Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it is distinguished from welding by the base metals not being melted during the joining process. In a soldering process, heat is applied to the parts to be joined, causing the solder to melt and be drawn into the joint by capillary action and to bond to the materials to be joined by wetting action. After the metal cools, the resulting joints are not as strong as the base metal, but have adequate strength, electrical conductivity, and water-tightness for many uses. Soldering is an ancient technique mentioned in the Bible and there is evidence that it was employed up to 5000 years ago in Mesopotamia.
A eutectic formulation has several advantages for soldering; chief among these is the coincidence of the liquidus and solidus temperatures, i.e. the absence of a plastic phase. This allows for quicker wetting out as the solder heats up, and quicker setup as the solder cools. A non-eutectic formulation must remain still as the temperature drops through the liquidus and solidus temperatures. Any differential movement during the plastic phase may result in cracks, giving an unreliable joint. Additionally, a eutectic formulation has the lowest possible melting point, which minimizes heat stress on electronic components during soldering.
Lead-free solders are suggested anywhere children may come into contact (since children are likely to place things into their mouths), or for outdoor use where rain and other precipitation may wash the lead into the groundwater. Common solder alloys are mixtures of tin and lead, respectively:
Lead-free solder alloys melt around 250 °C (482 °F), depending on their composition.
For environmental reasons, 'no-lead' solders are becoming more widely used. Unfortunately most 'no-lead' solders are not eutectic formulations, making it more difficult to create reliable joints with them. See complete discussion below; see also RoHS.
Other common solders include low-temperature formulations (often containing bismuth), which are often used to join previously-soldered assemblies without un-soldering earlier connections, and high-temperature formulations (usually containing silver) which are used for high-temperature operation or for first assembly of items which must not become unsoldered during subsequent operations. Specialty alloys are available with properties such as higher strength, better electrical conductivity and higher corrosion resistance.
Fluxes currently available include water-soluble fluxes (no VOC's required for removal) and 'no-clean' fluxes which are mild enough to not require removal at all. Performance of the flux needs to be carefully evaluated; a very mild 'no-clean' flux might be perfectly acceptable for production equipment, but not give adequate performance for a poorly-controlled hand-soldering operation.
Traditional rosin fluxes are available in non-activated (R), mildly activated (RMA) and activated (RA) formulations. RA and RMA fluxes contain rosin combined with an activating agent, typically an acid, which increases the wettability of metals to which it is applied by removing existing oxides. The residue resulting from the use of RA flux is corrosive and must be cleaned off the piece being soldered. RMA flux is formulated to result in a residue which is not significantly corrosive, with cleaning being preferred but optional.
The distinction between soldering and brazing is arbitrary, based on the melting temperature of the filler material. A temperature of 450 °C is usually used as a practical cut-off. Different equipment and/or fixturing is usually required since (for instance) a soldering iron generally cannot achieve high enough temperatures for brazing. Practically speaking there is a significant difference between the two processes—brazing fillers have far more structural strength than solders, and are formulated for this as opposed to maximum electrical conductivity. Brazed connections are often as strong or nearly as strong as the parts they connect, even at elevated temperatures.
"Hard soldering" or "silver soldering" (performed with high-temperature solder containing up to 40% silver) is also often a form of brazing, since it involves filler materials with melting points in the vicinity of, or in excess of, 450 °C. Although the term "silver soldering" is used much more often than "silver brazing", it may be technically incorrect depending on the exact melting point of the filler in use. In silver soldering ("hard soldering"), the goal is generally to give a beautiful, structurally sound joint, especially in the field of jewelry. Thus, the temperatures involved, and the usual use of a torch rather than an iron, would seem to indicate that the process should be referred to as "brazing" rather than "soldering", but the endurance of the "soldering" apellation serves to indicate the arbitrary nature of the distinction (and the level of confusion) between the two processes.
Induction soldering is a process which is similar to brazing. The source of heat in induction soldering is induction heating by high-frequency AC current. Generally copper coils are used for the induction heating. This induces currents in the part being soldered. The coils are usually made of copper or a copper base alloy. The copper rings can be made to fit the part needed to be soldered for precision in the work piece. Induction soldering is a process in which a filler metal (solder) is placed between the faying surfaces of (to be joined) metals. The filler metal in this process is melted at a fairly low temperature. Fluxes are a common use in induction soldering. This is a process which is particularly suitable for soldering continuously. The process is usually done with coils that wrap around a cylinder/pipe that needs to be soldered. Some metals are easier to solder than others. Copper, silver, and gold are easy. Iron and nickel are found to be more difficult. Because of their thin, strong oxide films, stainless steel and aluminum are a little more difficult. Titanium, magnesium, cast irons, steels, ceramics, and graphites can be soldered but it involves a process similar to joining carbides. They are first plated with a suitable metallic element that induces interfacial bonding.
Currently, mass-production printed circuit boards (PCBs) are mostly wave soldered or reflow soldered, though hand soldering of production electronics is also still standard practice for many tasks. In wave soldering, parts are temporarily adhered to the PCB with small dabs of adhesive, then the assembly is passed over flowing solder in a bulk container. Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to stick the components to their attachment pads, after which the assembly is heated by an infrared lamp, or (more commonly) by passing it through a carefully-controlled oven, or soldering with a hot air pencil. Since different components can be best assembled by different techniques, it is common to use two or more processes for a given PCB; the surface mounted parts may be reflow soldered, followed by a wave soldering process for the through-hole mounted components, with some of the bulkier parts hand-soldered on last.
For hand soldering of electronic components, the heat source tool should be selected to provide adequate heat for the size of joint to be completed. A 100 watt soldering iron may provide too much heat for printed circuit boards, while a 25 watt iron will not provide enough heat for large electrical connectors, joining copper roof flashing, or large stained-glass lead came. Using a tool with too high a temperature can damage sensitive components, but protracted heating by a tool that is too cool or under powered can also cause extensive heat damage.
Hand-soldering techniques require a great deal of skill to use on the finest pitch chip packages. In particular ball grid array (BGA) devices are notoriously difficult if not impossible to rework by hand.
For attachment of electronic components to a PCB, proper selection and use of flux helps prevent oxidation during soldering, which is essential for good wetting and heat transfer. The soldering iron tip must be clean and pre-tinned with solder to ensure rapid heat transfer. Components which dissipate large amounts of heat during operation are sometimes elevated above the PCB to avoid PCB overheating. After inserting a through-hole mounted component, the excess lead is cut off, leaving a length of about the radius of the pad. Plastic or metal mounting clips or holders may be used with large devices to aid heat dissipation and reduce joint stresses.
A heat sink may be used on the leads of heat sensitive components to reduce heat transfer to the component. This is especially applicable to germanium parts. (Note the heat sink will mean the use of more heat to complete the joint.) If all metal surfaces are not properly fluxed and brought above the melting temperature of the solder in use, the result will be an unreliable 'cold soldered' joint.
To simplify soldering, beginners are usually advised to apply the soldering iron and the solder separately to the joint, rather than the solder being applied direct to the iron. When sufficient solder is applied, the solder wire is removed. When the surfaces are adequately heated, the solder will flow around the joint. The iron is then removed from the joint.
Since non-eutectic solder alloys have a small plastic range, the joint must not be moved until the solder has cooled down through both the liquidus and solidus temperatures. Visually, a good solder joint will appear smooth and shiny, with the outline of the soldered wire clearly visible. A matte gray surface is a good indicator of a joint that was moved during soldering. Too little solder will result in a dry and unreliable joint; too much solder (the 'solder blob' very familiar to beginners) is not necessarily unsound, but tends to mean poor wetting. With some fluxes, flux residue remaining on the joint may need to be removed, using water, alcohol or other solvents compatible with the process. Excess solder and unconsumed flux and residue is sometimes wiped from the soldering iron tip between joints. The tip of the iron is kept wetted with solder ("tinned") when hot to minimise oxidation and corrosion of the tip itself.
Environmental legislation in many countries, and the whole of the European Community area, has led to a change in formulation of both solders and fluxes. Water soluble non-rosin based fluxes have been increasingly used since the 1980s so that soldered boards can be cleaned with water or water based cleaners. This eliminates hazardous solvents from the production environment, and effluent.
Since copper is an outstanding conductor of heat, and has a high heat capacity as well, large copper items like plumbing pipes and fittings require far more heat to solder effectively than an iron or gun can provide. The best choice for most plumbing jobs is a propane torch, though for large jobs MAPP gas is occasionally used.
As with all solder joints, all parts to be joined must be clean and oxide free. Internal and external wire brushes are available for the common pipe and fitting sizes; emery cloth and wire-wool are frequently used as well.
Because of the size of the parts involved, and the high activity and contaminating tendency of the flame, plumbing fluxes are typically much more chemically active, and more acidic, than electronic fluxes. Because plumbing joints may be done at any angle, even upside down, plumbing fluxes are generally formulated as pastes which stay in place better than liquids. Flux should be applied to all surfaces of the joint, inside and out. Flux residues should be removed after the joint is complete or they can, eventually, erode through the copper substrates and cause failure of the joint.
Many plumbing solder formulations are available, with different characteristics such as higher or lower melting temperature, depending on the specific requirements of the job. Building codes currently almost universally require the use of lead-free solder for potable water piping, though traditional tin-lead solder is still available. Some people maintain that the immediate risks of leaded solder are minimal, since minerals in municipal or well water supplies almost immediately coat the inside of the pipe, but studies have shown that lead-soldered plumbing pipes can result in elevated levels of lead in drinking water, which is particularly toxic to children. As with most heavy metals, lead poisoning is cumulative and can build up over many years.
Since copper pipe quickly conduct heat away from a joint, great care must be taken to ensure that the joint is properly heated through to obtain a good joint. After the joint is properly cleaned, fluxed and dry-fitted, the torch flame is applied to the thickest part of the joint, typically the fitting with the pipe inside it, with the solder applied on the opposite end of the joint. Dripping solder and flux, and hot parts, present a burn hazard to installers. When all the parts are heated through, the solder will melt and flow into the joint by capillary action. The torch may need to be moved around the joint to ensure all areas are wetted out; the molten solder will follow the heat of the torch around the joint. When the joint is properly wetted out, the solder and then the heat are removed, and while the joint is still very hot, it is usually wiped with a dry rag. This removes excess solder as well as flux residue before it cools down and hardens.
Pipes should be well flushed before drinking the water, to ensure that any flux residue from the inside of the joint has been removed.
More recently, electrically heated soldering irons are used. These are heated by a coil or ceramic heating element inside the tip of the iron. Different power ratings are available, and temperature can be controlled electronically. These characteristics allow longer beads to be run without interrupting the work to change tips. Soldering irons designed for electronic use are often effective though they are sometimes underpowered for the heavy copper and lead came used in stained glass work.
It is good practice to remove solder from a joint prior to resoldering—desoldering braids or vacuum desoldering equipment (solder suckers) can be used. Desoldering wicks contain plenty of flux that will lift the contamination from the copper trace and any device leads that are present. This will leave a bright, shiny, clean junction to be resoldered.
The lower melting point of solder means it can be melted away from the base metal, leaving it mostly intact though the outer layer will be "tinned" with solder. Flux will remain which can easily be removed by abrasive or chemical processes. This tinned layer will allow solder to flow into a new joint, resulting in a new joint, as well as making the new solder flow very quickly and easily.
Many new technical challenges have arisen with this endeavour. For instance, traditional lead-free solders have a significantly higher melting point than lead-based solders, which renders them unsuitable for use with heat-sensitive electronic components and their plastic packaging. To overcome this problem, solder alloys with a high silver content and no lead have been developed with a melting point slightly lower than many lead-free solders, but still much higher than traditional tin/lead solders.
Lead-free construction has also extended to components, pins, and connectors. Most of these pins used copper frames, and either lead, tin, gold or other finishes. Tin finishes are the most popular of lead-free finishes. Nevertheless, this brings up the issue of how to deal with tin whiskers. The current movement brings the electronics industry back to the problems solved in the 1960s by adding lead. JEDEC has created a classification system to help lead-free electronic manufacturers decide what provisions to take against whiskers, depending upon their application.
An improperly selected or applied flux can cause joint failure, or if not properly cleaned off the joint, may corrode the metals in the joint over time and cause eventual joint failure. Without flux the joint may not be clean, or may be oxidized, resulting in an unsound joint.
Movement of metals being soldered before the solder has cooled will cause a highly unreliable cracked joint.
Soldering torches are a type of soldering device that uses a flame rather than a soldering iron tip to heat solder. Soldering torches are often powered by butane and are available in sizes ranging from very small butane/oxygen units suitable for very fine but high-temperature jewelry work, to full-size oxy-fuel torches suitable for much larger work such as copper piping.
A soldering copper is a tool with a large copper head and a long handle, which is heated in a blacksmith's forge fire, and used to apply heat to sheet metal for soldering. Soldering coppers are sometimes used in auto bodywork, although body solder has been mostly superseded by non-metallic fillers.
Toaster ovens and hand held infrared lights have been used to reproduce production processes on a much smaller scale.
Bristle brushes are usually used to apply plumbing paste flux. For electronic work, flux-core solder is generally used, but additional flux may be used from a flux pen or dispensed from a small bottle with a syringe-like needle.
For PCB assembly and rework, alcohol and acetone are commonly used with cotton swabs or bristle brushes to remove flux residue. A heavy rag is usually used to remove flux from a plumbing joint before it cools and hardens. A fiberglass brush can also be used.
A heat sink, such as a crocodile clips, can also be used to prevent damaging heat-sensitive components while soldering.