Solder (/ˈsoʊldər/, /ˈsɒldər/ or in North America /ˈsɒdər/) 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". In fact, solder must be melted in order to adhere to and connect the pieces together, so a suitable alloy for use as solder will have a lower melting point than the pieces it is intended to join. Whenever possible, the solder should also be resistant to oxidative and corrosive effects that would degrade the joint over time. Solder that is intended for use in making electrical connections between electronic components also usually has favorable electrical characteristics.
Soft solder typically has a melting point range of 90 to 450 °C (190 to 840 °F; 360 to 720 K), and is commonly used in electronics, plumbing, and sheet metal work. Manual soldering uses a soldering iron or soldering gun. 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. 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, and with cores containing flux. It is also available as a paste or as a preformed foil shaped to match the workpiece, more suitable for mechanized mass-production. 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.
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.
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 inclusion of significant quantities of lead in most consumer electronics produced in the EU. 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, 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.
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. 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. Alloys commonly used for electrical soldering are 60/40 Sn-Pb, which melts at 188 °C (370 °F), and 63/37 Sn-Pb used principally in electrical/electronic work. 63/37 is a eutectic alloy of these metals, which:
- has the lowest melting point (183 °C or 361 °F) of all the tin-lead alloys; and
- 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). 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, 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.
In electronics, components on printed circuit boards (PCBs) are connected to the printed circuit, and hence to other components, by soldered joints. For miniaturized PCB joints with surface mount components, solder paste has largely replaced solid solder.
Lead-tin solders readily dissolve gold plating and form brittle intermetallics. 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.
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.
Flux-core solder 
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"), used for metal mending and plumbing, and rosin flux (sometimes called "passive flux"), used in electronics, where the corrosiveness of the vapors released when acid flux 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, 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 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 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.
|SnZnCu||200/300||no||Galvanite 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.||50||1||49|
|SnCuAg||226/260||no||KappFree 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.||95.5||.5||4|
|SnZnCu||200/222||no||Kapp Eco-Babbitt 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.||90||3||7|
|PbSn||268/302 275/302||Pb||no||Sn10, UNS L54520, ASTM10B. Balls for CBGA components, replaced by SnAgCu. Low cost and good bonding properties. Rapidly dissolves gold and silver, not recommended for those. Used for fabrication of car radiators and fuel tanks, for coating and bonding of metals for moderate service temperatures. Body solder. Has low thermal EMF, can be used as an alternative to Cd70 where parasitic thermocouple voltage has to be avoided.||10||90|
|PbSn||254/296||Pb||no||Used for fabrication of car radiators and fuel tanks, for coating and bonding of metals for moderate service temperatures. Body solder.||12||88|
|PbSn||227/288||Pb||no||Used for coating tubes and sheets and fabrication of car radiators. Body solder.||15||85|
|PbSn||183/280||Pb||no||Sn20, UNS L54711. Used for coating radiator tubes for joining fins.||20||80|
|PbSbSn||300||Pb||White Metal Capping. Used for locking mineshaft winding ropes into their tapered end sockets or 'capels'.||5||80||15|
|PbSn||183/266||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.||25||75|
|PbSn||185/255 183/257||Pb||no||Sn30, UNS L54280, crude solder for construction plumbing works, flame-melted, good for machine and torch soldering. Used for soldering car engine radiators. Used for machine, dip and hand soldering of plumbing fixtures and fittings. Superior body solder.||30||70|
|PbSn||253||Pb||no||"Plumber solder", for construction plumbing works||32||68|
|SnPbZn||177/288||Pb||no||Kapp GalvRepair Economical solder for repairing & joining most metals including Aluminum and cast Iron. Have been used for cast Iron and galvanized surface repair.||30||50||20|
|SnPbZn||230/275||Pb||no||Economical solder for repairing & joining most metals including Aluminum and cast Iron. Have been used for cast Iron and galvanized surface repair.||33||40||28|
|PbSn||187–230||Pb||no||PM 33, crude solder for construction plumbing works, flame-melted, temperature depends on additives||33||67|
|PbSn||183/250||Pb||no||Sn35. Used as a cheaper alternative of SnPb for wiping and sweating joints.||35||65|
|PbSn||183/238 183/247||Pb||no||Sn40, UNS L54915. For soldering of brass and car radiators. 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.||40||60|
|PbSn||183/227||Pb||no||For soldering radiator cores, roof seams, and for decorative joints.||45||55|
|SnPb||183/216 183–212||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. Low cost and good bonding properties. Rapidly dissolves gold and silver, not recommended for those. For wiping and assembling plumbing joints for non-potable water.||50||50|
|SnPbCu||183/215||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.||50||48.5||1.5|
|SnPb||183/190 183/188||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. Slightly cheaper than SnPb, often used instead for cost reasons as the melting point difference is insignificant in practice. On slow cooling gives slightly duller joints than SnPb.||60||40|
|SnPbCu||183/190||Pb||Cu2. Copper content increases hardness of the alloy and inhibits dissolution of soldering iron tips and part leads in molten solder.||60||38||2|
|SnPb||183||Pb||near||"Tinman's solder", used for tinplate fabrication work.||62||38|
|SnPb||183||Pb||yes||Sn63, 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. SnPb 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 SnPb.||63||37|
|SnPbP||183||Pb||yes||Sn63PbP. A special alloy for HASL machines. Addition of phosphorus reduces oxidation. Unsuitable for wave soldering as it may form metal foam.||63||37||P|
|SnPbCu||183||Pb||yes||Similar to SnPb. Copper content increases hardness of the alloy and inhibits dissolution of soldering iron tips and part leads in molten solder.||62||37||1|
|SnPb||183/213||Pb||no||formerly used for joints in food industry||90||10|
|SnPb||238||Pb||no||plumbing and heating||95||5|
|PbSnAg||286/301||Pb||no||For higher-temperature applications.||5.5||92||2.5|
|PbSnSb||Pb||no||Used for soldering iron and steel||12||80||8|
|PbSnAg||252/260||Pb||no||Used for soldering iron and steel||18||80||2|
|PbSnSb||Pb||no||General purpose solder. Antimony content improves mechanical properties but causes brittleness when soldering cadmium, zinc, or galvanized metals.||43.5||55||1.5|
|SnPbBi||144/163||Pb||no||Bi14. Good fatigue resistance combined with low melting point. Contains phases of tin and lead-bismuth. Useful for step soldering.||43||43||14|
|BiPbSn||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.||16||32||52|
|SnPbAg||179||Pb||yes||Sn62. Common in electronics. The strongest tin-lead solder. Appearance identical to Sn60Pb40 or Sn63Pb37. Crystals of AgSn 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. Not recommended for gold. General-purpose.||62||36||2|
|PbSnAg||268/290 267/299||Pb||no||Sn10, Pb88. Silver content reduces solubility of silver coatings in the solder. Not recommended for gold. Forms a eutectic phase, not recommended for operation above 120 °C.||10||88||2|
|PbSnAg||296/301 305/306||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 PbSn.||5||93.5||1.5|
|InAg||143||–||yes||Wettability and low-temperature malleability of indium, strength improved by addition of silver. Particularly good for cryogenic applications. Used for packaging of photonic devices.||3||97|
|InAg||143/237||–||no||Nearly as wettable and low-temperature malleable as indium. Large plastic range. Can solder silver, fired glass and ceramics.||10||90|
|InPb||156/165||Pb||no||Less gold dissolution and more ductile than lead-tin alloys. Used for die attachment, general circuit assembly and packaging closures.||25||75|
|InPb||160/174 165/175||Pb||no||In70. Suitable for gold, low gold-leaching. Good thermal fatigue properties.||30||70|
|InPb||174/185 173/181||Pb||no||In60. Low gold-leaching. Good thermal fatigue properties.||40||60|
|InPb||180/209 178/210||Pb||no||In50. 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. 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. Less gold dissolution and more ductile than lead-tin alloys. Good thermal fatigue properties.||50||50|
|InSn||118/125||–||no||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.||50||50|
|PbIn||250/264 240/260||Pb||no||In25. Low gold-leaching. Good thermal fatigue properties. Used for die attachment of e.g. GaAs dies. Used also for general circuit assembly and packaging closures. Less dissolution of gold and more ductile than tin-lead alloy.||75||25|
|Pb||yes||General purpose. Good physical properties.||70||18||12|
|SnPbIn||134/181||Pb||no||Good wettability. Not recommended for gold.||37.5||37.5||25|
|PbInAg||300/310||Pb||no||UNS L51510. Minimal leaching of gold, good thermal fatigue properties. Reducing atmosphere frequently used..||92.5||2.5||5|
|PbAg||305/364 304/343||Pb||no||Ag5.5, UNS L50180||94.5||5.5|
|PbAg||303 304 304/579||Pb||yes no||Ag2.5, 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. Torch solder.||97.5||2.5|
|SnPbAg||305||Pb||yes||Important for hybrid circuits assembly.||97.5||1||1.5|
|PbAgSn||309||Pb||yes||Ag1.5, ASTM1.5S. High melting point, used for commutators, armatures, and initial solder joints where remelting when working on nearby joints is undesirable. Silver content reduces solubility of silver coatings in molten solder. Not recommended for gold. 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.||1||97.5||1.5|
|PbSnAg||177–210||Pb||exceptional strength, silver gives it a bright long-lasting finish; ideal for stainless steel||45||54||1|
|SnAgSb||233||–||yes||Very high tensile strength. For die attachment. Very brittle. Old Motorola die attach solder.||65||25||10|
|SnAgCu||217/220 217/218||–||near||SAC305. 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 SnAg alloy. Nitrogen atmosphere can be used to reduce losses by dross formation. Dull, surface shows formation of dendritic tin crystals.||96.5||3||0.5|
|SnAgCu||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 SnAg alloy (designated e.g. SN96Ce). Nitrogen atmosphere can be used to reduce losses by dross formation. Dull, surface shows formation of dendritic tin crystals.||95.8||3.5||0.7|
|SnAgCu||217||–||yes||Determined by NIST to be truly eutectic.||95.6||3.5||0.9|
|SnAgCu||217||–||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 SnAg alloy (designated e.g. SN96Ce). Nitrogen atmosphere can be used to reduce losses by dross formation. Dull, surface shows formation of dendritic tin crystals.||95.5||3.8||0.7|
|SnAgCuSb||–||Preferred by the European IDEALS consortium for wave soldering.||95.25||3.8||0.7||0.25|
|SnAgCu||217||–||yes||Recommended by the US NEMI consortium for reflow soldering. Used as balls for BGA/CSP and CBGA components, a replacement for SnPb. Solder paste for rework of BGA boards. Alloy of choice for general SMT assembly.||95.5||3.9||0.6|
|SnAgCu||217||–||yes||SAC405. Lead-Free, Cadmium Free formulation designed specifically to replace Lead solders in Copper and Stainless Steel plumbing, and in electrical and electronic applications.||95.5||4||0.5|
|SnAg||221||–||yes||Sn96, Sn96.5, 96S. Fine lamellar structure of densely distributed AgSn. Annealing at 125 °C coarsens the structure and softens the solder. Creeps via dislocation climb as a result of lattice diffusion. Used as wire for hand soldering rework; compatible with SnCu, SnAgCu, SnAgCu, 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. Widely used. Strong lead-free joints. Silver content minimizes solubility of silver coatings. Not recommended for gold. 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. High tin content allows absorbing significant amount of gold without embrittlement.||96.5||3.5|
|SnAg||221–229||–||no||ASTM96TS. "Silver-bearing solder". Food service equipment, refrigeration, heating, air conditioning, plumbing. Widely used. Strong lead-free joints. Silver content minimizes solubility of silver coatings. Not recommended for gold.||96||4|
|SnAg||221/254||–||no||Widely 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.||95||5|
|SnAg||221/279||–||no||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.||94||6|
|SnAg||221/302||–||no||Produces 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. 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.||93||7|
|Sn||232||–||pure||Sn99. Good strength, non-dulling. Use in food processing equipment, wire tinning, and alloying. Susceptible to tin pest.||99.99|
|SnCu||227||–||yes||Sn99Cu1. Also designated as SnCu. Cheap alternative for wave soldering, recommended by the US NEMI consortium. Coarse microstructure with ductile fractures. Sparsely distributed CuSn. Forms large dendritic ß-tin crystals in a network of eutectic microstructure with finely dispersed CuSn. High melting point unfavorable for SMT use. Low strength, high ductility. Susceptible to tin pest. 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.||99.3||0.7||(Ni)|
|SnCuAg||217/228||–||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 SnAg alloy (designated e.g. SN96Ce). Nitrogen atmosphere can be used to reduce losses by dross formation. Dull, surface shows formation of dendritic tin crystals.||99||0.3||0.7|
|SnCu||227/250 232/332||–||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.||97||3|
|SnCuAg||228/314||–||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.||97||0.25||2.75|
|Zn||419||–||pure||For soldering aluminium. Good wettability of aluminium, relatively good corrosion resistance.||100|
|Bi||271||–||pure||Used as a non-superconducting solder in low-temperature physics. Does not wet metals well, forms a mechanically weak joint.||100|
|SnZn||199||–||yes||KappAloy9 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. UNS#: L91090||91||9|
|SnZn||199/260||–||no||KappAloy15 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.||85||15|
|ZnAl||382||–||yes||For soldering aluminium. Good wetting.||95||Al|
|SnBiAg||211/213||–||no||Do not use on lead-containing metallizations. U.S. Patent 5,439,639 (ICA Licensed Sandia Patent).||91.8||3.4||4.8|
|SnZn||199/316||–||no||KappAloy30 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.||70||30|
|SnZn||199/288||–||no||KappAloy20 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.||80||20|
|SnZn||199/343||–||no||KappAloy40 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.||60||40|
|PbSnZn||170/256||Pb||no||Poor wetting of aluminium. Poor corrosion rating.||34||63||3|
|PbCd||310?||Pb,Cd||?||For soldering aluminium. US patent 1,333,666.||92||8|
|SnBiPb||140/160||Pb||no||For low-temperature soldering of heat-sensitive parts, and for soldering in the vicinity of already soldered joints without their remelting.||48||20||32|
|SnZnBi||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.||89||3||8|
|SnZnIn||181/187||–||no||High dross due to zinc. Covered by U.S. Patent #5,242,658.||83.6||8.8||7.6|
|SnZnInBi||174/186||–||no||Lead-free. Corrosion concerns and high drossing due to zinc content.||86.5||3.5||4.5||5.5|
|SnInAg||204/205||–||Potential use in flip-chip assembly, no issues with tin-indium eutectic phase.||86.9||3.1||10|
|SnSb||235/240 232/240||–||no||Sb5, 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 SnAg. 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. High strength and bright finish. Use in air conditioning, refrigeration, some food containers, and high-temperature applications. Good wettability, good long-term shear strength at 100 °C. Suitable for potable water systems. Used for stained glass, plumbing, and radiator repairs.||95||5|
|SnAgCuSb||217–225 217||–||Ag03A. Patented by AIM alliance.||96.2||2.5||0.8||0.5|
|SnInAgBi||197–208||–||Patented by Matsushita/Panasonic.||88||3.5||0.5||8|
|BiSnAg||137/139 139/140||–||Addition of silver improves mechanical strength. Established history of use. Good thermal fatigue performance. Patented by Motorola.||42||1||57|
|BiSn||138||–||yes||Bi58. Reasonable shear strength and fatigue properties. Combination with lead-tin solder may dramatically lower melting point and lead to joint failure. Low-temperature eutectic solder with high strength. Particularly strong, very brittle. 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. Sensitive to shear rate. Good for electronics. Used in thermoelectric applications. Good thermal fatigue performance. 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.||42||58|
|Pb||no||In80. Compatible with gold, minimum gold-leaching. Resistant to thermal fatigue. Can be used in step soldering.||15||5||80|
|PbIn||195/225||Pb||no||In40. Low gold-leaching. Good thermal fatigue properties.||60||40|
|PbIn||270/280 260/275||Pb||no||In19. Low gold-leaching. Good thermal fatigue properties.||81||19|
|InSn||118||–||yes||In52. Suitable for the cases where low-temperature soldering is needed. Can be used for glass sealing. 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.||48||52|
|SnIn||118/131||–||no||very low tensile strength||52||48|
|SnPbCd||145||Pb,Cd||yes||General-purpose. Maintains creep strength well. Unsuitable for gold.||51.2||30.6||18.2|
|SnInAg||175/187||–||no||Similar mechanical properties with SnPb, SnPbAg and SnPb, suitable lead-free replacement. Contains eutectic Sn-In phase with melting point at 118 °C, avoid use above 100 °C.||77.2||2.8||20|
|BiPbSnIn||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. Also the ChipQuik desoldering alloy.||12||18||49||21|
|BiPbSnCd||70||Pb,Cd||yes||Cerrobend. Used in low-temperature physics as a solder.||13.3||26.7||50||10|
|BiPbInCdSn||47||Cd,Pb||yes||Cerrolow 117. Used as a solder in low-temperature physics.||8.3||22.6||44.7||19.1||5.3|
|BiPbSnCd||71||Pb,Cd||yes||Wood's metal, mostly used for casting.||12.5||25||50||12.5|
|BiPbSn||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.||22||28||50|
|BiSnIn||79/91||no||ChipQuik desoldering alloy, lead-free||30||56||14|
|CdAg||338/393||Cd||no||KappTec 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.||5||95|
|CdZn||265||Cd||yes||Medium temperature alloy that provide strong, corrosion-resistant joints on most metals. Also for soldering aluminium and die-cast zinc alloys. Used in cryogenic physics for ataching electrical potential leads to specimens of metals, as this alloy does not become superconductive at liquid helium temperatures.||17.5||82.5|
|CdZn||265/300||Cd||no||Medium 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.||30||70|
|CdZn||265/316||Cd||no||Medium 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.||40||60|
|CdZnAg||249/316||Cd||no||KappTecZ 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.||5||17||78|
|SnZnCd||176/260||Cd||no||KappRad Developed specifically to join and repair Aluminum and Aluminum/Copper radiators and heat exchangers. A lower melting point makes delicate repair work easier.||40||27||33|
|ZnCd||265/399||Cd||For soldering aluminium. Good wetting.||90||10|
|ZnCd||265/335||Cd||For soldering aluminium. Very good wetting.||60||40|
|CdSn||140/160||Cd||no||Cd70, thermal-free solder. Produces low thermal EMF joints in copper, does not form parasitic thermocouples. Used in low-temperature physics.||29.56||70.44|
|SnPbCd||145||Cd,Pb||Low melting temperature allows repairing pewter and zinc objects, including die-cast toys.||40||42||18|
|ZnSn||199/376||–||no||For soldering aluminium. Excellent wetting. Good strength.||30||70|
|ZnSn||199/341||–||no||For soldering aluminium. Good wetting.||40||60|
|ZnSn||382||–||yes?||For soldering aluminium. Excellent wetting.||5||95|
|AuSn||280||–||yes||Au80. 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. Forms a mixture of two brittle intermetallic phases, AuSn and AuSn. 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. One of the best materials for soldering in optoelectronic devices and components packaging. Low vapor pressure, suitable for vacuum work. Generally used in applications that require a melting temperature over 150 °C. Good ductility. Also classified as a braze.||20||80|
|AuSi||370/800||–||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.||98||Si|
|AuSi||370 363||–||yes||Au97. AuSi 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.||96.8||Si|
|AuGe||361 356||–||yes||Au88. Used for die attachment of some chips. The high temperature may be detrimental to the chips and limits reworkability.||87.5||Ge|
|AuIn||451/485||–||no||Au82. High-temperature, extremely hard, very stiff.||18||82|
|In||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. 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.||99.99|
|SnAgCuCr||217/1050||–||no||C-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 and other materials generally considered as difficult to solder, including aluminium, stainless steel, titanium, glass, ceramics.||90.7||3.6||0.7||Cr|
Notes on the above table
Temperature ranges for solidus and liquidus (the boundaries of the mushy state) are listed as solidus/liquidus.
In the Sn-Pb alloys, tensile strength increases with increasing tin content. Indium-tin alloys with high indium content have very low tensile strength.
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.
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/cm.
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.
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.
- Bismuth significantly lowers the melting point and improves wettability. In presence of sufficient lead and tin, bismuth forms crystals of SnPbBi 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 SnAgCu.
- Nickel can be added to the solder alloy to form a supersaturated solution to inhibit dissolution of thin-film under-bump metallization.
- 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.
- 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 SnPbAg phase with melting point at 179 °C, which moves to the board-solder interface, solidifies last, and separates from the board. 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 AgSn, 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.
- 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%.
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.
- 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.
- Iron – forms intermetallics, causes grittiness, but rate of dissolution is very low; readily dissolves in lead-tin above 427 °C.
- 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.
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-CuSn-CuSn-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.
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.
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.
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.
|Copper||CuSn, CuSn, CuSn, CuSn||CuIn, CuIn|
|Nickel||NiSn, NiSn, NiSn NiSn||NiIn, NiIn NiIn, NiIn|
|Silver||AgSn, AgSn||AgIn, AgIn|
|Gold||AuSn, AuSn AuSn, AuSn||AuPb, AuPb||AuIn, AuIn|
|Palladium||PdSn, PdSn, PdSn, PdSn, PdSn, PdSn||PdIn, PdIn, PdIn PdIn|
|Platinum||PtSn, PtSn, PtSn, PtSn, PtSn, PtSn||PtPb, PtPb PtPb||PtIn, PtIn, PtIn|
- CuSn – common on solder-copper interface, forms preferentially when excess of tin is available; in presence of nickel (Cu,Ni)Sn compound can be formed
- CuSn – common on solder-copper interface, forms preferentially when excess of copper is available, more thermally stable than CuSn, often present when higher-temperature soldering occurred
- NiSn – common on solder-nickel interface
- FeSn – very slow formation
- AgSn - at higher concentration of silver (over 3%) in tin forms platelets that can serve as crack initiation sites.
- AuSn – β-phase – brittle, forms at excess of tin. Detrimental to properties of tin-based solders to gold-plated layers.
- AuIn – forms on the boundary between gold and indium-lead solder, acts as a barrier against further dissolution of gold
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. 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-BO-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. 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.
The bonding between the glass or ceramics and the glass solder can be either covalent, or, more often, van der Waals. 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. 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. 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.
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.
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. Evacuated glazing windows are made of glass panels soldered together.
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.
Conductive glass solders can be also prepared.
A preform is a pre-made shape of solder specially designed for the application where it is to be used. Many methods are used to manufacture the solder preform, stamping being the most common. The solder preform may include the solder flux needed for the soldering process. This can be an internal flux, inside the solder preform, or external, with the solder preform coated.
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