The field of automotive electronics puts more emphasis in realizing high precision in the prediction of lifetime by having an understanding of the behavior of solder material, the effects of package’s warpage, and environmental conditions, particularly when mounted on electronic control units. A mix of shearing and bending load conditions, which originate from thermal mismatch between the materials and thermally induced warpage, may lead to an enhanced, high multiaxial stress state imposed onto solder connections in contrast to mounting the device on PCBs only where shear stresses may dominate the loading condition.
According to the theory, failure or damage in ductile materials can be associated with volume change of microscopic voids and their coalescence that are enhanced under hydrostatic tension. This takes place before the damage can be identified on a macroscopic scale. The dependency of these aspects on the tensile hydrostatic stress and effects on the ductility of materials can be linked to magnitude of dimensionless triaxiality factor.
In view of a number of literature sources, triaxiality factor is among the most critical factors that affect the initiation of ductile fracture. It has been reported extensively that the service life of ductile materials decreases under hydrostatic tension and high triaxiality values. It has also been recently reported that compressive hydrostatic stress state with high shearing load and low triaxiality values contribute to failure.
Marta Kuczynska and colleagues at the Robert Bosch GmbH in Germany presented an overview on the role of the stress state as well as stress triaxiality factor in solder joints lifetime prediction. They applied the principle of strain increment modification in order to understand the effect of multiaxial loads in an array of triaxiality-regimes in application of electronic packages plastic ball grid array (PBGA) and loss free packaging (LFPAK). Their research work is published in journal, Microelectronics Reliability.
The authors investigated two packaging types, the LFPAK and the PBGA through the finite element simulation on Board- and System-level. They then compared the damage prediction with experimental results. In the two packaging solder joints, the regimes of hydrostatic tension as well as compression in the course of the temperature cycles were investigated and compared to the distribution of equivalent von Mises stress, triaxiality factor, and stress intensity. They included multiaxial effects in both, fracture location and lifetime prediction.
For the case of LFPAK, there was a strong sensitivity to the PCB bending. This was correlated to a particular crack propagation near the IMC layer. Contrary to the free expanding PCB used in the laboratory tests, on the system level excessive bending was observed, where considerable changes in the location of crack initiation as well as its propagation path were observed.
In the case of PBGAs, the position of crack initiation remained unchanged for experiments on the package positioned on the free-expanding PCB, and when the package was mounted in a system-like set-up. In addition, both packages showed a high level of stress multiaxiality. For the LFPAK, the correction of multiaxial stress state through the triaxiality factor enabled the numerical localization of the damage initiation and the simulation results correlated well with the damage propagation observed in cross-sections.
M. Kuczynska, N. Schafet, U. Becker, B. Métais, A. Kabakchiev, P. Buhl, S. Weihe. The role of stress state and stress triaxiality in lifetime prediction of solder joints in different packages utilized in automotive electronics. Microelectronics Reliability, volume 74 (2017), pages 155–164.
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