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White-Light Device Measures Warpage of Printed Circuit Boards

Photonics Spectra
Jan 1997
Kathleen G. Tatterson

ATLANTA -- A new technique using a broadband white-light source will enable electronics manufacturers to measure the warpage of a printed circuit board design before it goes into mass production, according to scientists at the Georgia Institute of Technology who developed the process.
Led by Georgia Tech's Charles Ume, researchers incorporated the technology into a device called the TherMoiré. Electronic Packaging Services Ltd., Co. of Atlanta uses it to monitor design and manufacturing processes at Motorola, Ford Electronics and Lucent Technologies, among others.
Previously, the makers and users of printed circuit boards had no way to gauge how their devices warped in the high-temperature soldering environment, explained Dirk Zwemer, general manager at Electronic Packaging Services. "Manufacturers could measure warpage before and after the soldering process, but the high-temperature environment was not conducive to real-time measurement during processing," he said.
Engineers mount the specimen board underneath a glass grating inside an oven. A computer then simulates the soldering process by raising the temperature up to 230 °C. The technique models conditions for infrared-reflow, convective-reflow and wave-soldering processes using LabView software.

Capturing fringes
Through a large window in the top of the oven, engineers shine collimated white light on the grating and specimen. A shadow grating cast on the specimen surface forms an interference fringe pattern when observed through the glass grating using a Pulnix CCD camera (768 3 494-pixel resolution). The fringe pattern is captured using an S-VHS videocassette recorder and an Epix digital frame grabber for storage, display and conversion into 3-D color plots (see figure).
Although Georgia Tech's warpage detection technology will mainly impact the electronics industry, the company is also working with potential customers in the paper industry. The technique would use various processing steps to characterize paper flatness. "This is a potentially useful technology in any area where flatness of a large area with a range of a couple of mils is important," said Zwemer.
Of course, the ultimate solution to warpage would be to prevent it from happening at all. The Georgia Tech team is working on a system that would check the design before a board hits the prototype stage.
"The experimental capabilities from our previous work provide a grounding for the modeling work that's going on," he said. "With this technique, wafer board warpage would be determined before significant value is added."

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