Editing Solder (section)

=== 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}}
|}