Gentle Handling for Unnecessary Roughness

Brent D. Johnson

Rogers Corp. develops high-performance specialty materials for telecommunications electronics. The company's primary application is laminating copper foil to various dielectric substrates to insulate the layers on circuit boards. Ensuring a uniform roughness value is the key to making dielectric layers that insulate without impeding circuit performance. Process control is critical.

Scott Kennedy, development engineer, said the company used to analyze surfaces with a portable stylus profilometer. Although it was easy to use, it took a long time, he said. It also required contact, acting like a needle point that is dragged across the surface of the substrate. As the profilometer encountered surface changes, fluctuations in current indicated defects. However, these devices can damage fragile surfaces such as copper foil or plastics.

Another alternative was interferometric surface characterization, which provided even more detail but at a higher cost and required several days for analysis.

Now the Lasercheck handheld surface-roughness measurement gauge from Optical Dimensions LLC is improving Rogers' online surface analysis. The instrument uses a 660-nm diode laser to reflect light from the surface of the test material. The scattered light hits a photodiode array detector, whose elements are arranged to measure the angle and intensity of the beam. Two isolated photodiodes measure roughness contributions from surface spatial wavelengths in nearly the same bandwidth region that is measured by a mechanical profiler, the baseline instrument.

A significant benefit of the Lasercheck is that it requires no contact with a surface. Kennedy said it is easy to use, is portable and enables data-logging. "We can get immediate results and many more measurements," he said.

The device is not perfect, he added, noting that the operator must calibrate it separately for each surface; e.g., settings differ for measuring a ground or honed surface.