Ultraviolet light checks circuit-board flaws

"We had the idea to turn around the way automated optical inspection systems look at PCBs, but we needed someone to put hardware and software around it," Feld says. "We came up with what is essentially a PCB with UV-sensitive markings. The vision system sees this signature as light, while everything else is black," he explains.

FIGURE 2. The large white area in the image with the red border is the "blue dot" beneath a component. The visible dot means the part is missing. The red border indicates a failure. The green outlines show the locations of other blue dots, but a component is covering them, indicating that the parts are properly placed. The green lines are superimposed over the blue dots located between the leads of IC packages. Because the dots are visible, those locations are clear of solder or material; there is no short and the indicator is green.

The inspection points are built in at the same time that the boards are designed, and the boards are built by the board fabricator. Feld did not reveal all the details of the patented process, saying only that the board fabricator uses the Gerber file to "add our technology to the board."

Schwarzbach says another key requirement was a light source that is rich in the UV spectrum, safe because it presents a unified field of vision, and allows the least amount of light to pass through. "We needed a lens and lighting system that highlights the object of interest while de-emphasizing the background," he points out.

To reduce the effects of lens aberrations, InoSys chose a Computar TEC 55—a telecentric 55-mm lens—from CBC (America) Corp. (New York, NY). The lens overcomes the usual CCTV lens perspective aberrations by providing a depth of field that allows off-center objects to be the same size as those at the center. The other half of the lens and lighting tandem is a dual-circular ring light from StockerYale Inc. (Salem, NH). It uses black fluorescent bulbs to provide "the most uniform UV-rich lighting we could find," Schwarzbach says.

"Because of the research nature of the project initially, we chose a software package that gave us considerable flexibility in pre- and postprocessing of the [Original Solutions] algorithms," Schwarzbach says. InoSys chose Sherlock software from Coreco Inc. (St. Laurent, QC, Canada). Furthermore, because the inspection system had to interface with other machines in a shop-floor network, InoSys chose an open system architecture that uses a standard personal computer (PC), a PC-based frame-grabber board, and software.

The movable y-axis conveyor of the inspection system gathers the waiting board, moves it to the inspection location, clamps it to eliminate slip, and begins the calibration and inspection processes. Data are automatically logged to a database, and, depending on user needs, the system halts on a defect or simply logs the data. Upon completion of the inspection process, the system's conveyor moves the board to an offload position—either to a continuation conveyor for further processing or to a repair loop or queue.

The prototype is an off-line system, not intended for production-line use. This system has all of the features of the in-line machine, except that an operator must load and unload each panel of PCBs. It is appropriate for use in incoming inspection, quality control, or other audit areas of a factory. Schwarzbach says InoSys has a strategic alliance with PCT Automation Systems (Addison, IL), which makes transfer conveyors for PCBs that pick up, pass along, and position a board for inspection.