Experiments were carried out to investigate the effect of reversing the heat flux direction during cooling on the formation of voids during the reflow process. Under different upward and downward solidification conditions, 480 high-lead solder joints of flip-chip assemblies were processed. The solder samples were then microsectioned to determine the size and location of voids. The results show that reversing the flow direction during cooling has a significant effect on the final void formation. For the case of the melting direction from top (flip-chip side) to bottom (test board side), reversing the heat flux direction results in solidification direction from top to bottom. The percentage of defective bumps was found to be 28% and the volume of voids per defective bump was 1.5%. This is the best reflow methodology to minimize voids. Without reversing the heat flux the defective bumps were 80% with 4.0% void volume. In the case of solidification direction/melting direction from bottom to top, the percentage of defective bumps increases from 40% to 51%, accompanying a rise of the volume of voids from 3.0% to 3.7%.