Other Properties of Sn-3.5Ag

Young's Modulus vs. Temperature

Figure 22: Plot of Sn3.5Ag Young's Modulus E (MPa) versus temperature T (°K).

Young's modulus versus temperature is plotted in Figure 22, similar to Figure 9 in Wiese et al., 2001. The original source of the data was publications by Darveaux et al. (1995) and Lau & Pao (1997). The data was digitized and, for each dataset, we added a linear trendline and its equation so the temperature-dependence of Young's modulus is readily available. The temperature-dependent Young's modulus is entered 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 of use in stress/strain analysis programs or to convert Young's modulus E to a shear modulus, G, is Poisson's ratio. Lau et al. (2002) used a Poisson's ratio n = 0.4 in the finite element modeling of Sn3.5Ag assemblies. We were not able to locate any report with direct measurements or experimental determination of Poisson's ratios for Sn3.5Ag alloy.

Coefficients of Thermal Expansion (CTE)

Table 9: Sn3.5Ag CTE values.

Measured or quoted Coefficients of Thermal Expansion (CTEs) for the Sn3.5Ag alloy are given in Table 9. As expected, the CTE results show a slight temperature dependence. From the data in Table 9, an overall average value for the CTE of Sn3.5Ag is about 21.5 ppm/°C.

For comparison purposes, a value that is often quoted for the CTE of eutectic SnPb is 24 ppm/°C. The CTE of Sn-3.5Ag appears to be slightly lower than that of eutectic SnPb. This is beneficial to solder joint reliability, in general, since a lower CTE of the solder alloy reduces local CTE mismatches between the solder joint and the interconnected parts.

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:

• Solder density, in lb/in³ (or g/cm³).

• (g/cm³). = 7.5 (NIST Boulder database).

• Note that an error seems to have propagated through the literature since both the ITRI publication (No. 656) and the GE / DeVore handbook quote = 10.38 g/cm³ for Sn-3.5Ag.
• The NIST-quoted value is consistent with the rule of mixture calculation: = 96.5% _Sn + 3.5% _Ag, where the density of pure Sn and Ag are _Sn = 7.31 g/cm³ and _Ag = 10.5 g/cm³, gives: _Sn-3.5Ag = 7.42 g/cm³.
  

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

• = 431 mNm^-1 in air, g = 493 mNm^-1 in nitrogen (at 50°C above liquidus), after Glazer (1994) & quoted in NIST-Boulder database.

• Thermal conductivity, k in units of W/m°K, is also used for heat transfer analysis: NA.

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