Scattered Light Finds Defects in Wires

Daniel Woolls

Physicists in Spain and Argentina have developed a laser-based system designed to let makers of thin metallic wires detect and assess surface defects without shutting down the assembly line. The technique monitors the cone of light that is scattered from a wire when it is illuminated obliquely.

Wires with diameters of 50 to 2000 µm are found in everything from semiconductors to automobile air bags. Scratches, grooves, holes and other imperfections that wire can develop during manufacture can change its physical and chemical properties, so identifying such glitches is key to quality control.

In the experimental setup, the researchers illuminated steel and copper wires with a 675-nm diode laser. Projected onto a rotating screen, the scattered light cone created an annular image, which was recorded with a CCD camera. The presence of defects caused variations in the intensity of the ring, and by analyzing the variations, the researchers could determine both the location and nature of the defects.

Longitudinal defects such as scratches and protuberances were most evident, but holes and noses also were detectable. Although it could not quantify defect depth, making it difficult to disregard those that do not threaten performance, the system could analyze 1 to 2 cm of wire in a single image, suitable for in-line testing. Scanning-electron and atomic-force microscopy examine only about a millimeter at a time.

Eusebio Bernabeu of Universidad Complutense de Madrid, a member of the team that described the system in the Feb. 1 issue of Applied Optics, said it is the first designed for incorporation into wire manufacturing equipment. "Until now manufacturers had to stop their machines, withdraw a sample of wire and take it away to be tested," he said.

The setup is still in the experimental phase, but the team plans to test it in a factory in September and hopes to develop it commercially within 18 months.