New building blocks for LED lighting
Larry Maloney, Contributing Technical Editor -- Test & Measurement World, 9/1/2009 2:00:00 AM
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Smaller IC packages, ball-grid arrays, and a variety of solder pastes are prompting electronics engineers to look for better, more dependable lighting solutions for machine vision.
“The quality-control requirements in electronics for catching every single defect are very high,” said Brian Merz, sales engineering manager for StockerYale, which manufactures fluorescent, laser, and LED illumination sources. “In that industry, a lot of very smart engineers are working on inspection systems, and they put every possible solution through its paces.”
Merz, whose machine-vision experience also includes applications engineering work with lighting manufacturer CCS America and automation systems integrator Axis New England, sees an increasing interest in LED lighting for electronics applications. Not only do LED illuminators offer reliable and controllable illumination, but their designs can also be customized to fit the application.
 With COB packaging, LED chips can be placed on circuit boards in high-density arrays, creating illuminators that are compact, uniform, and extremely bright. |
Yet, because of their larger size and integrated optics, conventional surface-mount and through-hole LED illuminators sometimes fall short in tough electronics inspection applications, particularly when high intensity is required. To address that issue, more lighting companies now offer COB (chip-on-board) LED products. In these designs, LED chips are attached directly to the conductive tracks of the PCB (printed-circuit board).
In StockerYale’s patented COBRA (chip-on-board reflective-array) design, the LED semiconductor dies almost touch one another, providing a very high level of intensity—up to 1 million lux—in a very compact footprint. The efficiency of this line-scan illuminator is further enhanced by placing a miniature reflector around each LED, which focuses more of the light forward where it is needed.
“Customers in the electronics and flat-panel industries tell us that we make the brightest LED lights they’ve ever seen,” said Merz. “COB also offers great advantages in customization because you start with the most basic LED building blocks.”
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Merz pointed out that new COB designs also enhance LED longevity, which is already a prime reason why engineers switch to LEDs from other lighting options. For example, halogen bulbs typically last less than 3000 hr. In contrast, tests that StockerYale has conducted on its COBRA product have shown less than a 5% change in light output over 10,000 hr of service for the standard 630-nm model. Each individual COBRA Slim 100-mm substrate, containing well above 100 LEDs, also has its own temperature-monitoring system. If the unit overheats, then it automatically shuts down and an alarm signals a fault condition. Among other advantages, a simple 0–5-V control lets users adjust the brightness of all COBRA LEDs simultaneously.
As for costs, these COB-based LED lights occupy the middle ground between less-expensive fluorescent and halogen illuminators and costlier laser-based lighting. Yet, Merz predicts steady growth of COB LEDs, even in high-end electronics inspection. Said Merz: “IC and PCB inspection are definitely in our sweet spot, as are solar wafers and flat-panel displays.”
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