For double-sided assemblies, the solder joints on the topside of the board are inverted and reflowed again. During the second reflow, the components are held in place by the surface tension, which may prevent the components from failing off under the gravitational force. A method is needed to determine a component's candidacy for bottom-side attachment based on the component weight and total pad area. In this paper, a theoretical model was introduced to determine the critical value for component fall-off during the second reflow. Design of Experiments (DOE) and ANOVA analysis for lead-free solder boards were performed to examine the main process factors which have different effects on the component fall-off for different components, and comparison was made between lead-free and SnPb solders. Optical inspection and cross-sectioning were carried out for further investigation. The test results indicated no significant difference of C-g/P-a value between SnPb and lead-free solders.
