[SOLVED] PP layer over outmost trace: why?

Hi,

I would like to ask a question on trace impedance.

When reading a book Complete PCB Design Using OrCAD Capture and PCB Editor by Kraig Mitzner, on page 125, Table 6-3, I found formulas for “Embedded Microstrip Transmission Lines”, of which the screenshot is attached.


Why there is no plane or trace over the trace being calculated? Does it mean the trace (embedded microstrip) is actually on the top (or bottom) layer? Surface (top or bottom) traces are usually placed on the outside directly with a layer of green oil covering, but not like in the diagram that there is still another PP layer over it.

I personally haven’t seen this configuration. Does anyone have experience with it? What is the purpose for overlaying another layer of PP on top of the outmost trace? Is this for security consideration such that this would make “eavesdropping” trace signals with monitoring devices (oscilloscope, wave/protocol analyzer, etc.) more difficult?


Bob

Hi,

Micrcostrip is the configuration on the Top most (or bottom most) layer of an un-coated PCB- meaning there is no Solder mask covering on the trace. While Embedded microstrip is the same configuration with Coating over the trace. This will embed the outermost copper layer inside the dielectric material and that introduces a new variable H1 to impedance calculation.

cks3976,

I got it, thanks for the explanation.

Bob

This will embed the outermost copper layer inside the dielectric material and that introduces a new variable H1 to impedance calculation.

Click to expand...

Usually, the stripline copper won't be the outermost copper layer. It's just on an inner layer of a multilayer board with no copper above it. In usual understanding, the term microstrip involves a one-side open field. With ground plane on both sides, it's called embedded stripline. It's probably on the next page in the book.

There may be various reasons to use an embedded microstrip. It's a matter of transmission line impedance design, signal routing and more. Just presume, that it serves a purpose in some situations.

The assumption of "no copper" outer layers is at least unlikely. You loose the option to place components and connection pads on the surface. Also, throughplating basically involves a copper foil during processing. You can of course completely remove it, if you like. But why?