Editing Solder (section)

=== 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>