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SEPTEMBER 2024 I SMT007 MAGAZINE 83 Figure 12: Arrhenius plot of a solder alloy that can be used to predict the electromigration lifetime of a solder joint using that alloy under any temperature and current density condition. Figure 13: The linear curve was obtained by mea- suring the joint resistance at various chamber tem- peratures at very low currents at which the joule heating was known to be negligible. From the resistance measured when the oven was set at the temperature of interest for the electromigration study (100°C in this example) and the current raised to the current of interest of the electromigration study, the joule heating of the solder joint could be determined as illustrated in this figure. a polished specimen will show decreasing area fraction of the Bi-rich phase as the alloy ages. is seeming anomaly is because the Bi-rich phase with time tends to sink into the bulk, away from the polished surface. e transient resistance change of Sn-Bi sol- der as a result of a step change of current and/ or temperature is due to the Bi con- centration in the Sn-rich phase adjusting to the thermodynamic equilibrium value for the new tem- perature of the solder as per the sol- vus line of the Sn-Bi phase diagram. Electromigration experiments can be designed to collect data suit- able for determining the constants in the Black's equation or for deter- mining the Arrhenius plot using the Nernst-Einstein equation. e for- mer takes the Weibull approach requiring many specimens to obtain the mean time to failure under many different conditions to calculate the constants in the Black's equation; whereas, for the latter, one speci- men is enough to obtain an Arrhe- nius plot that can predict the electromigration life of a solder alloy under any application con- dition, though more specimens would improve the lifetime prediction. Figure 14: Arrhenius plot of Bi electromigration in eutectic Sn-Bi solder using the planar and the BTC approach.