This paper mainly describes pore formation characteristics in surface mount solder joints and its effect on joint strength. Two PCB boards with 14-lands on each board are used in shear strength test. Several solder pastes and reflow temperature profiles are chosen for the experiments. Porosity in solder joints is detected by X-ray radiographic inspection and its area fraction measured quantitatively by computer image analysis. From this work, it is found that pore formation has a great effect on the shear strength. Different solder pastes and reflow temperature profiles may cause different pore formation and joint strength of solder joints. The area fraction of pores in tested solder joints varies from 0 to 20%, and their shear strengths change from 40 to 17 MPa correspondingly. The expression, σp = σae-kP, is established to relate the shear strength, σp, to the area percentage of porosity, P, in surface mount solder joints.