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Unlike microstrip, stripline transmission lines are fully contained within a substrate – which is sandwiched between two ground planes. Due to the substrate impregnated nature of stripline, these transmission lines can only exist on internal routing layers and require a minimum of 3 board layers (2 ground planes and a routing layer). Though the image above shows the substrate as a single, contiguous layer, if you’re familiar with the pcb construction process, you’ll know that isn’t the case. The half of the substrate below the trace is part of the PCB laminate while the substrate above the trace is prepreg. The dielectric of both of these materials affects transmission line impedance. With proper material selection, both techniques can provide excellent performance up through the milimeter wave frequency range. Though, like many engineering decisions, there are tradeoffs associated with each technique. Based on the specifics of your design it’s up to you to make the right choice. Microstrip: Dielectric losses are less (when using identical materials) Cheaper and easier to manufacture Location of traces on top and bottom layers leads to easier debugging. Stripline: Greater isolation of transmission lines Supports more densely populated designs (traces are smaller, large number of internal layers possible) Requires stricter manufacturing tolerances.

Here the internet reference which Lenny - as usual - has forgotten http://www.bitweenie.com/listings/microstrip-vs-stripline/ or is Lenny=Nicola T. Or has Nicola taken the org text from Lenny.

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