buried trace impedance

John Larkin · 2006-12-19T23:18:49+00:00

Suppose we have a 6-layer pcb, FR4, 10 mil dielectrics for the sake ofargument. Ground plane is Layer 3. L1 traces are simple microstrip, 20 mils above the plane, so somethinglike Appcad gives the trace impedances, about 87 ohms for a 10 miltrace.If the L2 trace were pure microstrip, 10 mils wide above a 10 mil dielwith no overlay, the impedance would be 66.5 ohms.But the L2 traces are buried microstrip. They are 10 mils above theground plane but have 10 mils of epoxy glass on top. So the upperdielectric lowers the impedance. I coded an equation from a Motorola appnote, but like most simpleequations of this sort, it's not very good. For the situation above, a10 mil buried trace, it predicts an impedance *above* 66.5 ohms!Wadell's book suggests that the buried impedance will be about 0.85 ofthe non-overlay impedance, so my L2 trace will be maybe 56 ohms. Isthis about right?Anybody have or know know of a calculator app that does buried traces?John

Dec 19, 2006 21 Replies

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Fred Bartoli 19 years ago

John Larkin a écrit :

Hmm I thought it was $350... but just for the TL pairs. Otherwise I think it's free.

Thanks, Fred.

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qrk 19 years ago

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has a nice calculator, but really expensive.

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has a free impedance solver, but clumsy to use. You can use arbitrary shapes.

Mark

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