Alternate Stack Ups - Advantech SOM-ETX Series Design Manual

4.2 Alternate Stack Ups

When customers choose to use different stack-ups (number of layers, thickness,
trace width, etc.). However, the following key elements should be observed:
1. Final post lamination, post etching, and post plating dimensions should be
used for electrical model extractions.
2. All high-speed signals should reference solid ground planes through the
length of their routing and should not cross plane splits. To guarantee this,
both planes surrounding strip-lines should be GND.
3. Recommends that high-speed signal routing be done on internal, strip-line
layers.
High-speed routing on external layers should be minimized in order to avoid EMI.
Routing on external layers also introduces different delays compared to internal
layers. This makes it extremely difficult to do length matching if routing is done on
both internal and external layers.
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Advantech SOM-ETX Design Guide
Chapter 4 General Design Recommendations