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ISO Solder Alloy 721 (Sn96Ag2.5Bi1Cu0.5)

Solder alloy 721 is a tin-based soldering alloy. Cited properties are appropriate for the as-fabricated (no temper or treatment) condition. 721 is the ISO numeric designation for this material. Older literature may refer to this material as S-Sn96Ag2,5Bi1Cu0,5, but this is now discouraged.

It has a moderately low ductility among the tin-based solder alloys in the database.

The graph bars on the material properties cards below compare solder alloy 721 to: tin-based solder alloys (top), all alloys in the same category (middle), and the entire database (bottom). A full bar means this is the highest value in the relevant set. A half-full bar means it's 50% of the highest, and so on.

Mechanical Properties

Elastic (Young's, Tensile) Modulus

51 GPa 7.3 x 106 psi

Elongation at Break

17 %

Poisson's Ratio


Shear Modulus

19 GPa 2.7 x 106 psi

Tensile Strength: Ultimate (UTS)

50 MPa 7.3 x 103 psi

Thermal Properties

Latent Heat of Fusion

61 J/g

Melting Completion (Liquidus)

220 °C 420 °F

Melting Onset (Solidus)

210 °C 420 °F

Specific Heat Capacity

220 J/kg-K 0.053 BTU/lb-°F

Thermal Conductivity

56 W/m-K 32 BTU/h-ft-°F

Thermal Expansion

22 µm/m-K

Electrical Properties

Electrical Conductivity: Equal Volume

14 % IACS

Electrical Conductivity: Equal Weight (Specific)

17 % IACS

Otherwise Unclassified Properties


7.4 g/cm3 460 lb/ft3

Embodied Carbon

17 kg CO2/kg material

Embodied Energy

280 MJ/kg 120 x 103 BTU/lb

Common Calculations

Stiffness to Weight: Axial

3.8 points

Stiffness to Weight: Bending

17 points

Strength to Weight: Axial

1.9 points

Strength to Weight: Bending

4.1 points

Thermal Diffusivity

34 mm2/s

Thermal Shock Resistance

3.2 points

Alloy Composition

Tin (Sn)Sn 95 to 96.6
Silver (Ag)Ag 2.3 to 2.7
Bismuth (Bi)Bi 0.8 to 1.2
Copper (Cu)Cu 0.3 to 0.7
Antimony (Sb)Sb 0 to 0.1
Indium (In)In 0 to 0.1
Lead (Pb)Pb 0 to 0.070
Gold (Au)Au 0 to 0.050
Arsenic (As)As 0 to 0.030
Iron (Fe)Fe 0 to 0.020
Nickel (Ni)Ni 0 to 0.010
Cadmium (Cd)Cd 0 to 0.0020
Aluminum (Al)Al 0 to 0.0010
Zinc (Zn)Zn 0 to 0.0010

All values are % weight. Ranges represent what is permitted under applicable standards.

Followup Questions

Similar Alloys

Further Reading

ISO 9453: Soft solder alloys - Chemical compositions and forms