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Question about PCB stack up

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EDA_hg81

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If I used two grounds to sandwich a high speed signal layer.

Should I keep the distance of the signal layer to each ground be even or not ?

Thanks.
 

btbass

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What you are describing is a stripline. The width of the strip and the width of the sustrate determine the characteristic impedance of the strip, or track. The distance to each ground plane need not be equally spaced, but it will all effect the impedance.
 

    EDA_hg81

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EDA_hg81

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what is the advantage and disadvtange of even distance?

what is the advantage and disadvtange of uneven distance?


Thanks
 

btbass

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It is easier and cheaper to just keep the stack up with eaqual spacing. You can find stripline calculators on the net.
Some complex stack ups have different substrate materials between layers, but these are pretty specialised applications.
 

    EDA_hg81

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EDA_hg81

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One guy told me I should make the distance uneven since if I made them even the

impudence is going to be changed dramatically.

But I think it is not a big deal.

How do you think?

Added after 11 minutes:

I keep then even anyway.
 

electron_boy

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see, if the adjacent layers are reference layers say power or ground. the signal travels on the skin of the trace and if u have different dielectric thickness on either sides of the trace the signal transition will not be same for rise and fall.

it is accepted that the impedance is going to vary. but u also need to care about the capacitance. the trace to power and trace to ground capacitances are going to be different for the stack up that has different heights of dielectric on its either sides.
 

    EDA_hg81

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EDA_hg81

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I think the even distance is the best.

Thank you all so much.
 

Rame

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    EDA_hg81

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prakash13

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i will prefer even distance,if both are ground planes
 

    EDA_hg81

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kiddy

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I show you an Example:
4 layers stackup

Layer 1 signal : 0.5 oz
--------- Prepreg, 5 - 7mils
Layer 2 GND : 1oz
--------- : Laminate, 5 - 7mils
---------- : Core (Reference layer) Prepreg, 10mil
--------- : Laminate, 5 - 7mils
Layer 3 PWR Plane: 1oz
--------- Prepreg, 5 - 7mils
Layer 4 signal : 0.5 oz

Notes: Do not route highspeed signals across split planes.
 

    EDA_hg81

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ezpcb

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Do you need the high speed lines to be impedance controlled? No need to be that complicated. Just tell us which lines to be controled, we'll adjust the width and choose the right prepreg material to meet the impedance.
the impedance could be controled within +-5%.

Mike @ ezpcb.com
 

    EDA_hg81

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chinta

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even distances are always recommonded. otherwise, when you are routing the same signal on these two layers, the trace impedance will differ.
 

    EDA_hg81

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flypcb

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It seems you prefer to take an impedance control on your circuits. Yes, you may calculate the impedance value depends on strip widths and spacings. However, the pcb fab generally will do a second caculation based on their fab machine and fab flow. Anyway, don't worry, the difference normally is very small. All in all, ask the fab shop engineers for help is needed, or you probably meet some difficults when receiving the final prod.
 

    EDA_hg81

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eswar_babu77

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When you have an impedence controlled signals, it does not matter if the distance from an trace is even or uneven w.r.t the refernec plane untill u meet the requirement.

If is always good practice to follow the balanced pcb stackup.
 

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