Note that the worst situation may occur when both planes are equal,
In this PCB, they are more or less equal. It's an 8-layer stack (bottom/ Layer8=GND, Layer7=Signals, Layer6=GND, and .). But the signal traces in Layer7 have now continuous return path in both L8 and L6. Regards Amund . Fra: Gert Gremmen Sendt: 18. mars 2020 09:52 Til: Amund Westin <[email protected]>; [email protected] Emne: Re: [PSES] Stripline - discontinuity in adjacent ground planes Any signal flowing in the SIGNAL layer, will when getting to the load have to choose between 2 ground layers. (when ordinary vias are used) The signal layer with the lowest impedance is that where a good mutual coupling between SIGNAL current and GNDX is assured al along. So just verify the trace of SIGNAL in both GND's and you have your answer. Note that the worst situation may occur when both planes are equal, because the ground current may be cut in 2 parts and you lost easy control on what happens. Therefore use just 1 ground plane only. (in this 3 layer situation). Note that 1% corresponds to 40 dB, so if 99% flows where you want it to, 1% may choose the wrong route and create a possible EMI problem and 40 dB less is not much. Buried via may be he answer to your plane problem, but does increase the costs quite alot, where the single ground layer solution saves money. The situation changes in 6 or 8 layer PCBS. There you will couple to the closest groundplane using buried vias only. The generic answer is : control your return current, and avoid ambiguities. Gert Gremmen On 18-3-2020 9:38, Amund Westin wrote: Let's say, part of a pcb stack looks like this --- GND1 - SIGNAL - GND2 GND1 is solid copper fill SIGNAL layer consists of high frequency lines LVDS, USB3.0, etc. GND2 should be solid copper fill, but some traces are in that layer due to space utilization reason. These traces are crossing 90 degrees the traces in SIGNAL layer. These traces in the GND2 will create gaps and therefore problems for the return current for the high frequency lines I SINAL layer. But does it matter, as long as the GND1 layer is 100% filled, and there is no disruption of the return current path for the traces in the adjacent SIGNAL layer? Best regards Amund Westin - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected] <mailto:[email protected]> > All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) <http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected] <mailto:[email protected]> > Mike Cantwell <[email protected] <mailto:[email protected]> > For policy questions, send mail to: Jim Bacher <[email protected] <mailto:[email protected]> > David Heald <[email protected] <mailto:[email protected]> > -- Independent Expert on CE marking EMC Consultant Electrical Safety Consultant - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
