Abstract
In flip chip technology, under bump metallization (UBM) composing multi metallic thin-films is usually used to improve the bonding between solder and pad on chip. The Cr/Cu/Au UBM is commonly used to joint with SnPb solder. [1] However, the Sn-Cu intermetallic compounds (IMC) spalled rapidly from interface, due to the quick reaction between Cu and Sn. Therefore the Ni-based UBM, such as Ti/Cu/Ni and Al/Ni(V)/Cu, is advanced to replace Cr/Cu/Au UBM. The primary advantage of Ni-based UBM is slower reaction rate with Sn-Pb solder and Pb-free solder than Cu-based UBM, so the spalling of IMC would be eliminated. Nevertheless, sputtering a pure Ni target is more difficult because of magnetic interference. In order to overcome the magnetic interference, the Delco process which adds 7 at.% V into Ni target is applied. Only a few literatures were focused on the interface reaction between Al/Ni(V)/Cu UBM and solder. [2-7] Li et al. [3] indicated that (Cu,Ni) 6 Sn 5 attached well to the Ni(V) in the SnAgCu solder and Al/Ni(V)/Cu UBM system after multiple reflows. However it was revealed that the Ni(V) layer was gradually replaced by the Sn-patch with reflow. Jang and Duh [5] also reported the conformable results for Sn3.0Ag(0.5 or 1.5)Cu solder with Al/Ni(V)/Cu UBM. The Sn-patch was observed in Sn3.0Ag0.5Cu solder system with lower Cu concentration. More Cu was dissolved from Al/Ni(V)/Cu UBM, resulting in the formation of Sn-patch in Ni(V) layer. Most literatures for Al/Ni(V)/Cu UBM were concerning interfacial reaction, [2, 5-7] yet there was limited data on the mechanical properties. [2, 3] In fact, the mechanical testing methods, such as shear and pull test, would affect the packaging reliability, and the failure mechanism for reliability tests should be better understood. In this study, Sn3.0Ag0.5Cu solder bump was jointed on Ni(V)/Cu UBM and then aged at 125°C for 500-2000h. The heat treatment samples were employed to investigate the interfacial reaction and the mechanical properties between solder and UBM. (Cu,Ni) 6 Sn 5 IMC formed at the interface between solder and Ni(V) layer after assembly. The thickness of (Cu,Ni) 6 Sn 5 gradually increased to 4μm during aging at 200°C for 2000h. The rod-like (Cu,Ni) 6 Sn 5 IMC and fine Ag 3 Sn particles were not significantly changed even after 2000h. In addition, the Sn-patch was observed in the Ni(V) layer after 500h, and replaced the original Ni(V) layer. The mechanical properties for solder joints were estimated with a XYZTEC bond tester. Scanning electron microscope images of the crack surface after testing provided the information for failure modes analysis. The relations between interfacial reaction and mechanical properties would be discussed. Furthermore, the possible failure mechanism was proposed. © 2008 IEEE.