Temperature Gradient Induced Orientation Change of Bi Grains in Sn-Bi57-Ag0.7 Solder Joint

Sn-Bi-X solders are widely used in electronic packaging industry. However, thermomigration (TM) behaviors of Sn-Bi-X solder joints and the orientations change of Bi grains under the temperature gradient are rarely reported. In this study, Sn-Bi57-Ag0.7/Cu solder joints were used to conduct a TM test under a temperature gradient of 625 degrees C/cm for 400 h, and an isothermal aging test at 85 degrees C was also conducted for comparison. The microstructural evolution of Sn-Bi-X solder joints after reflow, TM and isothermal aging were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis (EPMA). The results indicated that the Sn/Bi areal ratio after TM did not change significantly whether at the hot end (from 46.78%/52.12% to 50.90%/48.78%) or at the cold end (from 50.25%/49.64% to 48.71%/51.16%) compared with that of as-reflowed samples due to the insufficient thermal energy. The thickness of intermetallic compound (IMC) after TM at hot end (2.49 mu m) was very close to that of the IMC at cold end (2.52 mu m), which was also close to that of the aged samples. In addition, the preferred orientations of Sn and Bi grains in different Sn-Bi-Ag solder joints resulting from different conditions (reflow, TM and isothermal aging) were characterized by electron backscatter diffraction (EBSD). The obtained results demonstrated that both Sn and Bi grains had no preferred orientation whether after reflow or isothermal aging, while the orientation of Bi grains of the sample after TM changed from random direction to c-axis ([0001] direction) parallel to the heat flow. Ag3Sn could hinder the change of orientation of Bi grains under the temperature gradient, and the corresponding mechanism was also systematically illuminated. This study firstly revealed the orientation change of Bi grains under the temperature gradient, which would have a profound guiding significance for enhancing the reliabilities of Sn-Bi-Ag solder joints.

Automated summary

This study investigates the thermomigration behaviors of Sn-Bi-X solder joints and the orientation change of Bi grains under the temperature gradient. The results show that thermomigration has little effect on the Sn/Bi ratio and the thickness of intermetallic compound (IMC). However, the orientation of Bi grains changes after thermomigration, and this change is hindered by Ag3Sn. This study provides valuable insights for enhancing the reliabilities of Sn-Bi-Ag solder joints.