The thermosonic bonding was investigated as a fluxless solder bump flip chip bonding method. The 100 ㎛-diameter Sn-3.5 mass%Ag solder bumps were formed on test flip chip by laser ball bonding process. The test flip chips were bonded to a TSM-coated glass substrate using thermosonic bonding method at different temperature, bonding load and ultrasonic power. The die shear strength was evaluated and fracture surfaces were examined with SEM. The Sn-3.5Ag solder flip chip bonding was possible at lower temperature than the melting point of the solder. The die shear strength increased with increasing bonding temperature, bonding load, and ultrasonic power. However, at excessive bonding load condition over 1.0 N/bump, the die shear strength decreased due to the fracture at UBM/ Si-chip interface. The bump height decreased with increasing bonding load but did not change with ultrasonic power.