Influences of the Solder Size on Growth of Interfacial CU6Sn5 and Mechanical Performance of Sn-3.0Ag-0.5Cu/(111)Cu Joints Subjected to Multiple Reflow Soldering

In heterogeneous integration with three-dimensional (3D) packaging, solder joints are used in different levels of packages, which have a wide range of sizes, for instance, from hundreds of microns in BGA joints to a few tens of microns in micro-bump joints. Thus far, it is not yet well understood how the change in joint size affects the growth of interfacial intermetallic compound (IMC) and reliability of joints. In this work, a series of Sn-3.0Ag-0.5Cu solder balls with diameter scaling down from
${450 \mu\mathrm{m}}$
to
${15\ \mu\mathbf{m}}$
were used to fabricate single-sided joints on monocrystalline (111)Cu by reflow soldering at 260°C for different reflow cycle times of 1, 2 and 4. The influences of solder ball sizes and reflow cycle times on interfacial IMC growth and shear strength of Sn-3.0Ag-0.SCu/(111)Cu joints were investigated systematically. Results show that interfacial
${\text{Cu}_{6}\text{Sn}_{5}}$
grains in Sn-3.0Ag-0.SCu/(111)Cu joints exhibit mainly scallop-like morphology. The grain size and thickness of the interfacial IMC layer in the joints change non-monotonically with decreasing solder ball diameter, and both of them reach the maximum at a solder ball size of
${200 \mu\mathrm{m}}$
. The ripening of IMC grains by grain boundary (GB) migration mechanism during IMC growth occurs as earlier as the size of solder joints reduces and the reflow cycle time increases. In addition, the shear strength of Sn-3.0Ag-0.SCu/(111)Cu joints increases with decrease of the solder ball diameter.