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Meshed Metal Structure In The Stub Region of PCB Vias As A Substitute of Antipads to Increase Stub Impedance and Improve Channel Insertion Losses

IP.com Disclosure Number: IPCOM000236786D
Publication Date: 2014-May-15
Document File: 4 page(s) / 460K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a meshed metallic structure implemented in the stub region of a printed circuit board, possibly in conjunction with other techniques, to reduce the stub effects.

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Meshed Metal Structure In The Stub Region of PCB Vias As A Substitute of Antipads to Increase Stub Impedance and Improve Channel Insertion Losses

Via stubs are detrimental for high-speed signal propagation, especially for thick printed circuit boards (PCB) (i.e. more than 15 layers). A via stub is formed when there is a

transmission line transitioning from one layer to another one of the PCBs. The residual metal (not part of the desired channel) of the plated via reaching the top or/and bottom surface of the card creates a λ/4 resonator that is going to strongly impact the high-speed path. (Figure 1)

To alleviate this problem, high-speed traces are routed on external layers (whenever

possible) so the stubs are not created at all or, for additional costs, the stubs can be backdrilled. Backdrilling is not always possible and leaves a residual portion of the stub (e.g., 15 to 30 mils long) affecting higher frequencies. Other techniques to mitigate stub resonance effects have been proposed including via terminations to match the stub, but these create issues at lower frequencies and are not always easy to implement.

Figure 1: Typical differential channel insertion losses with (solid red curve) and without via stub (dashed brown curve)

The solution is a meshed metallic structure implemented in the stub region, possibly in conjunction some of the techniques described above, to reduce the stub effects. Each ground (or power) via is connected the others using thin strips with a width comparable to the diameter of the vias. The portion of via stack-up not included in the stub region maintains the regular antipad structure. In addition, even for the layers where the stub is present but the high-speed vias are not present, solid metal is used.

Figure 2: Footprint example of the proposed meshed grid structure to be applied on the stub region only. This is a potential implementation for a 2-to-1 signal-ground pattern

with an offset.

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Figure 3: A three-dimensional (3D) view of the classical structure with regula...