The interfacial reactions between In49Sn solders and Ag thick films at temperatures ranging from 200°C to 350°C have been studied. The intermetallic compound formed at the Ag/In49Sn interface is Ag2In enveloped in a thin layer of AgIn2. Through the measurement of the thickness decrease of Ag thick films, it has been determined that the reaction kinetics of Ag2In has a linear relation to reaction time. Morphology observations indicated that the linear reaction of Ag2In was caused by the floating of Ag2In into the In49Sn solder as a result of the In49Sn solder penetrating into the porous Ag thick film. A sound joint can be obtained when a sufficient thickness of the Ag thick film (over 19.5 µm) reacts with the In49Sn solder. In this case, the tensile tested specimens fracture in the In49Sn matrix.