Abstract
Flip chip technology (FCT) or gold wire bonding with ball grid array interconnection has attracted a great deal of attention in today’s microelectronic packaging. In flip chip technology, one of the challenging issues is the material selection for under bump metallization (UBM). The Ni-based UBM is of interest in FCT owing to the lower growth rate of the Ni-Sn compound and limited spalling effect. In this study, the elemental distribution and related phase transformation between solders and Ni/Cu or Cu/Ni(V)/Al UBMs were investigated with the aid of microstructure evolution and quantitative analysis with an electron probe microanalyzer. In the Sn-3.5Ag/Ni/Cu joints reflowed at 260°C, only (Ni,Cu)3Sn4 intermetallic compound (IMC) formed at the solder/Ni interface. For the Sn-37Pb joints reflowed at 225°C for 1 ~ 10 cycles, the interfacial reaction was similar to Sn-3.5Ag joints. Nevertheless, another (Cu,Ni)6Sn5 IMC was observed in the joints reflowed either at 245°C after five cycles or at 265°C after three cycles. With the aid of microstructure evolution, quantitative analysis, elemental distribution between the solder and Ni/Cu UBM, it was revealed that Cu content in the solder near the solder/IMC interface played an important role for the formation of (Cu,Ni)6Sn5 IMC. Cu/Ni(V)/Al thin film UBM is currently applicable for flip chip technology when Sn-Ag-Cu solder is used. Interfacial reaction for the flip chip Sn-3.0Ag-(0.5 or 1.5)Cu joints with Cu/Ni(V)/Al UBM during aging at 150°C was also investigated. In the Sn-3.0Ag-0.5Cu joint, the Cu layer at the chip side was completely dissolved into solder, and Ni(V) layer was thus dissolved and reacted with solder to form (Cu,Ni)6Sn5 IMC. For the Sn-3.0Ag-1.5Cu joint, only some portion of Cu layer was dissolved, and the remained Cu layer reacted with solder to form Cu6Sn5 IMC. The Ni in Ni(V) layer was incorporated into the Cu6Sn5 IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. In general, formation and growth of intermetallic compounds play a major role in the reliability of solder joint. The formation of Cu-Sn or Ni-Sn IMCs have been observed at the interface of Sn-rich solders reacted with Cu or Ni substrates. In this study, nanoindentation technique was employed to investigate nanohardness and elastic modului of Cu6Sn5, Cu3Sn and Ni3Sn4 IMCs in the solder joints. The magnitude of hardness of IMCs was in the order Ni3Sn4 > Cu6Sn5 > Cu3Sn. Elastic modului of Cu6Sn5, Cu3Sn and Ni3Sn4 in the Sn-3.5Ag joints were 125 ± 6.8, 136 ± 5.9 and 143 ± 9.2 GPa, respectively. Besides, the influence of aging at 150°C up to 3000 h on the interfacial reaction of the Au wire bonded with Al-Cu pad was also investigated. To observe various IMCs with field-emission scanning electron microscope, polished samples were ion milled through precision etching and coating technique. With the aid of microstructure evolution, quantitative analysis, and Al-Au phase diagram, the mechanism of interfacial phase transformation between Au wire and Al-Cu pad was discussed and proposed.