GROUNDED: Your PCB, EMC/EMI, and SI Nuggets

If you want to pass EMC tests without excessive trouble, you need to ensure your PCB stackup is designed correctly.

Poor stackup choices are a common source of EMI issues, and they can derail your project faster than you might expect.

It’s not unusual to see stackups chosen by project stakeholders rather than the board designer.

I’ve observed this in many cases, and it’s typically driven by the assumption that reducing layers will cut production costs.

At first glance, it seems reasonable—fewer layers, lower expenses.

But here’s the reality: this approach often backfires.

A poorly planned stackup significantly reduces the chances of passing EMC tests.

Instead of generating revenue in the market, the project stalls in debugging phases.

Costs escalate as you iterate through PCB revisions and make repeated visits to the EMC lab.

In the meantime, clients wait for delivery while you assure them certification and production are just around the corner—sometimes for months or even years.

The initial savings from the stackup?

They’re long gone, overshadowed by delays and rework.

To illustrate the kind of issues I often see, consider this stackup from a recent project review:

- L1 - Signals

- L2 - Power (Mixed)

- L3 - Signals

- L4 - GND

- L5 - Signals

- L6 - Signals

- L7 - GND

- L8 - Signals

I’d rate this a 4 out of 10 for EMI performance (1 being high risk, 10 being excellent).

Several issues stand out. For example:

- Power Plane Placement (L2):

L2 is a mixed power plane, but it’s not adjacent to a reference/return plane (RRP). L4, the nearest GND, is separated by L3’s signals.

The energy here flows between the PWR and GND layers, L3 will certainly experience interference.

Power planes should be tightly paired with an RRP (e.g., L2 Power, L3 GND), not separated by signal layers.

Additionally, L2 serving as a reference for L1’s signals is far from ideal, especially if not executed properly.

These are basic EMI pitfalls that could have been avoided with a better stackup.

You don’t need to repeat these mistakes.

-Dario

P.S. If you’re still struggling to pass EMC tests, I’ve created an EMI Control Guide that details these and other mistakes that lead to EMC failures.

I explain the issues in simple engineering language and provide you with the practical solution.

You can get my new EMI Control Guide here: