Meaning
A fatigue life evaluation model relates the plastic strain amplitude to the number of cycles to failure in metals and solder joints. The Coffin-Manson equation predicts low-cycle fatigue, where plastic deformation dominates the fatigue process. This relationship is particularly useful for assessing electronic packages subjected to cyclic thermal loading.
It assumes that the number of cycles to failure decreases as the plastic strain range increases.
Predictive Power
Calculations rely on a power-law relationship between the plastic strain range and the fatigue life. This formula utilizes two materials-specific constants, the fatigue ductility coefficient and the fatigue ductility exponent. For many metals, the exponent lies between negative zero point five and negative zero point seven.
By determining the plastic strain range using finite element modeling, the expected operational life of the solder joints can be calculated.
Frequency Correction
Standard formulations are often modified to include the frequency of the thermal cycles and the maximum temperature. In many industrial applications, the modified version is preferred because it accounts for creep and stress relaxation during the dwell times. Solder joints at high temperatures exhibit time-dependent deformation, making the basic low-cycle fatigue formulation less accurate.
By incorporating frequency and temperature terms, the modified equation addresses the joint damage from creep and fatigue.
Application Boundary
High-cycle fatigue where elastic strain dominates lies outside this model. When the strain levels are low, other formulations like Basquin’s equation must be used instead. The model also assumes a constant temperature during the strain cycle, which is rarely the case in actual electronic boards.
