The effects of solder pastes and infra red reflow temperature profiles on the characteristics of porosity in solder joints are described. Porosity in solder joints can be detected by X-ray radiography. It was found that the composition and structure of solder pastes had the most significant effect on pore formation. However, a lower metal content and/or a higher heating rate did not necessarily cause a higher percentage of pores in solder joints. Results of experiments on pore formation processes during the whole infrared reflow soldering cycle show that high area fraction of pores in solder joints correspond to the peak temperatures in infrared reflow temperature profiles. To evaluate the thermal effect on the performance and structure of solder pastes, tests were also conducted using differential scanning calorimetry, thermogravimetric analysis (TGA) and weight loss in infrared reflow. It was found that the weight loss rate in the TGA curve and infrared reflow and floating speed of porosity are useful to predict the pore formation behaviour in the solder joint. It is recommended that an IR reflow process is chosen to fit with a suitable solder paste in order to decrease porosity in surface-mount solder joints.