Influence of Cu column under-bump-metallizations on current crowding and Joule heating effects of electromigration in flip-chip solder joints

Abstract

The electromigration behavior of SnAg solder bumps with and without Cu column under-bumpmetallizations (UBMs) has been investigated under a current density of 2.16 Â 10 4 A/cm 2 at 150 C. Different failure modes were observed for the two types of samples. In those without Cu column UBMs, when SnAg solder bumps that had implemented 2 lm Ni UBMs were current stressed at 2.16 Â 10 4 A/cm 2 , open failure occurred in the bump that had an electron flow direction from the chip side to the substrate side. However, in those with Cu column UBMs, cracks formed along the interface of Cu 6 Sn 5 intermetallic compounds and the solder on the substrate side in the Sn-3.0Ag-0.5Cu solder bump that had an electron flow direction from the substrate side to the chip side. A three-dimensional simulation of the current density distribution was performed in order to obtain a better understanding of the current crowding behavior in solder bumps. The current crowding effect was found to account for the void formation on both the chip and the substrate side for the two kinds of solder bumps. One more important finding, as confirmed by infrared microscopy, is that the alleviation of current crowding by Cu column UBMs also helped decrease the Joule heating effect in solder bumps during current stressing. Therefore, the measured failure time for the solder joints with Cu column UBMs appears to be much longer than that of the ones with the 2 lm Ni UBMs. V

Formation Mechanism of Porous Cu3Sn Intermetallic Compounds by High Current Stressing at High Temperatures in Low-Bump-Height Solder Joints

“…The intermetallic compounds (IMC) of Cu 3 Sn and Cu 6 Sn 5 are commonly formed in solder reactions on Cu under-bump-metallization (UBM). The Cu column-type UBM (a structure with a thicker Cu) was developed to alleviate both the Joule heating and current crowding effects in flip-chip solder joints under normal device operating conditions, having a longer lifetime [10].…”

Electromigration in microbumps with Cu-Sn intermetallic compounds

“…This phenomenon was barely found in the electromigration tests when solder was not consumed completely. [8][9][10][11] The mechanism of forming porous Cu 3 Sn was due to the phase transformation: Cu 6 Sn 5 →2 Cu 3 Sn+3 Sn. In the Sn-lack system, the Cu 6 Sn 5 would transform into Cu 3 Sn and released the Sn atoms.…”

Experimental and simulation analysis of concave-down resistance curve during electromigration in solder joints

“…[15][16][17] In addition, electron flow may enhance the dissolution of under bump metallization (UBM), causing extensive formation of intermetallic compounds (IMCs). [18][19][20][21][22] Therefore, Cu columns of about 50 lm thick were adopted as UBMs to relieve the currentcrowding effect. 23 Only a few papers had reported on the electromigration failure mechanism for solder joints with Cu column structures.…”