Soldering is a process in which two or more elements are joined by melting and placing a filler (solder) in the joint, where the filler has a lower melting point than the adjacent metal. Unlike welding, soldering does not involve melting the workpieces. In soldering, the metal of the workpiece is also not melted, but the filler is melted at a higher temperature than during soldering. In the past, almost all solders contained lead, but environmental and health issues increasingly mandated the use of lead-free alloys for electronic and hydraulic purposes.
Where is soldering used?
Soldering is used in hydraulics, electronics, and metalworking, from flashing to jewelry and musical instruments. In addition:
- Soldering provides fairly durable, but reversible connections between copper pipes in plumbing installations, as well as connections in sheet metal objects, such as food cans, roof processing, rain gutters, and car radiators.
- Jewelry parts, machine tools, and some cooling and hydraulic elements are often assembled and repaired in the process of 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. Electronic soldering connects electrical wiring with devices, and electronic components with printed circuit boards.
- Electronic connections can be soldered manually using a soldering iron. Automated methods, such as wave soldering or using ovens, can perform many connections on a complex printed circuit board in one operation, significantly reducing the production costs of electronic devices.
- Musical instruments, especially brass and woodwind instruments, utilize 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 soldered connection can be made with this material. Some metals are easier to solder than others. Copper, zinc, brass, silver, and gold are among the simpler ones. The next difficulties are iron, soft steel, and nickel, due to thin but strong layers of oxides. Stainless steel and some aluminum alloys are even more difficult to solder. Titanium, magnesium, iron, some high-carbon steels, ceramics, and graphite can be soldered, but this requires a process similar to carbide joining: they are first coated with an appropriate metal element, which causes interfacial bonding.
Laser soldering
Induction soldering
Laser soldering is a technique that involves using a 30–50 W laser to melt and solder an electrical joint. For this purpose, laser diode systems based on semiconductor junctions are used. Suzanne Jenniches patented laser soldering in 1980. Wavelengths typically range from 808 nm to 980 nm. The beam is delivered by a fiber optic to the workpiece with a fiber diameter of 800 µm and less. Since the beam exiting the end of the fiber spreads quickly, lenses are used to create the appropriate spot size on the workpiece at the correct working distance. A wire feeder is used to feed the solder. Both lead-tin and silver-tin materials can be soldered. Process recipes will vary depending on the alloy composition. For 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. A low power level can lead to incomplete wetting and void formation, which can weaken the joint.
Induction soldering utilizes high-frequency alternating current induction heating in the surrounding copper coil. This coil induces a current that generates heat in the soldered part due to the higher resistance of the joint compared to the surrounding metal (resistance heating). These copper coils can be shaped for soldering a specific component, to more accurately fit the joint. The solder (flux) is placed between the front surfaces, and the solder melts at a relatively low temperature. Fluxes are commonly used in induction soldering. This technique is particularly suitable for continuous soldering, in which case these coils wrap around a cylinder or tube that is to be soldered.
Infrared soldering with fiber optic focus
Infrared soldering with fiber optic focus is a technique in which multiple infrared sources are guided through fibers and then focused in one place where the joint is soldered.
Resistance soldering
Resistance soldering is a type of soldering in which the heat needed for the solder flow is generated by the flow of electric current through the soldering iron tip. When current flows through a resistive material, a certain level of heat is generated. By regulating the amount of conducted current and the encountered level of resistance, the amount of generated heat can be predetermined and controlled. Resistance soldering differs from conductive soldering, in which heat is generated in the element and then passed through the heat-conducting tip to the joint area.
Resistance soldering - process flow
A cold soldering iron needs time to reach its working temperature and must be kept at a high temperature between soldering joints. Heat transfer can be hampered if the tip is not properly moistened during use. With resistance soldering, intense heat can quickly be generated directly in the joint area in a strictly controlled manner. This allows for faster raising to the required solder melting temperature and minimizes the heat journey from the soldered joint, helping to minimize the risk of thermal damage to materials or components in the vicinity. Heat is only generated during the execution of each connection, making resistance soldering more energy-efficient. Resistance soldering equipment, unlike conductive irons, can be used for difficult soldering applications and hard soldering, where significantly higher temperatures may be required. This makes resistance comparable to flame soldering in some situations. When the required temperature can be achieved using a flame or resistance method, 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 a specialized soldering crucible, along with active solder containing an active element, most commonly titanium, zirconium, or chromium. Active elements, due to mechanical activation, react with the surface of materials commonly considered difficult to solder without prior 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 additive is gallium—usually introduced as a wetting promoter. Mechanical activation required for active soldering can be done by brushing (for example, with a wire brush or a steel shovel) or ultrasonic vibrations (20-60 kHz). It has been shown that active soldering effectively joins ceramics based on aluminum, titanium, silicon, graphite, and carbon nanotubes at temperatures below 450°C or in a protective atmosphere.
Traditional soldering vs. hard soldering
There are three forms of soldering, each requiring higher temperatures and increasing the strength of the connection:
- soft soldering, where tin and lead alloys are initially used as a filler;
- soldering with silver using alloys containing silver;
- welding using a brass alloy as a filler.
The flux stop used for any type of joint can be adjusted to change the melting temperature of the solder. Hard soldering differs significantly from gluing in that the glue binds directly to the surface of the workpiece at the joint, creating a connection that is both electrically conductive and airtight for air and liquids.
Soldering fluxes have a melting temperature below about 400 ° C (752 ° F), while silver soldering and hard soldering use higher temperatures, often requiring a flame or arc burner to achieve filler melting. 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 causes the solder to melt and bind with the item in a surface melting process called wetting. In stranded wire, the solder is drawn into the conductors between the strands by capillary action in a process called "wicking". Capillary action also occurs when objects are very close to each other or come into contact. 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 soldered connection provides an electrically conductive, water-resistant, and gas-tight connection.
Pros and cons of types of solder
Each type of solder has its advantages and disadvantages. Soft solder gets its name from the basic ingredient, soft lead. Soft soldering uses the lowest temperature (and therefore the least strain on the component), but it does not create 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 soldering used in jewelry, mechanics, and some hydraulic applications requires the use of a torch or other high-temperature source and is much stronger than soft soldering. Brazing provides the strongest non-welded connection, but also requires the highest temperature to melt the adhesive, a torch or other high-temperature source, and darkened glasses to protect the eyes from the glare of the white-hot work. Commonly used for repairing cast iron castings, wrought iron furniture, etc.