PCB Components: Via Hole

Vias are utilized to simplify trace routing around other components or when there is a high density of interconnections to be made (i.e. BGA packages). Just as a PCB trace had inductance and capacitance, so todoes a via. Generally these elements are ignored as the length of the vias
are typically very small relative to the rest of the trace. But, this Can cause issues if the signals are very high frequency (>100MHz) or have energy / harmonics at high frequencies.

The easiest way to minimize problems of a via is to simply not use them with signal traces. At the very least it should be minimized. If vias must be used, there are other issues to worry about that will be discussed later.

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.