Effect of solidification cooling rate on Ag3Sn formation and its morphological appearance in Sn-Ag based lead-free solder was investigated. Depends on cooling rate, three types of Ag3Sn compound with different morphologies may form by solidification. They are particle-like, needle-like, and platelike respectively. Small particle-like Ag3Sn in large amount was occurred by rapid cooling, where plate liked Ag3Sn was formed by slow cooling condition. Extremely slow cooling such as furnace cooling results large plate-like or pillar-like Ag3Sn to form, particularly in solder matrix adjacent to the solder/Cu interface. The enrichment of Sn in Cu 6Sn5 IMC layer formation at interface causes Sn depletion at the adjacent area near the interface. As a result, Ag atoms were enriched relatively which favored formation and growth of primary Ag3Sn in liquid phase.