Other "SAC" Properties

Young's Modulus vs. Temperature

Figure 34: Plot of SAC Young's Modulus E (MPa) versus temperature T (°K).

Young's modulus is given versus temperature in Table 15 and the data is plotted in Figure 34. For the two Sn-Ag-Cu alloys, the data was digitized from Figure 2 in Schubert et al., 2001. For Sn and Castin, the data was obtained from equations given in Kariya et al., 2001, and Neu et al., respectively. For each dataset, a linear trendline and its equation are shown in Figure 34.

The temperature-dependent Young's modulus is used as material properties in stress / strain analysis programs such as FEA codes. Young's modulus is also used to scale steady-state stresses in some creep rate models.

Poisson's Ratio

Another elastic property that is also used in stress/strain analysis programs or to convert Young's modulus to a shear modulus, G, is Poisson's ratio. Neu et al. reported a Poisson's ratio = 0.4 for Castin (after Whitelaw et al., 1999), however, we did not find literature values of Poisson's ratios for the other SAC alloys.

Coefficients of Thermal Expansion (CTE)

Table 16: CTE literature data.

Measured or quoted CTEs are given in Table 16. The CTE values given by Schubert et al. for the SAC alloys are lower than the CTE of 23.5 ppm/°C that is quoted for pure tin. The CTE of the Castin alloy is higher at 26.9 ppm/°C. A typical value quoted for the CTE of eutectic Sn-Pb is 24 ppm/°C.

Other Physical Properties

Other properties of interest for predictive modeling of solder joint geometry using, for example, a computer program such as Surface Evolver, are:

• The solder density, in lb/in³ (or g/cm³). Values quoted in the NIST-Boulder database are:

• (g/cm³). = 7.39 for Castin; 7.39, 7.44 for Sn-4Ag-0.5Cu; 7.5 for Sn-3.8Ag-0.7Cu (Multicore solder).

• The surface tension, , in units of mNm^-1, to be specified in terms of the soldering atmosphere (e.g., air, nitrogen…).

The thermal conductivity, k in units of W/m°K, is also used for heat transfer analysis:

• k = 57.26 W/m°K, quoted for Castin in the NIST-Boulder database.

Missing values of the above physical properties will be added to the material property database when the data is available.