Identification of essential factors causing solder bridging of right-angle solder interconnects in laser jet solder ball bonding process

Solder bridging is a commonly seen and serious processing defect in electronic packaging, which may lead to short circuit even absolute failure of electronic devices and products. Solder bridging may occur in all types of solder interconnects and in each of soldering processes during manufacturing journey, for example, in ball grid array solder interconnects and three-dimensional (3D) interconnects, as well as in wave soldering and reflow soldering assemblies. With the increasing trend of electronic products towards miniaturization and multifunction, the pitch and size of solder interconnects have been scaling down. The finer pitch and decreased size of solder joints greatly increase the difficulty of soldering process and the solder bridging defect is more likely to appear. Notably, the laser jet solder ball bonding (LJSBB) with the advantages of localized heating and higher energy inputting provides a novel soldering technology for 3D packaging, for instance, the right-angle solder interconnects and temperature-sensitive components. When fabricating the right-angle solder interconnects, the liquid solder ball is blown by the protected N2 flow and hardly stays firmly in place during the LJSBB process, thus the solder bridging occurs more often in the LJSBB process. Under such circumstances, it is necessary to identify the essential factors causing solder bridging of right-angle solder interconnects and seeking the improvement measures for mass production. In this study, the crucial factors causing solder bridging of right-angle Au/Sn-3.0Ag-0.5Cu/Au interconnects during LJSBB process have been investigated and identified. The results demonstrate that the poor wetting and the large displacement are the main reasons causing the solder bridging defect. The composition analysis and surface morphology characterization results manifest that the contamination of Au bonding pads and a thicker oxidation layer on the surface of solder balls significantly prolong the time of the metallurgical reaction between the solder and Au pads, which results in poor wetting of the molten solder and consequently the solder bridging defect. The verification experiments indicate that a large displacement obviously increases the waving probability of the liquid solder ball. Thus, the solder bridging defect occurs more easily. In addition, the impurities attached onto the nozzle can occasionally lead to the solder bridging defect in mass production.