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ISO Solder Alloy 121 (Pb85Sn15)

Solder alloy 121 is a lead-based soldering alloy. Cited properties are appropriate for the as-fabricated (no temper or treatment) condition. 121 is the ISO numeric designation for this material. Older literature may refer to this material as S-Pb85Sn15, but this is now discouraged.

It has a fairly low electrical conductivity among lead-based solder alloys. In addition, it has a fairly low embodied energy and a moderately low heat capacity.

The graph bars on the material properties cards below compare solder alloy 121 to: lead-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

Brinell Hardness


Elastic (Young's, Tensile) Modulus

21 GPa 3.1 x 106 psi

Elongation at Break

25 %

Poisson's Ratio


Shear Modulus

7.5 GPa 1.1 x 106 psi

Tensile Strength: Ultimate (UTS)

32 MPa 4.7 x 103 psi

Thermal Properties

Latent Heat of Fusion

29 J/g

Melting Completion (Liquidus)

290 °C 550 °F

Melting Onset (Solidus)

230 °C 440 °F

Specific Heat Capacity

140 J/kg-K 0.034 BTU/lb-°F

Thermal Conductivity

31 W/m-K 18 BTU/h-ft-°F

Thermal Expansion

28 µm/m-K

Electrical Properties

Electrical Conductivity: Equal Volume

8.8 % IACS

Electrical Conductivity: Equal Weight (Specific)

7.4 % IACS

Otherwise Unclassified Properties

Base Metal Price

80 % relative


11 g/cm3 670 lb/ft3

Embodied Carbon

5.6 kg CO2/kg material

Embodied Energy

93 MJ/kg 40 x 103 BTU/lb

Embodied Water

580 L/kg 69 gal/lb

Common Calculations

Stiffness to Weight: Axial

1.1 points

Stiffness to Weight: Bending

8.6 points

Strength to Weight: Axial

0.84 points

Strength to Weight: Bending

2.1 points

Thermal Diffusivity

20 mm2/s

Thermal Shock Resistance

3.9 points

Alloy Composition

Lead (Pb)Pb 83.4 to 85.5
Tin (Sn)Sn 14.5 to 15.5
Antimony (Sb)Sb 0 to 0.5
Bismuth (Bi)Bi 0 to 0.25
Silver (Ag)Ag 0 to 0.1
Indium (In)In 0 to 0.1
Copper (Cu)Cu 0 to 0.080
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 0.0050
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