Page 96 - Building A Succesful Board-Test Strategy

82 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

Figure 2-26 The shadow with nodes only on V1 and V3. (From Proceedings of
Nepcon West, 1998. Courtesy Agilent Technologies.)

Figure 2-27 Actual test results with limited access. (From Proceedings of Nepcon
West, 1998. Courtesy Agilent Technologies.)

Some complex boards also require a “modified” bed-of-nails to supplement
edge and test connectors. Differences between in-circuit and functional beds-of-
nails include the number of nails and their purpose. Whereas an in-circuit bed-of-
nails provides a nail for every circuit node, functional versions include nails only
at critical nodes that defy observation from the edge connector. Relying on only a
few nails avoids loading the circuit with excess capacitance, which would reduce
maximum reliable test speeds.