Solder (/ˈsoʊldə/,[1] /ˈsɒldə/[1] or USA /ˈsɒdər/[2]) is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece(s).
Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of 90 to 450 °C (190 to 840 °F). It is commonly used in electronics and plumbing. Alloys that melt between 180 and 190 °C (360 and 370 °F) are the most commonly used. By definition, using alloys with melting point above 450 °C (840 °F) is called 'hard soldering', 'silver soldering' or brazing. Soft solder can contain lead and/or flux but in many applications lead free solder is used. Perhaps the most common and most familiar form of solder is as a wire or rod, though plumbers often use bars of solder while jewelers often use solder in thin sheets which they cut into snippets. Solder can also come in a paste or as a preformed foil shaped to match the workpiece. 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]
Eutectic alloys melt at a single temperature. Non-eutectic alloys have markedly different solidus and liquidus temperature, and within that range they exist as a paste of solid particles in a melt of the lower-melting phase. The pasty state causes some problems during handling; it can however be exploited as it allows molding of the solder during cooling, e.g. for ensuring watertight joint of pipes, resulting in a so called 'wiped joint'.
With the reduction of the size of circuit board features, the size of interconnects shrinks as well. Current densities above 104 A/cm2 are often achieved and electromigration becomes a concern. At such current densities the Sn63Pb37 solder balls form hillocks on the anode side and voids on the cathode side; the increased content of lead on the anode side suggests lead is the primary migrating species.[4]
Contact with molten solder can cause 'solder embrittlement' of the materials, a type of liquid metal embrittlement.
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Tin/lead 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. At the retail level, the two most common alloys are 60/40 Tin/lead (Sn/Pb) which melts at 370 °F or 188 °C and 63/37 Sn/Pb used principally in electrical/electronic work. The 63/37 ratio is notable in that it is a eutectic mixture, which means:
In plumbing, a higher proportion of lead was used, commonly 50/50. This had the advantage of making the alloy solidify more slowly, so that it could be wiped over the joint to ensure watertightness, the pipes being physically fitted together before soldering. 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, however tin is protected by insoluble oxide. Since even small amounts of lead have been found detrimental to health,[5] 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 improves wetting properties of the alloy; lead itself has poor wetting characteristics. Tin however increases the cost of the solder. High-tin tin-lead alloys have limited use as the workability range can be provided by a cheaper high-lead alloy.[6]
In electronics, printed circuit boards use solder joints to mount components and create a circuit. For miniaturised PCB joints with surface mount components, solder paste has largely replaced solid solder.
Lead-tin solders readily dissolve gold plating and form brittle intermetallics.
Sn60Pb40 solder oxidizes on the surface with forming complex 4-layer structure: tin(IV) oxide on the surface, below it a layer of tin(II) oxide with finely dispersed lead, below a layer of tin(II) oxide with finely dispersed tin and lead, and the solder alloy itself underneath.[7]
Some alloys, namely of lead and to some degree tin, contain small but significant amounts of radioisotope impurities. The 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 lead containing alloys.[4][8]
On July 1, 2006 the European Union Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) came into effect prohibiting the intentional addition of lead to most consumer electronics produced in the EU. Manufacturers in the U.S. 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 Sn60/Pb40 and Sn63/Pb37 solder have melting points from 5 to 20 °C higher,[9] though solders with much lower melting points are available.
Drop-in replacements for silkscreen with solder paste soldering operations are available. Minor modification to the solder pots (e.g. titanium liners and/or impellers) used in wave-soldering operations may be desired to reduce maintenance costs associated with the increased tin-scavenging effects of high tin solders. Since the properties of lead-free solders are not as thoroughly known, they may therefore be considered less desirable for critical applications, like certain aerospace or medical projects. "Tin whiskers" were a problem with early electronic solders, and lead was initially added to the alloy in part to eliminate them.
Sn-Ag-Cu (Tin-Silver-Copper) 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 Sn-3.5Ag (wt.%) eutectic of 221 °C and the Sn-0.7Cu eutectic of 227 °C (recently revised by P. Snugovsky to Sn-0.9Cu). 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., Sn-3.5Ag-0.74Cu-0.21Zn (melting range of 217–220 ˚C) and Sn-3.5Ag-0.85Cu-0.10Mn (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.[10]
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, used for metal mending and plumbing, and rosin flux, used in electronics, where the corrosiveness of acid flux and vapors released when solder is heated would risk damaging delicate circuitry.
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, some light 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 non-acid flux embedded lengthwise inside it. As the solder melts onto the joint, it frees the flux and releases that on it as well.
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 away 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.
| Composition | Sn | Pb | Ag | Cu | Sb | Bi | In | Zn | Cd | Au | oth. | M.p. °C S/L |
Toxic | Eutectic | Comments |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Pb98Sn2 | 2 | 98 | 316/322[11] | Pb | no | Non-critical sealing and joining. Body solder. | |||||||||
| Pb97Sn3 | 3 | 97 | 314/320[12] | Pb | no | Sn3[12] | |||||||||
| Pb96Sn4 | 4 | 96 | 299/310[11] | Pb | no | Used for coating steel and copper, to provide resistance against mild acids and seawater. | |||||||||
| Pb95Sn5 | 5 | 95 | 308/312[13] 301/314[4] | Pb | no | Sn5, UNS L54320, ASTM5A, ASTM5B, Indalloy 171.[14] Low cost and good bonding properties. Used for coating steel and copper. Used in both SMT (Surface-mount technology) and through-hole electronics. Rapidly dissolves gold and silver, not recommended for those.[15] useful for high-temperature service and step soldering. Remains ductile at very low temperatures, can be used for parts subject to vibration at cryogenic applications. Pb93.5Sn5Ag1.5 provides superior wetting and better strength.[16] | |||||||||
| Pb93Sn7 | 7 | 93 | 288/308[11] | Pb | no | Used for coating steel to provide corrosion resistance, allows subsequent soldering. | |||||||||
| Pb90Sn10 | 10 | 90 | 268/302[13] 275/302[12] | Pb | no | Sn10, UNS L54520, ASTM10B, Indalloy 159. Balls for CBGA components, replaced by SnAg3.9Cu0.6.[9] Low cost and good bonding properties. Rapidly dissolves gold and silver, not recommended for those.[15] Used for fabrication of car radiators and fuel tanks, for coating and bonding of metals for moderate service temperatures. Body solder.[11] Has low thermal EMF, can be used as an alternative to Cd70 where parasitic thermocouple voltage has to be avoided.[17] | |||||||||
| Pb88Sn12 | 12 | 88 | 254/296[11] | Pb | no | Used for fabrication of car radiators and fuel tanks, for coating and bonding of metals for moderate service temperatures. Body solder. | |||||||||
| Pb85Sn15 | 15 | 85 | 227/288[11] | Pb | no | Used for coating tubes and sheets and fabrication of car radiators. Body solder. | |||||||||
| Pb80Sn20 | 20 | 80 | 183/280[12] | Pb | no | Sn20, UNS L54711. Used for coating radiator tubes for joining fins.[11] | |||||||||
| Pb75Sn25 | 25 | 75 | 183/266[13] | Pb | no | Crude 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.[11] | |||||||||
| Pb70Sn30 | 30 | 70 | 185/255[13] 183/257[12] | Pb | no | Sn30, UNS L54280, crude solder for construction plumbing works, flame-melted, good for machine and torch soldering.[18] Used for soldering car engine radiators. Used for machine, dip and hand soldering of plumbing fixtures and fittings. Superior body solder.[11] | |||||||||
| Pb68Sn32 | 32 | 68 | 253 | Pb | no | "Plumber solder", for construction plumbing works[19] | |||||||||
| Pb68Sn30Sb2 | 30 | 68 | 2 | 185/243[12] | Pb | no | Pb68 | ||||||||
| Pb67Sn33 | 33 | 67 | 187–230 | Pb | no | PM 33, crude solder for construction plumbing works, flame-melted, temperature depends on additives | |||||||||
| Pb65Sn35 | 35 | 65 | 183/250[12] | Pb | no | Sn35. Used as a cheaper alternative of Sn60Pb40 for wiping and sweating joints.[11] | |||||||||
| Pb60Sn40 | 40 | 60 | 183/238[13] 183/247[12] | Pb | no | Sn40, UNS L54915. For soldering of brass and car radiators.[18] 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.[11] | |||||||||
| Pb55Sn45 | 45 | 55 | 183/227[11] | Pb | no | For soldering radiator cores, roof seams, and for decorative joints. | |||||||||
| Sn50Pb50 | 50 | 50 | 183/216[13] 183–212[12] | Pb | no | Sn50, 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.[10][19] Low cost and good bonding properties. Rapidly dissolves gold and silver, not recommended for those.[15] For wiping and assembling plumbing joints for non-potable water.[11] | |||||||||
| Sn50Pb49Cu1 | 50 | 49 | 1 | 183/215[12] | Pb | no | Cu1 | ||||||||
| Sn50Pb48.5Cu1.5 | 50 | 48.5 | 1.5 | 183/215[20] | Pb | no | Savbit, 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.[16] | ||||||||
| Sn60Pb40 | 60 | 40 | 183/190[13] 183/188[12] | Pb | near | Sn60, 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.[15] 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.[16] | |||||||||
| Sn60Pb38Cu2 | 60 | 38 | 2 | 183/190[12][21] | Pb | Cu2. Copper content increases hardness of the alloy and inhibits dissolution of soldering iron tips and part leads in molten solder. | |||||||||
| Sn60Pb39Cu1 | 60 | 39 | 1 | Pb | no | ||||||||||
| Sn62Pb38 | 62 | 38 | 183 | Pb | near | "Tin man's solder"[19] | |||||||||
| Sn63Pb37 | 63 | 37 | 182 183[22] | Pb | yes | Sn63, ASTM63A, ASTM63B. Common in electronics; exceptional tinning and wetting properties, also good for stainless steel. One of 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.[15] 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.[16] | |||||||||
| Sn63Pb37P0.0015–0.04 | 63 | 37 | P | 183[23] | Pb | yes | Sn63PbP. A special alloy for HASL machines. Addition of phosphorus reduces oxidation. Unsuitable for wave soldering as it may form metal foam. | ||||||||
| Sn62Pb37Cu1 | 62 | 37 | 1 | 183[21] | Pb | yes | Similar to Sn63Pb37. Copper content increases hardness of the alloy and inhibits dissolution of soldering iron tips and part leads in molten solder. | ||||||||
| Sn70Pb30 | 70 | 30 | 183/193[13] | Pb | no | Sn70 | |||||||||
| Sn90Pb10 | 90 | 10 | 183/213[12] | Pb | no | formerly used for joints in food industry | |||||||||
| Sn95Pb5 | 95 | 5 | 238 | Pb | no | plumbing and heating | |||||||||
| Pb92Sn5.5Ag2.5 | 5.5 | 92 | 2.5 | 286/301[21] | Pb | no | For higher-temperature applications. | ||||||||
| Pb80Sn12Sb8 | 12 | 80 | 8 | Pb | no | Used for soldering iron and steel[19] | |||||||||
| Pb80Sn18Ag2 | 18 | 80 | 2 | 252/260[12] | Pb | no | Used for soldering iron and steel[19] | ||||||||
| Pb79Sn20Sb1 | 20 | 79 | 1 | 184/270 | Pb | no | Sb1 | ||||||||
| Pb55Sn43.5Sb1.5 | 43.5 | 55 | 1.5 | Pb | no | General purpose solder. Antimony content improves mechanical properties but causes brittleness when soldering cadmium, zinc, or galvanized metals.[19] | |||||||||
| Sn43Pb43Bi14 | 43 | 43 | 14 | 144/163[13] | Pb | no | Bi14, Indalloy 97. Good fatigue resistance combined with low melting point. Contains phases of tin and lead-bismuth.[24] Useful for step soldering. | ||||||||
| Sn46Pb46Bi8 | 46 | 46 | 8 | 120/167[12] | Pb | no | Bi8 | ||||||||
| Bi52Pb32Sn16 | 16 | 32 | 52 | 96 | Pb | yes? | 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.[24] | ||||||||
| Bi46Sn34Pb20 | 34 | 20 | 46 | 100/105[12] | Pb | no | Bi46 | ||||||||
| Sn62Pb36Ag2 | 62 | 36 | 2 | 179[13] | Pb | yes | Sn62. 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.[10][16][24] Not recommended for gold.[15] General-purpose. | ||||||||
| Sn62.5Pb36Ag2.5 | 62.5 | 36 | 2.5 | 179[13] | Pb | yes | |||||||||
| Pb88Sn10Ag2 | 10 | 88 | 2 | 268/290[13] 267/299[25] | Pb | no | Sn10, Pb88, Indalloy 228. Silver content reduces solubility of silver coatings in the solder. Not recommended for gold.[15] Forms a eutectic phase, not recommended for operation above 120 °C. | ||||||||
| Pb90Sn5Ag5 | 5 | 90 | 5 | 292[13] | Pb | yes | |||||||||
| Pb92.5Sn5Ag2.5 | 5 | 92.5 | 2.5 | 287/296[13] 299/304[12] | Pb | no | Pb93, Indalloy 151. Similar to Indalloy 165. | ||||||||
| Pb93.5Sn5Ag1.5 | 5 | 93.5 | 1.5 | 296/301[13] 305/306[12] | Pb | no | Pb94, 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.[16] | ||||||||
| Pb95.5Sn2Ag2.5 | 2 | 95.5 | 2.5 | 299/304[13] | Pb | no | |||||||||
| In97Ag3 | 3 | 97 | 143[26] | – | yes | Indalloy 290. Wettability and low-temperature malleability of indium, strength improved by addition of silver. Particularly good for cryogenic applications. Used for packaging of photonic devices. | |||||||||
| In90Ag10 | 10 | 90 | 143/237[27] | – | no | Indalloy 3. Nearly as wettable and low-temperature malleable as indium. Large plastic range. Can solder silver, fired glass and ceramics. | |||||||||
| In75Pb25 | 25 | 75 | 156/165[15] | Pb | no | Less gold dissolution and more ductile than lead-tin alloys. Used for die attachment, general circuit assembly and packaging closures.[15] | |||||||||
| In70Pb30 | 30 | 70 | 160/174[13] 165/175[12][28] | Pb | no | In70, Indalloy 204. Suitable for gold, low gold-leaching. Good thermal fatigue properties. | |||||||||
| In60Pb40 | 40 | 60 | 174/185[13] 173/181[12] | Pb | no | In60, Indalloy 205. Low gold-leaching. Good thermal fatigue properties. | |||||||||
| In50Pb50 | 50 | 50 | 180/209[15] 178/210[12] | Pb | no | In50, Indalloy 7. 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.[24] 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.[29] Less gold dissolution and more ductile than lead-tin alloys.[15] Good thermal fatigue properties. | |||||||||
| In50Sn50 | 50 | 50 | 118/125[30] | – | no | Indalloy 1, Cerroseal 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.[31] | |||||||||
| In70Sn15Pb9.6Cd5.4 | 15 | 9.6 | 70 | 5.4 | 125[32] | Pb,Cd | Indalloy 13 | ||||||||
| Pb75In25 | 75 | 25 | 250/264[15] 240/260[33] | Pb | no | In25, Indalloy 10. Low gold-leaching. Good thermal fatigue properties. Used for die attachment of e.g. GaAs dies.[29] Used also for general circuit assembly and packaging closures. Less dissolution of gold and more ductile than tin-lead alloy.[15] | |||||||||
| Sn70Pb18In12 | 70 | 18 | 12 | 162[13] 154/167[34] |
Pb | yes | Indalloy 9. General purpose. Good physical properties. | ||||||||
| Sn37.5Pb37.5In25 | 37.5 | 37.5 | 25 | 134/181[15] | Pb | no | Good wettability. Not recommended for gold.[15] | ||||||||
| Pb90In5Ag5 | 90 | 5 | 5 | 290/310[13] | Pb | no | |||||||||
| Pb92.5In5Ag2.5 | 92.5 | 2.5 | 5 | 300/310[13] | Pb | no | UNS L51510, Indalloy 164. Minimal leaching of gold, good thermal fatigue properties. Reducing atmosphere frequently used.. | ||||||||
| Pb92.5In5Au2.5 | 92.5 | 5 | 2.5 | 300/310[12] | Pb | no | In5 | ||||||||
| Pb94.5Ag5.5 | 94.5 | 5.5 | 305/364[12] 304/343[35] | Pb | no | Ag5.5, UNS L50180, Indalloy 229 | |||||||||
| Pb95Ag5 | 95 | 5 | 305/364[36] | Pb | no | Indalloy 175 | |||||||||
| Pb97.5Ag2.5 | 97.5 | 2.5 | 303[13] 304[12] 304/579[37] | Pb | yes no | Ag2.5, UNS L50132, Indalloy 161. 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.[38] Torch solder. | |||||||||
| Sn97.5Pb1Ag1.5 | 97.5 | 1 | 1.5 | 305 | Pb | yes | Important for hybrid circuits assembly.[10] | ||||||||
| Pb97.5Ag1.5Sn1 | 1 | 97.5 | 1.5 | 309[13] | Pb | yes | Ag1.5, ASTM1.5S, Indalloy 165. High melting point, used for commutators, armatures, and initial solder joints where remelting when working on nearby joints is undesirable.[18] Silver content reduces solubility of silver coatings in molten solder. Not recommended for gold.[15] 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. | ||||||||
| Pb54Sn45Ag1 | 45 | 54 | 1 | 177–210 | Pb | exceptional strength, silver gives it a bright long-lasting finish; ideal for stainless steel[18] | |||||||||
| Pb96Ag4 | 96 | 4 | 305 | Pb | high-temperature joints[18] | ||||||||||
| Pb96Sn2Ag2 | 2 | 96 | 2 | 252/295[12] | Pb | Pb96 | |||||||||
| Sn61Pb36Ag3 | 61 | 36 | 3 | Pb | [10] | ||||||||||
| Sn56Pb39Ag5 | 56 | 39 | 5 | Pb | [10] | ||||||||||
| Sn98Ag2 | 98 | 2 | – | [10] | |||||||||||
| Sn65Ag25Sb10 | 65 | 25 | 10 | 233 | – | yes | Indalloy 209. Very high tensile strength. For die attachment. Very brittle. Old Motorola die attach solder. | ||||||||
| Sn96.5Ag3.0Cu0.5 | 96.5 | 3 | 0.5 | 217/220 217/218[12][39] | – | near | SAC305, Indalloy 256, SN97C. Predominantly used in Japan. 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 (designated e.g. SN97Ce). Nitrogen atmosphere can be used to reduce losses by dross formation. Dull, surface shows formation of dendritic tin crystals. | ||||||||
| Sn95.8Ag3.5Cu0.7 | 95.8 | 3.5 | 0.7 | 217–218 | – | near | SN96C-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. | ||||||||
| Sn95.6Ag3.5Cu0.9 | 95.6 | 3.5 | 0.9 | 217 | – | yes | Determined by NIST to be truly eutectic. | ||||||||
| Sn95.5Ag3.8Cu0.7 | 95.5 | 3.8 | 0.7 | 217[40] | – | almost | SN96C. 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. | ||||||||
| Sn95.25Ag3.8Cu0.7Sb0.25 | 95.25 | 3.8 | 0.7 | 0.25 | – | Preferred by the European IDEALS consortium for wave soldering. | |||||||||
| Sn95.5Ag3.9Cu0.6 | 95.5 | 3.9 | 0.6 | 217[41] | – | yes | Indalloy 252. Recommended 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.[9] Alloy of choice for general SMT assembly. | ||||||||
| Sn95.5Ag4Cu0.5 | 95.5 | 4 | 0.5 | 217[42] | – | yes | Indalloy 246. Prior-art use makes it patent-free. | ||||||||
| Sn96.5Ag3.5 | 96.5 | 3.5 | 221[13] | – | yes | Sn96, Sn96.5, 96S, Indalloy 121. Fine lamellar structure of densely distributed Ag3Sn. Annealing at 125 °C coarsens the structure and softens the solder.[9] Creeps via dislocation climb as a result of lattice diffusion.[8] 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.[9] Widely used. Strong lead-free joints. Silver content minimizes solubility of silver coatings. Not recommended for gold.[15] 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.[16] High tin content allows absorbing significant amount of gold without embrittlement.[43] | |||||||||
| Sn96Ag4 | 96 | 4 | 221–229 | – | no | ASTM96TS. "Silver-bearing solder". Food service equipment, refrigeration, heating, air conditioning, plumbing.[18] Widely used. Strong lead-free joints. Silver content minimizes solubility of silver coatings. Not recommended for gold.[15] | |||||||||
| Sn95Ag5 | 95 | 5 | 221/240[15] | – | no | Sn95. Widely used. Strong lead-free joints. Silver content minimizes solubility of silver coatings. Not recommended for gold.[15] | |||||||||
| Sn95Ag4Cu1 | 95 | 4 | 1 | – | |||||||||||
| Sn | 99.99 | 232 | – | pure | Sn99. Good strength, non-dulling. Use in food processing equipment, wire tinning, and alloying.[18] Susceptible to tin pest. | ||||||||||
| Sn99.3Cu0.7 | 99.3 | 0.7 | (Ni) | 227 | – | yes | Indalloy 244, Sn99Cu1. Also designated as Sn99Cu1. Cheap alternative for wave soldering, recommended by the US NEMI consortium. Coarse microstructure with ductile fractures. Sparsely distributed Cu6Sn5.[44] Forms large dendritic β-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.[8] 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.[45] An example with 0.05% Ni is designated SN100C. The properties degrade with dissolved copper; at above 0.85% the alloy tends to form bridges between part leads. At above 0.9% Cu needles of copper-tin intermetallic precipitate and settle at the bottom of the solder bath. The alloy attacks steel less than the tin-silver-copper alloys, allowing use of stainless steel solder pots. Slower wetting than Sn63Pb37.[46] | ||||||||
| Sn99Cu0.7Ag0.3 | 99 | 0.3 | 0.7 | 217/228[47] | – | no | SCA, 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. | ||||||||
| Sn97Cu3 | 97 | 3 | 227/250[48] 232/332[11] | – | 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. | ||||||||||
| Sn97Cu2.75Ag0.25 | 97 | 0.25 | 2.75 | 228/314[11] | – | 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. | |||||||||
| Zn100 | 100 | 419 | – | pure | For soldering aluminium. Good wettability of aluminium, relatively good corrosion resistance.[49] | ||||||||||
| Bi100 | 100 | 271 | – | pure | Used as a non-superconducting solder in low-temperature physics. Does not wet metals well, forms a mechanically weak joint.[31] | ||||||||||
| Sn91Zn9 | 91 | 9 | 199 | – | yes | Indalloy 201. Cheaper alloy, prone to corrosion and oxidation. Recommended for soldering aluminium.[19] Fair wetting of aluminium, fair corrosion rating.[38] Room temperature tensile strength twice of SnPb37. High drossing. Solder paste has short shelf-life. | |||||||||
| Zn95Al5 | 95 | Al5 | 382 | – | yes | For soldering aluminium. Good wetting.[49] | |||||||||
| Sn91.8Bi4.8Ag3.4 | 91.8 | 3.4 | 4.8 | 211/213[50] | – | no | Indalloy 249. Do not use on lead-containing metallizations. U.S. Patent 5,439,639 (ICA Licensed Sandia Patent). | ||||||||
| Sn70Zn30 | 70 | 30 | 199/311 | – | no | For soldering of aluminium. Good wetting.[38] | |||||||||
| Pb63Sn35Sb2 | 35 | 63 | 2 | 185/243[12] | Pb | no | Sb2 | ||||||||
| Pb63Sn34Zn3 | 34 | 63 | 3 | 170/256 | Pb | no | Poor wetting of aluminium. Poor corrosion rating.[38] | ||||||||
| Pb92Cd8 | 92 | 8 | 310? | Pb,Cd | ? | For soldering aluminium. US patent 1,333,666.[51] | |||||||||
| Sn48Bi32Pb20 | 48 | 20 | 32 | 140/160[21] | Pb | no | For low-temperature soldering of heat-sensitive parts, and for soldering in the vicinity of already soldered joints without their remelting. | ||||||||
| Sn89Zn8Bi3 | 89 | 3 | 8 | 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.[52] | |||||||||
| Sn83.6Zn7.6In8.8 | 83.6 | 8.8 | 7.6 | 181/187[53] | – | no | Indalloy 226. High dross due to zinc. Covered by U.S. Patent #5,242,658. | ||||||||
| Sn86.5Zn5.5In4.5Bi3.5 | 86.5 | 3.5 | 4.5 | 5.5 | 174/186[54] | – | no | Indalloy 231. Lead-free. Corrosion concerns and high drossing due to zinc content. | |||||||
| Sn86.9In10Ag3.1 | 86.9 | 3.1 | 10 | 204/205[55] | – | Indalloy 254. Potential use in flip-chip assembly, no issues with tin-indium eutectic phase. | |||||||||
| Sn95Ag3.5Zn1Cu0.5 | 95 | 3.5 | 0.5 | 1 | 221L[52] | – | no | ||||||||
| Sn95Sb5 | 95 | 5 | 235/240[13] 232/240[12] | – | no | Sb5, ASTM95TA, Indalloy 133. 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.[8] High strength and bright finish. Use in air conditioning, refrigeration, some food containers, and high-temperature applications.[18] Good wettability, good long-term shear strength at 100 °C. Suitable for potable water systems. Used for stained glass, plumbing, and radiator repairs. | |||||||||
| Sn97Sb3 | 97 | 3 | 232/238[56] | – | no | Indalloy 131 | |||||||||
| Sn99Sb1 | 99 | 1 | 232/235[57] | – | no | Indalloy 129 | |||||||||
| Sn99Ag0.3Cu0.7 | 99 | 0.3 | 0.7 | – | |||||||||||
| Sn96.2Ag2.5Cu0.8Sb0.5 | 96.2 | 2.5 | 0.8 | 0.5 | 217–225 217[12] | – | Ag03A. Patented by AIM alliance. | ||||||||
| Sn88In8.0Ag3.5Bi0.5 | 88 | 3.5 | 0.5 | 8 | 197–208 | – | Patented by Matsushita/Panasonic. | ||||||||
| Bi57Sn42Ag1 | 42 | 1 | 57 | 137/139 139/140[58] | – | Indalloy 282. Addition of silver improves mechanical strength. Established history of use. Good thermal fatigue performance. Patented by Motorola. | |||||||||
| Bi58Sn42 | 42 | 58 | 138[13][15] | – | yes | Bi58, Indalloy 281, Indalloy 138, Cerrothru. Reasonable shear strength and fatigue properties. Combination with lead-tin solder may dramatically lower melting point and lead to joint failure.[24] Low-temperature eutectic solder with high strength.[15] Particularly strong, very brittle.[13] 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.[52] Sensitive to shear rate. Good for electronics. Used in thermoelectric applications. Good thermal fatigue performance.[59] 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.[31] | |||||||||
| Bi58Pb42 | 42 | 58 | 124/126[60] | Pb | Indalloy 67 | ||||||||||
| In80Pb15Ag5 | 15 | 5 | 80 | 142/149[12] 149/154[61] |
Pb | no | In80, Indalloy 2. Compatible with gold, minimum gold-leaching. Resistant to thermal fatigue. Can be used in step soldering. | ||||||||
| Pb60In40 | 60 | 40 | 195/225[12] | Pb | no | In40, Indalloy 206. Low gold-leaching. Good thermal fatigue properties. | |||||||||
| Pb70In30 | 70 | 30 | 245/260[12] | Pb | no | In30 | |||||||||
| Sn37.5Pb37.5In26 | 37.5 | 37.5 | 26 | 134/181[12] | Pb | no | In26 | ||||||||
| Sn54Pb26In20 | 54 | 26 | 20 | 130/154[12] 140/152[62] | Pb | no | In20, Indalloy 532 | ||||||||
| Pb81In19 | 81 | 19 | 270/280[12] 260/275[63] | Pb | no | In19, Indalloy 150. Low gold-leaching. Good thermal fatigue properties. | |||||||||
| In52Sn48 | 48 | 52 | 118 | – | yes | In52, Indalloy 1E. Suitable for the cases where low-temperature soldering is needed. Can be used for glass sealing.[52] 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. | |||||||||
| Sn52In48 | 52 | 48 | 118/131[13] | – | no | very low tensile strength | |||||||||
| Sn58In42 | 58 | 42 | 118/145[64] | – | no | Indalloy 87 | |||||||||
| Sn51.2Pb30.6Cd18.2 | 51.2 | 30.6 | 18.2 | 145[65] | Pb,Cd | yes | Indalloy 181. General-purpose. Maintains creep strength well. Unsuitable for gold. | ||||||||
| Sn77.2In20Ag2.8 | 77.2 | 2.8 | 20 | 175/187[66] | – | no | Indalloy 227. Similar 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. | ||||||||
| In74Cd26 | 74 | 26 | 123[67] | Cd | yes | Indalloy 253. | |||||||||
| In61.7Bi30.8Cd7.5 | 30.8 | 61.7 | 7.5 | 62[68] | Cd | yes | Indalloy 18 | ||||||||
| Bi47.5Pb25.4Sn12.6Cd9.5In5 | 12.6 | 25.4 | 47.5 | 5 | 9.5 | 57/65[69] | Pb,Cd | no | Indalloy 140 | ||||||
| Bi48Pb25.4Sn12.8Cd9.6In4 | 12.8 | 25.4 | 48 | 9.6 | 61/65[70] | Pb,Cd | no | Indalloy 147 | |||||||
| Bi49Pb18Sn15In18 | 15 | 18 | 49 | 18 | 58/69[71] | Pb | no | Indalloy 21 | |||||||
| Bi49Pb18Sn12In21 | 12 | 18 | 49 | 21 | 58 | Pb | yes | Cerrolow 136. Slightly expands on cooling, later shows slight shrinkage in couple hours afterwards. Used as a solder in low-temperature physics.[31] | |||||||
| Bi50.5Pb27.8Sn12.4Cd9.3 | 12.4 | 27.8 | 50.5 | 9.3 | 70/73[72] | Pb,Cd | no | Indalloy 22 | |||||||
| Bi50Pb26.7Sn13.3Cd10 | 13.3 | 26.7 | 50 | 10 | 70 | Pb,Cd | yes | Cerrobend. Used in low-temperature physics as a solder.[31] | |||||||
| Bi44.7Pb22.6In19.1Cd5.3Sn8.3 | 8.3 | 22.6 | 44.7 | 19.1 | 5.3 | 47 | Cd,Pb | yes | Indalloy 117, Cerrolow 117. Used as a solder in low-temperature physics.[31] | ||||||
| In60Sn40 | 40 | 60 | 113/122[13] | – | no | ||||||||||
| In51.0Bi32.5Sn16.5 | 16.5 | 32.5 | 51 | 60.5 | – | yes | Field's metal | ||||||||
| Bi49.5Pb27.3Sn13.1Cd10.1 | 13.1 | 27.3 | 49.5 | 10.1 | 70.9 | Pb,Cd | yes | Lipowitz Metal | |||||||
| Bi50.0Pb25.0Sn12.5Cd12.5 | 12.5 | 25 | 50 | 12.5 | 71 | Pb,Cd | yes | Wood's metal, mostly used for casting. | |||||||
| Bi50.0Pb31.2Sn18.8 | 18.8 | 31.2 | 50 | 97 | Pb | no | Newton's metal | ||||||||
| Bi50Pb28Sn22 | 22 | 28 | 50 | 109 | Pb | no | Rose's metal. It was used to secure cast iron railings and balusters in pockets in stone bases and steps. Does not contract on cooling. | ||||||||
| Cd95Ag5 | 5 | 95 | 338/393 340/395[73] |
Cd | Indalloy 185. melts at 338 °C, flows at 393 °C; for high-temperature applications, for soldering aluminium to itself or to other metals.[19] Braze 053. For medium-strength joints. For low-temperature brazing. | ||||||||||
| Cd82.5Zn17.5 | 17.5 | 82.5 | 265 | Cd | For soldering aluminium and die-cast zinc alloys.[19] Used in cryogenic physics for ataching electrical potential leads to specimens of metals, as this alloy does not become superconductive at liquid helium temperatures.[31] | ||||||||||
| Zn90Cd10 | 90 | 10 | 265/399 | Cd | For soldering aluminium. Good wetting.[49] | ||||||||||
| Zn60Cd40 | 60 | 40 | 265/335 | Cd | For soldering aluminium. Very good wetting.[49] | ||||||||||
| Cd70Sn30 | 29.56 | 70.44 | 140/160[12] | Cd | no | Cd70, thermal-free solder. Produces low thermal EMF joints in copper, does not form parasitic thermocouples. Used in low-temperature physics.[31] | |||||||||
| Sn50Pb32Cd18 | 50 | 32 | 18 | 145[12] | Cd,Pb | Cd18 | |||||||||
| Sn40Pb42Cd18 | 40 | 42 | 18 | 145[74] | Cd,Pb | LT145. Low melting temperature allows repairing pewter and zinc objects, including die-cast toys. | |||||||||
| Zn70Sn30 | 30 | 70 | 199/376 | – | no | For soldering aluminium. Excellent wetting.[38] Good strength. | |||||||||
| Zn60Sn40 | 40 | 60 | 199/341 | – | no | For soldering aluminium. Good wetting.[49] | |||||||||
| Zn95Sn5 | 5 | 95 | 382 | – | yes? | For soldering aluminium. Excellent wetting.[38] | |||||||||
| Sn90Au10 | 90 | 10 | 217[75] | – | yes | Indalloy 238. | |||||||||
| Au80Sn20 | 20 | 80 | 280 | – | yes | Au80, Indalloy 182, Premabraze 800. 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.[76] Forms a mixture of two brittle intermetallic phases, AuSn and Au5Sn.[77] 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.[29] Low vapor pressure, suitable for vacuum work. Generally used in applications that require a melting temperature over 150°C.[78] Good ductility. Also classified as a braze. | |||||||||
| Au98Si2 | 98 | Si2 | 370/800[12] | – | Au98, Indalloy 194. 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. | ||||||||||
| Au96.8Si3.2 | 96.8 | Si3.2 | 370[12] 363[79] | – | yes | Au97, Indalloy 184.[76] 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.[44] | |||||||||
| Au87.5Ge12.5 | 87.5 | Ge12.5 | 361 356[12] | – | yes | Au88, Indalloy 183. Used for die attachment of some chips.[13] The high temperature may be detrimental to the chips and limits reworkability.[29] | |||||||||
| Au82In18 | 18 | 82 | 451/485[12] | – | no | Au82, Indalloy 178. High-temperature, extremely hard, very stiff. | |||||||||
| In100 | 99.99 | 157 | – | pure | In99. 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.[80] 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.[31] |
Temperature ranges for solidus and liquidus (the boundaries of the mushy state) are listed as solidus/liquidus.[13]
In the Sn-Pb alloys tensile strength increases with increasing tin content. Indium-tin alloys with high indium content have very low tensile strength.[13]
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 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.[38]
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.
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.[81]
The solidifying behavior depends on the alloy composition. Pure metals solidify at a sharply defined 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 mixture of solid particles in a molten eutectic is referred to as mushy state. Even a relatively small proportion of solids in the liquid can dramatically lower its fluidity.[45]
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.
Different elements serve different roles in the solder alloy:
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.[84]
Many different intermetallic compounds are formed during solidifying of solders and during their reactions with the soldered surfaces. Some of the phases are:[84]
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, and/or be susceptible to 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.[85]
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.[8]
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.[86]
Two processes play role in a solder joint formation: interaction between substrate and molten solder, and solid-state growth of intermetallic compounds. The base metal dissolves in the molten solder in amount depending on its solubility in the solder. The active constituent of the solder reacts with the base metal with 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.[87]
| Tin | Lead | Indium | |
|---|---|---|---|
| Copper | Cu4Sn, Cu6Sn5, Cu3Sn, Cu3Sn8 | 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 |
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.[88] 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. They are used for sealing of electronic components.[89] 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.[90]
The bonding between the glass or ceramics and the glass solder can be either covalent, or, more often, van der Waals.[91] 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.[92] 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.[93] 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 and/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.[94]
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 to lay 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.[89]
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.[95] Evacuated glazing windows are made of glass panels soldered together.[96]
A glass solder is used for e.g. 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.[97]
Conductive glass solders can be also prepared.
A preform is a specially designed shape of solder adjusted to the application where it is used.[98] Although there are many methods that are used to manufacture the solder preform, stamping is probably the most common method. The solder preform can be manufactured including the solder flux needed for the soldering process. This can be an internal flux, inside the solder preform, or external, where the solder preform is coated.[99]