Homologous temperature, solders

However, it must be appreciated that the fatigue of Pb-free solders takes place at high homologous temperatures. As such, mechanisms... 

If a material is used at a homologous temperature above 0.6, creep deformation is typically significant. Although usage of a material with a Th > 0.6 is usually avoided, the electronic industry uses Sn-based solder at Ty 0.9 (Fig. 1). Thus, the main deformation regime in soft solder joints is creep. 

Because of the high homologous temperatures, the stress-strain relations for tin-based solders are much affected by strain rate except at very low stresses and strains. As already discussed, this presents difficulties in determining the modulus from a stress-strain curve. Fig. 3(a) from Ref 3 gives a set of tensile stress-strain curves for Sn-3.5Ag solder at three different strain rates. The elastic line is also given. Deviation from this line is evident even at = 0.0002. The deviation becomes less as the strain rate is increased. 

The creep behavior of as-cast, Sn-0.5Cu, and several other solders (Sn-3.5Ag, Sn-3.8Ag-0.7Cu) was investigated at 75°C and compared to eutectic Sn-Pb [76]. In terms of stress and time to rupture, the Sn-0.5Cu alloy behaves similar to eutectic Sn-Pb over the range of rupture lives up to 1000 hr. The Ag-containing alloys exhibit a creep resistance which typically is a hundred and a thousand folds greater than observed for Sn-Ag and Sn-Ag-Cu, respectively. The superior creep resistance of Ag-containing alloys is also retained when the alloys are tested at a constant homologous temperature [76]. 

Table 1 lists typical homologous temperature ranges experienced by solders and some engineering alloys during service. Homologous temperature is the ratio of a test or service temperature to the melting temperature, both in absolute temperature. As can be seen from the data in Table 1,... 

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