The effect of reflow temperature and time on the formation and growth kinetics of Intermetallic Compounds (IMCs) between Sn-0.7Cu -0.4Co eutectic solder and ENIG/Cu substrate finish

Soldering with lead-free tin-based solder alloys demands substantially higher processing temperatures compared to conventional tin-lead solders, resulting in both significantly greater growth rates of Intermetallic Compounds (IMCs) and dissolution of surface finish layers. In this paper, the interfacial reactions between a new lead-free solder composition, eutectic Sn-0.7Cu- 0.4Co, and Electroless Nickel/ Immersion Gold (ENIG)/Cu surface finish have been investigated as a function of reflow temperature and reflow time. Three different soldering temperatures were used, namely, 240, 250 and 260degC, while the reflow time was varied between 2, 5, 10, 30, 60 and 120 min, respectively. The microstructure of the solder joints was observed by means of Scanning Electron Microscopy (SEM) and the chemical composition of the different phases was analyzed by means of Energy Dispersive X-ray (EDX), in order to study the relationship between interface IMC formation and reflow process parameters. The results show that the IMCs in the bulk solder matrix are different from those built at the interface between the solder and the surface finish layer. In the bulk solder only CoSn₂ IMCs were found. The reaction between the molten solder and electroless Ni layer resulted in formation of a ternary (Cu, Ni)₆Sn₅ IMC at the interface. This interfacial IMC layer was continuous, and with a faceted morphology. The thickness of the IMC layer as a function of both reflow time and temperature was also investigated. The apparent activation energy was also obtained by using one reflow process with different reflow temperatures and times.