Fatigue strength of electroplated copper thin films under uni-axial stress

Fatigue strength of electroplated copper thin films was measured under uni-axial stress. Two kinds of electroplated films were prepared for the fatigue test. One was a commercial film mainly used for interconnections in printed wiring boards. The other film was grown on a stainless steel substrate by using acid copper sulfate bath without any additive agent. The micro texture of each film was observed by using SEM (Scanning Electro Microscope) and SIM (Scanning Ion Microscope). It was found that the micro texture of each film was quite different with each other. The mechanical properties such as the yield stress, fracture elongation and Young's modulus of each film changed significantly from those of bulk copper depending on their micro structure. The low-cycle fatigue strength also varied drastically with each other, while the high-cycle fatigue strength was almost same. The fracture surfaces were observed by SEM after the fatigue test. It was found that there were two fracture modes under the fatigue test. One was a typical ductile fracture, and another was brittle one even under the fatigue load higher than its yield stress. The crack seemed to propagate through the grains when the ductile fracture occurred. On the other, the crack seemed to propagate along grain boundaries of columnar grains when the brittle fracture occurred. These results clearly indicated that the fatigue strength of electroplated copper thin films varies depending on their micro structure.