Let’s say you have circuit board assembly that contains red and green LEDs. Automated optical inspection can tell you if the LEDs are present and oriented correctly. But how tell if a green LED has been accidentally swapped with the red one?
Let’s say you have circuit board assembly that contains red and green LEDs. Automated optical inspection can tell you if the LEDs are present and oriented correctly. In-circuit testing can tell you if the LEDs have been soldered correctly; there are no bridges or opens to prevent current from flowing in and out of the components. However, neither technology can reliably tell you if the green LED has been accidentally swapped with the red one, nor can they tell you if the LEDs are putting out as much light as they should.
To measure the color and intensity of the light emitted by diodes, test engineers add photoelectric sensors to the fixture for in-circuit testing. Ideally, these sensors should be located directly over the LEDs. Unfortunately, this has become increasingly difficult to accomplish. Like every other electronic component, LEDs are getting smaller, and they are being placed very close together on boards.
To get around that problem, test engineers have been forced to pull the sensors off the fixture. Instead, fiber-optic light pipes are connected to the fixture to channel light from each LED to its corresponding sensor. Although this technique is effective, it adds complexity to the fixture. Besides test development, engineers have to be concerned with shielding, polishing, mounting and bending the light pipes. In addition, as the LED gets smaller and the light pipe gets thinner, alignment of the fiber with the LED becomes more critical for obtaining accurate results.
In September, Test Coach Corp. (Hoffman Estates, IL) introduced a new device for LED testing that is so small and thin that it can be plugged directly into an in-circuit test fixture, obviating the need for light pipes.
Called the Ultra FINN, the device combines a custom-packaged, four-band color sensor with a microprocessor to optimize all the functions necessary to distinguish the color and intensity of a light source or LED. The device is only 31 millimeters long, including the power and output leads. The sensor face is just 3.5 millimeters wide and 5.8 millimeters tall.
The tiny sensor detects 2,000 different hues between blue and red, covering the complete visible, ultraviolet and infrared spectra. It tests most LEDs in less than 10 milliseconds.
During a test, the LED is activated and the Ultra FINN outputs a frequency, in kilohertz, that quantifies the LED’s color. The pulse width of the same signal quantifies the brightness of the LED. This allows engineers to verify the performance of LEDs with a simple stand-alone meter or integrate the process with the automated test equipment, explains Kevin Schmitt, vice president of engineering and head of product development at Test Coach. That flexibility lets engineers simplify the process and reduce overall test time.
The device works with any LED-bright or dim, diffused or nondiffused, surface-mount or through-hole. It operates on 5 VDC and fits into a custom sleeve that installs easily into a drilled hole in a test fixtures with a press fit.
For more information, call 847-885-4880 or visit www.testcoachcorp.com.
Assembly Innovations: Tiny Sensor Checks LEDs
January 1, 2007