Degradation Mechanisms Of Chip Scale Package Solder Interconnects By Electrical Loading

In microelectronic packaging, components are often mounted on a substrate throughthe solder interconnection that provides both mechanical support and electricalconnection. As the trend of miniaturization and high perfomance in electronic productsgoes forward, the package density increased continuously and the size of the solderconnection becomes smaller and smaller. The miniaturization of the solder connectionwill inevitablly place challenges on the reliability of the solder connection.Electromigration damage was found in solder joints by creating voids, hillocks andinterfacial failures. The microstructure defect including vacancies and voids may affectmechanical properties of solder joints. Meanwhile, the electric current in the solderintereconnects generated great joule heat and the temperature of components is raised. Asa result, the solder joints are subjected to both electric field and thermal field. In thestudy, we investigated the failure of CSP solder interconnects under combined effect ofelectromigration and thermomigration as also.One kind of commercial CSP interconnects was used for the experiment,Cross-sectional microstructural evolution of Sn-3.8Ag-0.7Cu solder joints, mechanism ofthe polarity effect,and the failure of Al trace were studied. After direct electric was appliedto the SnAgCu solder joints in the flip-chip configuration, the temperature of chip side wasnotably higher than PCB side and a huge thermal gradient exsisted in solder joints. Underelectromigration, the Sn and Cu atoms in the solder diffused from the cathode to the anode,however, under thermomigration, Cu atoms diffused to the PCB side while Sn atomsdiffused to the chip side. The atoms diffusion was enhanced if the direction of both drivingforces were the same. The atoms diffusion was inhibited by decreasing the temperature andthermal gradient in the solder joints, as a result the failure of the solder interconnects wassuppressed. the electromigration of Al trace led the temperature of CSP interconnects risesharply, as the primary responsible for melting solders. Take Al trace replacement for Cutrace, electromigration experiment shows that the polarity effect appeared in thecross-sectional of solder joint, electromigration of the soders lead to melting the balls.