In-Line Test Structures and Non-destructive Characterization of Electromigration-Driven Phase Evolution in Microscale Solder Joints

Power efficiency and performance requirements are driving the trend towards smaller interconnects in 3D packaging. Also, significant ongoing effort is aimed at finding alternatives to SnAgCu (SAC) solder alloys, either for low-temperature reflow or high reliability. Therefore, there is a need to characterize such alloys at dimensions relevant to their applications. Typically, such characterization involves testing the devices, followed by cross-sectioning and polishing to observe failure mechanisms. The drawback of this method is that the material removal due to polishing may reduce the amount of data that can be obtained. Also, cross-sectioning the devices renders them unusable for further testing, thus creating a discontinuity in the experiments. In this study, a procedure is outlined for fabricating devices termed in-line microjoints, featuring multiple Cu–solder–Cu junctions in either a straight line or a serpentine arrangement, enabling non-destructive characterization of the solder. Using this procedure, devices can be made with any commercially available solder paste, which enables testing of various alloys. The electromigration of Bi in SnBi low-temperature solder is characterized non-destructively and the diffusivity of Bi is reported at 75°C, 90°C, and 105°C. The reported diffusivity values align with the existing literature on Bi diffusivity. We also report a transient phenomenon in SnBi solder joints, where the initial rate of resistance change is non-linear, followed by a transition to a linear rate. Lastly, tests on the serpentine structures show lower Bi accumulation in regions where current crowding is expected, which is counter-intuitive.