Comprehensive Comparative Analysis of Microstructure of Sn-Ag-Cu (SAC) Solder Joints by Traditional Reflow and Thermo-Compression Bonding (TCB) Processes

Abstract

Thermal-compression bonding (TCB) process has been considered to successfully fabricate the 3D stacked interconnection with ultra-fine pitch I/O bumped thin flip-chip and achieve package miniaturization, such as high bandwidth memory (HBM) modules that exceed capabilities of traditional thermal reflow process. Until now, a conventional reflow process has been the de facto solder interconnection method in microelectronic packaging. The alternative, TCB process has not been employed commonly for solder interconnection method as TCB-bonded solder joints have yet failed to prove their reliabil-ity such as upon thermal cycling and electro-migration (EM). A significant difference in solder reflow (i.e. melting and solidi-fication) technique may affect solder grain and crystallographic characteristics, thus affecting the reliability performance of 3D package modules. In addition, there has not been any fundamental and comparative study between the conventional reflow bonding and TCB processes. Also, the degradation mechanism of TCB-processed flip-chip packages upon EM failure has not been clearly defined. Here we show that the Cu6Sn5 intermetallic compound (IMC) morphology at Sn-Ag-Cu (96.5 wt./3.0 wt./0.5 wt.% SAC-305) solder joint interfaces and the texture of β-Sn crystallographic orientation of SAC-305 solder joint upon TCB-utilized flip-chip fabrication drives premature EM failure. We found that the lack of the IMC layer of TCB-processed solder joints facilitates the diffusion of Cu and Ni atoms at the solder-Cu bond pad interfaces, and furthermore the thermomigration (TM) caused by Joule heating at the TCB interface could provide synergistic impact on the EM. In ad-dition, through statistical studies using EBSD results, higher lattice coherency of β-Sn crystallographic texture was observed for TCB-processed solder joint that adds to a much faster diffusion of Cu and Ni atoms during the EM test, which is respon-sible for leaving defects both within the solder and at the interfaces of the joint. Our results demonstrate an insightful deriv-ing force of EM from the solder joint the microstructure and the crystal orientation that allows for juxtaposition of two most widely used bonding technologies, reflow bonding and TCB.