IWLPC (Wafer-Level Packaging) Conference Proceedings


Authors: Mark Huang and Yong Poo Chia et al.
Company: Micron Semiconductor Asia Pte. Ltd.
Date Published: 9/17/2007   Conference: IWLPC (Wafer-Level Packaging)

Abstract: The demand for high-performance, high-density, smaller-sized, and multifunctional portable electronic devices has fueled the development of three-dimensional (3-D) packaging, system-in-a-package (SiP), and wafer-level packaging (WLP). Copper-pillar interconnect is a popular interposing option for these packaging methods (as opposed to conventional high-lead flip chip bonding processes) because of its small pillar size and high thermal and electrical conductivities. These advantages enable copper-pillar interconnect to be used for fine-pitch, high-frequency, and high-density flip-chip-on-module (FCOM) packages. However, the challenge associated with the technology is how to control the formation of brittle intermetallic compounds and to effectively prevent copper migration during bonding and reliability testing.

This paper focuses on Ni/Au-capped and Sn-capped copper-pillar interconnects using lead-free solder pastes (Sn-Ag-Cu [SAC]) in FCOM packages. The copper-pillar devices were flipped over and bonded to SAC-covered Cu/Ni/Au electrodes on printed circuit boards (PCB) or modules, followed by multiple reflows at 265°C. Both finishes can protect Cu-pillar bumps from oxidation in an oxygen-contained environment. After 1st reflow at 265 ºC, both systems can form a solid interconnection with the additional help of SAC soldering. The copper pillars are embedded into SAC alloys and can most likely improve FCOM reliability. Reliability testing inclusive of thermal cycle (TC), autoclave (AC), high-temperature storage (HTS), and thermal shock (TS) shows that Sn-capped Cu-pillar systems are more robust than Ni/Au-capped Cu-pillar systems. The different intermetallic interconnecting mechanisms are also discussed.

Keywords: Flip-chip-on-module, Copper-pillar interconnects, intermetallics, Sn-Ag-Cu alloy

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