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Design >> RF Design >> Best choice of layers in a multilayer PCB
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Message started by syntax on Mar 28^th, 2007, 4:58am

Title: Best choice of layers in a multilayer PCB
Post by syntax on Mar 28^th, 2007, 4:58am

Having to design a circuit up to 2 GHz, and having a 4 layers PCB of 360, 800, 360 micron dielectrics, what is best choice for top to ground thickness (360, 1160, or 1500 micron?)
Thanks for suggestions

Title: Re: Best choice of layers in a multilayer PCB
Post by mg777 on Mar 28^th, 2007, 9:44am

The answer depends on your application, the maximum trace length & the layer materials.

Say your board material is FR-4 for all layers. For a 50 Ohm single ended microstrip on the 14 mil layer you can expect a loss of about 0.1 dB/inch at 2 GHz, mostly arising from the dielectric. The 32 mil substrate will exhibit a slightly lower loss - a second order effect, since an ideal TEM line has a dielectric loss that is independent of geometry, depending only on frequency and tan(δ). However the 50 Ohm line width will be ~ double that in the 14 mil case, which can impact routing. Edge-coupled differential lines are more susceptible to odd mode dielectric loss as the electric field lines get scrunched up between the traces, and the current crowds the sides of the conductors.

In short, at 2 GHz you don't have any serious problems. The same FR-4 for a 36 inch 10 Gbps serial backplane would become a formidable challenge when you throw in connectors.

M.G.Rajan
www.eecalc.com