Electro-Migration And Material Transport In Solder Joints
Authors: Mathias Nowottnick and Andrej Novikov, and Andreas Fix Company: University of Rostock and Robert Bosch GmbH Date Published: 10/16/2011
Abstract: Above the critical recrystallization temperature, which is about 0.4 homologue temperature for metals and metal alloys, the mobility of atoms increases rapidly also for solid bodies. The homologue temperature of common solder joints, typically based on tin, is higher than 0.6 already at room temperature and more than 0.8 for high operating temperatures of 125°C, e.g. for automotive application. The transport of atoms can be caused by electrical fields or the directed flow of electrons in a metal. Such electro-migration effects are well known and investigated for chip metallization and wire bonds. The current density could be extremely high, because of the small size of the cross sectional area of wire bonds, made of aluminum or gold. Current densities up to 106 A/cm² are possible, which can cause a migration of metal atoms toward the electron flow. In comparison with wire bonds is the homologue temperature of solder joints much higher, so migration effects could be expected already for the average current density of 104 A/cm² depending on temperature. But also the natural electrical potentials of different metals can cause significant field strength, e.g. between copper and nickel metallization in the same solder joint. This contribution shows the test results of solder joints with different metallization and electronic assemblies, stressed with different current densities at 125°C ambient temperature. A metallographic analysis shows the causing failure effects and weak spots of assemblies.
electromigration, flip-chip, current density, reliability