The Ames lab research team at Iowa State University, US have published results showing that small additions of zinc, manganese or aluminium can stop formation of Ag3Sn intermetallics in a SAC lead-free solder and so prevent embrittlement in thermal ageing.

This is particularly important for applications such as under-the-hood automotive electronics.

Using calorimetry and microstructure measurements Anderson and his colleagues have demonstrated that adding 0.21Zn, 0.10Mn and 0.05Al can reduce undercooling below that for SAC3595 alloy and eliminate Ag3Sn blades.

Although initial as-soldered shear strength of solder with 0.05Al and 0.21Zn was reduced, all joints failed at this strength after thermal ageing.

The team also observed a 211'C melting onset for Mn-containing alloys, suggesting discovery of a new quaternary eutectic.

The work is published in a special lead-free soldering edition of Journal of Electronic Materials in December 2009.