These alloys are frequently called "fusible" alloys because they are easily melted at a relatively low temperature compared to most solder alloys. These alloys are made from two or more of the following elements: Bismuth, Tin, Lead, Cadmium, and Indium.
Each of these ingredients is used to impart specific characteristics and properties to the alloy. The alloys can be selected in proportions to make binary, tertiary alloys that melt well below their alloying elements. They are either eutectic, which means they melt at a single temperature (like a pure metal) or non-eutectic, which means they have a melting range. This range is defined as a liquidus - the temperature above which they are completely liquid, and a solidus - the temperature below which they are completely solid. The temperatures in between these are defined as the "pasty range".
The characteristics of these alloys include: low vapor pressure, good thermal conductivity, ease of handling, high liquid fluidity, ability to be reused, and controlled thermal dimensional properties. With this last item, an alloy can be made with minimal solidification shrinkage. Unlike most metals, pure bismuth expands 3.3% of volume on changing from liquid to solid. This expansion is employed in alloys containing bismuth to offset solidification shrinkage. An alloy containing more than 55% of bismuth expand and those with less than 48% contract with solidification. Those in between alloy exhibit little change in volume. The change in volume due to cooling is simple linear shrinkage, but some alloys show changes in the structure of the alloy, which permit castings made of these alloys to have dimensions, the same or larger than those of the mold they were cast into.
Some of these dimensional changes can continue to occur for as long as 1000 hours after solidification. Most of these alloys stop growing after 24 hours.
Applications of Low Melt Alloys
|Safety Appliances||Fire sprinklers, Boiler plugs, releasing automatic fire doors, etc.|
|Foundry Practice||Making patterns, proof casting, fusible cores, etc.|
|Radiation Shielding||Casting permanent molds for protection.|
|Bonding & Sealing||Soldering to glass and temperature sensitive devices. Prevents a thin wall tube from buckling on sharp bending radii.|
|Tube Bending||Prevents a thin wall tube from buckling on sharp bending radii.|
|Electroforming||To produce expendable mandrels for complicated shapes.|
Low temperature alloys, or fusible links, are widely used in many industries. Because of their unique characteristics, they are finding more uses for these alloys in electronic assemblies.
Low temperature alloys contain two or more of the following elements: bismuth, tin, lead, silver, cadmium and/or indium.
Specific characteristics of these alloys include low vapor pressure, good thermal conductivity, ease in reflow, high liquid fluidity and controlled dimensional properties.
Low Melt Alloys
|Constituents -Weight Percent||Melting Points|
|Constituents -Weight Percent|