Editing Solder

{{Short description|Alloy used to join metal pieces}}
{{about|the material|the process|Soldering|the settlement in Karelia|Sodder, Republic of Karelia}}
{{redirect|Sodder|the family's disappearance|Sodder children disappearance}}

[[File:Solder on spool.jpeg|thumbnail|upright=1.3|Solder, [[Ductility|drawn out]] to a diameter of 1.6 mm and wound/bent around a spool]]
[[File:Solderedjoint.jpg|thumb|A soldered joint used to attach a wire to a through-pin of a component on the rear of a [[printed circuit board]] (not a customary application of such joints)]]

'''Solder''' ({{IPAc-en|UK|ˈ|s|ɒ|l|d|ə|,_|ˈ|s|ə|ʊ|l|d|ə}};<ref name="OED">{{Cite encyclopedia |url=http://www.lexico.com/definition/solder |archive-url=https://web.archive.org/web/20220110071207/https://www.lexico.com/definition/solder |url-status=dead |archive-date=January 10, 2022 |title=solder |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}</ref> [[North American English|NA]]: {{IPAc-en|ˈ|s|ɒ|d|ər}})<ref>{{cite encyclopedia |title=solder |encyclopedia=[[Oxford American Dictionary]] |publisher=[[Oxford University Press]] |year=1980}}</ref> is a [[fusible alloy|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.

Soft solder typically has a melting point range of {{convert|90|to|450|C|F K|-1}},<ref>Frank Oberg, Franklin D. Jones, Holbrook L. Horton, Henry H. Ryffel eds. (1988) ''Machinery's Handbook 23rd Edition'' Industrial Press Inc., p. 1203. {{ISBN|0-8311-1200-X}}</ref> and is commonly used in [[electronics]], [[plumbing]], and sheet metal work. [[Alloy]]s that melt between {{convert|180|and|190|C|F K|-1}} are the most commonly used. Soldering performed using alloys with a melting point above {{convert|450|C|F K|-1}} 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 (chemistry)|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 (metallurgy)|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.<ref name="Ogunseitan-2007">{{cite journal | last1 = Ogunseitan | first1 = Oladele A. | year = 2007 | title = Public health and environmental benefits of adopting lead-free solders | journal = Journal of the Minerals, Metals and Materials Society | volume = 59 | issue = 7| pages = 12–17 | doi=10.1007/s11837-007-0082-8| bibcode = 2007JOM....59g..12O | s2cid = 111017033 }}</ref>

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. [[Jeweler]]s often use solder in thin sheets, which they cut into snippets.

==Etymology==
The word solder comes from the [[Middle English]] word {{lang|enm|soudur}}, via [[Old French]] {{lang|fro|solduree}} and {{lang|fro|soulder}}, from the [[Latin]] {{lang|la|solidare}}, meaning "to make solid".<ref>{{OEtymD|solder}}</ref>

== Composition ==

=== Lead-based ===
{{anchor|lead solder}}
[[File:60-40 Solder.jpg|thumb|{{chem2|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 strength|tensile]] and [[shear strength]]s. Lead mitigates the formation of [[tin whiskers]],<ref name="Jiang-2019"/> though the precise mechanism for this is unknown.<ref>{{cite web|url=http://nepp.nasa.gov/Whisker/background/index.htm|title=Basic Info on Tin Whiskers|website=nepp.nasa.gov|access-date=27 March 2018}}</ref> 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 coating]]s.<ref>{{cite web |url=http://www.dfrsolutions.com/uploads/white-papers/WP_SnWhisker.pdf|title=A New (Better) Approach to Tin Whisker Mitigation|author1=Craig Hillman |author2=Gregg Kittlesen |author3=Randy Schueller  |name-list-style=amp |publisher=DFR Solutions |access-date=23 October 2013 }}</ref> Alloys commonly used for electrical soldering are 60/40&nbsp;Sn-Pb, which melts at {{convert|188|°C|°F}},<ref>[http://www.farnell.com/datasheets/315929.pdf Properties of Solders]. farnell.com.</ref> and 63/37&nbsp;Sn-Pb used principally in electrical/electronic work. The latter mixture is a [[eutectic point|eutectic]] alloy of these metals, which:
# has the lowest melting point ({{convert|183|°C|°F|disp=or}}) of all the tin-lead alloys; and
# the melting point is truly a ''point''&nbsp;— 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]].<ref>{{cite web |url=https://www.gpo.gov/fdsys/pkg/USCODE-2010-title42/pdf/USCODE-2010-title42-chap6A-subchapXII.pdf|title=U.S. Code: Title 42. The Public Health and Welfare|publisher=govinfo.gov|page=990}}</ref> 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]],<ref>{{cite journal|doi=10.1056/NEJM199001113220203|pmid=2294437|year=1990|author=H.L. Needleman |display-authors=et al.|title=The long-term effects of exposure to low doses of lead in childhood. An 11-year follow-up report|volume=322|issue=2|pages=83–8|journal=The New England Journal of Medicine|doi-access=free}}</ref> 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|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.<ref>{{cite book|url=https://books.google.com/books?id=Sg9fAVdf8WoC&pg=PA538|title=Alloying: understanding the basics |publisher=ASM International|page=538|author=Joseph R. Davis|isbn=978-0-87170-744-4|year=2001}}</ref>

Lead-tin solders readily dissolve [[gold]] plating and form brittle intermetallics.<ref name="Manko-2001" />
60/40&nbsp;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.<ref>{{cite book|url=https://books.google.com/books?id=6F3TYrNmbjAC&pg=PA45|title=Lead finishing in semiconductor devices: soldering|publisher=World Scientific|page=45|author=A. C. Tan|isbn=978-9971-5-0679-7|year=1989}}</ref>

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 error]]s. [[Polonium-210]] is especially troublesome; [[lead-210]] [[beta decay]]s to [[bismuth-210]] which then beta decays to polonium-210, an intense emitter of [[alpha particle]]s. [[Uranium-238]] and [[thorium-232]] are other significant contaminants of alloys of lead.<ref name="Datta-2005">{{cite book|url=https://books.google.com/books?id=qj6OJ_jTcg8C&pg=PA196|author1=Madhav Datta |author2=Tetsuya Ōsaka |author3=Joachim Walter Schultze |title=Microelectronic packaging |page=196|publisher=CRC Press|year=2005|isbn=978-0-415-31190-8}}</ref><ref name="Puttlitz-2004">{{cite book|url=https://books.google.com/books?id=H75TywRXUK4C&pg=PA541|title=Handbook of lead-free solder technology for microelectronic assemblies|publisher=CRC Press|page=541|author1=Karl J. Puttlitz |author2=Kathleen A. Stalter |isbn=978-0-8247-4870-8|year=2004}}</ref>

=== {{anchor|Lead-free solder}}Lead-free ===
[[File:Pure tin solder.JPG|thumb|Pure tin solder wire]]
[[File:Propane torch soldering copper pipe.jpg|thumb|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&nbsp;Sn-Pb solder have melting points from 50 to 200&nbsp;°C higher,<ref name="Ganesan-2006">{{cite book|url=https://books.google.com/books?id=z4Ha0AYdon4C&pg=PA110 |title=Lead-free electronics |publisher=Wiley|author1=Sanka Ganesan |author2=Michael Pecht |isbn=978-0-471-78617-7|year=2006|page=110}}</ref> though there are also solders with much lower melting points. Lead-free solder typically requires around 2% flux by mass for adequate wetting ability.<ref>{{cite web |url=https://www.kester.com/Portals/0/Documents/Knowledge%20Base/Lead-free-Handsoldering.Final_.4.19.06.pdf |title=Lead-free Hand-soldering{{snd}}Ending the Nightmares |website=Kester |author=Peter Biocca |date=19 April 2006 |access-date=20 October 2019}}</ref>

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 is prohibited in critical applications, such as [[aerospace]], military and medical projects, because joints are likely to suffer from metal fatigue failure under stress (such as that from thermal expansion and contraction). Although this is a property that conventional leaded solder possesses as well (like any metal), the point at which stress fatigue will usually occur in leaded solder is substantially above the level of stresses normally encountered.

[[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 ({{convert|217|C|F|disp=semicolon}}), which is below the 22/78&nbsp;Sn-Ag (wt.%) eutectic of {{convert|221|C|F}} and the 99.3/0.7&nbsp;Sn-Cu eutectic of {{convert|227|C|F}}.<ref name="Zhao-2019"/> 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 array]]s. Examples of these four-element compositions are 18/64/14/4&nbsp;tin-silver-copper-zinc (Sn-Ag-Cu-Zn) (melting range 217–220&nbsp;°C) and 18/64/16/2&nbsp;tin-silver-copper-[[manganese]] (Sn-Ag-Cu-Mn; melting range of 211–215&nbsp;°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 layers 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 silver are suitable; tin-free alloys are also a choice, though their wetting ability 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.<ref name="Manko-2001">{{cite book|url=https://books.google.com/books?id=MvSMg5HC1YcC&pg=PA164|title=Solders and soldering: materials, design, production, and analysis for reliable bonding|publisher=McGraw-Hill Professional|page=164|author=Howard H. Manko|isbn=978-0-07-134417-3|year=2001}}</ref>

=== Hard solder ===
[[File:Silver solder.jpg|thumb|Silver solders (Ag/Cu/Zn) marked with their different hardness. From no.1="repair" to no.5="enameling".]]
[[File:Gold solder.jpg|thumb|Gold solders (Au/Ag/Cu/Zn) marked with their different hardness. From no.1=lowest temp to no.3=highest temp.]]
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 [[Metallurgical assay|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", "easy" and "repair". [[Vitreous enamel|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 or repair solder are 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 [[welding|welded]]. The alloys used for these purposes contain a high proportion of silver (up to 40%), and may also contain [[cadmium]].

=== Alloys ===
{{prose|section|date=August 2020}}
{{Main|Solder alloys}}

Different elements serve different roles in the solder alloy:
* [[Antimony]] is added to increase strength without affecting wettability. Prevents tin pest. Should be avoided on zinc, cadmium, or galvanized metals as the resulting joint is brittle.<ref name="Kaushish-2008">{{cite book|url=https://books.google.com/books?id=1ZOXXV9LdcwC&pg=PA378|title=Manufacturing Processes |publisher=PHI Learning Pvt. Ltd.|page=378|author=Kaushish|isbn=978-81-203-3352-9|year=2008}}</ref>
* [[Bismuth]] significantly lowers the melting point and improves wettability. In presence of sufficient lead and tin, bismuth forms crystals of {{chem2|Sn16Pb32Bi52}} with melting point of only 95&nbsp;°C, which diffuses along the grain boundaries and may cause a joint failure at relatively low temperatures. A high-power part pre-tinned with an alloy of lead can therefore desolder under load when soldered with a bismuth-containing solder. Such joints are also prone to cracking. Alloys with more than 47% Bi expand upon cooling, which may be used to offset thermal expansion mismatch stresses. Retards growth of [[tin whiskers]]. Relatively expensive, limited availability.
* [[Copper]] improves resistance to thermal cycle fatigue, and improves [[wetting]] properties of the molten solder. It also slows down the rate of dissolution of copper from the board and part leads in the liquid solder. Copper in solders forms intermetallic compounds. Supersaturated (by about 1%) solution of copper in tin may be employed to inhibit dissolution of thin-film under-bump metallization of [[ball grid array|BGA]] chips, e.g. as {{chem2|Sn94Ag3Cu3}}.<ref name="Zhao-2019"/><ref name="King-Ning-2007" />
* [[Nickel]] can be added to the solder alloy to form a supersaturated solution to inhibit dissolution of thin-film under-bump metallization.<ref name="King-Ning-2007" /> In tin-copper alloys, small addition of Ni (<0.5 wt%) inhibits the formation of voids and interdiffusion of Cu and Sn elements.<ref name="Zhao-2019"/> Inhibits copper dissolution, even more in synergy with bismuth. Nickel presence stabilizes the copper-tin intermetallics, inhibits growth of pro-eutectic β-tin dendrites (and therefore increases fluidity near the melting point of copper-tin eutectic), promotes shiny bright surface after solidification, inhibits surface cracking at cooling; such alloys are called "nickel-modified" or "nickel-stabilized". Small amounts increase melt fluidity, most at 0.06%.<ref>{{cite web|url=https://www.aimsolder.com/sites/default/files/the_fluidity_of_the_ni-modified_sn-cu_eutectic_lead-free_solder_white_paper.pdf |title=The Fluidity of the Ni-Modified Sn-Cu Eutectic Lead-free Solder |access-date=2019-09-07}}</ref> Suboptimal amounts may be used to avoid patent issues. Fluidity reduction increase hole filling and mitigates bridging and icicles.
* [[Cobalt]] is used instead of nickel to avoid patent issues in improving fluidity. Does not stabilize intermetallic growths in solid alloy.
* [[Indium]] lowers the melting point and improves ductility. In presence of lead it forms a ternary compound that undergoes phase change at 114&nbsp;°C. Very high cost (several times of silver), low availability. Easily oxidizes, which causes problems for repairs and reworks, especially when oxide-removing flux cannot be used, e.g. during GaAs die attachment. Indium alloys are used for cryogenic applications, and for soldering gold as gold dissolves in indium much less than in tin. Indium can also solder many nonmetals (e.g. glass, mica, alumina, magnesia, titania, [[zirconia]], porcelain, brick, concrete, and marble). Prone to diffusion into semiconductors and cause undesired doping. At elevated temperatures easily diffuses through metals. Low vapor pressure, suitable for use in vacuum systems. Forms brittle intermetallics with gold; indium-rich solders on thick gold are unreliable. Indium-based solders are prone to corrosion, especially in presence of [[chloride]] ions.<ref>{{cite book|author=I. R. Walker|title=Reliability in Scientific Research: Improving the Dependability of Measurements, Calculations, Equipment, and Software|url=https://books.google.com/books?id=zhX_5hccFWMC&pg=PA160|date=2011|publisher=Cambridge University Press|isbn=978-0-521-85770-3|pages=160–}}</ref>
* [[Lead]] is inexpensive and has suitable properties. Worse wetting than tin. Toxic, being phased out. Retards growth of tin whiskers, inhibits tin pest. Lowers solubility of copper and other metals in tin.
* [[Silver]] provides mechanical strength, but has worse ductility than lead. In absence of lead, it improves resistance to fatigue from thermal cycles. Using SnAg solders with HASL-SnPb-coated leads forms {{chem2|SnPb36Ag2}} phase with melting point at 179&nbsp;°C, which moves to the board-solder interface, solidifies last, and separates from the board.<ref name="Ganesan-2006" /> Addition of silver to tin significantly lowers solubility of silver coatings in the tin phase. In eutectic tin-silver (3.5% Ag) alloy and similar alloys (e.g. SAC305) it tends to form platelets of {{chem2|Ag3Sn}}, which, if formed near a high-stress spot, may serve as initiating sites for cracks and cause poor shock and drop performance; silver content needs to be kept below 3% to inhibit such problems.<ref name="King-Ning-2007">King-Ning Tu (2007) ''Solder Joint Technology – Materials, Properties, and Reliability''. Springer. {{ISBN|978-0-387-38892-2}}</ref> High ion mobility, tends to migrate and form short circuits at high humidity under DC bias. Promotes corrosion of solder pots, increases dross formation.
* [[Tin]] is the usual main structural metal of the alloy. It has good strength and wetting. On its own it is prone to [[tin pest]] and growth of [[tin whiskers]]. Readily dissolves silver, gold and to less but still significant extent many other metals, e.g. copper; this is a particular concern for tin-rich alloys with higher melting points and reflow temperatures.
* [[Zinc]] lowers the melting point and is low-cost. However, it is highly susceptible to corrosion and oxidation in air, therefore zinc-containing alloys are unsuitable for some purposes, e.g. wave soldering, and zinc-containing solder pastes have shorter shelf life than zinc-free. Can form brittle Cu-Zn intermetallic layers in contact with copper. Readily oxidizes which impairs wetting, requires a suitable flux.
* [[Germanium]] in tin-based lead-free solders influences formation of oxides; at below 0.002% it increases formation of oxides. Optimal concentration for suppressing oxidation is at 0.005%.<ref>{{cite web|url=http://www.balverzinn.com/downloads/DESOXY_RSN_GB.pdf|title=Balver Zinn Desoxy RSN|website=balverzinn.com|access-date=27 March 2018|archive-date=7 July 2011|archive-url=https://web.archive.org/web/20110707210258/http://www.balverzinn.com/downloads/DESOXY_RSN_GB.pdf|url-status=dead}}</ref> Used in e.g. Sn100C alloy. Patented.
* [[Rare-earth element]]s, when added in small amounts, refine the matrix structure in tin-copper alloys by segregating impurities at the grain boundaries. However, excessive addition results in the formation of tin whiskers; it also results in spurious rare earth phases, which easily oxidize and deteriorate the solder properties.<ref name="Zhao-2019"/>
* [[Phosphorus]] is used as antioxidant to inhibit dross formation. Decreases fluidity of tin-copper alloys.

=== Impurities ===
{{original research|section|date=August 2020}}

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.<ref name="Pecht-1993" />
* [[Aluminium]]&nbsp;– little solubility, causes sluggishness of solder and dull gritty appearance due to formation of oxides. Addition of antimony to solders forms Al-Sb intermetallics that are segregated into [[dross]]. Promotes embrittlement.
* [[Antimony]]&nbsp;– added intentionally, up to 0.3% improves wetting, larger amounts slowly degrade wetting. Increases melting point.
* [[Arsenic]]&nbsp;– forms thin intermetallics with adverse effects on mechanical properties, causes dewetting of brass surfaces
* [[Cadmium]]&nbsp;– causes sluggishness of solder, forms oxides and tarnishes
* [[Copper]]&nbsp;– most common contaminant, forms needle-shaped intermetallics, causes sluggishness of solders, grittiness of alloys, decreased wetting
* [[Gold]]&nbsp;– easily dissolves, forms brittle intermetallics, contamination above 0.5% causes sluggishness and decreases wetting. Lowers melting point of tin-based solders. Higher-tin alloys can absorb more gold without embrittlement.<ref name="Dead Link">{{cite web |url=http://www.aimsolder.com/specialty-materials-division-solders-photonic-packing |title=Solder selection for photonic packaging |access-date=20 August 2016|date=2013-02-27 }}</ref>
* [[Iron]]&nbsp;– forms intermetallics, causes grittiness, but rate of dissolution is very low; readily dissolves in lead-tin above 427&nbsp;°C.<ref name="Manko-2001" />
* [[Lead]]&nbsp;– causes Restriction of Hazardous Substances Directive compliance problems at above 0.1%.
* [[Nickel]]&nbsp;– causes grittiness, very little solubility in Sn-Pb
* [[Phosphorus]]&nbsp;– forms tin and lead [[phosphide]]s, causes grittiness and dewetting, present in electroless nickel plating
* [[Silver]]&nbsp;– often added intentionally, in high amounts forms intermetallics that cause grittiness and formation of pimples on the solder surface, potential for embrittlement
* [[Sulfur]]&nbsp;– forms lead and tin [[sulfide]]s, causes dewetting
* [[Zinc]]&nbsp;– in melt forms excessive dross, in solidified joints rapidly oxidizes on the surface; zinc oxide is insoluble in fluxes, impairing repairability; copper and nickel barrier layers may be needed when soldering brass to prevent zinc migration to the surface; potential for embrittlement

Board finishes vs wave soldering bath impurities buildup:
* HASL, lead-free (Hot Air Level): usually virtually pure tin. Does not contaminate high-tin baths.
* HASL, leaded: some lead dissolves into the bath
* ENIG (Electroless Nickel Immersion Gold): typically 100-200 microinches of nickel with 3-5 microinches of gold on top. Some gold dissolves into the bath, but limits exceeding buildup is rare.
* Immersion silver: typically 10–15 microinches of silver. Some dissolves into the bath, limits exceeding buildup is rare.
* Immersion tin: does not contaminate high-tin baths.
* OSP (Organic solderability preservative): usually imidazole-class compounds forming a thin layer on the copper surface. Copper readily dissolves in high-tin baths.<ref>[https://floridacirtech.com/wp-content/uploads/2018/10/SN100C-Technical-Guide.pdf SN100C Technical Guide]. floridacirtech.com</ref>

== Flux {{anchor|Flux core solder}} ==
[[File:Rosin core electrical solder.JPG|thumb|right|Electrical solder with an integrated rosin core, visible as a dark spot in the cut end of the solder wire.]]
{{Main|Flux (metallurgy)}}
[[Flux (metallurgy)|Flux]] is a [[reducing agent]] designed to help [[redox|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]] [[solvent]]s. 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.<ref name="Sweatman-2006">{{cite web |url=http://www.aimsolder.com/sites/default/files/the_fluidity_of_the_ni-modified_sn-cu_eutectic_lead-free_solder_white_paper.pdf |title=The Fluidity of the Ni-Modified Sn-Cu Eutectic Lead-Free Solder|author=Keith Sweatman & Tetsuro Nishimura |website=Nihon Superior Co., Ltd|date=2006}}</ref>

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 [[intermetallics|intermetallic compounds]] are formed during solidifying of solders and during their reactions with the soldered surfaces.<!-- todo: add properties --><ref name="Pecht-1993">{{cite book|url=https://books.google.com/books?id=Mu0o3X_k7p8C&pg=PA18|title=Soldering processes and equipment |publisher=Wiley-IEEE|page=18|author=Michael Pecht|isbn=978-0-471-59167-2|year=1993}}</ref> 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 {{chem2|Cu\sCu3Sn\sCu6Sn5\sSn}}. 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.<ref>{{cite book|url=https://books.google.com/books?id=HgqTNwFkfeEC&pg=PA51|title=The Mechanics of solder alloy interconnects|publisher=Springer|page=51|author1=D. R. Frear |author2=Steve Burchett |author3=Harold S. Morgan |author4=John H. Lau |isbn=978-0-442-01505-3|year=1994}}</ref>

Some example interactions include:
* Gold and palladium readily dissolve in solders. Copper and nickel tend to form intermetallic layers during normal soldering profiles. Indium forms intermetallics as well.
* Indium-gold intermetallics are brittle and occupy about 4 times more volume than the original gold. Bonding wires are especially susceptible to indium attack. Such intermetallic growth, together with thermal cycling, can lead to failure of the bonding wires.<ref>[http://nepp.nasa.gov/wirebond/literatures/na-gsfc-2004-01.pdf Indium Solder Encapsulating Gold Bonding Wire Leads to Fragile Gold-Indium Compounds and an Unreliable Condition that Results in Wire Interconnection Rupture]. GSFC NASA Advisory]. (PDF). Retrieved on 2019-03-09.</ref>
* Copper plated with nickel and gold is often used. The thin gold layer facilitates good solderability of nickel as it protects the nickel from oxidation; the layer has to be thin enough to rapidly and completely dissolve so bare nickel is exposed to the solder.<ref name="Puttlitz-2004" />
* Lead-tin solder layers on copper leads can form copper-tin intermetallic layers; the solder alloy is then locally depleted of tin and form a lead-rich layer. The Sn-Cu intermetallics then can get exposed to oxidation, resulting in impaired solderability.<ref>{{cite book|url=https://books.google.com/books?id=GTq6w21s2cwC&pg=PA397 |title=Modern solder technology for competitive electronics manufacturing|publisher=McGraw-Hill Professional|page=397|author=Jennie S. Hwang|isbn=978-0-07-031749-9|year=1996}}</ref>
* {{chem2|Cu6Sn5}}&nbsp;– common on solder-copper interface, forms preferentially when excess of tin is available; in presence of nickel, {{chem2|(Cu,Ni)6Sn5}} compound can be formed<ref name="Zhao-2019"/><ref name="Jiang-2019"/>
* {{chem2|Cu3Sn}}&nbsp;– common on solder-copper interface, forms preferentially when excess of copper is available, more thermally stable than {{chem2|Cu6Sn5}}, often present when higher-temperature soldering occurred<ref name="Zhao-2019"/><ref name="Jiang-2019"/>
* {{chem2|Ni3Sn4}}&nbsp;– common on solder-nickel interface<ref name="Zhao-2019"/><ref name="Jiang-2019"/>
* {{chem2|FeSn2}}&nbsp;– very slow formation
* {{chem2|Ag3}}Sn&nbsp;- at higher concentration of silver (over 3%) in tin forms platelets that can serve as crack initiation sites.
* {{chem2|AuSn4}}&nbsp;– β-phase&nbsp;– brittle, forms at excess of tin. Detrimental to properties of tin-based solders to gold-plated layers.
* {{chem2|AuIn2}}&nbsp;– forms on the boundary between gold and indium-lead solder, acts as a barrier against further dissolution of gold

{| class="wikitable"
|+ Matrix of intermetallic solder compounds
|-
! !! [[Tin]] !! [[Lead]] !! [[Indium]]
|-
| [[Copper]] || {{chem2|Cu4Sn}}, '''{{chem2|Cu6Sn5}}''', '''{{chem2|Cu3Sn}}''', {{chem2|Cu3Sn8}}<ref name="Zhao-2019"/> || || {{chem2|Cu3In}}, {{chem2|Cu9In4}}
|-
| [[Nickel]] || {{chem2|Ni3Sn}}, {{chem2|Ni3Sn2}}, '''{{chem2|Ni3Sn4}}''' {{chem2|NiSn3}} || || {{chem2|Ni3In}}, {{chem2|NiIn}} {{chem2|Ni2In3}}, {{chem2|Ni3In7}}
|-
| [[Iron]] || {{chem2|FeSn}}, {{chem2|FeSn2}} || ||
|-
| [[Indium]] || {{chem2|In3Sn}}, {{chem2|InSn4}} || {{chem2|In3Pb}} || –
|-
| [[Antimony]] || {{chem2|SbSn}} || ||
|-
| [[Bismuth]] || || {{chem2|BiPb3}} ||
|-
| [[Silver]] || {{chem2|Ag6Sn}}, {{chem2|Ag3Sn}} || || {{chem2|Ag3In}}, {{chem2|AgIn2}}
|-
| [[Gold]] || '''{{chem2|Au5Sn}}''', '''{{chem2|AuSn}}''' {{chem2|AuSn2}}, '''{{chem2|AuSn4}}''' || {{chem2|Au2Pb}}, {{chem2|AuPb2}} || {{chem2|AuIn}}, {{chem2|AuIn2}}
|-
| [[Palladium]] || {{chem2|Pd3Sn}}, {{chem2|Pd2Sn}}, {{chem2|Pd3Sn2}}, {{chem2|PdSn}}, {{chem2|PdSn2}}, {{chem2|PdSn4}} || || {{chem2|Pd3In}}, {{chem2|Pd2In}}, {{chem2|PdIn}}, {{chem2|Pd2In3}}
|-
| [[Platinum]] || {{chem2|Pt3Sn}}, {{chem2|Pt2Sn}}, {{chem2|PtSn}}, {{chem2|Pt2Sn3}}, {{chem2|PtSn2}}, {{chem2|PtSn4}} || {{chem2|Pt3Pb}}, {{chem2|PtPb}} {{chem2|PtPb4}} || {{chem2|Pt2In3}}, {{chem2|PtIn2}}, {{chem2|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 [[glass]]es to other glasses, [[ceramic]]s, [[metal]]s, [[semiconductor]]s, [[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 {{cvt|450|and|550|C|-1}}.

== See also ==
* [[Body solder]]
* [[Graping]]
* [[Restriction of Hazardous Substances Directive]]
* [[Solderability]]
* [[Solder alloys]]
* [[Solder mask]]
* [[Solder fatigue]]
* [[Wood's metal]]
* [[Solder paste]]

== References ==

{{reflist|refs=
<ref name="Zhao-2019">{{cite journal|year=2019|volume=20|issue=1|pages=421–444|author=Meng Zhao, Liang Zhang, Zhi-Quan Liu, Ming-Yue Xiong, and Lei Sun|title=Structure and properties of Sn-Cu lead-free solders in electronics packaging|journal=Science and Technology of Advanced Materials|doi=10.1080/14686996.2019.1591168|pmid=31489052|pmc=6711112|bibcode=2019STAdM..20..421Z}} {{open access}}</ref>
<ref name="Jiang-2019">{{cite journal|year=2019|volume=20|issue=1|pages=876–901|author=Nan Jiang |title=Reliability issues of lead-free solder joints in electronic devices|journal=Science and Technology of Advanced Materials|doi=10.1080/14686996.2019.1640072|pmid=31528239|pmc=6735330|bibcode=2019STAdM..20..876J}} {{open access}}</ref>
}}

== External links ==
{{commons category|Solder wires}}
* {{cite EB1911|wstitle=Solder|volume=25 |page=374 |short=x}}
* [http://www.metallurgy.nist.gov/phase/solder/solder.html Phase diagrams of different types of solder alloys]

[[Category:Fusible alloys]]
[[Category:Soldering]]
[[Category:Lead alloys]]