Cross-interaction of under-bump metallurgy and surface finish in flip-chip solder joints

Abstract

The cross-interaction of the under-bump metallurgy (UBM)/solder interface and the solder/surface-finish interface in flip-chip solder joints was investigated. In this study, the UBM on the chip side was a single layer of Cu (8.5 µm), and the surface finish on the substrate side was a 0.2-µm Au layer over 5-µm Ni. It was shown that, after two reflows, the Ni layer of the surface finish had been covered with (Cu 1Ϫx Ni x ) 6 Sn 5 . This shows that the effect of cross-interaction of the two interfaces is important even during the reflow stage. During subsequent solidstate aging at 115°C, 135°C, and 155°C, the formation of (Cu 1Ϫx Ni x ) 6 Sn 5 over the Ni layer was found to have the effect of reducing the Ni consumption rate. At the same time, the Cu consumption rate of the UBM was accelerated. The results of this study show that the selection of the UBM and the surface finish has to be considered together because the cross-interaction of the two interfaces plays an important role.

Kinetic analysis of the interfacial reactions in Ni/Sn/Cu sandwich structures

“…The potential formation of a Ni 3 Sn 4 phase at the Sn/Ni interface was prevented due to the fast arrival of Cu atoms from the Cu side and the formation of a Cu 6 Sn 5 phase. 3,5,10 As discussed in the Introduction section, Wang and Liu have already described the driving force behind the rapid diffusion of Cu from the Cu side to the Ni side. 3 From the above, we realize that the force driving the diffusion of Cu through the molten Sn is very much dependent on the Ni content in the molten Sn near the Sn/(Cu,Ni) 6 Sn 5 interface.…”

“…Recently, it has been pointed out that the two interfacial reactions indeed would occur during the reflow process. [3][4][5][6][7] Wang and Liu reported that when Ni and Cu foils were joined by Pb-free Sn-rich SnAg solder, the Cu foil would dissolve into the molten solder and the dissolved Cu atoms would diffuse toward the Ni foil across the molten solder. The arriving Cu atoms would then form a ternary (Cu,Ni) 6 Sn 5 compound on the Ni foil.…”

Effects of Electromigration on Interfacial Reactions in the Ni/Sn-Zn/Cu Solder Interconnect

“…The interfacial reactions in the Ni/Snbased solder/Cu combination after annealing were reported and attributed to the difference in the Cu or Ni concentrations between the Ni and the Cu pads. [1][2][3][4][5][6] Under such conditions, the interfacial reactions were greatly accelerated and the thermal stability of the Cu UBM degraded due to the formation of (Cu,Ni) 6 Sn 5 intermetallic compounds (IMCs). 1,2,5,6 Electromigration behavior of the Ni/Sn-based solder/Cu combination has also been studied.…”

“…[1][2][3][4][5][6] Under such conditions, the interfacial reactions were greatly accelerated and the thermal stability of the Cu UBM degraded due to the formation of (Cu,Ni) 6 Sn 5 intermetallic compounds (IMCs). 1,2,5,6 Electromigration behavior of the Ni/Sn-based solder/Cu combination has also been studied. [7][8][9][10][11][12] Kim et al 7 found that separation of the Pb-rich and Sn-rich phases occurred in Sn-Pb flip-chip bumps when the electrons traveled from the Ni side to the Cu side, while a void was formed under the Cu 6 Sn 5 IMC layer in the reverse current direction.…”

Interfacial reactions and mechanical properties of ball-grid-array solder joints using Cu-cored solder balls

“…The Ni-Sn intermetallic compound formed at the solder/Ni interface was replaced by a Cu-Sn compound, e.g., (Cu,Ni) 6 Sn 5 . Tsai et al 30 also reported the cross-interaction between under-bump metallization and surface finish in a flip-chip solder joint.…”