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A soldered joint used to attach a wire to the pin of a component on the rear of a printed circuit board.

Solder is a fusible metal alloy with a melting point or melting range of 90 to 450 degree Celsius (190 to 840 °F), used in a process called soldering where it is melted to join metallic surfaces. It is especially useful 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 brazing. Solder can contain lead and/or flux but in many applications solder is now lead free.

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

Contents

Lead solder

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 Sn/Pb which melts at 370 °F or 188 °C and 63/37 Sn/Pb used principally in electrical work. The 63/37 ratio is notable in that it is a eutectic mixture, which means:

  1. It has the lowest melting point (183 °C or 361.4 °F) of all the tin/lead alloys; and
  2. The melting point is truly a point — not a range.

At a eutectic composition, the liquid solder solidifies at a single temperature. Tin/lead solder solidifies to fine grains of nearly pure lead and nearly pure tin phases, there are no tin/lead intermetallics and no solid solution of tin in lead or lead in tin, as can be seen from a tin/lead equilibrium diagram.[1]

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,[2] 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.)

In electronics, the traditional use of solder was to fortify mechanically made electrical contacts, e.g. two solid copper wires twisted together. This was in part due to the higher electrical resistance of solder versus copper.[3] Printed circuit boards use solder joints to mount components and create a circuit, also replacing the use of solid solder with solder paste.

Tin (Sn) [%] Lead (Pb) [%] Melting point [°C] Comment
25 75 271 crude solder for construction plumbing works, flame-melted
30 70 262 crude solder for construction plumbing works, flame-melted
33 67 180-230 PM 33, crude solder for construction plumbing works, flame-melted, temperature depends on additives
40 60 240 for soldering of brass
50 50 220 for soldering of brass, electricity meters, gas meters, formerly also tin cans
60 40 190 for electronics
63 37 182 for electronics
90 10 220 formerly used for joints in food industry

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.

Hard solder

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

In silversmithing or jewelry making, special hard solders are used that will pass 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 "enamelling", "hard", "medium" and "easy". Enamelling solder has a high melting point, close to that of the material itself, to prevent the joint desoldering during firing in the enamelling 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.

Flux-core solder

A tube of multicore electronics solder used for manual soldering - the flux is contained in five cores within the solder itself

Flux is a reducing agent designed to help reduce (return oxidized metals to their metallic state) 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 vapours 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 deionised 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.

Lead-free solder

A coil of lead-free solder wire

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. California recently adopted a RoHS law[4] and China has a version as well. 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 5–20 °C higher,[citation needed] 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 solders are used by two thirds of Japanese manufacturers for reflow and wave soldering, and by about ¾ companies for hand soldering. The widespread use of this popular Pb-free solder alloy family is based on the reduced melting point of the Sn-Ag-Cu ternary eutectic reaction (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 reaction 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.
    • SnAg3.0Cu0.5, tin with 3% silver and 0.5% copper, has a melting point of 217 to 220 °C and is 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.
    • SnAg3.5Cu0.7 is another commonly used alloy, with melting point of 217-218 °C.
    • SnAg3.5Cu0.9, with melting point of 217 °C, is determined by NIST to be truly eutectic.
    • SnAg3.8Cu0.7, with melting point 217-218 °C, is preferred by the European IDEALS consortium for reflow soldering.
    • SnAg3.8Cu0.7Sb0.25 is preferred by the European IDEALS consortium for wave soldering.
    • SnAg3.9Cu0.6, with melting point 217-223 °C, is recommended by the US NEMI consortium for reflow soldering.
    • 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).
  • SnCu0.7, with melting point of 227 °C, is a cheap alternative for wave soldering, recommended by the US NEMI consortium.
  • SnZn9, with melting point of 199 °C, is a cheaper alloy but is prone to corrosion and oxidation.
  • SnZn8Bi3, with melting point of 191-198 °C, is also prone to corrosion and oxidation due to its zinc content.
  • SnSb5, tin with 5% of antimony, is the US plumbing industry standard. Its melting point is 232-240 °C. It displays good resistance to thermal fatigue and good shear strength.
  • SnAg2.5Cu0.8Sb0.5 melts at 217-225 °C and is patented by AIM alliance.
  • SnIn8.0Ag3.5Bi0.5 melts at 197 to 208 °C and is patented by Matsushita/Panasonic.
  • SnBi57Ag1 melts at 137-139 °C and is patented by Motorola.
  • SnBi58 melts at 138 °C.
  • SnIn52 melts at 118 °C and is suitable for the cases where low-temperature soldering is needed.

Different elements serve different roles in the solder alloy:

  • Silver provides mechanical strength, but has worse ductility than lead. In absence of lead, it improves resistance to fatigue from thermal cycles.
  • 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.
  • Bismuth significantly lowers the melting point and improves wettability. In presence of sufficient lead and tin, bismuth forms crystals of Sn16Pb32Bi52 with melting point of only 95 °C, which diffuses along the grain boundaries and may cause a joint failure at relatively low temperatures. A high-power part pre-tinned with an alloy of lead can therefore desolder under load when soldered with a bismuth-containing solder.
  • Indium lowers the melting point and improves ductility. In presence of lead it forms a ternary compound that undergoes phase change at 114 °C.
  • 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.
  • Antimony is added to increase strength without affecting wettability.

Glass solder

Glass solders are used to join glasses to other glasses, ceramics, metals, semiconductors, mica, and other materials. 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. metalization layers on chips or ceramic substrates) occurs. The usual temperature of achieving flowing and wetting is between 450-550 °C.

Two types of glass solders are used: vitreous, and devitrifying. Vitreous solders retain their amorphous structure during remelting, can be reworked repeately, 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 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.[5] 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.[6] 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.[7]

The bonding between the glass or ceramics and the glass solder can be either covalent, or, more often, van der Waals.[8] The seal can be leaktight; 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.[9] Glass solders are frequently used for glass-to-metal seals and glass-ceramic-to-metal seals.

Glass solders are available as fritpowder with grain size of 60 or less 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.[10] 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.[11]

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 choosed 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.[6]

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.[12] Evacuated glazing windows are made of glass panels soldered together.[13]

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.[14]

Conductive glass solders can be also prepared.

See also

References

  1. ^ "Internet Microscope for Schools : Micrographs : Lead". Pwatlas.mt.umist.ac.uk. http://pwatlas.mt.umist.ac.uk/internetmicroscope/micrographs/microstructures/more-metals/lead.html. Retrieved 2008-09-22. 
  2. ^ N Engl J Med. 1990 Jan 11;322 (2):83-8 PMID 2294437 (P,S,G,E,B) The long-term effects of exposure to low doses of lead in childhood. An 11-year follow-up report.
  3. ^ Principles of soldering By Giles Humpston, David M. Jacobson (Google Books result)
  4. ^ http://www.dtsc.ca.gov/HazardousWaste/EWaste
  5. ^ http://books.google.com/books?id=c2YxCCaM9RIC&pg=PA239&dq=%22glass+solders%22&lr=&as_drrb_is=q&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=&num=50&as_brr=3&cd=5#v=onepage&q=%22glass%20solders%22&f=false
  6. ^ a b http://books.google.com/books?id=-Ll6qjWB-RUC&pg=PA51&dq=%22solder+glass%22&lr=&num=50&as_brr=3&cd=5#v=onepage&q=%22solder%20glass%22&f=false
  7. ^ http://books.google.com/books?id=dzNPGZqQY3wC&pg=PA40&dq=%22solder+glass%22&lr=&num=50&as_brr=3&cd=14#v=onepage&q=%22solder%20glass%22&f=false
  8. ^ http://books.google.com/books?id=CRt97aa7cnMC&pg=PA389&dq=%22glass+solder%22&lr=&num=50&as_brr=3&cd=23#v=onepage&q=%22glass%20solder%22&f=false
  9. ^ http://books.google.com/books?id=sdKAPJh5RgQC&pg=PA273&dq=%22glass+solder%22&lr=&num=50&as_brr=3&cd=5#v=onepage&q=%22glass%20solder%22&f=false
  10. ^ http://books.google.com/books?id=v5q4Hje3iFgC&pg=PA30&dq=%22borate+glass%22&lr=&num=50&as_brr=3&cd=18#v=onepage&q=%22borate%20glass%22&f=false
  11. ^ http://books.google.com/books?id=7VNfvKjlzYAC&pg=PA8&dq=%22glass+solder%22&lr=&num=50&as_brr=3&cd=2#v=onepage&q=%22glass%20solder%22&f=false
  12. ^ http://books.google.com/books?id=2obIbuqA43UC&pg=PA374&dq=%22glass+solders%22&lr=&as_drrb_is=q&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=&num=50&as_brr=3&cd=8#v=onepage&q=%22glass%20solders%22&f=false
  13. ^ http://books.google.com/books?id=_btgLVQUVL0C&pg=PT96&dq=%22solder+glass%22&lr=&num=50&as_brr=3&cd=67
  14. ^ http://books.google.com/books?id=wuD5JHZ7ajIC&pg=PA266&dq=%22glass+solders%22&lr=&as_drrb_is=q&as_minm_is=0&as_miny_is=&as_maxm_is=0&as_maxy_is=&num=50&as_brr=3&cd=2#v=onepage&q=%22glass%20solders%22&f=false

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1911 encyclopedia

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Simple English

[[File:|thumb|A use of solder: holding a wire to a circuit board]] Solder is a metal or alloy that melts at a low temperature. There are two types of solder; soft solder (easily melting alloys that are melted with soldering irons and used for electronics and electrical work), and hard solder (high melting alloys or metals that are melted with a torch). The use of solder is called soldering.

File:Ex Lead
Lead-free solder

There are two main types of soft solder; lead solder and lead-free solder. Lead solders have about 60% (or 63%) tin and 40% (or 37%) lead in them. They are toxic because they have lead in them. They melt around 185°C. In plumbing, a 50% tin and 50% lead mixture was used. This was thought to be safe until it was seen that the lead was coming into the water. Now lead solder is illegal for water. Lead solder was once used for food cans. After many years of standing, the cans were poisoning people who ate the food in the cans because the lead was coming into the food. Lead solder is still used in electronics. It is cheap, which is why it used to be popular.

In 2006 the European Union, as well as China and California, banned the use of lead in consumer products. This made lead solder illegal for use in electronic devices in some places. Lead-free solder was then made. Many lead-free solders have tin, silver, and copper in them. They melt around 217°C. Sometimes indium is added to the solder to make it better, although indium is very expensive.

Many times when a metal is being soldered, it oxidizes, making a layer of metal oxide that does not hold solder. Flux is added to react with the metal oxide and turn it back into the metal again, helping the solder to stick. Rosin is a common flux. Some electronics makers use fluxes that can be washed away with water. Some solders have a flux core, where the flux is inside the solder.

Another type of solder is used to connect glass to other things. They melt around 450-550°C.


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