Investigation Of Electromigration Failure Of Solder Joints

Driven by the high-performance, multi-function and miniaturization ofelectronic devices, High-density packing technology has a rapid development in thepast decade. The I/O number on the single chip has increased dramatically, thecurrent density passed the joint reach10⁴A/cm². electromiaration-induced failure inthe solder bumps become an unavoidable issue. the solder consists of only limitednumber of crystal grains and the β-Sn has the anisotropic characteristic. Therefore,the study at the electromigtation reliability has important practical signifsicance.In this paper, the electromigration mechanism of the solder joint weresystematically investigated. Firstly，the failure mechanism of the BGA bumpsunder electric-thermal coupling field and intermetallic compound evolutionmechanism were discussed, then, EBSD technology was employed to investigatethe grain characteristics in the Sn3.0Ag0.5Cu and Sn0.7Cu joints, the relationshipbetween electromigration failure mechanism and grain orientation was alsoelaborated, finally,3D packageing modle was builded to calculate the atoms fluxdivergences of the solder considering electromigration, thermolmigration and stressmigration, the effect of material properties and structure parameters on atoms fluxdivergences was discussed.The results showed that there exsit current crowing in the bump, The growth ofthe IMC had the polarity effect, at the cathode the IMC dissolved rapidly, some bigpits appeared due to the fast migration of Cu and Ni, eventually formed the creakers,lots of dendritic （CuNi）6Sn5gathered at the anode; current density was the keyfactors to the IMC growth at the cathode, when the current density was over thecritical value, EM force accelerated the IMC dissolution, Cu and Ni atoms migratedto the anode, thickness of IMC decreased, when the current density was low, thethickness of IMC increased; stress gradient was builded up in the solder due to thedirectional migration of the atoms, which pushed the Cu atoms away from theanode’s Cu₃Sn/Cu interface and accelerated the formation of kirkendall void at theanode; The separation of the Pb-rich phase and Sn-rich phase was clearly, Pb atomsmigrated in the electrons’ moving direction, while the Sn atoms were piled up in thedownward direction; The degradation mechanisms was closely related to Sn-grain orientation at the interface in multi-grain Sn0.7Cu joints, the Cu plate hollow andcrack at the cathode caused by the rapid dissolution of cathode IMC and Cuinterstitial diffusion to anode lead to electrical failure at an early stage, when the caxis roughly aligned with the electrical current; Single-grain structure with layeredtwinning usually appeared in Sn3.0Ag0.5Cu solders, most of them had c axisroughly at a right angle relative to the current direction, in which the rates of IMCdissolution was very slow. After EM, the number of grain increased with sizereduced, twin orientation relationship weakened and misorientation angledistribution changed uniform. The effect of electromigration was dominatedcompared with thermomigration and stress migration in3D FEA model, theSnAgCu solder seemed more resistant to electromigration failure than the SnPbsolder. The crowing factor and temperature increased when the trace width reduced,the solder were more easier to failure, the maximum current density andtemperature reduced when the size of solder increased. the solders were more stableunder EM stressing.