Bismuth and Indium Low-Melting Alloys
These are the fusible alloys — bismuth, indium, tin, lead and cadmium combined in eutectics that melt anywhere from 47 °C (Cerrolow-117) to 157 °C (pure indium). Bismuth-rich grades expand slightly as they freeze and are brittle, which suits fixturing, tube-bending fillers and fusible links; indium-rich grades are soft, ductile down to cryogenic temperatures and wet glass and ceramics, which suits thermal interfaces and seals. In electronics Sn42Bi58 at 138 °C is the low-temperature solder for heat-sensitive parts and step soldering.
Popular low-melting alloy grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| Sn42Bi58 | 42 | 62 | 8.56 | Good | Poor | Low-temperature electronics solder, 138 °C |
| Bi57Sn42Ag1 | 40 | 62 | 8.57 | Fair | Poor | Tougher Sn-Bi joint for step soldering |
| In52Sn48 | 7 | 12 | 7.3 | Poor | Not recommended | Soldering glass, quartz and ceramics, 118 °C |
| In100 | 1 | 3 | 7.31 | Poor | Very good | Thermal interfaces and cryogenic seals |
| Bi32.5In51Sn16.5 | 8 | 15 | 7.88 | Poor | Poor | Cadmium-free 62 °C links and casting |
| Bi50Pb25Sn25 | 18 | 30 | 9.7 | Fair | Not recommended | Fixturing and tube filling, melts in hot water |
| Cerrolow-117 | — | 29 | 9.16 | Poor | Not recommended | Lowest practical melting point, 47 °C |
Choose a low-melting alloy for
Soldering LEDs, sensors and plastic-bodied components that cannot take 220 °C, second- and third-level step soldering, thermal fuses and sprinkler links that must release at a set temperature, casting a block around a thin or oddly shaped part to hold it for machining, filling tube before a tight bend, indium foil between a chip and a heat sink, and metal seals that stay ductile at liquid-nitrogen temperature.
Look elsewhere when
The joint sees service above about 80 °C (SAC305 or Sn63Pb37 — bismuth alloys lose most of their strength well below their melting point), it will be dropped or vibrated (Sn42Bi58 is brittle; use Bi57Sn42Ag1 or a tin-based solder), the assembly might later be reworked with tin-lead (bismuth plus lead forms a 96 °C phase), or the fixture alloy contains cadmium and the shop cannot handle it — Bi50Pb25Sn25 and Bi32.5In51Sn16.5 are the cadmium-free substitutes.
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Low-melting alloy by application
- Heat-sensitive electronics
- Sn42Bi58 paste at a 165–180 °C reflow peak; Bi57Sn42Ag1 where the board will be flexed or dropped. Do not mix with lead-bearing finishes.
- Fusible links and thermal fuses
- Bi32.5In51Sn16.5 at 62 °C or Bi50Pb25Sn25 at 94–98 °C cast as a slug or wire; Cerrolow-117 where the release point must be 47 °C. The link is sized so the alloy carries no load until it melts.
- Workholding and tube bending
- Bi50Pb25Sn25 or Bi50Pb25Sn12.5Cd12.5 poured around the part or into the tube, machined or bent, then melted out in water at 90–100 °C and reused.
Working with low-melting alloy
- Process
- Sn42Bi58 reflows at a 165–180 °C peak with a mild flux; In52Sn48 and In100 are soldered at 150–200 °C with an indium-specific flux or ultrasonically without one. Fusible alloys for fixturing are melted in a thermostatic pot 30–50 °C above liquidus and poured into a cavity or around the part; most bismuth-rich grades expand slightly — up to about 0.5 % — as they solidify and age, so they lock into the mould rather than shrinking away from it.
- Joint reliability
- Sn-Bi joints are brittle below about 60 °C and creep quickly above 80 °C — keep them out of vibration and off components that self-heat. Any lead in the joint (a tin-lead finish, a mixed rework) forms a Sn-Pb-Bi phase that melts at 96 °C. Indium joints are ductile to cryogenic temperature but soft (about 2 MPa yield) and corrode in humid air unless coated.
- Fixturing, tube bending and fusible links
- Cast the alloy around a thin turbine blade or optic, machine the block as one solid, then release in hot water; the alloy is recovered and reused. Fill tube with the same alloy before bending to hold the section round, then melt it out. Fusible links and sprinkler heads use a solder chosen for a release temperature 20–30 °C above the highest ambient, cast as a slug in a spring-loaded holder.
- Regulatory and handling
- Cadmium is restricted under RoHS to 0.01 % and lead to 0.1 %, so the classic Cerro and Wood’s-metal compositions are industrial fixture alloys, not product materials; Bi-In-Sn and Bi-Sn grades are the compliant choices. Indium is scarce and priced accordingly, so it is used as thin foil and preforms. Fixture alloy pots should be covered to keep out solder scrap — a little tin-lead lowers the melting point unpredictably.
Other solder & low-melting alloy series
Bismuth and Indium Low-Melting Alloys — frequently asked
What is low-melting alloy?
These are the fusible alloys — bismuth, indium, tin, lead and cadmium combined in eutectics that melt anywhere from 47 °C (Cerrolow-117) to 157 °C (pure indium). Bismuth-rich grades expand slightly as they freeze and are brittle, which suits fixturing, tube-bending fillers and fusible links; indium-rich grades are soft, ductile down to cryogenic temperatures and wet glass and ceramics, which suits thermal interfaces and seals. In electronics Sn42Bi58 at 138 °C is the low-temperature solder for heat-sensitive parts and step soldering.
Which grade of low-melting alloy should I use?
Sn42Bi58 for low-temperature electronics soldering and Bi57Sn42Ag1 where the joint needs more ductility. In52Sn48 melts at 118 °C and bonds to glass, quartz and metallized ceramics; In100 is the thermal-interface and cryogenic-seal metal, and In97Ag3 adds a little strength. For fixturing and tube bending, a cadmium-free Bi-Pb-Sn alloy such as Bi50Pb25Sn25 melts in boiling water; Bi32.5In51Sn16.5 melts at 62 °C and is the cadmium-free choice for links and demonstrations. Bi50Pb25Sn12.5Cd12.5 and Cerrolow-117 melt lower still but contain cadmium and lead, which restricts them to industrial use. Ga68In20Sn12 is liquid at room temperature and is a heat-transfer fluid, not a solder.
Which grade of low-melting alloy is the strongest?
Sn89Zn8Bi3 has the highest yield strength in the series at 45 MPa; Bi58Sn42 follows at 42 MPa. Values are for each grade's default condition.
Which grade of low-melting alloy is easiest to machine?
Sn42Bi58 (machinability good, 55/100), then Bi42.5Pb37.7Sn11.3Cd8.5 (45/100).
Sn42Bi58 or SAC305 for LED boards?
Sn42Bi58 lets you reflow at 165–180 °C instead of 235–250 °C, which protects the phosphor, lens and plastic packages, and it saves energy. The cost is a joint that is brittle when cold and weak above 80 °C, so it is wrong for LEDs that run hot or boards that flex; Bi57Sn42Ag1 recovers some ductility. If the LED package is rated for lead-free reflow, SAC305 gives the longer-lived joint.
Which alloy melts in hot water?
Any of the Bi-Pb-Sn grades: Bi50Pb25Sn25 melts at 94–98 °C, Bi50Pb25Sn12.5Cd12.5 at about 70 °C, Bi32.5In51Sn16.5 at 62 °C and Cerrolow-117 at 47 °C. For workholding pick the highest melting point the water bath can release, because the higher-melting grades are also stronger and cheaper; for a demonstration or a fusible link pick the release temperature you need.