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.
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. Or we can custom make a shape for your specific application.
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").
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.
For 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 Indium5.5LT is the best choice for these alloys.
Features of Indium5.5LT include:
- Clear residue
- Exceptional wetting in air reflow
- Halogen-free
- Low activation temperature (specifically designed for BiSn alloys)
Indium5.5LT 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 Indium5.5LT 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.
Indium5.5LT Product Data Sheet
Indium Corporation engineers are eager to discuss your application or requirement. Send your bismuth inquiry now.
