Solder

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Spool of solder, 1.6 mm diameter Solder on spool.jpeg
Spool of solder, 1.6 mm diameter
A soldered joint used to attach a wire to the pin of a component on the rear of a printed circuit board Solderedjoint.jpg
A soldered joint used to attach a wire to the pin of a component on the rear of a printed circuit board

Solder ( UK: /ˈsɒldə,ˈsəʊldə/ ; [1] NA: /ˈsɒdər/ ) [2] is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable for use as solder should have a lower melting point than the pieces to be joined. The solder should also be resistant to oxidative and corrosive effects that would degrade the joint over time. Solder used in making electrical connections also needs to have favorable electrical characteristics.

Contents

Soft solder typically has a melting point range of 90 to 450 °C (190 to 840 °F; 360 to 720 K), [3] and is commonly used in electronics, plumbing, and sheet metal work. Alloys that melt between 180 and 190 °C (360 and 370 °F; 450 and 460 K) are the most commonly used. Soldering performed using alloys with a melting point above 450 °C (840 °F; 720 K) is called "hard soldering", "silver soldering", or brazing.

In specific proportions, some alloys are eutectic — that is, the alloy's melting point is the lowest possible for a mixture of those components, and coincides with the freezing point. Non-eutectic alloys can have markedly different solidus and liquidus temperatures, as they have distinct liquid and solid transitions. Non-eutectic mixtures often exist as a paste of solid particles in a melted matrix of the lower-melting phase as they approach high enough temperatures. In electrical work, if the joint is disturbed while in this "pasty" state before it fully solidifies, a poor electrical connection may result; use of eutectic solder reduces this problem. The pasty state of a non-eutectic solder can be exploited in plumbing, as it allows molding of the solder during cooling, e.g. for ensuring watertight joint of pipes, resulting in a so-called "wiped joint".

For electrical and electronics work, solder wire is available in a range of thicknesses for hand-soldering (manual soldering is performed using a soldering iron or soldering gun), and with cores containing flux. It is also available as a room temperature paste, as a preformed foil shaped to match the workpiece which may be more suited for mechanized mass-production, or in small "tabs" that can be wrapped around the joint and melted with a flame where an iron isn't usable or available, as for instance in field repairs. Alloys of lead and tin were commonly used in the past and are still available; they are particularly convenient for hand-soldering. Lead-free solders have been increasing in use due to regulatory requirements plus the health and environmental benefits of avoiding lead-based electronic components. They are almost exclusively used today in consumer electronics. [4]

Plumbers often use bars of solder, much thicker than the wire used for electrical applications, and apply flux separately; many plumbing-suitable soldering fluxes are too corrosive (or conductive) to be used in electrical or electronic work. Jewelers often use solder in thin sheets, which they cut into snippets.

Etymology

The word solder comes from the Middle English word soudur, via Old French solduree and soulder, from the Latin solidare, meaning "to make solid". [5]

Composition

Lead-based

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Sn60Pb40 solder 60-40 Solder.jpg
Sn60Pb40 solder

Tin-lead (Sn-Pb) solders, also called soft solders, are commercially available with tin concentrations between 5% and 70% by weight. The greater the tin concentration, the greater the solder’s tensile and shear strengths. Lead mitigates the formation of tin whiskers, [6] though the precise mechanism for this is unknown. [7] Today, many techniques are used to mitigate the problem, including changes to the annealing process (heating and cooling), addition of elements like copper and nickel, and the application of conformal coatings. [8] Alloys commonly used for electrical soldering are 60/40 Sn-Pb, which melts at 188 °C (370 °F), [9] and 63/37 Sn-Pb used principally in electrical/electronic work. The latter mixture is a eutectic alloy of these metals, which:

  1. has the lowest melting point (183 °C or 361 °F) of all the tin-lead alloys; and
  2. the melting point is truly a point — not a range.

In the United States, since 1974, lead is prohibited in solder and flux in plumbing applications for drinking water use, per the Safe Drinking Water Act. [10] Historically, a higher proportion of lead was used, commonly 50/50. This had the advantage of making the alloy solidify more slowly. With the pipes being physically fitted together before soldering, the solder could be wiped over the joint to ensure water tightness. Although lead water pipes were displaced by copper when the significance of lead poisoning began to be fully appreciated, lead solder was still used until the 1980s because it was thought that the amount of lead that could leach into water from the solder was negligible from a properly soldered joint. The electrochemical couple of copper and lead promotes corrosion of the lead and tin. Tin, however, is protected by insoluble oxide. Since even small amounts of lead have been found detrimental to health as a potent neurotoxin, [11] lead in plumbing solder was replaced by silver (food-grade applications) or antimony, with copper often added, and the proportion of tin was increased (see lead-free solder).

The addition of tin—more expensive than lead—improves wetting properties of the alloy; lead itself has poor wetting characteristics. High-tin tin-lead alloys have limited use as the workability range can be provided by a cheaper high-lead alloy. [12]

Lead-tin solders readily dissolve gold plating and form brittle intermetallics. [13] 60/40 Sn-Pb solder oxidizes on the surface, forming a complex 4-layer structure: tin(IV) oxide on the surface, below it a layer of tin(II) oxide with finely dispersed lead, followed by a layer of tin(II) oxide with finely dispersed tin and lead, and the solder alloy itself underneath. [14]

Lead, and to some degree tin, as used in solder contains small but significant amounts of radioisotope impurities. Radioisotopes undergoing alpha decay are a concern due to their tendency to cause soft errors. Polonium-210 is especially troublesome; lead-210 beta decays to bismuth-210 which then beta decays to polonium-210, an intense emitter of alpha particles. Uranium-238 and thorium-232 are other significant contaminants of alloys of lead. [15] [16]

Lead-free

Pure tin solder wire Pure tin solder.JPG
Pure tin solder wire
Soldering copper pipes using a propane torch and lead-free solder Propane torch soldering copper pipe.jpg
Soldering copper pipes using a propane torch and lead-free solder

The European Union Waste Electrical and Electronic Equipment Directive and Restriction of Hazardous Substances Directive were adopted in early 2003 and came into effect on July 1, 2006, restricting the inclusion of lead in most consumer electronics sold in the EU, and having a broad effect on consumer electronics sold worldwide. In the US, manufacturers may receive tax benefits by reducing the use of lead-based solder. Lead-free solders in commercial use may contain tin, copper, silver, bismuth, indium, zinc, antimony, and traces of other metals. Most lead-free replacements for conventional 60/40 and 63/37 Sn-Pb solder have melting points from 50 to 200 °C higher, [17] though there are also solders with much lower melting points. Lead-free solder typically requires around 2% flux by mass for adequate wetting ability. [18]

When lead-free solder is used in wave soldering, a slightly modified solder pot may be desirable (e.g. titanium liners or impellers) to reduce maintenance cost due to increased tin-scavenging of high-tin solder.

Lead-free solder may be less desirable for critical applications, such as aerospace and medical projects, because its properties are less thoroughly known.

Tin-silver-copper (Sn-Ag-Cu, or SAC) solders are used by two-thirds of Japanese manufacturers for reflow and wave soldering, and by about 75% of companies for hand soldering. The widespread use of this popular lead-free solder alloy family is based on the reduced melting point of the Sn-Ag-Cu ternary eutectic behavior (217 °C, 423 °F), which is below the 22/78 Sn-Ag (wt.%) eutectic of 221 °C (430 °F) and the 59/41 Sn-Cu eutectic of 227 °C (441 °F). [19] The ternary eutectic behavior of Sn-Ag-Cu and its application for electronics assembly was discovered (and patented) by a team of researchers from Ames Laboratory, Iowa State University, and from Sandia National Laboratories-Albuquerque.

Much recent research has focused on the addition of a fourth element to Sn-Ag-Cu solder, in order to provide compatibility for the reduced cooling rate of solder sphere reflow for assembly of ball grid arrays. Examples of these four-element compositions are 18/64/14/4 tin-silver-copper-zinc (Sn-Ag-Cu-Zn) (melting range 217–220 °C) and 18/64/16/2 tin-silver-copper-manganese (Sn-Ag-Cu-Mn; melting range of 211–215 °C).

Tin-based solders readily dissolve gold, forming brittle intermetallic joins; for Sn-Pb alloys the critical concentration of gold to embrittle the joint is about 4%. Indium-rich solders (usually indium-lead) are more suitable for soldering thicker gold layer as the dissolution rate of gold in indium is much slower. Tin-rich solders also readily dissolve silver; for soldering silver metallization or surfaces, alloys with addition of silvers are suitable; tin-free alloys are also a choice, though their wettability is poorer. If the soldering time is long enough to form the intermetallics, the tin surface of a joint soldered to gold is very dull. [13]

Hard solder

Hard solders are used for brazing, and melt at higher temperatures. Alloys of copper with either zinc or silver are the most common.

In silversmithing or jewelry making, special hard solders are used that will pass assay. They contain a high proportion of the metal being soldered and lead is not used in these alloys. These solders vary in hardness, designated as "enameling", "hard", "medium" and "easy". Enameling solder has a high melting point, close to that of the material itself, to prevent the joint desoldering during firing in the enameling process. The remaining solder types are used in decreasing order of hardness during the process of making an item, to prevent a previously soldered seam or joint desoldering while additional sites are soldered. Easy solder is also often used for repair work for the same reason. Flux is also used to prevent joints from desoldering.

Silver solder is also used in manufacturing to join metal parts that cannot be welded. The alloys used for these purposes contain a high proportion of silver (up to 40%), and may also contain cadmium.

Alloys

Different elements serve different roles in the solder alloy:

Impurities

Impurities usually enter the solder reservoir by dissolving the metals present in the assemblies being soldered. Dissolving of process equipment is not common as the materials are usually chosen to be insoluble in solder. [25]

Board finishes vs wave soldering bath impurities buildup:

Flux

Electrical solder with an integrated rosin core, visible as a dark spot in the cut end of the solder wire. Rosin core electrical solder.JPG
Electrical solder with an integrated rosin core, visible as a dark spot in the cut end of the solder wire.

Flux is a reducing agent designed to help reduce (return oxidized metals to their metallic state) metal oxides at the points of contact to improve the electrical connection and mechanical strength. The two principal types of flux are acid flux (sometimes called "active flux"), containing strong acids, used for metal mending and plumbing, and rosin flux (sometimes called "passive flux"), used in electronics. Rosin flux comes in a variety of "activities", corresponding roughly to the speed and effectiveness of the organic acid components of the rosin in dissolving metallic surface oxides, and consequently the corrosiveness of the flux residue.

Due to concerns over atmospheric pollution and hazardous waste disposal, the electronics industry has been gradually shifting from rosin flux to water-soluble flux, which can be removed with deionized water and detergent, instead of hydrocarbon solvents. Water soluble fluxes are generally more conductive than traditionally used electrical / electronic fluxes and so have more potential for electrically interacting with a circuit; in general it is important to remove their traces after soldering. Some rosin type flux traces likewise should be removed, and for the same reason.

In contrast to using traditional bars or coiled wires of all-metal solder and manually applying flux to the parts being joined, much hand soldering since the mid-20th century has used flux-core solder. This is manufactured as a coiled wire of solder, with one or more continuous bodies of inorganic acid or rosin flux embedded lengthwise inside it. As the solder melts onto the joint, it frees the flux and releases that on it as well.

Operation

The solidifying behavior depends on the alloy composition. Pure metals solidify at a certain temperature, forming crystals of one phase. Eutectic alloys also solidify at a single temperature, all components precipitating simultaneously in so-called coupled growth. Non-eutectic compositions on cooling start to first precipitate the non-eutectic phase; dendrites when it is a metal, large crystals when it is an intermetallic compound. Such a mixture of solid particles in a molten eutectic is referred to as a mushy state. Even a relatively small proportion of solids in the liquid can dramatically lower its fluidity. [28]

The temperature of total solidification is the solidus of the alloy, the temperature at which all components are molten is the liquidus.

The mushy state is desired where a degree of plasticity is beneficial for creating the joint, allowing filling larger gaps or being wiped over the joint (e.g. when soldering pipes). In hand soldering of electronics it may be detrimental as the joint may appear solidified while it is not yet. Premature handling of such joint then disrupts its internal structure and leads to compromised mechanical integrity.

Intermetallics

Many different intermetallic compounds are formed during solidifying of solders and during their reactions with the soldered surfaces. [25] The intermetallics form distinct phases, usually as inclusions in a ductile solid solution matrix, but also can form the matrix itself with metal inclusions or form crystalline matter with different intermetallics. Intermetallics are often hard and brittle. Finely distributed intermetallics in a ductile matrix yield a hard alloy while coarse structure gives a softer alloy. A range of intermetallics often forms between the metal and the solder, with increasing proportion of the metal; e.g. forming a structure of Cu−Cu3Sn−Cu6Sn5−Sn. Layers of intermetallics can form between the solder and the soldered material. These layers may cause mechanical reliability weakening and brittleness, increased electrical resistance, or electromigration and formation of voids. The gold-tin intermetallics layer is responsible for poor mechanical reliability of tin-soldered gold-plated surfaces where the gold plating did not completely dissolve in the solder.

Two processes play a role in a solder joint formation: interaction between the substrate and molten solder, and solid-state growth of intermetallic compounds. The base metal dissolves in the molten solder in an amount depending on its solubility in the solder. The active constituent of the solder reacts with the base metal with a rate dependent on the solubility of the active constituents in the base metal. The solid-state reactions are more complex – the formation of intermetallics can be inhibited by changing the composition of the base metal or the solder alloy, or by using a suitable barrier layer to inhibit diffusion of the metals. [29]

Some example interactions include:

Matrix of intermetallic solder compounds
Tin Lead Indium
Copper Cu4Sn, Cu6Sn5, Cu3Sn, Cu3Sn8 [19] Cu3In, Cu9In4
Nickel Ni3Sn, Ni3Sn2, Ni3Sn4NiSn3Ni3In, NiInNi2In3, Ni3In7
Iron FeSn, FeSn2
Indium In3Sn, InSn4In3Pb
Antimony SbSn
Bismuth BiPb3
Silver Ag6Sn, Ag3SnAg3In, AgIn2
Gold Au5Sn, AuSnAuSn2, AuSn4Au2Pb, AuPb2AuIn, AuIn2
Palladium Pd3Sn, Pd2Sn, Pd3Sn2, PdSn, PdSn2, PdSn4Pd3In, Pd2In, PdIn, Pd2In3
Platinum Pt3Sn, Pt2Sn, PtSn, Pt2Sn3, PtSn2, PtSn4Pt3Pb, PtPbPtPb4Pt2In3, PtIn2, Pt3In7

Preform

A preform is a pre-made shape of solder specially designed for the application where it is to be used. Many methods are used to manufacture the solder preform, stamping being the most common. The solder preform may include the solder flux needed for the soldering process. This can be an internal flux, inside the solder preform, or external, with the solder preform coated.

Similar substances

Glass solder is used to join glasses to other glasses, ceramics, metals, semiconductors, mica, and other materials, in a process called glass frit bonding. The glass solder has to flow and wet the soldered surfaces well below the temperature where deformation or degradation of either of the joined materials or nearby structures (e.g., metallization layers on chips or ceramic substrates) occurs. The usual temperature of achieving flowing and wetting is between 450 and 550 °C (840 and 1,020 °F).

See also

Related Research Articles

Tin Chemical element, symbol Sn and atomic number 50

Tin is a chemical element with the symbol Sn and atomic number 50. Tin is a silvery-coloured metal.

Brazing High-temperature soldering; metal-joining technique by high-temperature molten metal filling

Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.

The white metals are a series of often decorative bright metal alloys used as a base for plated silverware, ornaments or novelties, as well as any of several lead-based or tin-based alloys used for things like bearings, jewellery, miniature figures, fusible plugs, some medals and metal type. The term is also used in the antiques trade for an item suspected of being silver, but not hallmarked.

Flux (metallurgy)

In metallurgy, a flux is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.

Wave soldering

Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the circuit board makes contact with this wave, the components become soldered to the board. Wave soldering is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued onto the surface of a printed circuit board (PCB) by placement equipment, before being run through the molten solder wave. Wave soldering is mainly used in soldering of through hole components.

Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology. Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.

Intermetallic Type of metallic alloy

An intermetallic is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, with good high-temperature mechanical properties. They can be classified as stoichiometric or nonstoichiometic intermetallic compounds.

Gold plating

Gold plating is a method of depositing a thin layer of gold onto the surface of another metal, most often copper or silver, by chemical or electrochemical plating. This article covers plating methods used in the modern electronics industry; for more traditional methods, often used for much larger objects, see gilding.

Reflow soldering Attachment of electronic components

Reflow soldering is a process in which a solder paste is used to temporarily attach one or thousands of tiny electrical components to their contact pads, after which the entire assembly is subjected to controlled heat. The solder paste reflows in a molten state, creating permanent solder joints. Heating may be accomplished by passing the assembly through a reflow oven, under an infrared lamp, or (unconventionally) by soldering individual joints with a desoldering hot air pencil.

The solderability of a substrate is a measure of the ease with which a soldered joint can be made to that material. Good solderability requires wetting of the substrate by the solder.

Dip soldering

Dip soldering is a small-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The solder wets to the exposed metallic areas of the board, creating a reliable mechanical and electrical connection.

Glass-to-metal seal

Glass-to-metal seals are a very important element of the construction of vacuum tubes, electric discharge tubes, incandescent light bulbs, glass encapsulated semiconductor diodes, reed switches, pressure tight glass windows in metal cases, and metal or ceramic packages of electronic components.

Colored gold Various colors of gold obtained by alloying gold with other elements

Pure gold is slightly reddish yellow in color, but colored gold in various other colors can be produced by alloying gold with other elements.

Soldering Process of joining metal pieces with heated filler metal

Soldering is a process in which two or more items are joined by melting and putting a filler metal (solder) into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involve melting the work pieces. In brazing, the work piece metal also does not melt, but the filler metal is one that melts at a higher temperature than in soldering. In the past, nearly all solders contained lead, but environmental and health concerns have increasingly dictated use of lead-free alloys for electronics and plumbing purposes.

Tin-silver-copper, is a lead-free (Pb-free) alloy commonly used for electronic solder. It is the main choice for lead-free surface-mount technology (SMT) assembly in the industry, as it is near eutectic, with adequate thermal fatigue properties, strength, and wettability. Lead-free solder is gaining much attention as the environmental effects of lead in industrial products is recognized, and as a result of Europe's RoHS legislation to remove lead and other hazardous materials from electronics. Japanese electronics companies have also looked at Pb-free solder for its industrial advantages.

Materials for use in vacuum

Materials for use in vacuum are materials that show very low rates of outgassing in vacuum and, where applicable, are tolerant to bake-out temperatures. The requirements grow increasingly stringent with the desired degree of vacuum to be achieved in the vacuum chamber. The materials can produce gas by several mechanisms. Molecules of gases and water can be adsorbed on the material surface. Materials may sublimate in vacuum. Or the gases can be released from porous materials or from cracks and crevices. Traces of lubricants, residues from machining, can be present on the surfaces. A specific risk is outgassing of solvents absorbed in plastics after cleaning.

Bismuth–indium

The elements bismuth and indium have relatively low melting points when compared to other metals, and their alloy bismuth–indium (Bi–In) is classified as a fusible alloy. It has a melting point lower than the eutectic point of the tin–lead alloy. The most common application of the Bi-In alloy is as a low temperature solder, which can also contain, besides bismuth and indium, lead, cadmium, and tin.

Wiped joint Form of soldered joint used

A wiped joint is a form of soldered joint used to join lead pipework.

Solder alloys

Solder is a metallic material that is used to connect metal workpieces. The choice of specific solder alloys depends on their melting point, chemical reactivity, mechanical properties, toxicity, and other properties. Hence a wide range of solder alloys exist, and only major ones are listed below. Since early 2000s the use of lead in solder alloys is discouraged by several governmental guidelines in the European Union, Japan and other countries, such as Restriction of Hazardous Substances Directive and Waste Electrical and Electronic Equipment Directive.

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