Research on fatigue properties of micron scale copper bonding wires

Copper bonding wires are frequently used to connect to Micro-Electro-Mechanical System (MEMS) devices. Mechanical properties of copper wire are crucial to the reliability of MEMS system. The paper reported a symmetrical bending fatigue test on micron scale copper bonding wires. The test is based on the phenomenon that a micro-cantilever can be set into self-excited vibration between two electrodes under DC voltage. The results demonstrate that the yield strength, ultimate tensile strength and Young's modulus of copper wires with diameter of 20 μm are higher than those with a diameter of 30 μm and 40 μm, which significantly performs size effect. In fatigue test, the number of cycles to failure is 10⁴~10⁷. Under the same stress condition, fatigue strength (N=10⁶) of copper wires (d=20 μm, 30 μm, 40 μm) is 140 MPa, 97 MPa, 70 MPa respectively. The tensile fracture surface is the chisel-shaped peak, and the surface of the fracture appears many spaced strip drawing traces. The fatigue fracture surface is flat. Two cracks almost simultaneously originate from the surface, and the final rupture region is just like a narrow sheet.