Power modules are packages that contain various semiconductor devices, including MOSFETs, IGBTs, and diodes, and are widely used as a basic component for supporting electrical infrastructure, including power conversion devices, industrial equipment, railways, and automobiles. Thermal strains occur at the solder joints of power modules due to differences in the linear expansion coefficient between each member resulting from temperature changes that accompany the power being turned on and off. It is known that this repeated thermal strain causes fatigue cracks to occur and propagate in the solder joints, ultimately leading to failure. Fatigue life can range anywhere from 10,000 to 1 million cycles. The Manson-Coffin law based on the inelastic strain range has been used as a method for estimating the fatigue life of solder materials. However, the Manson-Coffin law has been applied mainly to low-cycle fatigue with a life of 10,000 cycles or less and may not be applicable for lives of 10,000 to 1 million cycles. In recent years, a fatigue life law based on inelastic strain energy density as proposed by Morrow has been applied to solder materials. Considering that strain energy is required for the initiation and propagation of fatigue cracks, accumulated damage can be understood more accurately compared with strains that have no physical meaning.

In this study, the effectiveness of the fatigue life law based on inelastic strain energy density was compared with the conventional law based on inelastic strain range.

First, the fatigue properties of Sn-Ag-Cu solder alloy were investigated by torsional fatigue test with strain control. It was found that the stress–strain hysteresis loop arising from inelastic deformation occurred even under a low strain load with a fatigue life of about 1 million cycles. Therefore, as an extension of the low-cycle fatigue test, evaluation was made using inelastic strain range and inelastic strain energy density.

Experimental results show that when fatigue life was evaluated in the inelastic strain range, the fatigue ductility exponent differed between the low-cycle region and high-cycle region after tens of thousands of cycles and was large in the low-cycle region and small in the high-cycle region. However, when fatigue life was evaluated using inelastic strain energy density, a single power law was found over a wide range from the low-cycle region to the high-cycle region, and the validity of the fatigue life law based on inelastic strain energy density was confirmed.

Next, a simple prediction method for the fatigue life law based on inelastic strain energy density was examined by taking the physical background into account.

Two material constants of the fatigue life law based on the inelastic strain energy density were estimated from the stress–strain curve for a monotonic load. The fatigue coefficient corresponding to the intercept was correlated with the inelastic strain energy density at breakage, and the fatigue exponent corresponding to the slope was correlated with the inelastic strain energy density due to work hardening, which is effective for fatigue damage. The above approach showed that the two material constants estimated from the stress–strain curve for a monotonic load were very close to the fatigue test results.


Fatigue Life Law Based on Inelastic Strain Energy Density of Solder Alloys and its Simple Prediction Method

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Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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