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78 SMT007 MAGAZINE I SEPTEMBER 2024 Transient Behavior of Solder Joint Resistance During Step Change in Temperature and/or Current Figure 8a shows a typical behavior of a sol- der joint resistance during step changes in tem- perature and/or current. As an example, at the one-hour mark, the current through a solder joint in an oven at 60°C is suddenly raised from 0.1 to 1.47 A. ere is a steep rise in the sol- quite depleted of the Bi-rich phase particles as shown in Figure 6. e three stages of electro- migration are shown graphically at constant current density and temperature for some sim- ilar solder joint specimens in Figure 7. Figure 6: The third stage is when a continuous layer of Bi-rich phase has formed on the anode side and has grown with time to such an extent that the region near the cathode feeding Bi to the anode has been quite depleted of the Bi-rich phase. The lighter phase in the scanning electron micrograph is the Bi-rich phase that has accumu- lated at the anode side of the solder. Figure 7: Resistance change of various specimens at a constant current density and constant temper- ature showing the three stages of electromigration. Figure 8: a) Transient behavior of eutectic Sn-Bi solder resistance due to the shape of the solvus line; b) Sn-Bi equilibrium phase diagram.