Growth of an intermetallic compound (IMC) plays a critical role in the reliability of flip-chip solder joints. It has been found that IMC growth is accelerated on the anode and inhibited on the cathode during electromigration (EM), although there are discrepancies in the relevant literature. In this study, Cu/Sn3.0Ag0.5Cu/Cu solder joints were examined by EM under a current density of 1 × 104 A/cm2 at 150 °C; an aging test was conducted at 150 °C to compare the EM results. Cu atoms are the dominant diffusion species that migrate from the cathode to the anode during EM. The increase in IMC thickness at the anode is divided into two stages: during stage 1, the IMC thickness first grows with a gradual linear tendency, followed by retarded IMC growth. During stage 2, the EM driving force dominates IMC growth because of the high IMC thickness after stage 1, inducing a lower Cu concentration gradient and decreasing the driving force of the chemical potential. At the cathode, the IMC thickness fluctuates between two threshold values. This phenomenon is attributed to the competition between the effects of chemical potential and EM on IMC growth. In the initial stage, the chemical potential is sufficiently large to induce considerable Cu flux from the Cu underbump metallization into the Sn solder, inducing IMC growth. When the IMC becomes too thick to significantly reduce the chemical potential, a substantial number of Cu atoms migrate via EM, reducing the IMC thickness at the cathode. However, when IMC thickness decreases, the chemical potential again increases and enhances IMC growth. Therefore, thresholds 1 and 2 may be set based on changes observed in the IMC thickness at the cathode during EM. These findings elucidate the polarity effect in intermetallic compounds, and its effect on the stability of solder joints.