Intermetallic compound formation and mechanical property of SN-CU-XCR/CU lead-free solder joint

The shear strength and interface characteristics of a new Pb-free solder, Sn-0.7Cu-0.2Cr (in wt.%) alloy, were estimated to evaluate its suitability for automobile electronics modules. The mother alloy of this environmentally friendly Sn-0.7Cu-0.2Cr solder, which has a moderate melting temperature of about 231 °C, was fabricated first, and 300 μm-diameter solder balls were made from this mother alloy. Then, prototype modules were fabricated by reflow using the solder balls. A shear test was performed, and the interface microstructures were observed after aging under various conditions. For comparison, the commercial Sn-3.0Ag-0.5Cu and Sn-0.7Cu solders were also tested. After aging at 125 and 150 °C, the shear strength of the Sn-0.7Cu-0.2Cr solder joint was lower than that of the Sn-3.0Ag-0.5Cu joint but higher than that of the Sn-0.7Cu joint. An irregular intermetallic compound (IMC) layer also grew, and its average thickness was measured for each aging condition. The thickness of the IMC layer of the Sn-0.7Cu-0.2Cr solder joint was smaller than those of the other two solder joints. From these results, it is expected that the new solder will be more reliable than other commercial solders.