Nucleation and electromigration-induced grain rotation in an SABI333 solder joint

Abstract

Single-grain or tricrystal joints are often observed in Sn-based Pb-free solder alloys, such as Sn3.5Ag and Sn3.0Ag0.5Cu, and Sn dendrites tend to grow in the [110] and [1 1 ¯ 0 ] directions in these solder joints. In this study, electron backscattering diffraction was used to quantitatively characterize the grain orientations of a reflowed line-type Sn3.0Ag3.0Bi3.0In (SABI333) solder joint. The SABI333 solder joint was composed of polycrystals, and polycrystals with misorientations of more than 15° were separated by high-angle grain boundaries. It was reasonable to assume that these orientations arose from Sn dendrite deviations from the preferred directions. In addition, double twinning of Sn was observed in the SnAgCu solder alloy, and a polycrystal SABI333 solder joint was formed by spreading from the predominant double tricrystals (with five orientations in the preferred [110] and [1 1 ¯ 0 ] growth directions). Additionally, during electromigration (EM), the near-pad-interface small grains separated by high-angle grain boundaries in the SABI333 solder joint tended to rotate, and the rotation caused the c-axis to be perpendicular to the current direction. Furthermore, no detectable segregation of Ag or increase in the thickness of the interfacial intermetallic compounds (IMCs) occurred in the SABI333 solder joint after EM for 168 h. These results confirm that the Sn grain orientations in the joints significantly influence the reliability of the solder joints under service conditions that promote EM and that polycrystal SABI333 solder joints may have longer EM service lifetimes than Sn3.5Ag or Sn3.0Ag0.5Cu joints.