Sn-Ag alloy system has been regarded as one of the most promising lead-free solder to substitute conventional Sn-Pb eutectic solder. But the formation of bulk Ag3Sn intermetallic compounds (IMCs) during reflow and post heattreatment significantly influences the performance of the solder joints. With an effort to clarify its microstructuralevolution as a function of slow cooling rates, the fraction of bulk IMCs within the slowly solidified Sn-4.0 wt pctAg solder was investigated by standard metallographic and compared with that detected by thermal analysis. It wasfound that the bulk IMCs fraction determined by thermal analysis corresponds quite well with the microstructureobservation results. In accordance with the conventional solidification theory, the lower the applied cooling rate, thefewer the amount of bulk Ag3Sn IMCs formed in Sn-4.0 wt pct Ag alloy. In addition, Vickers hardness measurementresults indicated that the relative coarse eutectic Ag3Sn IMCs distributing in the lamellar eutectic structure favoredthe improvement of the mechanical performance. Sn-Ag alloy system has been considered as one of the most promising lead-free solder to conventional conventional Sn-Pb eutectic solder. But the formation of bulk Ag3Sn intermetallic compounds (IMCs) during reflow and post heat treatment significant influences the performance of the solder joints. With an effort to clarify its microstructuralevolution as a function of slow cooling rates, the fraction of bulk IMCs within the slowly solidified Sn-4.0 wt pctAg solder was investigated by standard metallographic and compared with that detected by thermal analysis. It wasfound that the bulk IMCs fraction determined by thermal analysis quite well with the microstructureobservation results. In accordance with the conventional solidification theory, the lower the applied cooling rate, the amount of bulk Ag3Sn IMCs formed in Sn-4.0 wt pct Ag alloy. Vickers hardness measurement results indicate that the relative coarse eutectic Ag3Sn IMCs distributing in the lamella r eutectic structure favoredthe improvement of the mechanical performance.