Amorphous silicon x-ray detectors find PCB flaws
By Ann R. Thryft, Contributing Technical Editor -- Test & Measurement World, 2/1/2009 2:00:00 AM
Long used in medical imaging applications, amorphous silicon x-ray detection is moving into the in-line inspection of PCBs (printed-circuit boards). Amorphous silicon, which is among the most common materials used in digital x-ray flat-panel detector technology, acts as a photodiode that converts x-ray energy into an electrical signal. “Digital x-ray detectors provide high-speed, high-throughput, real-time industrial inspection, applicable to the nondestructive testing of PCB boards,” said Jeffrey Foote, VP strategic marketing and business development for PerkinElmer's digital imaging group.
 Detectors such as the XRD 1621 N ES provide high-speed inspection for nondestructive testing of PCBs. Courtesy of PerkinElmer. |
Bringing this digital technology into industrial environments proved to be more difficult than it first appeared, said Foote. Although medical and industrial applications demand similar productivity, throughput, and efficiency, the two environments have major differences. “The hurdles we faced included continuous use of the equipment, more rugged operating conditions, and the need for higher x-ray doses,” said Foote.
PerkinElmer and other vendors first adapted amorphous silicon digital x-ray detectors for aerospace and automotive manufacturing and inspection environments. “We changed the detector to make the machine more rugged and to make the amorphous silicon substrate last longer in these higher-energy environments,” said Foote.
The next step was to adapt the detectors to PCB inspection. “Since the parts are so small, you need very high resolution, but you also need a much larger dynamic range, or contrast resolution,” said Foote. “The boards are complex and dense and there are multiple layers, so you need to take image slices in three dimensions, taking 2-D images at multiple depths.”
On the other hand, PCBs don't need as high an x-ray energy level as automotive applications, so the lower doses for PCBs are less damaging to the detector. “But other challenges, such as high-speed throughput, are critical,” said Foote. PerkinElmer's new amorphous silicon digital x-ray detectors offer twice the output speed of its previous designs, up to 30 fps, and feature 16-bit contrast resolution.
The higher contrast resolution enables the detectors to see smaller, subtler defects in components. The size and complexity of PCBs requires a larger field of view, said Foote. The new detector's bigger, 41-cm field of view makes it useful for inspecting either one large board or several smaller ones at a time. A 41-cm field of view is also helpful for inspecting wafers, since they are increasing in size.
Prices of faster flat-panel detectors have come down recently as the technology has become more available, said Foote. Consequently, these digital detectors are beginning to compete with analog image intensifiers, based on CCD cameras, for the inline inspection of PCBs and wafers.
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It looks very interesting...
By the way, what is the resolution of the detector?
Can I change the resolution?
How sensitive is it, can I see each layer into details: fine lines of 10 micron for example?
Is there any limit to the number of layers I could distonguish?
Thanks
Uri
Gold Uri - 2009-11-2 10:13:00 EST
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