SOLDERING

'Soldering' is the process in which two metals are joined together by means of a third metal or alloy having a relatively low melting point. 'Soft soldering' is characterized by the value of the melting point of the third metal or alloy, which is below 450°C.^[1] The third metal or alloy used in the process is called solder.
Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it is distinguished from welding since the base metal is not 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 that has been used practically as long as humans have been making items out of metal.

Contents

Applications

Methods

Solders

Flux

Basic soldering techniques

Electronic components (PCB's)

Pipe/Mechanical soldering

Stained Glass Soldering

Desoldering and Resoldering

Lead-free Electronic Soldering

Soldering defects

Common Tools

Soldering processes

See also

Notes

External links

Applications

The most frequent application of soldering is assembling electronic components to printed circuit boards (PCBs). Another common application is making permanent but reversible connections between copper pipes in plumbing systems. Joints in sheet-metal objects such as food cans, roof flashing, rain gutters and automobile radiators have also historically been soldered, and occasionally still are. Jewelry and small mechanical parts are often assembled by soldering. Soldering is used to join lead came and copper foil in stained glass work. Soldering can also be used to effect a semi-permanent patch for a leak in a container or cooking vessel.

Methods

Soldering operations can be performed with hand tools, one joint at a time, or ''en masse'' on a production line. Hand soldering is typically performed with a soldering iron (for fine work) or gun (for larger items), a torch (again large or small depending on the size of the work), or occasionally a hot-air pencil. These techniques require a great deal of skill to use on the newest fine-pitch chip packages; in particular ball grid array (BGA) devices are notoriously difficult if not impossible to rework by hand.
Currently, mass-production printed circuit boards (PCBs) are almost always wave soldered or reflow soldered. In wave soldering, parts are temporarily adhered to the PCB with small dabs of adhesive, then the assembly is passed over a small fountain in a bulk container of molten solder. Reflow soldering is a somewhat newer 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. 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.
Note that the distinction between soldering and brazing is arbitrary, with the only difference being 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.

Solders

Main articles: Solder

Soldering filler materials are available in many different alloys for differing applications. In electronics assembly, the eutectic alloy of 63% tin and 37% lead (or 60/40, which is almost identical in performance to the eutectic) has been the alloy of choice. Other alloys are used for plumbing, mechanical assembly, and other applications.
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 the soldering process.
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.

Flux

In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder, for example, attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Secondarily, flux acts as a wetting agent in the soldering process, reducing the surface tension of the molten solder and causing it to better wet out the parts to be joined.
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.

Basic soldering techniques

Electronic components (PCB's)

The heat source tool should be selected to provide adequate heat for the size of joint to be completed. A soldering gun at 100 watts output may provide too much heat for printed circuit boards, while a 23 watt iron may not provide enough heat for joining copper roof flashing or large stained-glass lead came. Using a tool with too high a temperature can damage sensitive components, but a tool that is too cool can cause an extended heating-up period which can also cause extensive damage.
For attachment of electronic components to a PCB, proper selection and use of flux is the best way to ensure that all solder pads and device terminals remain clean and oxide-free, 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. The devices must be mounted on the circuit board properly. Components which dissipate large amounts of heat during operation are sometimes elevated above the PCB a few millimeters to allow proper cooling. After inserting a through-hole mounted component, the excess leads can be cut leaving only a length equal to the radius of the pad. Plastic mounting clips or holders may be used with large devices to reduce mounting stresses.
A heat sink may be used on the leads of sensitive components to reduce heat transfer to the component (notice that the heat sink will require the use of MORE heat to complete the joint!). If ALL copper 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 achieve a properly heated joint, the soldering iron and the solder must be applied separately to the surfaces to be joined, rather than the iron being applied directly to the solder. When the surfaces are adequately heated up, the solder will melt and flow into the joint. The solder supply is then removed from the joint, followed by the heat source. Even distribution of solder throughout the joint gives a mechanically sound and electrically conductive joint. Since non-eutectic solder alloys have a small plastic range, the joint must not be disturbed 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. 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 will not allow proper inspection of the joint. Depending on the flux used, any 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 residues must be wiped from the soldering iron tip between joints, but the tip of the iron must be kept wetted with solder ("tinned") constantly when hot to prevent oxidation and vaporization of the tip itself.
Environmental legislation in many countries, and the whole of the European Community area, have led to a change in formulation of both solders and fluxes. Water soluble non-rosin based fluxes have been increasingly used since the 1980's so that soldered boards can be cleaned with water or water based cleaners. This eliminates hazardous solvents from the production environment, and effluent.

Pipe/Mechanical soldering

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. With the large surface areas and open flame heating usually involved, mechanical pre-cleaning is almost always advised in this sort of process. 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 almost universally require the use of lead-free solder these days, though traditional tin-lead solder is still generally available at hardware stores for DIY'ers. 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 can so 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, flux'ed 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. Hands and other body parts should never be put directly below the joint -- extremely hot liquid flux and even solder frequently drip from the joint. 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.

Stained Glass Soldering

Historically, stained glass soldering tips were copper, heated by placing in a charcoal-burning brazier. Multiple tips were used; when one tip cooled down from use, it was placed back in the brazier of charcoal and the next tip was used.
More recently, electrically heated soldering irons are used. These consist of coil or ceramic heating elements 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 to a heated soldering tip. Soldering irons designed for electronic use are often effective though sometimes a bit underpowered for the heavy copper and lead came used in stained glass work.
As usual, many formulations of solders are available. 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 solders for stained glass are mixtures of tin and lead, respectively:

★ 63/37: melts between 180-185°C

★ 60/40: melts between 183–190°C

★ 50/50: melts between 185–215°C

★ lead-free solder: melts around 250°C

Desoldering and Resoldering

Used solder contains some of the dissolved base metals and is unsuitable for reuse in making new joints. Once the solder's capacity for the base metal has been achieved it will no longer properly bond with the base metal, usually resulting in a brittle cold solder joint with a crystalline appearance.
It is good practice to remove solder from a joint prior to resoldering—desoldering wicks or vacuum desoldering equipment 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.

Lead-free Electronic Soldering

More recently environmental legislation has specifically targeted the wide use of lead in the electronics industry. The RoHS directives in Europe require many new electronic circuit boards to be lead free by 1 July 2006, mostly in the consumer goods industry, but in some others as well.
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 traditional 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.

Soldering defects

Various problems may arise in the soldering process which lead to joints which are non-functional either immediately or after a period of use. The most common defect when hand-soldering results from the parts being joined not exceeding the solder's liquidus temperature, resulting in a "cold solder" joint. 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.

Common Tools

Hand-soldering tools include the electric soldering iron, which has a variety of tips available ranging from blunt to very fine to chisel heads for hot-cutting plastics, and the soldering gun, which typically provides more power, giving faster heat-up and allowing larger parts to be soldered. Hot-air guns and pencils allow rework of component packages which cannot easily be performed with irons and guns. Torches 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. Toaster ovens and handheld 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.
Wire brushes, wire wool and emery cloth are commonly used to prepare plumbing joints for connection. Electronic joints rarely require mechanical cleaning.
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 glassfibre brush can also be used.
For electronic work, solder wick and vacuum-operated "solder suckers" are used to undo solder connections.

Soldering processes

★ Wave soldering

★ Reflow soldering

★ Infrared soldering

★ Induction soldering - An overview of soldering with induction and a collection of Application Notes

★ Ultrasonic soldering

★ Dip soldering

★ Furnace soldering

★ Iron soldering

★ Resistance soldering

★ Torch soldering

★ Silver soldering/Brazing

See also

★ Desoldering

★ Solder

★ Link Lock

Notes

1. The Basics of Soldering, , Armin, Rahn, John Wiley & Sons, 1993,

External links

★ Basic soldering guide

★ RoHS Directive 2002/95/EC on the restriction of the use of certain hazardous substances in electrical and electronic equipment.

★ ELFNET (European Lead Free Soldering Network, a website where will be decided what the replacements will be for the present day's lead-tin alloy

★ European Association for Brazing and Soldering - A detailed technical library and information about soldering and brazing.

★ Induction soldering - An overview of soldering with induction and a collection of Application Notes

★ American Welding Society Brazing and Soldering Forums A technical discussion group focused on brazing and soldering.

★ soldering silver jewellery An illustrative and practical guide to soldering silver jewellery.