4 layers with no solid ground
To cut cost, a design with a 1 GHz processor and DDR4 must go on a 4-layer board. The layout lead proposes signal / signal / power / signal, with no solid ground layer. How do you respond?
| Level | ●●● onsite design / senior |
| Track | General board |
| Really testing | Trade-off honesty — a trick question. |
Going deeper
Extra explanation that is not in the book: worked numbers, where people go wrong, and how to say it in the room.
What the standard 4-layer board really looks like. A typical 1.6 mm board has thin prepreg between L1–L2 and L3–L4 and a thick core in the middle. So L2 and L3 are far apart — there is almost no plane capacitance between them — and the outer layers each reference the plane right below them. With 0.1–0.2 mm prepreg, a 50 Ω microstrip is roughly 7–15 mil wide.
Turning the push-back into numbers. Show the lead what changes: the return path for every DDR net, the crosstalk between adjacent signal layers, the extra decoupling capacitors, and the EMC risk. Then compare with the cost delta of the right stack — often zero, because a 4-layer board costs the same whatever you put on each layer.
Where people go wrong
- Accepting the stack silently, then fighting EMC failures for months.
- Pushing back without an alternative — always offer "same cost, different layer assignment".
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-02 in the subject. Good questions get answered on this page, and corrections go into the next printing.
Related
← PCB-01 Stack-up for a mixed-signal boardPCB-03 Analog/digital partitioning →