Influence of heating direction on BGA solder balls structure

The soldering of BGA packages is well known process. Despite the increasing complexity of PCBs and diversity of BGA package footprints, further optimization and improvement of soldering is required. In this paper, the influence of heat flow direction during reflow soldering on the structure of ball joint was described. The SAC305 solder alloy was used for ball joint investigation under the no-clean solder flux condition. Tested samples were made from FR4 material with ENIG as a surface finish. The heating was generated by infrared heaters placed on the top, bottom and both sides of the sample. The substantial part of this work was focused on the characterization the solder joint structure with the respect to the intermetallic compounds formation. SEM and optical microscope images of cross-sectioned solder joints were used to characterize the quality of the boundary formation. Shear test method was used to evaluate the impact of intermetallic layer and inner solder joint structure on the mechanical properties of joints. This research helps to determine the influence of the direction of heat flux, i.e. direction of heating of the infrared heater used in the reflow soldering, for the formation of the solder ball joint on BGA package (primarily for rework).