Intermetallic Formation of Copper Pillar With Sn–Ag–Cu for Flip-Chip-On-Module Packaging

Copper pillar interconnects are a popular interposing option due to the advantages of small pillar size and good thermal and electrical performance, making copper pillar interconnects very useful for high-frequency and high-density flip-chip-on-module (FCOM) packages. However, the challenges associated with the technology include controlling the formation of brittle intermetallic compounds (IMC) and weak interfaces during heat-related processes, and preventing copper migration during bonding and reliability testing. As the reliability of the joint is significantly affected by the property of the surface finish, it is important to understand the influence of different surface finishes on the reliability of copper pillar interconnections. This paper focuses on Ni/Au-capped, Sn-capped, Sn-2.5Ag-capped, and organic solderability preservative (OSP)-capped copper pillar interconnections with lead-free Sn-3.0Ag-0.5Cu solder paste in FCOM packages. The types, morphology, and distribution of IMC formed in the bulk solder, the copper pillar/SAC, and copper pad/ENIG/SAC interfaces during multiple reflows ( 265degC ) and reliability testing thermal cycle (TC), autoclave (AC), high-temperature storage (HTS), and thermal shock [(TS)] were investigated using a scanning electron microscope with energy dispersive X-ray (SEM-EDX). The feasibility and reliability of these copper-pillar FCOM systems were also compared and evaluated. The reliability results show that OSP-capped copper pillar interconnects are the best interposing option in terms of reliability and performance.