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76 SMT007 MAGAZINE I SEPTEMBER 2024 Stages of Sn-Bi Alloy Electromigration e stages of Sn-Bi electromigration are illus- trated in Figure 4 5 . During the first stage, the alloy electrical resistance takes a dip because of two phenomena, both of which were explained earlier. One is the microstructural evolu- tion towards equilibrium from point b on the phase diagram to point c (Figure 1), decreas- ing the Bi content of the Sn-rich phase matrix and therefore making it more conductive. e other is the microstructural coarsening, shown in Figure 5, with the larger Bi-rich phase parti- cles growing at the expense of the finer Bi-rich phase particles, making the alloy electrically more conductive by providing straighter paths for the electron to traverse from cathode side to the anode. e second stage—the linear stage—of Sn-Bi electromigration occurs when the Bi-rich phase particles' growth mechanism subsides, and Bi begins to form a near continuous layer at the anode end of the solder joint structure, dominating the electrical resistance of the sol- der joint. During this stage there is a near-orig- inal microstructure feeding the Bi atoms accu- mulating at the anode end. e steady sup- ply of Bi atoms results in a near-linear rise in the alloy resistance with time. e third stage, the parabolic stage, occurs when the cathode region feeding Bi atoms to the anode side gets e other micrographs in Figure 3 show what is beneath the top surface of the specimen. Fig- ures 3b, 3c, and 3d are micrographs of the sur- faces revealed when 1.5 and 10 µm of the sol- der was ground and polished away from the surface. e temporal decrease of the area fraction of the Bi-rich phase at the surface of eutectic alloy specimens occurs even though, as explained in the previous section, the overall volume fraction of the Bi-rich phase can only increase with time as the alloy tends towards thermodynamic equilibrium. is observation of the area fraction of the Bi-rich phase decreasing at the free surface of Sn-Bi solder points to the care one must take in drawing conclusions from observing the micro- structure of Sn-Bi alloy at the polished surface. e combination of high homologous temper- ature of Sn-Bi alloys and the supposedly high surface tension of the interface between Bi- rich phase and air compared to that of Sn-rich phase and air causes the Bi-rich phase particles to move away from the free surface of the alloy. Over time, even at temperature as low as the room temperature, the surfaces of Sn-Bi alloys become somewhat depleted of Bi-rich phase particles. But it should be noted and empha- sized that thermodynamics does not allow the decrease of the Bi-rich phase volume fraction due to aging at room temperature. e Bi-rich phase volume fraction can only increase when a Sn-Bi alloy is aged at room temperature. Figure 4: Stages of Sn-Bi electromigration. Figure 5: During the first stage of electromigration, the microstructure coarsens, from that shown in (a) to that shown in (b). In the coarsened microstruc- ture, the electron current finds a straighter path in the more conductive Sn-rich phase matrix to flow from the cathode to the anode. The solder joint resistance, thus, takes a dip in the first stage of electromigration.