Agilent Technologies has developed bead probe methodology for in-circuit test of high-speed and high-density printed circuit boards (PCBs). Bead probe technology addresses the needs of electronics contract manufacturers for accurate testing of increasingly complex PCBs, it says.
As board circuitry gets smaller, traditional bed-of-nails test solutions become problematic. Because on-board test target sizes are shrinking, this approach is no longer capable of reliably contacting hyper-small test targets, which have an unacceptable performance impact on high-frequency signals. Likewise, the boundary-scan test approach cannot solve this problem because it does not offer 100% coverage of all defects.
Bead probe technology circumvents these issues by placing very small, hemi-ellipsoid beads of solder directly onto a board's copper traces. A bead probe, which is only a few mils tall, protrudes above the solder mask. When partially flattened by a fixture-based, flat-faced target probe, Agilent says that it gives the excellent, low-contact resistance needed for testing. The target probe (that replaces the spear-point nail) is wide enough to ensure reliable contact using today's 35 mil 'cannot miss' dimensions.
In addition, bead probes are easily fabricated, using the same solder masking/stenciling steps in use today. No unusual or extra steps are necessary. "Bead probes are particularly well suited for highly dense layouts or gigabit signals and, according to our tests, have a negligible impact on circuit performance during normal operation," said Kenneth Parker, engineer/scientist for Agilent's Electronic Manufacturing Test Division. "Bead probes let us approach an ideal of layout-independent, test-point placement - a great benefit in high-density, high-frequency design that will revolutionise the normally adversarial relationship between board designers and test engineers."
Bead probe technology is being tested by Agilent's high-volume manufacturing partner. Full licensing of the technology is expected to be available by mid-2005.
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