Solder

A soldered joint used to attach a wire to the pin of a component on the rear of a printed circuit board
Spool of solder, 1.6 mm diameter

Solder (/ˈsoʊldər/,[1] /ˈsɒldər/[1] or in North America /ˈsɒdər/[2]) is a fusible metal alloy used to create a permanent bond between metal workpieces. The word solder comes from the Middle English word soudur, via Old French solduree and soulder, from the Latin solidare, meaning "to make solid".[3] In fact, solder must first be melted in order to adhere to and connect the pieces together after cooling, which requires that an alloy suitable for use as solder have a lower melting point than the pieces being 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),[4] 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 can become eutectic — that is, their melting point is the same as their freezing point, and the alloy's melting point is lower than that of either component. Non-eutectic alloys have markedly different solidus and liquidus temperatures, and within that range they exist as a paste of solid particles in a melt of the lower-melting phase. In electrical work, if the joint is disturbed in the pasty state before it has solidified totally, 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 soldering iron or soldering gun), and with cores containing flux. It is also available as a paste, as a preformed foil shaped to match the workpiece, more suitable for mechanized mass-production, or in small "tabs" that can be wrapped around the joint and melted with a flame, for field repairs where an iron isn't usable or available. 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.[5]

Plumbers often use bars of solder, much thicker than the wire used for electrical applications. Jewelers often use solder in thin sheets, which they cut into snippets.

Lead-free solder

Pure tin solder wire
Soldering copper pipes using a propane torch and lead-free solder

On July 1, 2006 the European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) came into effect, 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 5 to 20 °C higher,[6] though there are also solders with much lower melting points.

It may be desirable to use minor modification of the solder pots (e.g. titanium liners or impellers) used in wave-soldering, to reduce maintenance cost due to increased tin-scavenging of high-tin solder.

Lead-free solder may be less desirable for critical applications, such as aerospace and medical projects, because its properties are less thoroughly known.

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 (217 ˚C), which is below the 22/78 Sn-Ag (wt.%) eutectic of 221 °C and the 59/41 Sn-Cu eutectic of 227 °C (recently revised by P. Snugovsky to 53/47 Sn-Cu). 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 selection of 4th element additions to Sn-Ag-Cu to provide compatibility for the reduced cooling rate of solder sphere reflow for assembly of ball grid arrays, e.g., 18/64/14/4 Tin-Silver-Copper-Zinc (Sn-Ag-Cu-Zn) (melting range of 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 intermetallics; 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 layer 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 silvers are suitable; tin-free alloys are also a choice, though their wettability 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.[7]

Lead solder

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 and shear strengths. Historically, lead has been widely believed to mitigate the formation of tin whiskers, though the precise mechanism for this is unknown.[8] 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 inclusion of conformal coatings.[9] Alloys commonly used for electrical soldering are 60/40 Sn-Pb, which melts at 188 °C (370 °F),[10] and 63/37 Sn-Pb used principally in electrical/electronic work. 63/37 is a 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, lead is prohibited in solder and flux in plumbing applications for drinking water use, per the Safe Drinking Water Act (SDWA).[11] 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,[12] 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.[13]

Before recent requirements for lead-free products, electronic components on printed circuit (printed wiring) boards (PCBs) were connected to the etched-copper-foil circuit, and hence to other components, by soldered joints created either by hand with a soldering iron using flux-core eutectic or near-eutectic (60/40) wire solder, or by a wave soldering machine which applies a flux beforehand. However, boards using the finest-pitch surface mount technology usually use solder paste applied to each location to be soldered before the component is placed on the board.

Lead-tin solders readily dissolve gold plating and form brittle intermetallics.[7] 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 radioisotope impurities. Radioisotopes undergoing alpha decay are a concern due to their tendency to cause soft errors. Polonium-210 is especially problematic; lead-210 beta decays to 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]

Flux-core solder

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 atmospheric pollution and hazardous waste disposal, the electronics industry has been gradually shifting from rosin flux to water-soluble flux, which can be removed with deionized water and detergent, instead of hydrocarbon solvents.

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.

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 silversmithing or 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 or rouge 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.

Solder alloys

CompositionM.P. S/L (°C)ToxicEutecticCommentsSnPbAgCuSbBiInZnCdAuOth.














Sn50Zn49Cu1 200/300[17]NoGalvanite Lead-free galvanizing solder formulation designed specifically for high quality repairs to galvanized steel surfaces. Simple, effective and easy to use, in both manufacturing and field applications. Metallurgically bonds to the steel, for a seamless protective barrier.[17] 50149
Sn95.5Cu4Ag0.5 226/260[18]NoKappFree provides good joint strength, vibration resistance, and thermal cycle fatigue resistance in both piping and electrical products as opposed to tin-lead solders. Higher working temperature. Wets well to brass, copper, and stainless steel. Good electrical conductivity.[18] 95.50.54
Sn90Zn7Cu3 200/222[19]NoKapp Eco-Babbitt[19] Commonly used in capacitor manufacturing as protective coating to shield against electromotive force (EMF) and electromagnetic interference (EMI) with the specified performance of the capacitor, to prevent current and charge leakage out of and within the layers of the capacitor, and to prevent the development of electron flows within the coating material itself, that would diminish capacitor performance, coating, and capacitor life.[19] 9037
Pb90Sn10 268/302[20] 275/302[21]PbNoSn10, UNS L54520, ASTM10B. Balls for CBGA components, replaced by Sn95.5Ag3.9Cu0.6.[6] Low cost and good bonding properties. Rapidly dissolves gold and silver, not recommended for those.[22] Used for fabrication of car radiators and fuel tanks, for coating and bonding of metals for moderate service temperatures. Body solder.[23] Has low thermal EMF, can be used as an alternative to Cd70 where parasitic thermocouple voltage has to be avoided.[24] 1090
Pb88Sn12 254/296[23]PbNoUsed for fabrication of car radiators and fuel tanks, for coating and bonding of metals for moderate service temperatures. Body solder. 1288
Pb85Sn15 227/288[23]PbNoUsed for coating tubes and sheets and fabrication of car radiators. Body solder. 1585
Pb80Sn20 183/280[21]PbNoSn20, UNS L54711. Used for coating radiator tubes for joining fins.[23] 2080
Pb80Sb15Sn5 300PbWhite Metal Capping. Used for locking mineshaft winding ropes into their tapered end sockets or 'capels'.[25] 58015
Pb75Sn25 183/266[20]PbNoCrude solder for construction plumbing works, flame-melted. Used for soldering car engine radiators. Used for machine, dip and hand soldering of plumbing fixtures and fittings. Superior body solder.[23] 2575
Pb70Sn30 185/255[20] 183/257[21]PbNoSn30, UNS L54280, crude solder for construction plumbing works, flame-melted, good for machine and torch soldering.[26] Used for soldering car engine radiators. Used for machine, dip and hand soldering of plumbing fixtures and fittings. Superior body solder.[23] 3070
Pb68Sn32 253PbNo"Plumber solder", for construction plumbing works[27] 3268
Pb68Sn30Sb2 185/243[21]PbNoPb68 30682
Sn30Pb50Zn20 177/288[28]PbNoKapp GalvRepair Economical solder for repairing & joining most metals including Aluminum and cast Iron. Have been used for cast Iron and galvanized surface repair.[28] 305020
Sn33Pb40Zn28 230/275[28]PbNoEconomical solder for repairing & joining most metals including Aluminum and cast Iron. Have been used for cast Iron and galvanized surface repair.[28] 334028
Pb67Sn33 187–230PbNoPM 33, crude solder for construction plumbing works, flame-melted, temperature depends on additives 3367
Pb65Sn35 183/250[21]PbNoSn35. Used as a cheaper alternative of Sn60Pb40 for wiping and sweating joints.[23] 3565
Pb60Sn40 183/238[20] 183/247[21]PbNoSn40, UNS L54915. For soldering of brass and car radiators.[26] For bulk soldering, and where wider melting point range is desired. For joining cables. For wiping and joining lead pipes. For repairs of radiators and electrical systems.[23] 4060
Pb55Sn45 183/227[23]PbNoFor soldering radiator cores, roof seams, and for decorative joints. 4555
Sn50Pb50 183/216[20] 183–212[21]PbNoSn50, UNS L55030. "Ordinary solder", for soldering of brass, electricity meters, gas meters, formerly also tin cans. General purpose, for standard tinning and sheetmetal work. Becomes brittle below −150 °C.[7][27] Low cost and good bonding properties. Rapidly dissolves gold and silver, not recommended for those.[22] For wiping and assembling plumbing joints for non-potable water.[23] 5050
Sn50Pb48.5Cu1.5 183/215[29]PbNoSavbit, Savbit 1, Sav1. Minimizes dissolution of copper. Originally designed to reduce erosion of the soldering iron tips. About 100 times slower erosion of copper than ordinary tin/lead alloys. Suitable for soldering thin copper platings and very thin copper wires.[30] 5048.51.5
Sn60Pb40 183/190[20] 183/188[21]PbNearSn60, ASTM60A, ASTM60B. Common in electronics, most popular leaded alloy for dipping. Low cost and good bonding properties. Used in both SMT and through-hole electronics. Rapidly dissolves gold and silver, not recommended for those.[22] Slightly cheaper than Sn63Pb37, often used instead for cost reasons as the melting point difference is insignificant in practice. On slow cooling gives slightly duller joints than Sn63Pb37.[30] 6040
Sn60Pb38Cu2 183/190[21][31]PbCu2. Copper content increases hardness of the alloy and inhibits dissolution of soldering iron tips and part leads in molten solder. 60382
Sn60Pb39Cu1 PbNo 60391
Sn62Pb38 183PbNear"Tinman's solder", used for tinplate fabrication work.[27] 6238
Sn63Pb37 183[32]PbYesSn63, ASTM63A, ASTM63B. Common in electronics; exceptional tinning and wetting properties, also good for stainless steel. One of the most common solders. Low cost and good bonding properties. Used in both SMT and through-hole electronics. Rapidly dissolves gold and silver, not recommended for those.[22] Sn60Pb40 is slightly cheaper and is often used instead for cost reasons, as the melting point difference is insignificant in practice. On slow cooling gives slightly brighter joints than Sn60Pb40.[30] 6337
Sn63Pb37P0.0015–0.04 183[33]PbYesSn63PbP. A special alloy for HASL machines. Addition of phosphorus reduces oxidation. Unsuitable for wave soldering as it may form metal foam. 6337P
Sn62Pb37Cu1 183[31]PbYesSimilar to Sn63Pb37. Copper content increases hardness of the alloy and inhibits dissolution of soldering iron tips and part leads in molten solder. 62371
Sn70Pb30 183/193[20]PbNoSn70 7030
Sn90Pb10 183/213[21]PbNoformerly used for joints in food industry 9010
Sn95Pb5 238PbNoplumbing and heating 955
Pb92Sn5.5Ag2.5 286/301[31]PbNoFor higher-temperature applications. 5.5922.5
Pb80Sn12Sb8 PbNoUsed for soldering iron and steel[27] 12808
Pb80Sn18Ag2 252/260[21]PbNoUsed for soldering iron and steel[27] 18802
Pb79Sn20Sb1 184/270PbNoSb1 20791
Pb55Sn43.5Sb1.5 PbNoGeneral purpose solder. Antimony content improves mechanical properties but causes brittleness when soldering cadmium, zinc, or galvanized metals.[27] 43.5551.5
Sn43Pb43Bi14 144/163[20]PbNoBi14. Good fatigue resistance combined with low melting point. Contains phases of tin and lead-bismuth.[34] Useful for step soldering. 434314
Sn46Pb46Bi8 120/167[21]PbNoBi8 46468
Bi52Pb32Sn16 96Pbyes?Bi52. Good fatigue resistance combined with low melting point. Reasonable shear strength and fatigue properties. Combination with lead-tin solder may dramatically lower melting point and lead to joint failure.[34] 163252
Bi46Sn34Pb20 100/105[21]PbNoBi46 342046
Sn62Pb36Ag2 179[20]PbYesSn62. Common in electronics. The strongest tin-lead solder. Appearance identical to Sn60Pb40 or Sn63Pb37. Crystals of Ag3Sn may be seen growing from the solder. Extended heat treatment leads to formation of crystals of binary alloys. Silver content decreases solubility of silver, making the alloy suitable for soldering silver-metallized surfaces, e.g. SMD capacitors and other silver-metallized ceramics.[7][30][34] Not recommended for gold.[22] General-purpose. 62362
Sn62.5Pb36Ag2.5 179[20]PbYes 62.5362.5
Pb88Sn10Ag2 268/290[20] 267/299[35]PbNoSn10, Pb88. Silver content reduces solubility of silver coatings in the solder. Not recommended for gold.[22] Forms a eutectic phase, not recommended for operation above 120 °C. 10882
Pb90Sn5Ag5 292[20]PbYes 5905
Pb92.5Sn5Ag2.5 287/296[20] 299/304[21]PbNoPb93. 592.52.5
Pb93.5Sn5Ag1.5 296/301[20] 305/306[21]PbNoPb94, HMP alloy, HMP. Service temperatures up to 255 °C. Useful for step soldering. Also can be used for extremely low temperatures as it remains ductile down to −200 °C, while solders with more than 20% tin become brittle below −70 °C. Higher strength and better wetting than Pb95Sn5.[30] 593.51.5
Pb95.5Sn2Ag2.5 299/304[20]PbNo 295.52.5
In97Ag3 143[36]–YesWettability and low-temperature malleability of indium, strength improved by addition of silver. Particularly good for cryogenic applications. Used for packaging of photonic devices. 397
In90Ag10 143/237[37]–NoNearly as wettable and low-temperature malleable as indium. Large plastic range. Can solder silver, fired glass and ceramics. 1090
In75Pb25 156/165[22]PbNoLess gold dissolution and more ductile than lead-tin alloys. Used for die attachment, general circuit assembly and packaging closures.[22] 2575
In70Pb30 160/174[20] 165/175[21][38]PbNoIn70. Suitable for gold, low gold-leaching. Good thermal fatigue properties. 3070
In60Pb40 174/185[20] 173/181[21]PbNoIn60. Low gold-leaching. Good thermal fatigue properties. 4060
In50Pb50 180/209[22] 178/210[21]PbNoIn50. Only one phase. Resoldering with lead-tin solder forms indium-tin and indium-lead phases and leads to formation of cracks between the phases, joint weakening and failure.[34] On gold surfaces gold-indium intermetallics tend to be formed, and the joint then fails in the gold-depleted zone and the gold-rich intermetallic.[39] Less gold dissolution and more ductile than lead-tin alloys.[22] Good thermal fatigue properties. 5050
In50Sn50 118/125[40]–NoCerroseal 35. Fairly well wets glass, quartz and many ceramics. Malleable, can compensate some thermal expansion differences. Low vapor pressure. Used in low temperature physics as a glass-wetting solder.[41] 5050
In70Sn15Pb9.6Cd5.4 125[42]Cd, Pb 159.6705.4
Pb75In25 250/264[22] 240/260[43]PbNoIn25. Low gold-leaching. Good thermal fatigue properties. Used for die attachment of e.g. GaAs dies.[39] Used also for general circuit assembly and packaging closures. Less dissolution of gold and more ductile than tin-lead alloy.[22] 7525
Sn70Pb18In12 162[20]
154/167[44]		PbYesGeneral purpose. Good physical properties. 701812
Sn37.5Pb37.5In25 134/181[22]PbNoGood wettability. Not recommended for gold.[22] 37.537.525
Pb90In5Ag5 290/310[20]PbNo 9055
Pb92.5In5Ag2.5 300/310[20]PbNoUNS L51510. Minimal leaching of gold, good thermal fatigue properties. Reducing atmosphere frequently used.. 92.52.55
Pb92.5In5Au2.5 300/310[21]PbNoIn5 92.552.5
Pb94.5Ag5.5 305/364[21] 304/343[45]PbNoAg5.5, UNS L50180 94.55.5
Pb95Ag5 305/364[46]PbNo 955
Pb97.5Ag2.5 303[20] 304[21] 304/579[47]Pbyes noAg2.5, UNS L50132. Used during World War II to conserve tin. Poor corrosion resistance; joints suffered corrosion in both atmospheric and underground conditions, all had to be replaced with Sn-Pb alloy joints.[48] Torch solder. 97.52.5
Sn97.5Pb1Ag1.5 305PbYesImportant for hybrid circuits assembly.[7] 97.511.5
Pb97.5Ag1.5Sn1 309[20]PbYesAg1.5, ASTM1.5S. High melting point, used for commutators, armatures, and initial solder joints where remelting when working on nearby joints is undesirable.[26] Silver content reduces solubility of silver coatings in molten solder. Not recommended for gold.[22] Standard PbAgSn eutectic solder, wide use in semiconductor assembly. Reducing protective atmosphere (e.g. 12% hydrogen) often used. High creep resistance, for use at both elevated and cryogenic temperatures. 197.51.5
Pb54Sn45Ag1 177–210Pbexceptional strength, silver gives it a bright long-lasting finish; ideal for stainless steel[26] 45541
Pb96Ag4 305Pbhigh-temperature joints[26] 964
Pb96Sn2Ag2 252/295[21]PbPb96 2962
Sn61Pb36Ag3 Pb[7] 61363
Sn56Pb39Ag5 Pb[7] 56395
Sn98Ag2 –[7] 982
Sn65Ag25Sb10 233–YesVery high tensile strength. For die attachment. Very brittle. Old Motorola die attach solder. 652510
Sn96.5Ag3.0Cu0.5 217/220 217/218[21][49]–NearSAC305. It is the JEITA recommended alloy for wave and reflow soldering, with alternatives SnCu for wave and SnAg and SnZnBi for reflow soldering. Usable also for selective soldering and dip soldering. At high temperatures tends to dissolve copper; copper buildup in the bath has detrimental effect (e.g. increased bridging). Copper content must be maintained between 0.4–0.85%, e.g. by refilling the bath with Sn97Ag3 alloy. Nitrogen atmosphere can be used to reduce losses by dross formation. Dull, surface shows formation of dendritic tin crystals. 96.530.5
Sn95.8Ag3.5Cu0.7 217–218–NearSN96C-Ag3.5 A commonly used alloy. Used for wave soldering. Usable also for selective soldering and dip soldering. At high temperatures tends to dissolve copper; copper buildup in the bath has detrimental effect (e.g. increased bridging). Copper content must be maintained between 0.4–0.85%, e.g. by refilling the bath with Sn96.5Ag3.5 alloy (designated e.g. SN96Ce). Nitrogen atmosphere can be used to reduce losses by dross formation. Dull, surface shows formation of dendritic tin crystals. 95.83.50.7
Sn95.6Ag3.5Cu0.9 217–YesDetermined by NIST to be truly eutectic. 95.63.50.9
Sn95.5Ag3.8Cu0.7 217[50]–NearSN96C. Preferred by the European IDEALS consortium for reflow soldering. Usable also for selective soldering and dip soldering. At high temperatures tends to dissolve copper; copper buildup in the bath has detrimental effect (e.g. increased bridging). Copper content must be maintained between 0.4–0.85%, e.g. by refilling the bath with Sn96.2Ag3.8 alloy (designated e.g. SN96Ce). Nitrogen atmosphere can be used to reduce losses by dross formation. Dull, surface shows formation of dendritic tin crystals. 95.53.80.7
Sn95.25Ag3.8Cu0.7Sb0.25 –Preferred by the European IDEALS consortium for wave soldering. 95.253.80.70.25
Sn95.5Ag3.9Cu0.6 217[51]–YesRecommended by the US NEMI consortium for reflow soldering. Used as balls for BGA/CSP and CBGA components, a replacement for Sn10Pb90. Solder paste for rework of BGA boards.[6] Alloy of choice for general SMT assembly. 95.53.90.6
Sn95.5Ag4Cu0.5 217[52]–YesSAC405. Lead-Free, Cadmium Free formulation designed specifically to replace Lead solders in Copper and Stainless Steel plumbing, and in electrical and electronic applications.[18] 95.540.5
Sn96.5Ag3.5 221[20]–YesSn96, Sn96.5, 96S. Fine lamellar structure of densely distributed Ag3Sn. Annealing at 125 °C coarsens the structure and softens the solder.[6] Creeps via dislocation climb as a result of lattice diffusion.[16] Used as wire for hand soldering rework; compatible with SnCu0.7, SnAg3Cu0.5, SnAg3.9Cu0.6, and similar alloys. Used as solder spheres for BGA/CSP components. Used for step soldering and die attachment in high power devices. Established history in the industry.[6] Widely used. Strong lead-free joints. Silver content minimizes solubility of silver coatings. Not recommended for gold.[22] Marginal wetting. Good for step soldering. Used for soldering stainless steel as it wets stainless steel better than other soft solders. Silver content does not suppress dissolution of silver metallizations.[30] High tin content allows absorbing significant amount of gold without embrittlement.[53] 96.53.5
Sn96Ag4 221–229–NoASTM96TS. "Silver-bearing solder". Food service equipment, refrigeration, heating, air conditioning, plumbing.[26] Widely used. Strong lead-free joints. Silver content minimizes solubility of silver coatings. Not recommended for gold.[22] 964
Sn95Ag5 221/254[54]–NoWidely used. Strong lead-free joints. Silver content minimizes solubility of silver coatings. Not recommended for gold. Produces strong and ductile joints on Copper and Stainless Steel. The resulting joints have high tolerance to vibration and stress, with tensile strengths to 30,000 psi on Stainless.[54] 955
Sn94Ag6 221/279[54]–NoProduces strong and ductile joints on Copper and Stainless Steel. The resulting joints have high tolerance to vibration and stress, with tensile strengths to 30,000 psi on Stainless.[54] 946
Sn93Ag7 221/302[54]–NoProduces strong and ductile joints on Copper and Stainless Steel. The resulting joints have high tolerance to vibration and stress, with tensile strengths to 31,000 psi on Stainless.[54] Audio industry standard for vehicle and home theater speaker installations. Its 7% Silver content requires a higher temperature range, but yields superior strength and vibration resistance.[55] 937
Sn95Ag4Cu1 – 9541
Sn 232–PureSn99. Good strength, non-dulling. Use in food processing equipment, wire tinning, and alloying.[26] Susceptible to tin pest. 100
Sn99.3Cu0.7 227–YesSn99Cu1. Also designated as Sn99Cu1. Cheap alternative for wave soldering, recommended by the US NEMI consortium. Coarse microstructure with ductile fractures. Sparsely distributed Cu6Sn5.[56] Forms large dendritic ß-tin crystals in a network of eutectic microstructure with finely dispersed Cu6Sn5. High melting point unfavorable for SMT use. Low strength, high ductility. Susceptible to tin pest.[16] Addition of small amount of nickel increases its fluidity; the highest increase occurs at 0.06% Ni. Such alloys are known as nickel modified or nickel stabilized.[57] 99.30.7(Ni)
Sn99Cu0.7Ag0.3 217/228[58]–NoSCA, SAC, or SnAgCu. Tin-silver-copper alloy. Relatively low-cost lead-free alloy for simple applications. Can be used for wave, selective and dip soldering. At high temperatures tends to dissolve copper; copper buildup in the bath has detrimental effect (e.g. increased bridging). Copper content must be maintained between 0.4–0.85%, e.g. by refilling the bath with Sn96.2Ag3.8 alloy (designated e.g. SN96Ce). Nitrogen atmosphere can be used to reduce losses by dross formation. Dull, surface shows formation of dendritic tin crystals. 990.30.7
Sn97Cu3 227/250[59] 232/332[23]–For high-temperature uses. Allows removing insulation from an enameled wire and applying solder coating in a single operation. For radiator repairs, stained glass windows, and potable water plumbing. 973
Sn97Cu2.75Ag0.25 228/314[23]–High hardness, creep-resistant. For radiators, stained glass windows, and potable water plumbing. Excellent high-strength solder for radiator repairs. Wide range of patina and colors. 970.252.75
Zn100 419–PureFor soldering aluminium. Good wettability of aluminium, relatively good corrosion resistance.[60] 100
Bi100 271–PureUsed as a non-superconducting solder in low-temperature physics. Does not wet metals well, forms a mechanically weak joint.[41] 100
Sn91Zn9 199[61]–YesKappAloy9 Designed specifically for Aluminum-to-Aluminum and Aluminum-to-Copper soldering. It has good corrosion resistance and tensile strength. Lies between soft solder and silver brazing alloys, thereby avoiding damage to critical electronics and substrate deformation and segregation. Best solder for Aluminum wire to Copper busses or Copper wire to Aluminum busses or contacts.[61] UNS#: L91090 919
Sn85Zn15 199/260[61]–NoKappAloy15 Designed specifically for Aluminum-to-Aluminum and Aluminum-to-Copper soldering. It has good corrosion resistance and tensile strength. Lies between soft solder and silver brazing alloys, thereby avoiding damage to critical electronics and substrate deformation and segregation. Has a wide plastic range this makes it ideal for hand soldering Aluminum plates and parts, allowing manipulation of the parts as the solder cools.[61] 8515
Zn95Al5 382–YesFor soldering aluminium. Good wetting.[60] 95Al5
Sn91.8Bi4.8Ag3.4 211/213[62]–NoDo not use on lead-containing metallizations. U.S. Patent 5,439,639 (ICA Licensed Sandia Patent). 91.83.44.8
Sn70Zn30 199/316[61]–NoKappAloy30 For soldering of aluminium. Good wetting. Used extensively in spray wire form for capacitors and other electronic parts. Higher temperature and higher tensile strength compared to 85Sn/15Zn and 91Sn/9Zn.[61] 7030
Sn80Zn20 199/288[61]–NoKappAloy20 For soldering of aluminium. Good wetting. Used extensively in spray wire form for capacitors and other electronic parts. Higher temperature and higher tensile strength compared to 85Sn/15Zn and 91Sn/9Zn.[61] 8020
Sn60Zn40 199/343[61]–NoKappAloy40 For soldering of aluminium. Good wetting. Used extensively in spray wire form for capacitors and other electronic parts. Higher temperature and higher tensile strength compared to 85Sn/15Zn and 91Sn/9Zn.[61] 6040
Pb63Sn35Sb2 185/243[21]PbNoSb2 35632
Pb63Sn34Zn3 170/256PbNoPoor wetting of aluminium. Poor corrosion rating.[48] 34633
Pb92Cd8 310?Cd, Pb?For soldering aluminium. US patent 1,333,666.[63] 928
Sn48Bi32Pb20 140/160[31]PbNoFor low-temperature soldering of heat-sensitive parts, and for soldering in the vicinity of already soldered joints without their remelting. 482032
Sn89Zn8Bi3 191–198–Prone to corrosion and oxidation due to its zinc content. On copper surfaces forms a brittle Cu-Zn intermetallic layer, reducing the fatigue resistance of the joint; nickel plating of copper inhibits this.[64] 8938
Sn83.6Zn7.6In8.8 181/187[65]–NoHigh dross due to zinc. Covered by U.S. Patent #5,242,658. 83.68.87.6
Sn86.5Zn5.5In4.5Bi3.5 174/186[66]–NoLead-free. Corrosion concerns and high drossing due to zinc content. 86.53.54.55.5
Sn86.9In10Ag3.1 204/205[67]–Potential use in flip-chip assembly, no issues with tin-indium eutectic phase. 86.93.110
Sn95Ag3.5Zn1Cu0.5 221L[64]–No 953.50.51
Sn95Sb5 235/240[20] 232/240[21]–NoSb5, ASTM95TA. The US plumbing industry standard. It displays good resistance to thermal fatigue and good shear strength. Forms coarse dendrites of tin-rich solid solution with SbSn intermetallic dispersed between. Very high room-temperature ductility. Creeps via viscous glide of dislocations by pipe diffusion. More creep-resistant than SnAg3.5. Antimony can be toxic. Used for sealing chip packagings, attaching I/O pins to ceramic substrates, and die attachment; a possible lower-temperature replacement of AuSn.[16] High strength and bright finish. Use in air conditioning, refrigeration, some food containers, and high-temperature applications.[26] Good wettability, good long-term shear strength at 100 °C. Suitable for potable water systems. Used for stained glass, plumbing, and radiator repairs. 955
Sn97Sb3 232/238[68]–No 973
Sn99Sb1 232/235[69]–No 991
Sn99Ag0.3Cu0.7 – 990.30.7
Sn96.2Ag2.5Cu0.8Sb0.5 217–225 217[21]–Ag03A. Patented by AIM alliance. 96.22.50.80.5
Sn88In8.0Ag3.5Bi0.5 197–208–Patented by Matsushita/Panasonic. 883.50.58
Bi57Sn42Ag1 137/139 139/140[70]–Addition of silver improves mechanical strength. Established history of use. Good thermal fatigue performance. Patented by Motorola. 42157
Bi58Sn42 138[20][22]–YesBi58. Reasonable shear strength and fatigue properties. Combination with lead-tin solder may dramatically lower melting point and lead to joint failure.[34] Low-temperature eutectic solder with high strength.[22] Particularly strong, very brittle.[20] Used extensively in through-hole technology assemblies in IBM mainframe computers where low soldering temperature was required. Can be used as a coating of copper particles to facilitate their bonding under pressure/heat and creating a conductive metallurgical joint.[64] Sensitive to shear rate. Good for electronics. Used in thermoelectric applications. Good thermal fatigue performance.[71] Established history of use. Expands slightly on casting, then undergoes very low further shrinkage or expansion, unlike many other low-temperature alloys which continue changing dimensions for some hours after solidification.[41] 4258
Bi58Pb42 124/126[72]Pb 4258
In80Pb15Ag5 142/149[21]
149/154[73]		PbNoIn80. Compatible with gold, minimum gold-leaching. Resistant to thermal fatigue. Can be used in step soldering. 15580
Pb60In40 195/225[21]PbNoIn40. Low gold-leaching. Good thermal fatigue properties. 6040
Pb70In30 245/260[21]PbNoIn30 7030
Sn37.5Pb37.5In26 134/181[21]PbNoIn26 37.537.526
Sn54Pb26In20 130/154[21] 140/152[74]PbNoIn20 542620
Pb81In19 270/280[21] 260/275[75]PbNoIn19. Low gold-leaching. Good thermal fatigue properties. 8119
In52Sn48 118–YesIn52. Suitable for the cases where low-temperature soldering is needed. Can be used for glass sealing.[64] Sharp melting point. Good wettability of glass, quartz, and many ceramics. Good low-temperature malleability, can compensate for different thermal expansion coefficients of joined materials. 4852
Sn52In48 118/131[20]–Novery low tensile strength 5248
Sn58In42 118/145[76]–No 5842
Sn51.2Pb30.6Cd18.2 145[77]Cd, PbYesGeneral-purpose. Maintains creep strength well. Unsuitable for gold. 51.230.618.2
Sn77.2In20Ag2.8 175/187[78]–NoSimilar mechanical properties with Sn63Pb37, Sn62Pb36Ag2 and Sn60Pb40, suitable lead-free replacement. Contains eutectic Sn-In phase with melting point at 118 °C, avoid use above 100 °C. 77.22.820
In74Cd26 123[79]CdYes 7426
In61.7Bi30.8Cd7.5 62[80]CdYes 30.861.77.5
Bi47.5Pb25.4Sn12.6Cd9.5In5 57/65[81]Pb, CdNo 12.625.447.559.5
Bi48Pb25.4Sn12.8Cd9.6In4 61/65[82]Cd, PbNo 12.825.4489.6
Bi49Pb18Sn15In18 58/69[83]PbNo 15184918
Bi49Pb18Sn12In21 58PbYesCerrolow 136. Slightly expands on cooling, later shows slight shrinkage in couple hours afterwards. Used as a solder in low-temperature physics.[41] Also the ChipQuik desoldering alloy.[84] 12184921
Bi50.5Pb27.8Sn12.4Cd9.3 70/73[85]Pb, CdNo 12.427.850.59.3
Bi50Pb26.7Sn13.3Cd10 70Pb, CdYesCerrobend. Used in low-temperature physics as a solder.[41] 13.326.75010
Bi44.7Pb22.6In19.1Cd5.3Sn8.3 47Cd, PbYesCerrolow 117. Used as a solder in low-temperature physics.[41] 8.322.644.719.15.3
In60Sn40 113/122[20]–No 4060
In51.0Bi32.5Sn16.5 60.5–YesField's metal 16.532.551
Bi49.5Pb27.3Sn13.1Cd10.1 70.9Cd, PbYesLipowitz Metal 13.127.349.510.1
Bi50.0Pb25.0Sn12.5Cd12.5 71Cd, PbYesWood's metal, mostly used for casting. 12.5255012.5
Bi50.0Pb31.2Sn18.8 97PbNoNewton's metal 18.831.250
Bi50Pb28Sn22 109PbNoRose's metal. It was used to secure cast iron railings and balusters in pockets in stone bases and steps. Does not contract on cooling. 222850
Bi56Sn30In14 79/91NoChipQuik desoldering alloy, lead-free[86] 305614
Cd95Ag5 338/393[87]CdNoKappTec General purpose solder that will join all solderable metals except Aluminum. High temperature, high strength solder. It is used in applications where alloys melting higher than soft solders are required, but the cost and strength of Silver-brazing alloys is not necessary.[87] 595
Cd82.5Zn17.5 265[88]CdYesMedium temperature alloy that provide strong, corrosion-resistant joints on most metals.[88] Also for soldering aluminium and die-cast zinc alloys.[27] Used in cryogenic physics for attaching electrical potential leads to specimens of metals, as this alloy does not become superconductive at liquid helium temperatures.[41] 17.582.5
Cd70Zn30 265/300[88]CdNoMedium temperature alloy that provide strong, corrosion-resistant joints on most metals. Works especially well on Aluminum-to-Aluminum and Aluminum-to-Copper joints, with excellent corrosion resistance and superior strength in high vibration and high stress applications in electronics, lighting and electrical products.[88] 3070
Cd60Zn40 265/316[88]CdNoMedium temperature alloy that provide strong, corrosion-resistant joints on most metals. Works especially well on Aluminum-to-Aluminum and Aluminum-to-Copper joints, with excellent corrosion resistance and superior strength in high vibration and high stress applications in electronics, lighting and electrical products.[88] 4060
Cd78Zn17Ag5 249/316[89]CdNoKappTecZ High temperature, high strength solder that may be used on most metals, but works extremely well on Aluminum, Copper and Stainless Steel. It has a high tolerance to vibration and stress, and good elongation for use on dissimilar metals. Above its liquidus of 600 °F, this solder is extremely fluid and will penetrate the closest joints.[89] 51778
Sn40Zn27Cd33 176/260[90]CdNoKappRad[90] Developed specifically to join and repair Aluminum and Aluminum/Copper radiators and heat exchangers. A lower melting point makes delicate repair work easier.[90] 402733
Zn90Cd10 265/399CdFor soldering aluminium. Good wetting.[60] 9010
Zn60Cd40 265/335CdFor soldering aluminium. Very good wetting.[60] 6040
Cd70Sn30 140/160[21]CdNoCd70, thermal-free solder. Produces low thermal EMF joints in copper, does not form parasitic thermocouples. Used in low-temperature physics.[41] 29.5670.44
Sn50Pb32Cd18 145[21]Cd, PbCd18 503218
Sn40Pb42Cd18 145[91]Cd, PbLow melting temperature allows repairing pewter and zinc objects, including die-cast toys. 404218
Zn70Sn30 199/376–NoFor soldering aluminium. Excellent wetting.[48] Good strength. 3070
Zn60Sn40 199/341–NoFor soldering aluminium. Good wetting.[60] 4060
Zn95Sn5 382–yes?For soldering aluminium. Excellent wetting.[48] 595
Sn90Au10 217[92]–Yes 9010
Au80Sn20 280–YesAu80. Good wetting, high strength, low creep, high corrosion resistance, high thermal conductivity, high surface tension, zero wetting angle. Suitable for step soldering. The original flux-less alloy, does not need flux. Used for die attachment and attachment of metal lids to semiconductor packages, e.g. kovar lids to ceramic chip carriers. Coefficient of expansion matching many common materials. Due to zero wetting angle requires pressure to form a void-free joint. Alloy of choice for joining gold-plated and gold-alloy plated surfaces. As some gold dissolves from the surfaces during soldering and moves the composition to non-eutectic state (1% increase of Au content can increase melting point by 30 °C), subsequent desoldering requires higher temperature.[93] Forms a mixture of two brittle intermetallic phases, AuSn and Au5Sn.[94] Brittle. Proper wetting achieved usually by using nickel surfaces with gold layer on top on both sides of the joint. Comprehensively tested through military standard environmental conditioning. Good long-term electrical performance, history of reliability.[39] One of the best materials for soldering in optoelectronic devices and components packaging.[95] Low vapor pressure, suitable for vacuum work. Generally used in applications that require a melting temperature over 150 °C.[96] Good ductility. Also classified as a braze. 2080
Au98Si2 370/800[21]–Au98. A non-eutectic alloy used for die attachment of silicon dies. Ultrasonic assistance is needed to scrub the chip surface so a eutectic (3.1% Si) is reached at reflow. 98Si2
Au96.8Si3.2 370[21] 363[97]–YesAu97.[93] AuSi3.2 is a eutectic with melting point of 363 °C. AuSi forms a meniscus at the edge of the chip, unlike AuSn, as AuSi reacts with the chip surface. Forms a composite material structure of submicron silicon plates in soft gold matrix. Tough, slow crack propagation.[56] 96.8Si3.2
Au87.5Ge12.5 361 356[21]–YesAu88. Used for die attachment of some chips.[20] The high temperature may be detrimental to the chips and limits reworkability.[39] 87.5Ge12.5
Au82In18 451/485[21]–NoAu82. High-temperature, extremely hard, very stiff. 1882
In100 157–PureIn99. Used for die attachment of some chips. More suitable for soldering gold, dissolution rate of gold is 17 times slower than in tin-based solders and up to 20% of gold can be tolerated without significant embrittlement. Good performance at cryogenic temperatures.[98] Wets many surfaces incl. quartz, glass, and many ceramics. Deforms indefinitely under load. Does not become brittle even at low temperatures. Used as a solder in low-temperature physics, will bond to aluminium. Can be used for soldering to thin metal films or glass with an ultrasonic soldering iron.[41] 100
Sn90.7Ag3.6Cu0.7Cr5 217/1050[99]–NoC-Solder. Lead-free, low-temperature soldering alloy for joining of various carbon materials including carbon fibres and carbon nanotube fibres in both carbon-carbon and carbon-metal arrangements. Produces mechanically strong and electrically conductive bonds. Provides wetting of carbon[100] and other materials generally considered as difficult to solder, including aluminium, stainless steel, titanium, glass, ceramics.[101] 90.73.60.7Cr5

Notes on the above table

Temperature ranges for solidus and liquidus (the boundaries of the mushy state) are listed as solidus/liquidus.[20]

In the Sn-Pb alloys, tensile strength increases with increasing tin content. Indium-tin alloys with high indium content have very low tensile strength.[20]

For soldering semiconductor materials, e.g. die attachment of silicon, germanium and gallium arsenide, it is important that the solder contains no impurities that could cause doping in the wrong direction. For soldering n-type semiconductors, solder may be doped with antimony; indium may be added for soldering p-type semiconductors. Pure tin and pure gold can be used.[48]

Various fusible alloys can be used as solders with very low melting points; examples include Field's metal, Lipowitz's alloy, Wood's metal, and Rose's metal.

Properties

The thermal conductivity of common solders ranges from 32 to 94 W/(m·K), and the density from 9.25 to 15.00 g/cm3.[102][103]

Material Thermal conductivity[103]
(W/m·K)		 Melting point[103]
(°C)
Sn-37Pb (eutectic)50.9183
Sn-2.8Ag-20.0In53.5175–186
Sn-2.5Ag-0.8Cu-0.5Sb57.26215–217
Pb-5Sn63310
Lead (Pb)35.0327.3
Tin (Sn)73.0231.9
Aluminum (Al)240660.1
Copper (Cu)393–4011083
FR-41.7

Solidifying

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.[57]

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.

Alloying element roles

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.[27]
  • Bismuth significantly lowers the melting point and improves wettability. In presence of sufficient lead and tin, bismuth forms crystals of Sn16Pb32Bi52 with melting point of only 95 °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 lowers the melting point, 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. Forms intermetallic compounds. May promote growth of tin whiskers. 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.[104]
  • Nickel can be added to the solder alloy to form a supersaturated solution to inhibit dissolution of thin-film under-bump metallization.[104]
  • Indium lowers the melting point and improves ductility. In presence of lead it forms a ternary compound that undergoes phase change at 114 °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.[105]
  • 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.[6] Addition of silver to tin significantly lowers solubility of silver coatings in the tin phase. In eutectic tin-silver (3.5% Ag) alloy it tends to form platelets of Ag3Sn, which, if formed near a high-stress spot, may serve as initiating sites for cracks; silver content needs to be kept below 3% to inhibit such problems.[104]
  • 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, tin cry, 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%.[106]

Impurities in solders

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.[107]

  • 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.
  • Antimony – added intentionally, up to 0.3% improves wetting, larger amounts slowly degrade wetting
  • 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.[53]
  • Iron – forms intermetallics, causes grittiness, but rate of dissolution is very low; readily dissolves in lead-tin above 427 °C.[7]
  • 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
  • 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

Intermetallics in solders

Many different intermetallic compounds are formed during solidifying of solders and during their reactions with the soldered surfaces.[107]

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.

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.[108]

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.[16]

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.[109]

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.[110]

TinLeadIndium
CopperCu4Sn, Cu6Sn5, Cu3Sn, Cu3Sn8Cu3In, Cu9In4
NickelNi3Sn, Ni3Sn2, Ni3Sn4 NiSn3Ni3In, NiIn Ni2In3, Ni3In7
IronFeSn, FeSn2
IndiumIn3Sn, InSn4In3Pb–
AntimonySbSn
BismuthBiPb3
SilverAg6Sn, Ag3SnAg3In, AgIn2
GoldAu5Sn, AuSn AuSn2, AuSn4Au2Pb, AuPb2AuIn, AuIn2
PalladiumPd3Sn, Pd2Sn, Pd3Sn2, PdSn, PdSn2, PdSn4Pd3In, Pd2In, PdIn Pd2In3
PlatinumPt3Sn, Pt2Sn, PtSn, Pt2Sn3, PtSn2, PtSn4Pt3Pb, PtPb PtPb4Pt2In3, PtIn2, Pt3In7
  • Cu6Sn5 – common on solder-copper interface, forms preferentially when excess of tin is available; in presence of nickel (Cu,Ni)6Sn5 compound can be formed
  • Cu3Sn – common on solder-copper interface, forms preferentially when excess of copper is available, more thermally stable than Cu6Sn5, often present when higher-temperature soldering occurred
  • Ni3Sn4 – common on solder-nickel interface
  • FeSn2 – very slow formation
  • Ag3Sn - at higher concentration of silver (over 3%) in tin forms platelets that can serve as crack initiation sites.
  • AuSn4 – β-phase – brittle, forms at excess of tin. Detrimental to properties of tin-based solders to gold-plated layers.
  • AuIn2 – forms on the boundary between gold and indium-lead solder, acts as a barrier against further dissolution of gold

Glass solder

Glass solders are 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.

Two types of glass solders are used: vitreous, and devitrifying. Vitreous solders retain their amorphous structure during remelting, can be reworked repeatedly, and are relatively transparent. Devitrifying solders undergo partial crystallization during solidifying, forming a glass-ceramic, a composite of glassy and crystalline phases. Devitrifying solders usually create a stronger mechanical bond, but are more temperature-sensitive and the seal is more likely to be leaky; due to their polycrystalline structure they tend to be translucent or opaque.[111] Devitrifying solders are frequently "thermosetting", as their melting temperature after recrystallization becomes significantly higher; this allows soldering the parts together at lower temperature than the subsequent bake-out without remelting the joint afterwards. Devitrifying solders frequently contain up to 25% zinc oxide. In production of cathode ray tubes, devitrifying solders based on PbO-B2O3-ZnO are used.

Very low temperature melting glasses, fluid at 200–400 °C, were developed for sealing applications for electronics. They can consist of binary or ternary mixtures of thallium, arsenic and sulfur.[112] Zinc-silicoborate glasses can also be used for passivation of electronics; their coefficient of thermal expansion must match silicon (or the other semiconductors used) and they must not contain alkaline metals as those would migrate to the semiconductor and cause failures.[113]

The bonding between the glass or ceramics and the glass solder can be either covalent, or, more often, van der Waals.[114] The seal can be leak-tight; glass soldering is frequently used in vacuum technology. Glass solders can be also used as sealants; a vitreous enamel coating on iron lowered its permeability to hydrogen 10 times.[115] Glass solders are frequently used for glass-to-metal seals and glass-ceramic-to-metal seals.

Glass solders are available as frit powder with grain size below 60 micrometers. They can be mixed with water or alcohol to form a paste for easy application, or with dissolved nitrocellulose or other suitable binder for adhering to the surfaces until being melted.[116] The eventual binder has to be burned off before melting proceeds, requiring careful firing regime. The solder glass can be also applied from molten state to the area of the future joint during manufacture of the part. Due to their low viscosity in molten state, lead glasses with high PbO content (often 70–85%) are frequently used. The most common compositions are based on lead borates (leaded borate glass or borosilicate glass). Smaller amount of zinc oxide or aluminium oxide can be added for increasing chemical stability. Phosphate glasses can be also employed. Zinc oxide, bismuth trioxide, and copper(II) oxide can be added for influencing the thermal expansion; unlike the alkali oxides, these lower the softening point without increasing of thermal expansion.

Glass solders are frequently used in electronic packaging. CERDIP packagings are an example. Outgassing of water from the glass solder during encapsulation was a cause of high failure rates of early CERDIP integrated circuits. Removal of glass-soldered ceramic covers, e.g., for gaining access to the chip for failure analysis or reverse engineering, is best done by shearing; if this is too risky, the cover is polished away instead.[117]

As the seals can be performed at much lower temperature than with direct joining of glass parts and without use of flame (using a temperature-controlled kiln or oven), glass solders are useful in applications like subminiature vacuum tubes or for joining mica windows to vacuum tubes and instruments (e.g., Geiger tube). Thermal expansion coefficient has to be matched to the materials being joined and often is chosen in between the coefficients of expansion of the materials. In case of having to compromise, subjecting the joint to compression stresses is more desirable than to tensile stresses. The expansion matching is not critical in applications where thin layers are used on small areas, e.g., fireable inks, or where the joint will be subjected to a permanent compression (e.g., by an external steel shell) offsetting the thermally introduced tensile stresses.[112]

Glass solder can be used as an intermediate layer when joining materials (glasses, ceramics) with significantly different coefficient of thermal expansion; such materials cannot be directly joined by diffusion welding.[118] Evacuated glazing windows are made of glass panels soldered together.[119]

A glass solder is used, e.g., for joining together parts of cathode ray tubes and plasma display panels. Newer compositions lowered the usage temperature from 450 to 390 °C by reducing the lead(II) oxide content down from 70%, increasing the zinc oxide content, adding titanium dioxide and bismuth(III) oxide and some other components. The high thermal expansion of such glass can be reduced by a suitable ceramic filler. Lead-free solder glasses with soldering temperature of 450 °C were also developed.

Phosphate glasses with low melting temperature were developed. One of such compositions is phosphorus pentoxide, lead(II) oxide, and zinc oxide, with addition of lithium and some other oxides.[120]

Conductive glass solders can be also prepared.

Preform

A preform is a pre-made shape of solder specially designed for the application where it is to be used.[121] 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.[122]

See also

References

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Bibliography

  • Sanka Ganesan; Michael Pecht (2006). Lead-free electronics. Wiley. ISBN 978-0-471-78617-7.
  • Wikisource "Solder". Encyclopædia Britannica. 25 (11th ed.). 1911. p. 374.
  • Phase diagrams of different types of solder alloys
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