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A precise creep tester was established to in situ measure the micro strain of the solder under different temperatures and certain loads. The ball grid array samples with the diameter of 0.76 mm Sn-3Ag-0.5Cu (SAC305) solder bumps were tested under the shear strength ranged from 7 MPa up to 14 MPa and the heating temperatures from 24°C up to 100°C. Creep constitutive equation of the steady-state creep, together with its stress exponent n, creep activation energy Q and structure constant A were determined by employing the power law creep. The coarsening Ag3Sn particles were observed with the temperature rising up under a certain load.