The liquid structure of Sn-based lead-free solders and the correlative effect in liquid-solid interfacial reaction

Two lead-free solder alloys, Sn-0.7Cu and Sn-2Cu (wt.%), have been examined using X-ray diffraction method. The liquid structure of Sn-0.7Cu is similar to that of pure Sn. A pre-peak has been found in the low Q part on the structure factor S(Q) for Sn-2Cu tested under 260°C, but it disappeared finally when the testing temperature reached 400°C. The interfacial reactions between liquid solders and Cu substrates at 260°C were also studied. The results show that the IMC layer at Sn-2Cu/Cu joint is thicker than that at Sn-0.7Cu/Cu interface. The correlative effect of liquid structure on phase evolution in the solder joints are analyzed and discussed.