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Electrolytic tough pitch copper (known as ETP, CDA110, or copper alloy no. 110) is a material used in the manufacture of external leads for hybrid integrated circuits. Cracking during the formation of 90° bends and the relatively poor cyclic fatigue behavior of this material are identified as due to embrittlement caused by second-phase oxide inclusions. Optical and SEM examinations of failures led to the following model for embrittlement. Under the applied loads, voids nucleate at the second-phase inclusions; continued loading causes the voids to grow and coalesce, resulting in a fracture. This model is consistent with the published literature. Cyclic fatigue behavior, i.e., the median number of cycles to failure (MCTF), was found to vary inversely with the oxygen concentration. Over the range 10-650 ppm oxygen (by weight), the fatigue results could be represented by [100/MCTF] = 0.0228 [ppm oxygen] + 8.32. For both CDA110 and oxygen-free copper (CDA102 or copper alloy no. 102) the MCTF was found to vary inversely with the percent of bond deformation over the 10-60-percent range. Therefore, all cyclic fatigue comparisons among the various CDA110 and CDA102 specimens were made at a common bond deformation, chosen to be 35 percent. Oxygen-free copper with ~10 ppm oxygen was found to have about twice the MCTF as CDA110 with ~400 ppm oxygen.