Bismuth (Bi) is being used more and more as a replacement for lead in solder alloys because it is non-toxic. 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.
- 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 #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.
Forms of Bismuth
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.
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 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").
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
- 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.
Below is a list of the most common bismuth alloys.
|42||46Bi 34Sn 20Pb||96°C||96°C||Low temperature eutectic solder. Can be used on the same metallizations as SnPb solders. Lowest temperature alloy available as a solder paste.|
|281||58Bi 42Sn||138°C||138°C||Eutectic Pb-Free version. Low melting temperature that is good for step soldering applications.|
|282||57Bi 42Sn 1Ag||140°C||139°C||More malleable and ductile than #281.|
|97||43Sn 43Pb 14Bi||163°C||144°C||Good general purpose step soldering alloy.|
|281-338||60Sn 40Bi||170°C||138°C||General purpose Pb-Free alloy with good physical properties.|