A Study on Copper Pillar Interconnect in Flip-Chip-On-Module Packaging

Increasing demand for high-density flip-chip packaging and lead-free solder technology has led to the development of fine-pitch copper pillar and lead-free solder Sn-Ag-Cu (SAC). However, the challenges associated with the technology include controlling the of brittle intermetallic compounds (IMCs) and preventing copper migration during bonding and reliability testing. Because the reliability of a flip-chip-on-module (FCOM) joint is significantly affected by the property of the surface finish, it is important to understand the influence that different surface finishes have on the reliability of the copper pillar interconnections. This paper focuses on organic solderability preservative (OSP)-capped and Sn-2.5Ag-capped copper pillar with lead- free Sn-3.0Ag-0.5Cu solder paste in FCOM packages. The types and morphology of IMCs formed in SAC solder and the interface between copper pillar, SAC solder, and Cu/Ni/Au bond pads on printed circuit boards (PCB) during multiple reflows (265degC) and reliability tests inclusive of thermal cycle B (T/C-B), autoclave (AC), high temperature storage (HTS) and thermal shock (TS) are systematically investigated using scanning electron microscope with energy dispersive X-ray (SEM-EDX). The reliability of copper-pillar interconnections is also evaluated. Different interconnection mechanisms are discussed. The results prove that OSP-capped copper pillar interconnect has better reliability performance.