Soldering is a group of joining processes which produces coalescence of material by heating them to a suitable temperature and by using a filler metal having a liquidus not exceeding 450oC and below the solidus of the base metals.

The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction. Solder is a filler metal used in soldering which has a liquidus not exceeding 450oC. It is normally a nonferrous alloy.

The temperature of 450oC is the temperature that differentiates soldering from brazing. This arbitrary number was selected many years ago and is universally accepted. Most of the factors involved with brazing apply to soldering.

Like brazing and other joining processes, soldering involves several fields of science, including mechanics, chemistry and metallurgy. Soldering is a simple operation, consisting of the relative placements of the parts to be joined, wetting the surfaces with molten solder, and allowing the solder to cool until it has solidified.

The mechanism for joining by soldering involves three closely related factors: 1) wetting, 2) alloying, and 3) capillary attraction.

Soldering in the field of electronics is in many respects different from soldering in other branches of industry. Although the physical principles of all soldering (and brazing) processes are the same, the features specific to their use in electronics are so numerous that it is possible to speak of soldering in electronics as a separate subject.

Soldering has several clear advantages over competitive joining techniques:

* The solder forms itself by the nature of the flow, wetting, and subsequent crystallization process, even when the heat and the solder are not directed precisely to the places to be soldered. Because the solder does not adhere to insulating materials, it often can be applied in excess quantities, in contrast to conductive adhesives. The soldering temperature is relatively low, so there is no need for the heat to be applied locally as in welding.
* Soldering allows considerably freedom in the dimensioning of joints, so that it is possible to obtain good results even if variety of components is used on the same product.
* The soldered connections can be disconnected if necessary, thus facilitating repair
* The equipment for both manual soldering and machine soldering is relatively simple
* The soldering process can be easily automated, offering the possibility of in-line arrangements of soldering machines with other equipment

Mass soldering by wave, drag, or dip machines has been the preferred method for making high-quality, reliable connections for many decades. Despite the appearance of new connecting systems, it still retains this position. Correctly controlled, soldering is one of the least expensive methods for fabricating electrical connections. Incorrectly controlled, it can be one of the most costly processes, not because of the initial costs, but because of the many far-reaching effects of poor workmanship.
Soldering Methods
Iron soldering (INS) was the earliest soldering method. The hand-held soldering iron used in this process continues to be popular. It is the most cost-effective tool for:

* prototypes and short production jobs
* rework of defective solder joints and/or components
* soldering of components that are too delicate or specialized to solder using mass-production techniques

Torch Soldering. Torch soldering (TS) utilizes a fuel gas flame as the heat source in the soldering process. The fuel gas is mixed with either air or oxygen to produce the flame, which is applied to the materials to be soldered until the assembly reaches the proper soldering temperature. Solder filler metal, which melts at temperatures below 450oC, is added to the assembly to bond it. Successful torch soldering is accomplished when parts are clean and fit together closely, and when oxides are not excessive.

Furnace and Infrared Soldering. Furnace soldering (FS) encompasses a group of re-flow soldering techniques in which the parts to be joined and pre-placed filler metal are put in furnace and then heated to the soldering temperature.

Dip Soldering. Dip soldering (DS) is accomplished by submerging parts to be joined into a molten solder bath. The molten bath can be any suitable filler metal, but the selection is usually confined to the lower melting point elements. The most common dip soldering operations use zinc-aluminum and tin-lead solders.

Resistance Soldering. Resistance soldering (RS) is a soldering process in which the heat needed to melt the solder is developed by the resistance of the material when a large electrical current is supplied. Resistance soldering can be applied to electrically conductive materials that allow the passage of electric current. The process can be used for selective spot soldering of small components, for the soldering of closely placed parts on an assembly, or for heat restriction when necessary. It is similar in many ways to resistance brazing.

Laser Soldering (LS). Industrial lasers are able to deliver large amounts of heat with great precision and without contact, making them ideal for application that have either a destructive nature, such as cutting or drilling, or a constructive nature, such as soldering or annealing. Laser soldering uses the well-focused, highly controlled beam to deliver energy to a desired location for a precisely measured length of time. The main advantage of laser soldering is a non-contact procedure.

Hot Gas Soldering. Hot gas soldering is a process that is commonly used in applications where the work-piece thermal mass is small and the melting temperature of the solder is relatively low. The electronics industry utilizes hot gas soldering to reflow or melt solder in localized areas on circuit assemblies.

Induction Soldering. Induction or radio frequency heating are versatile means of providing heat to the joint area. Heating is caused by electrical resistance to eddy currents induced in the work piece. These currents are induced by the rapidly changing magnetic field generated by a coil supplied with an alternating current. The eddy current is generated at the surface of the work-piece (skin effect) and diminishes toward the interior. The advantages of induction heating include the ability to supply heat uniformly over the entire joint area while maintaining a localized temperature rise so heat sensitive materials or devices neighboring the joint area are not damaged.
Soldering of Aluminum and Aluminum Alloys
Soldering of aluminum and aluminum alloys is relatively simple. Compared with brazing, soldering presents advantages such as little loss of base metal temper, minimal distortion, and easy removal of flux. Aluminum is soldered at a minimum 110oC below the solidus temperature of the base metal.

Wetting and spreading are affected by the presence of an oxide at the surface of the joint. The nature of the aluminum oxide is different from alloy to alloy. The heat-treatable alloys present a more tenacious oxide that is more adherent and more difficult to remove. The oxides of the non-heat-treatable alloys are less tenacious and easier to remove. The action of a corrosive flux is enough to disrupt and displace the oxide layer in a non-heat treatable aluminum alloy. However, the oxide layer of a heat-treatable alloy must be removed by chemical or mechanical means before soldering.

Solderability of aluminum alloys is influenced by alloying elements of the base metal. Generally, purer aluminum alloys are easily soldered. Analysis shows that alloys of groups 1xxx, 3xxx and 6xxx have good solderabilities.

Solderability is primarily affected by two alloying elements, magnesium and silicon. The presence of magnesium in an aluminum alloy not only reduces the wettability (more than 1.0 wt%) but also increases the intergranular penetration (more than 0.5 wt% Mg). Magnesium content up to 1.0 wt% does not reduce the flux effectiveness, so it does not affect wetting and spreading of the molten filler metal. Alloys with less than 1.0 wt% Mg can be soldered with all flux types. Between 1.0 and 1.5 wt% Mg the low-temperature organic-type flux is not effective to remove the surface oxide of the faying surface. When the amount of magnesium exceeds 1.5 wt%, the corrosive flux does not work either.

Silicon plays the same role. Thus, if the amount of silicon is higher than 4.0 wt%, all flux types are ineffective. To solve this problem, a fluxless technique such as abrasion or ultrasonic soldering should be utilized.

Typical solders used for aluminum are Zn, ZnCd, SnZn, SnPb, SnCd, SnZn. Solders that melt below 260oC are called low-melting-point solders. Those that melt between 260 and 370oC are called intermediate-melting-point solders. Those that melt between 370 and 440oC are called high-melting-point solders.

Aluminum alloys can be soldered to all usual metals and nonmetallic materials. However, loss of corrosion resistance should be expected. The high-melting-point solders are suitable for soldering mild steel, stainless steel, nickel, copper, brass, zinc, and silver directly to aluminum. Magnesium, titanium, zirconium, niobium, tantalum, molybdenum and tungsten may be soldered if they are plated with a solderable metal coating such as silver.
Soldering of Copper and Copper Alloys
Copper and copper alloys are among the most frequently soldered engineering materials. The copper oxide is easily disrupted and displaced by most flux types. The presence of alloying elements such as beryllium, chromium, silicon, and aluminum modifies the nature of the oxide, making it more tenacious. For these alloys, a special flux is recommended to remove the oxide from the surface and enhance the solderability of these base metal groups.

The most common solders for copper are tin- or lead-base solders. Tin can react with copper and form two intermetallic phases, Cu6Sn5 and Cu3Sn, at the solid-liquid interface.