Effects of a thermal-ultrasonic stress relaxation process on the residual stress, mechanical properties and microstructures of C19400 copper alloy strips

With the development of 5G communication, more demanding requirements have been specified for copper alloy strips applied in lead frames. Thin copper alloy strips with high residual stress always warp during the etching process, so they do not meet the requirements of industry. Therefore, it is essential to reduce the residual stress in these strips. In this work, a novel effective technique suitable for thin strips, the thermal-ultrasonic stress relaxation (TUSR) method, which integrates conventional thermal stress relaxation (TSR) and ultrasonic stress relaxation (USR), is proposed. The equipment used to conduct TUSR and TSR experiments on C19400 copper alloy was produced in this laboratory. The residual stress evolution was investigated using X-ray diffraction (XRD). The microstructure evolution of the strips was analyzed by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The mechanism of the relaxation of residual stress was revealed. The maximum residual stress relaxation values in the transverse and rolling directions after TUSR treatment were 80.56 and 89.39%, respectively, which were better than the results obtained after TSR treatment. After TUSR treatment, the grain sizes were smaller, and the percentage of low-angle grain boundaries (LAGBs) was higher than that before the treatment. The dislocation morphology was converted to ordered dislocation cells and dislocation walls and macroscopic plastic deformation of the grains released elastic strain energy, which was the main reason for residual stress relaxation in the C19400 alloy strips. In addition, the increase in tensile strength was mainly due to the enhancement of a precipitate pinning effect and grain refinement. These findings can provide guidance for future work related to the relaxation of residual stress in copper alloy strips for lead frames.