Meaning
Failure mode in materials subjected to simultaneous cyclic temperature changes and constant mechanical stress. Thermal fatigue creep occurs in power electronics and engine components where the differing expansion rates of bonded materials create internal strain. Over time, this combination of stressors leads to the slow deformation of the material, followed by the growth of micro-cracks that compromise the joint.
Mechanism Analysis
Plastic deformation accumulates during the dwell times at high temperatures when the material is most susceptible to flow. In the context of thermal fatigue creep, the mechanical strain is not fully reversed when the temperature drops, leading to a permanent change in the shape of the solder or sinter layer. The damage is cumulative, with each thermal cycle adding to the total strain until the material reaches its rupture point.
Material Selection
Alloys with high recrystallization temperatures provide better resistance to this type of degradation.
Testing Standard
Engineers use power cycling tests to accelerate the aging process and determine the expected life of the device. These specialized protocols fluctuate the current through the silicon chip to generate internal heat while maintaining a fixed ambient temperature in the test chamber. This data tracks the progression of thermal fatigue creep before total failure.
