Top-layer vs inner-layer routing
When do you route high-speed signals on the outer layer vs an inner layer?
| Level | ● screening |
| Track | General board |
| Really testing | Microstrip vs stripline in practice. |
Going deeper
Extra explanation that is not in the book: worked numbers, where people go wrong, and how to say it in the room.
Worked example — loss and impedance on the outside. At several GHz, a microstrip often has lower loss than a stripline of the same width, because part of the field is in air. But solder mask lowers its impedance a few ohms and adds loss, and plating makes the copper thickness less predictable — so outer-layer impedance tolerance is wider.
EMC in one sentence. A trace on the outside is an antenna with a ground plane under it; a trace between two planes is shielded on both sides.
A practical routing rule. Breakouts from a BGA to a nearby connector: keep them on top and short — no vias, no stubs. Anything long, or anything near the enclosure edge or a cable: take it inside.
Where people go wrong: routing a fast clock on the top layer along the board edge — it radiates well and it is in the ESD path.
Errata
No corrections known for this question. All errata
Ask about this question
Stuck on a step, or think something is wrong? Send it with PCB-04 in the subject. Good questions get answered on this page, and corrections go into the next printing.
Related
← PCB-03 Analog/digital partitioningPCB-05 HDI and via types →