Stack-up for a mixed-signal board
Design a stack-up for a board with high-speed interfaces (MIPI, USB), analog circuits and a power section.
| Level | ●●● onsite design / senior |
| Track | General board |
| Really testing | Reference planes, loop areas, plane capacitance. |
Free full answer — one of 12 questions answered in full on this site.
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Say this first: Every signal layer next to a solid reference plane; high-speed signals on layers with continuous ground references (stripline inside for EMC, microstrip for short breakouts); a closely spaced power/ground pair for plane capacitance; symmetric stack for warpage.
Example 8 layers: L1 signal/parts · L2 GND · L3 high-speed signal · L4 PWR · L5 GND · L6 signal · L7 GND · L8 signal/parts. Analog: placed in its own region, with its own local ground return paths over the same plane (no split), filtered supply. Power stage: wide pours on outer layers with many vias, its hot loop on the top layer.
Going deeper
Why the power/ground pair is close. Two planes 3 mil apart in FR4 give about 300 pF per square inch. That capacitance has almost no inductance, so it does the decoupling above ~100 MHz that discrete caps can't.
Quick impedance sense. In FR4, a 50 Ω microstrip is roughly twice as wide as its height above the plane: about 8 mil wide on a 4 mil prepreg. Thin dielectrics → narrow traces → denser routing, but more loss per inch.
Why symmetric. Copper and dielectric must mirror around the centre, or the board bows in reflow — which shows up later as BGA opens (see PCB-07).
Where people go wrong
- Two signal layers next to each other with no plane between them — broadside crosstalk.
- Routing a fast signal referenced to a power plane, then changing layers to a ground-referenced layer with no stitching cap nearby.
More like this in the book
This is PCB-01 from Hardware Engineer Interview Questions. The book has 8 questions on pcb layout, stack-up & manufacturing and 141 in all, each answered the same way: what to say first, how to explain it, the numbers worth quoting and the follow-ups that come next.
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