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The power and ground layer for signal lines routing in 4 layer PCB

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contagiouseddie

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Hi Guys,

I'm starting my 1st 4 layer PCB. I'm a bit divided on signal lines routing on the layers. I'm not sure on how much can we use the power & ground layer for signal lines routing. I know I should avoid using both of them as they are supposed to be dedicated ground & power plane respectively. But, around how many percentage of these 2 layers can be used sparingly for signal lines?

Thank you guys. :D
 

sigint

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4 layer pcb signal

It depends on what your signal requirements are, is it a high speed board with controlled impedance, or is it a consumer application with slow digital and possibly analog, if the later is the case you may route as much as you want on the power plane, keeping in mind that continuity must be kept and a generous amount of copper must be available from the power source to the higher current devices - the ground plane is better left untouched or used only as a last resort - if it is a high speed board, it's better to add layers rather than compromise your planes - common sense applies.

SiGiNT
 

Rame

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Azulykit

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Re: 4 Layer PCb Routing

Estimates will vary, but I usually consider anything above 2 GHz as the region where RF considerations become significant issues. Wavelengths are on the order of PCB sizes and transmission line effects are seen.

Shielding and via's also start to be significant.
 

contagiouseddie

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Re: 4 Layer PCb Routing

I'm adding another 2 layer to the board, so now it's 6 layer.
The circuit has external memory interface (SDRAM) but running at 48 MHz. The microcontroller runs at 55 MHz. I guess 6 layer is enough for this project. The ground and power plane will solely for themselves only. I'm using the book "High Speed Digital Design - A Handbook of Black Magic" by Howard Johnson as a guide and also some other reference. Important analog signal lines and sensitive signal lines I route them 1st.

Thanks for the advice.
 

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