Elastic Modulus and Coefficient of Thermal Expansion of 91.84Sn-3.33Ag-4.83Bi Pb-Free Solder

Elastic modulus ( E ) and coefficient of thermal expansion (CTE) properties were measured for 91.84Sn-3.33Ag-4.83Bi solder (wt.%, abbreviated Sn-Ag-Bi) as a function of temperature ( T ) in the range of −50°C to 200°C. Test specimens were in the as-fabricated condition or post-aged for 24 h at 125°C or 150°C. The 91.25Sn-2.25Ag-0.5Cu-6.0Bi alloy was similarly tested, but only in the as-fabricated condition. Dynamic (oscillatory) mechanical testing measured E ( T ). Dilatometry documented CTE versus T . The Sn-Ag-Bi E ( T ) ranged from 48 ± 1 GPa at −50°C to 20 ± 4 GPa at 195°C. The values were insensitive to the aging treatments. The slope, Δ E ( T )/Δ T , of the as-fabricated Sn-Ag-Bi alloy revealed three minima at −41°C, −20°C and 105°C and a valley/peak combination at 146°C. Only the −20°C minimum persisted after the aging treatments, which resulted from short-range ordering between the Bi and Sn atoms. The minima at −41°C and 105°C were attributed to Bi redistribution by re-solutionization and precipitation, while the valley/peak artifact at 146°C was likely caused by non-equilibrium Bi-Sn regions. The Sn-Ag-Cu-Bi E ( T ) ranged from 50 ± 2 GPa at −50°C to 22 ± 3 GPa at 195°C. The Sn-Ag-Cu-Bi alloy did not exhibit the minima and peak/valley behaviors in its Δ E ( T )/Δ T plots. The CTE values of the as-fabricated Sn-Ag-Bi alloy ranged from 14 ± 5 ppm/°C at −50°C to 24 ± 4 ppm/°C at 195°C and were unaffected by the aging treatments. The Sn-Ag-Cu-Bi showed comparable CTE values representing the as-fabricated. These findings allow the Sn-Ag-Bi properties of E ( T ) and CTE( T ) to be considered as surrogates for the Sn-Ag-Cu-Bi alloy in both phenomenological analyses and computational models.