Meaning
Permanent strain that accumulates over time under a constant mechanical load belongs to the category of rate dependent plastic flow. Engineers analyze creep deformation to predict the long term structural integrity of solder joints exposed to sustained stresses at elevated operating temperatures. The process occurs at high homologous temperatures, typically above half the melting point of the alloy in kelvins, where atomic diffusion becomes highly active.
It continues until the material achieves rupture or the cross section can no longer support the applied load.
Physical Mechanism
Thermally activated movement of dislocations and vacancies governs the progressive strain rate within the crystalline lattice. This movement enables the metal to deform at stress levels far below its yield strength as thermal energy assists the mechanical load in overcoming structural barriers. In lead free tin alloys, the phenomenon often starts at room temperature due to the low melting point of the metal.
Structural Damage
Microscopic voids form along the grain boundaries as atoms migrate away from regions of high compressive stress toward regions of high tensile stress. These vacancies eventually coalesce into microcracks that weaken the bulk material. This structural degradation decreases the load bearing capability of the assembly.
Asymmetric Behavior
Low stress levels produce a linear relationship between strain rate and stress, whereas higher stresses cause an exponential increase in the rate of flow. The rate also depends heavily on grain size, with fine grained alloys flowing more rapidly due to the greater density of paths for atomic diffusion. Large grain structures offer superior resistance to this slow structural collapse.
