Bismuth (Bi) is being used more and more as a replacement for lead in solder alloys because it is non-toxic.
Bismuth Solder Features & Benefits
Although the melting temperature of pure bismuth is 271°C, the addition of bismuth will lower the melting temperature of most metals it is alloyed with. Bismuth compounds are also widely used in medical and cosmetic applications.
Features
The most popular lead-free bismuth solder alloys are Indalloy® 281 (58Bi 42Sn) which melts at 138°C and Indalloy® 282 (57Bi42Sn1Ag) which melts at 140°C. The addition of the 1% Ag makes the alloy more malleable. Both alloys can be used for step soldering applications. After the initial joints are made with a standard SAC alloy (220°C) subsequent soldering operations can be done using the bismuth alloys.
Indalloy® 281 and Indalloy® 282 have joint properties similar to those of tin-lead solders, with superior fatigue and copper dissolution characteristics.
Bismuth is the most diamagnetic and the least thermally conductive of all metals
In addition to being non-toxic it does not oxidize as readily as lead does.
Bismuth, in elemental, alloyed, or chemical compound forms has numerous applications1:
Bismuth has also been used in solders. The fact that bismuth and many of its alloys expand slightly when they solidify make them ideal for this purpose.1
Many bismuth alloys have low melting points and are widely used for fire detection and suppression system safety devices.
In the early 1990s, research began to evaluate bismuth as a nontoxic replacement for lead in various applications.
As noted above, bismuth has been used in solders; its low toxicity will be especially important for solders to be used in food processing equipment and copper water pipes.
Bismuth compounds are used in cosmetics, medicines, and in medical procedures. As the toxicity of lead has become more apparent in recent years, alloy uses for bismuth metal as a replacement for lead have become an increasing part of bismuth's commercial importance.
Elemental bismuth is one of very few substances of which the liquid phase is denser than its solid phase (water being the best-known example). Because bismuth expands on freezing, it was long an important component of low-melting typesetting alloys, which needed to expand to fill printing molds.
Bismuth is used as an alloying agent in production of malleable irons.
Bismuth is used as a dense material for fishing sinkers.1
Bismuth is used as a replacement for lead in shot and bullets. The UK, U.S., and many other countries now prohibit the use of lead shot for the hunting of wetland birds, as many birds are prone to lead poisoning due to mistaken ingestion of lead (instead of small stones and grit) to aid digestion. Bismuth-tin alloy shot is one alternative that provides similar ballistic performance to lead. (Another less expensive but also more poorly performing alternative is "steel" shot, which is actually soft iron.). Bismuth core bullets are also starting to appear for use in indoor shooting ranges, where fine particles of lead from bullets impacting the backstop can be a chronic toxic inhalant problem. Owing to bismuth's crystalline nature, the bismuth bullets shatter into a non-toxic powder on impact, making recovery and recycling easy. The lack of malleability does, however, make bismuth unsuitable for use in expanding hunting bullets.1
Bismuth as a pure element is very brittle. When bismuth is added to tin or lead the bismuth works to reduce the melting temperature of the resulting alloy and the tin or lead works to reduce the brittleness of the alloyed material.
There are four common forms of bismuth-contained material and each of them has special considerations in the manufacturing process.
Solder Wire
In solid wire form, diameters start at .254mm (.010") and can go up to 6.35mm (.250") or larger. Consideration must be given to packaging since unspooling the wire can cause it to break. Also, the larger diameter wires are manufactured as straight rods because spooling is not possible due to the brittleness. Generally, bismuth-based alloys cannot be flux cored.
Solder Preforms
Solder preforms are generally solid pieces of metal that are designed to bond two surfaces together using an exact, repeatable amount of solder each time. Preforms are available as discs, squares, rectangles, washers or frames and can generally be flux coated. Custom shapes are also available.
The keys to specifying preforms are:
Start with the shape that you need.
Then calculate the volume of solder you need for a good solder joint. Generally the thickness can be adjusted to allow you to take advantage of existing tooling.
BiSn and BiPb alloys are the most workable alloys and will give the most flexibility of design.
The optimal thickness range for bismuth contained alloys is between .254mm (.010") and .508mm (.020").
Indium Corporation engineers are eager to discuss your application or requirement.
Solder Ribbon
Solder ribbon is very similar to solder preforms where lengths of ribbon are packaged on spools and automatic cutting equipment cuts exact sizes for automated placement. The same rules apply for preform specification.
Solder Paste
The minimum temperature for a solder paste is 96°C, generally the lower limit for flux activation. The bismuth containing alloys that melt below 96°C are generally used in fuse applications where a flux is not required. The eutectic alloy of 58Bi 42Sn (Indalloy® 281) melts at 138°C and is one of the more popular bismuth contained alloys.
Low-temperature alloys (including bismuth-containing and indium-containing alloys) need special flux vehicles that have activators lower than standard SMT pastes. Indium Corporation's Indium5.7LT is the best choice for these alloys.
Features of Indium5.7LT include:
Clear residue
Exceptional wetting in air reflow
Halogen-free
Low activation temperature (specifically designed for BiSn alloys)
Indium5.7LT solder paste with Indalloy® 281 (58Bi 42Sn) and Indalloy® 282 (57Bi 42Sn 1Ag) has been designed for applications requiring low temperature soldering.
The low activation temperature of the Indium5.7LT in combination with the low melt point solders 58Bi 42Sn and 57Bi 42Sn 1Ag feature exceptional wetting in an air reflow, clear residue and good mechanical, thermal and electrical properties.
57Bi 42Sn 1Ag has the added benefits of higher tensile strengths and good fracture energy, with creep resistance and is ideal for step soldering applications.
