PCB Design – How Are Transfer Vias Used?

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I watched a video on via basics by PhilsLab (Video: https://www.youtube.com/watch?v=WPT96w3eLAM), in which he discusses transfer vias. He presents the following picture to explain why you should place them:

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He then provides an example in a PCB software:

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Questions

  1. I want to place transfer vias on signal traces when they switch layers (as mentioned in his video.) I'm uncertain if I understand the concept correctly. Let's say I have a 6-layer PCB. On the first layer, I place signal traces, and on layer 6, I also place signal traces. Now, if I need to transition from layer 1 to layer 6, I would position the GND via as close as possible to the switching via. Do I also need to have a GND layer on the first layer? In Picture 1, the GND via doesn't seem to be connected to the signal layers. It is only connected to the GND layer. Based on this, my understanding is that I should replicate this setup. What is your opinion? Below is an image illustrating how I implemented this in KiCad V7.

enter image description here

  1. Should I apply this technique to a 6-layer board?

Best Answer

do I also need to have a GND layer on the first layer?

It depends on what kind of transmission line you are making.

For a microstrip line, the ground is on the layer "below" the signal trace. It's on layer 2 for a signal on layer 1 or on layer 5 for a signal on layer 6.

For stripline, ground is on layers above and below the signal. If the signal is on layer 4, then ground will be on layers 3 and 5.

For coplanar line, ground is on the same layer as the signal. Sometimes it's also on the layer below, forming coplanar line with conductor backing.

Whichever kinds of lines you use (and the two lines being connected might not be the same type), your "transfer via" needs to connect the ground layer(s) used by the first signal line to the ground layer(s) used by the second signal line.

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