Emerson COM Express Carrier Design Manual page 93

The second structure is the stripline. In a stripline, a trace or pair of traces is sandwiched between two
reference planes, as shown in Figure 40. If the traces are exactly halfway between the reference planes,
then the stripline is said to be symmetric or balanced. Usually the traces are a lot closer to one of the
planes than the other (often because there is another orthogonal trace layer, which is not shown in
Figure 40). In this case, the striplines are said to be asymmetric or unbalanced. Inner layer traces on
multi-layer PCBs are usually asymmetric striplines.
Before proceeding with a carrier board layout, designers should decide on a PCB stack-up and on trace
parameters, primarily the trace-width and differential-pair spacing. It is harder to change the
differential impedance of a trace pair after layout work is done than it is to change the impedance of a
single-ended signal. This is because the geometric factors that have the biggest impact on the
impedance of a single-ended trace are H1 and W1. See Figure 39, Figure 40, Table 27.
Both H1 and W1 can be manipulated slightly by the PCB vendor. The differential impedance of a trace
pair depends primarily on H1, W1, and the pair pitch. A PCB vendor can easily manipulate H1 and W1,
but changing the pair pitch cannot generally be done at fabrication time. It is more important for the
PCB designer and the Project Engineer to determine the routing parameters for differential pairs ahead
of time.
Work with a PCB vendor on a suitable board stack-up and do your own homework using a PCB
impedance calculator. An easy-to-use and comprehensive calculator is available from Polar
Instruments (www.polarinstruments.com). Many PCB vendors use software from Polar Instruments for
their calculations. Polar Instruments offers an impedance calculator on a low cost, per-use basis. To
find this, search the Web for a "Polar Instruments subscription". Alternatively, impedance calculators
are included in many PCB layout packages, although these are often incomplete when it comes to
differential-pair impedances. There also are quite a few free impedance calculators available on the
Web. Most are very basic, but they can be useful.
Figure 39 Microstrip Cross Section
Figure 40 Stripline Cross Section
COM Express Carrier Type 2
Design Guide
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