

Induction soldering uses high-frequency AC induction heating in a surrounding copper coil. This coil induces a current that generates heat in the part to be soldered due to the higher resistance of the joint compared to the surrounding metal (resistance heating). These copper coils can be shaped for the soldering of a given component, in order to more accurately fit the joint. The binder (solder) is placed between the faces, and the solder melts at a fairly low temperature. Fluxes are commonly used in induction soldering. This technique is particularly suitable for continuous soldering, in which case these coils wrap around the cylinder or tube to be soldered.

Infrared soldering with a fiber optic focus

Fiber-focused infrared soldering is a technique in which multiple infrared sources are guided through the fibers and then focused at a single point where the joint is soldered.

Resistance soldering

Resistance soldering is soldering in which the heat needed for the solder to flow is generated by the flow of an electric current through the soldering iron tip. As the current flows through the resistive material, a certain level of heat is generated. By adjusting the amount of current conducted and the level of resistance encountered, the amount of heat generated can be determined and controlled in advance. Resistance soldering differs from wired soldering, in which heat is generated in the component and then passed through a conductive tip to the joint area. 

Resistance soldering – the process flow 

A cold soldering iron needs time to reach operating temperature and must be kept at a high temperature between the solder joints. Heat transfer can be inhibited if the tip is not properly moistened during use. With resistance soldering, intense heat can be generated quickly directly in the joint area in a tightly controlled manner. This allows the solder to rise to the required melting temperature more quickly and minimizes heat travel away from the solder joint, which helps minimize the risk of thermal damage to materials or components in the environment. Heat is generated only while each joint is being made, making resistance soldering more energy efficient. Resistance soldering equipment, unlike conductive irons, can be used for difficult soldering and brazing applications where much higher temperatures may be required. This makes resistance comparable to flame soldering in some situations. When the required temperature can be achieved by flame or resistance methods, the resistance heat is more localized due to direct contact, while the flame spreads, potentially heating a larger area.

Active soldering

Fluxless soldering using a conventional soldering iron, ultrasonic soldering iron or specialized soldering crucible and an active solder containing an active element, usually titanium, zirconium or chromium. Active elements, thanks to mechanical activation, react with the surface of materials commonly considered difficult to solder without preliminary metallization. Active solders can be protected from excessive oxidation of their active element by adding rare earth elements with a higher affinity for oxygen (usually cerium or lanthanum). Another common addition is gallium-usually introduced as a wetting promoter. The mechanical activation required for active welding can be accomplished by brushing (for example, with a wire brush or steel shovel) or ultrasonic vibration (20-60 kHz). Active brazing has been shown to effectively bond ceramics based on aluminum, titanium, silicon, graphite and carbon nanotubes at temperatures below 450°C or in a protective atmosphere.

Classic soldering versus brazing

There are three forms of brazing, each of which requires higher temperatures and increases the strength of the joint:

  • soft soldering, which initially uses tin and lead alloys as a binder;
  • silver brazing using silver-containing alloys;
  • welding using a brass alloy as filler.

The binder alloy used for each type of weld can be adjusted to change the melting point of the binder. Brazing differs significantly from bonding in that the adhesive binds directly to the surface of the workpiece at the junction, creating a joint that is both electrically conductive and air- and liquid-tight.

Solder has a melting point below about 400°C (752°F), while silver soldering and brazing use higher temperatures, often requiring a flame or arc torch to achieve filler melt. Soft solder is usually an alloy (usually containing lead) with a liquidus temperature below 350°C (662°F).

In this soldering process, heat is applied to the parts to be joined, which melts the solder and bonds it to the workpiece in a surface alloying process called wetting. In wire bonding, solder is drawn into the conductors between the links by capillary action in a process called „seepage.” Capillary action also occurs when objects are very close together or come into contact with each other. The tensile strength of the joint depends on the adhesive used; in electrical welding, the added solder has little tensile strength, so it is recommended to twist or fold the wires before soldering to provide some mechanical strength to the joint. A good solder joint provides an electrically conductive, waterproof and gas-tight connection.

Advantages and disadvantages of solder types 

Each type of solder has its own advantages and disadvantages. Soft solder takes its name from the basic ingredient, soft lead. Soft solder uses the lowest temperature (and therefore the least stress on the component), but does not form a strong bond and is not suitable for mechanical applications. It is also not suitable for high temperature applications, as it will lose strength and eventually melt. Silver brazing used in jewelry, mechanical and some plumbing applications requires the use of a torch or other high temperature source and is much stronger than soft soldering. Soldering provides the strongest unsoldered connection, but also requires the highest temperature to melt the glue, a flashlight or other source of high heat, and tinted glasses to protect the eyes from the glare of the white-hot work. Commonly used to repair cast iron, wrought iron furniture, etc.