Microstructural evolution of intermetallic compounds between crystalline/amorphous Cobalt-Phosphorous coatings and lead-free solders

Abstract

In the advanced microelectronic packaging, Cu/Sn joints require high-quality diffusion barriers to inhibit Cu–Sn intermetallic compounds (IMCs) growth and improve the interface brittleness. Cobalt-Phosphorous (Co–P) alloy coatings are ideal candidates due to their good wettability, promising diffusion resistance and low interfacial brittleness. In this work, the microstructural evolution of IMCs between crystalline/amorphous Co–P coatings and lead-free solders during the solid-state diffusion were systemically investigated. The microstructure, phase distribution, and grain characteristics of coatings and IMCs were carefully characterized by SEM, EPMA, EBSD, and TEM. It was found that the consumption rate of crystalline Co–P, 21.2 nm/h, was significantly slower than that of amorphous Co–P, 96.4 nm/h, and exhibited an excellent diffusion resistance, which was attributed to the nanocrystalline structure and the P-rich layer which performed the diffusion barriers for Sn and Cu atoms. Compared with Ni, amorphous Co–P coatings could better improve the interfacial brittleness with soft and ductile Co–Sn compounds (hardness of 3.0 GPa) growing with the rapid diffusion of Co, and the toughened Co–Sn–P attributing to the diffusion of Sn. Kinetic analysis showed that the growth rate-controlling process of Co–Sn IMCs was jointly controlled by interfacial reaction and volume diffusion, and the effect of the interfacial reaction is more pronounced in the amorphous system, which leads to coarse Co–Sn grains that are beneficial to the softness and ductility of IMCs. Crystalline and amorphous Co–P coatings can be applied to the surface treatment for die pads and print-circuit-board pads according to their own characteristics, respectively.

Keywords

• Cobalt-phosphorus
• Solid-state diffusion
• Intermetallic compounds
• Lead-free solder
• Interfacial brittleness