Interfacial Structure and Mechanical Properties of Lead-Free Bi-Containing Solder/Cu Microelectronic Interconnects

Previous studies have shown that Bi segregation at the Cu3Sn/Cu interface in eutectic 42Sn-58Bi/Cu interconnects induces brittle fracture along this interface after aging at 120°C for 7 days. To verify whether Bi can still be applied in lead-free solder alloys, compact tension tests were conducted on Sn-10Bi-0.7Cu/Cu joints in as-reflowed and aged (at 180°C for 17 days) conditions, and their interfacial structure was studied by scanning electron microscopy (SEM). After aging the joints at 180°C for 17 days, the average thickness of intermetallic compound (IMC) between the solder and Cu substrate in the interconnect increased from 4 ± 1 μm to 25 ± 1 μm, the average fracture toughness dropped from 0.777 ± 0.002 MPa m1/2 to 0.297 ± 0.001 MPa m1/2, while the interconnects did not fracture in fragile fashion along the Cu3Sn/Cu interface. No Bi segregation was found at the Cu3Sn/Cu interface in the Sn-10Bi-0.7Cu/Cu interconnect aged at 180°C for 17 days, while Bi particles were found at the counterpart interface in the eutectic 42Sn-58Bi/Cu joint aged at 180°C for 2 days. Cu3Sn formed by consumption of Cu6Sn5 during IMC growth in the interconnect. The Bi segregation was caused because the content of Bi dissolved in Cu6Sn5 was higher than the maximum solubility of Cu3Sn in the eutectic 42Sn-58Bi/Cu interconnect. Bi precipitation and segregation occurred when the Cu6Sn5 IMC turned into Cu3Sn IMC. Electron probe microanalysis revealed that the amount of Bi dissolved in the Cu6Sn5 IMC in the solder joint depended on the Bi content in the solder alloy. The test results revealed that lead-free solder alloys with Bi content less than 10 wt.% will not lead to Bi segregation, which would greatly degrade the solder/Cu joint.