Wednesday, September 3, 2008

Test Points, Vias and Pads

Signal vias affect the overall loss and jitter budgets. Each via pair may contribute 0.25 dB of loss in some corner cases. Vias may limit the achievable maximum routing length.

A maximum of four via pairs can be used on a TX differential pair. A maximum of two via pairs can be used on an RX differential pair. Vias should have a pad size of 25 mils or less, and a finished hole size of 14 mils or less. Two vias must be placed as a symmetric pair in the same location.

Test points (which can be vias, pads or components) and probe pads should be placed symmetrically in series. Stubs should not be introduced on differential pairs. Refer below for illustrations of correct and incorrect placements.

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