Effects of solder volume and size on microstructures and mechanical properties of Sn-3.0Ag-0.5Cu solder joints

With the miniaturization and high-density integration of semiconductor packages, the importance of joint properties of microbumps and solder balls is increasing. The microstructure and mechanical properties of the joints were evaluated according to the size of the solder balls, which were made of Sn-3.0Ag-0.5Cu (wt%)-the most commonly used joint material. A specially prepared printed circuit board substrate that was surface-treated with an organic solderability preservative and solder balls of four different diameters (600, 450, 300, and 200 mu m) were employed. After the reflow process, the microstructures of the joint interfaces were examined; the results indicated that scallop-shaped Cu6Sn5 intermetallic compounds (IMCs) were generated at all diameters. As the diameter of the solder-balls increased, the number of IMCs created in the joint per unit area increased and the average size of the IMCs decreased. The thickness of the IMCs created at the joint interface decreased as the diameter increased. The mechanical properties were evaluated via solder-ball shear tests, and the results indicated that as the solder-ball diameter increased, the shear force increased; however, the shear strength, that is, the shear force divided by the unit area, decreased. The results of this study suggest that when solder joints of different sizes experience the same thermal history via the reflow process, the size and thickness of the interfacial IMCs may change, and these metallurgical changes may affect the mechanical properties and reliability of joints.