Editing Solder (section)

== 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>