Homologous temperature

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Homologous temperature expresses the temperature of a material as a fraction of its melting point temperature using the Kelvin scale. For example, the homologous temperature of lead at room temperature is approximately .50 (T_H = T/T_mp = 298K/601K = .50).

Solder (m.pt 183°C = 456K) at 0.85Tm or 115°C (= 388K), would thus be expected to have comparable properties to copper (m.pt 1085°C = 1358K) at 0.85Tm or 881°C (= 1154K).

In electronics applications, where circuits typically operate over a –55°C®+125°C range, eutectic tin-lead (Sn63) solder is working at 0.48®0.87Tm. From this we can deduce that solder will have limited mechanical strength (as a bulk material) and be within the ‘creep range’. This is borne out by its comparatively low values for tensile strength, shear strength and modulus of elasticity.

Copper, on the other hand, has a much higher melting point, so foils are working at only 0.16®0.29Tm, and their properties are little affected by temperature.