Comprehensive comparative analysis of microstructure of Sn–Ag–Cu (SAC) solder joints by traditional reflow and thermo-compression bonding (TCB) processes

Thermo-compression bonding (TCB) process has been considered an emerging alternative to the conventional reflow process because of its significantly fast bonding time due to rapid ramping and its ability to fabricate ultra-fine pitch 3D stacked interconnections to achieve package miniaturization, as is necessary in high bandwidth memory (HBM) modules. Since the TCB-process bonded joints are usually followed by additional solder reflow processes for bonding of other components, no attention has been solely dedicated to the reliability of TCB-only bonded joints. In addition, there has not been any fundamental and comparative studies between the conventional reflow bonding and the TCB process. A recent study showed that the TCB joints could be vulnerable to thermal cycling and electro-migration (EM), and therefore risk exhibiting early failures. A significant difference in bonding processes between the two processes may affect solder grain and crystallographic characteristics, thus affecting the reliability performance. Here, we show that the Cu₆Sn₅ intermetallic compound (IMC) morphology at Sn–Ag–Cu (SAC-305) solder joint interfaces and the β-Sn crystallographic orientation of the solder joint could explain for the premature EM failure in TCB-utilized solder joints. The effect of the IMC layer of TCB-processed solder joints on the diffusion of Cu and Ni atoms at the solder/bond pad interfaces is presented. Furthermore, the thermomigration (TM) caused by Joule heating for TCB-utilized solder joints is discussed. Lastly, statistical studies using electron backscatter diffraction (EBSD) results reveal the higher lattice coherency of β-Sn crystallographic texture for TCB-processed solder joints.