Abstract
In this study, 96.5 Sn-3.0 Ag-0.5 Cu solder balls were joined to Au/Ni-P/Cu pads by a thermal reflow process. The interface reactions, solderablity and mechanical reliability of the solder joints were investigated. At the interfaces between the solder balls and pads, intermetallic compounds are formed. With increasing reflow temperature from 220 to 260 °C, the thickness of the intermetallic compound layer increased from 1.1 to 2.5 μm, and the wetting force of molten solder to the pads consequently increased from 0.4 to 0.5 mN. The shear strength of the solder joints decreased from 9.8 to 7.5 MPa because a thick intermetallic compound layer resulted in brittle fracture. The mechanical properties of constituent phases at the solder joints, including primary Sn, Sn-Ag 3 Sn eutectic phase, intermetallic compound layer and Ni-P underlayer, were measured by nanoindentation. The elastic modulus and hardness of the intermetallic compound layer were obtained as high as about 181 and 6.5 GPa, respectively. Cracks were formed around the indented regions and verified the brittleness of the intermetallic compound layer. © 2009 Elsevier B.V. All rights reserved.