Editing Solder (section)

=== {{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>