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Review Article

Appl. Mech. Rev. 2017;69(4):040801-040801-14. doi:10.1115/1.4037177.

In flat rolling, the lateral position of the product in the rolling mill and the camber (curvature of the product centerline seen in top view) are key process variables. We explore how their evolution can be analytically modeled based on nonlinear geometric relations, material derivatives, balance equations, constitutive equations for the material flow in the roll gap, and a change of coordinates to obtain a time-free formulation. Based on example problems, we verify the developed novel model and further illustrate the mechanisms behind it. Finally, a literature review on models in this field reveals that there is not yet a consensus on the correct analytical model of the evolution of the camber in flat rolling. The literature review shows that most published models are special cases of the model developed in this paper.

Commentary by Dr. Valentin Fuster
Appl. Mech. Rev. 2017;69(4):040802-040802-15. doi:10.1115/1.4037462.

Due to the restriction of lead-rich solder and the miniaturization of electronic packaging devices, lead-free solders have replaced lead-rich solders in the past decades; however, it also brings new technical problems. Reliability, fatigue, and drop resistance are of concern in the electronic industry. The paper provides a comprehensive survey of recent research on the methodologies to describe the mechanical behavior of lead-free solders. In order to understand the fundamental mechanical behavior of lead-free solders, the visco-plastic characteristics should be considered in the constitutive modeling. Under mechanical and thermal cycling, fatigue is related to the time to failure and can be predicted based on the analysis to strain, hysteresis energy, and damage accumulation. For electronic devices with potential drop impacts, drop resistance plays an essential role to assess the mechanical reliability of solder joints through experimental studies, establishing the rate-dependent material properties and proposing advanced numerical techniques to model the interconnect failure. The failure mechanisms of solder joints are complicated under coupled electrical-thermal-mechanical loadings, the increased current density can lead to electromigration around the current crowding zone. The induced void initiation and propagation have been investigated based on theoretical approaches to reveal the effects on the mechanical properties of solder joints. To elucidate the dominant mechanisms, the effects of current stressing and elevated temperature on mechanical behavior of lead-free solder have been reviewed. Potential directions for future research have been discussed.

Commentary by Dr. Valentin Fuster

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