Solder

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Spool of solder, 1.6 mm diameter
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 (UK: /ˈsɒldə, ˈsəʊldə/;[1] NA: /ˈsɒdər/)[2] is a 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 90 to 450 °C (190 to 840 °F; 360 to 720 K),[3] and is commonly used in electronics, plumbing, and sheet metal work. Alloys that melt between 180 and 190 °C (360 and 370 °F; 450 and 460 K) are the most commonly used. Soldering performed using alloys with a melting point above 450 °C (840 °F; 720 K) is called "hard soldering", "silver soldering", or brazing.

In specific proportions, some alloys are

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

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.[4]

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.

Jewelers
often use solder in thin sheets, which they cut into snippets.

Etymology

The word solder comes from the Middle English word soudur, via Old French solduree and soulder, from the Latin solidare, meaning "to make solid".[5]

Composition

Lead-based

Sn60Pb40 solder

eutectic
alloy of these metals, which:

  1. has the lowest melting point (183 °C or 361 °F) of all the tin-lead alloys; and
  2. the melting point is truly a point — 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

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,[11] 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
).

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.[12]

Lead-tin solders readily dissolve gold plating and form brittle intermetallics.[13] 60/40 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.[14]

Lead, and to some degree tin, as used in solder contains small but significant amounts of

bismuth-210 which then beta decays to polonium-210, an intense emitter of alpha particles. Uranium-238 and thorium-232 are other significant contaminants of alloys of lead.[15][16]

Lead-free

Pure tin solder wire
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 Sn-Pb solder have melting points from 50 to 200 °C higher,[17] though there are also solders with much lower melting points. Lead-free solder typically requires around 2% flux by mass for adequate wetting ability.[18]

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.

-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 arrays. Examples of these four-element compositions are 18/64/14/4 tin-silver-copper-zinc (Sn-Ag-Cu-Zn) (melting range 217–220 °C) and 18/64/16/2 tin-silver-copper-manganese (Sn-Ag-Cu-Mn; melting range of 211–215 °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.[13]

Hard solder

Hard solders are used for brazing, and melt at higher temperatures. Alloys of copper with either zinc or silver are the most common.

In

jewelry making, special hard solders are used that will pass 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" and "easy". 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 is 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 welded. The alloys used for these purposes contain a high proportion of silver (up to 40%), and may also contain cadmium.

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.[20]
  • 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 BGA chips, e.g. as Sn94Ag3Cu3.[19][21]
  • Nickel can be added to the solder alloy to form a supersaturated solution to inhibit dissolution of thin-film under-bump metallization.[21] In tin-copper alloys, small addition of Ni (<0.5 wt%) inhibits the formation of voids and interdiffusion of Cu and Sn elements.[19] 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%.[22] 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.
  • 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.[23]
  • 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 SnPb36Ag2 phase with melting point at 179 °C, which moves to the board-solder interface, solidifies last, and separates from the board.[17] 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 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.[21] 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 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%.[24] Used in e.g. Sn100C alloy. Patented.
  • Rare-earth elements, 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.[19]
  • Phosphorus is used as antioxidant to inhibit dross formation. Decreases fluidity of tin-copper alloys.

Impurities

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.[25]

  • Aluminium – 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 – added intentionally, up to 0.3% improves wetting, larger amounts slowly degrade wetting. Increases melting point.
  • Arsenic – forms thin intermetallics with adverse effects on mechanical properties, causes dewetting of brass surfaces
  • Cadmium – causes sluggishness of solder, forms oxides and tarnishes
  • Copper – most common contaminant, forms needle-shaped intermetallics, causes sluggishness of solders, grittiness of alloys, decreased wetting
  • Gold – 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.[26]
  • Iron – forms intermetallics, causes grittiness, but rate of dissolution is very low; readily dissolves in lead-tin above 427 °C.[13]
  • Lead – causes Restriction of Hazardous Substances Directive compliance problems at above 0.1%.
  • Nickel – causes grittiness, very little solubility in Sn-Pb
  • Phosphorus – forms tin and lead phosphides, causes grittiness and dewetting, present in electroless nickel plating
  • Silver – often added intentionally, in high amounts forms intermetallics that cause grittiness and formation of pimples on the solder surface, potential for embrittlement
  • Sulfur – forms lead and tin sulfides, causes dewetting
  • Zinc – 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.[27]

Flux

Electrical solder with an integrated rosin core, visible as a dark spot in the cut end of the solder wire.

Flux is a reducing agent designed to help 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

deionized water and detergent, instead of hydrocarbon solvents
. 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.[28]

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

intermetallic compounds are formed during solidifying of solders and during their reactions with the soldered surfaces.[25]
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 Cu−Cu3Sn−Cu6Sn5−Sn. 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.[29]

Some example interactions include:

Matrix of intermetallic solder compounds
Tin Lead Indium
Copper Cu4Sn, Cu6Sn5, Cu3Sn, Cu3Sn8[19] Cu3In, Cu9In4
Nickel Ni3Sn, Ni3Sn2, Ni3Sn4 NiSn3 Ni3In, NiIn Ni2In3, Ni3In7
Iron FeSn, FeSn2
Indium In3Sn, InSn4 In3Pb
Antimony SbSn
Bismuth BiPb3
Silver Ag6Sn, Ag3Sn Ag3In, AgIn2
Gold Au5Sn, AuSn AuSn2, AuSn4 Au2Pb, AuPb2 AuIn, AuIn2
Palladium Pd3Sn, Pd2Sn, Pd3Sn2, PdSn, PdSn2, PdSn4 Pd3In, Pd2In, PdIn, Pd2In3
Platinum Pt3Sn, Pt2Sn, PtSn, Pt2Sn3, PtSn2, PtSn4 Pt3Pb, PtPb PtPb4 Pt2In3, PtIn2, 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 glasses to other glasses, ceramics, metals, semiconductors, 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 450 and 550 °C (840 and 1,020 °F).

See also

References

  1. ^ "solder". Lexico UK English Dictionary. Oxford University Press. Archived from the original on January 10, 2022.
  2. ^ "solder". Oxford American Dictionary. Oxford University Press. 1980.
  3. .
  4. ^ Harper, Douglas. "solder". Online Etymology Dictionary.
  5. ^
    PMID 31528239. Open access icon
  6. ^ "Basic Info on Tin Whiskers". nepp.nasa.gov. Retrieved 27 March 2018.
  7. ^ Craig Hillman; Gregg Kittlesen & Randy Schueller. "A New (Better) Approach to Tin Whisker Mitigation" (PDF). DFR Solutions. Retrieved 23 October 2013.
  8. ^ Properties of Solders. farnell.com.
  9. ^ "U.S. Code: Title 42. The Public Health and Welfare" (PDF). govinfo.gov. p. 990.
  10. PMID 2294437
    .
  11. .
  12. ^ .
  13. .
  14. .
  15. ^ .
  16. ^ .
  17. ^ Peter Biocca (19 April 2006). "Lead-free Hand-soldering – Ending the Nightmares" (PDF). Kester. Retrieved 20 October 2019.
  18. ^
    PMID 31489052.{{cite journal}}: CS1 maint: multiple names: authors list (link) Open access icon
  19. .
  20. ^
  21. ^ "The Fluidity of the Ni-Modified Sn-Cu Eutectic Lead-free Solder" (PDF). Retrieved 2019-09-07.
  22. .
  23. ^ "Balver Zinn Desoxy RSN" (PDF). balverzinn.com. Retrieved 27 March 2018.
  24. ^ .
  25. ^ "Solder selection for photonic packaging". 2013-02-27. Retrieved 20 August 2016.
  26. ^ SN100C® Technical Guide. floridacirtech.com
  27. ^ Keith Sweatman & Tetsuro Nishimura (2006). "The Fluidity of the Ni-Modified Sn-Cu Eutectic Lead-Free Solder" (PDF). Nihon Superior Co., Ltd.
  28. .
  29. ^ 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.
  30. .

External links

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