Interfacial reaction and IMC growth between Bi-containing Sn0.7Cu solders and Cu substrate during soldering and aging

Intermetallic compounds (IMCs) formations of Bi-containing Sn0.7Cu/Cu interface were studied for soldering at four levels of temperatures (240, 260, 280 and 300 °C) and isothermal aging at 150 °C. It was found that addition of Bi into the Sn0.7Cu eutectic solder caused the excessive formations of IMCs during the soldering reaction and thereafter in aging condition. The interfacial IMC layer was composed of Cu6Sn5 and Cu3Sn layers after liquid soldering for Sn0.7Cu/Cu, Sn0.7Cu0.7Bi/Cu and Sn0.7Cu1.3Bi/Cu joints. With the increasing soldering temperature, the thickness of IMC layers (including Cu6Sn5 and Cu3Sn IMCs) increased linearly. After various days of aging, a comparatively planar IMC layer at the solder/Cu interface was observed than that of the as-soldered joints. The growth of IMC layer during aging for Sn0.7Cu, Sn0.7Cu0.7Bi and Sn0.7Cu1.3Bi solders followed the diffusion control mechanism. And the interfacial IMC layers were thicker with higher Bi content in solder alloy since the Bi could result in more chemical bonds between Cu atoms or between Cu and Sn atoms to be broken, which made more Cu and Sn atoms activated.