Non-destructive micro analysis of electromigration failures in solder microbumps using 3D X-ray computed tomography

Three-dimensional (3D) X-ray was employed to characterize the electromigration (EM) induced failures (voids, cracks, and necking) in 30 mu m Cu/Sn/Ni/Cu solder joints. The 3D X-ray is capable of mapping the positions and evolution of voids and morphology change in the solder joints caused by EM. Such an approach offers an alternatively non-destructive assessment for the reliability of large-area solder arrays. The EM-induced failures of hundreds of solder joints could be captured and characterized. The results show that large voids mostly formed near the entrance of electron flow. Two-step change of solder electrical resistance was found: a gradual increase (10.6% after 5.1 h) at the beginning EM stage due to slow nucleation of newly individual voids and a sharp increase (10.6% to 22.2% within 0.8 h) caused by the significant growth and coalescence of multiple voids and necking. We found that the total number of solder joints with obvious voids in plan-view CT images was larger than that in side-view micrographs indicating their better practical implementation. Additionally, we found that the total contact area of the solder joints gradually decreased due to solder necking after EM tests. It further expanded and merged with neighboring voids to generate cracks near solder edges.