5 Common Mistakes in 4-Layer PCB Routing

Let’s be honest—moving from a simple 2-layer board to a 4-layer stack-up feels like upgrading from a bicycle to a sports bike. Suddenly, you have dedicated power planes, better signal integrity, and way more room to route.

But here’s the trap: Just because you have more layers doesn’t mean your design is automatically better. In fact, I see engineers making the same routing mistakes over and over again, which leads to EMI issues, noisy signals, and failed prototypes.

Whether you are designing your first STM32 board or a complex IoT gateway, avoiding these five mistakes will save you a lot of debugging time (and money).

1. Treating the Ground Plane like Swiss Cheese

We love 4-layer boards because they give us a dedicated Layer 2 Ground Plane (GND). It acts like a shield and a return path for your signals.

The mistake? Cutting it up.

If you route a few signal traces on Layer 2 just because you "ran out of space" on Layer 1, you are creating slots in your ground plane. When a high-speed signal on Layer 1 tries to find its return path directly underneath, it hits that slot and has to loop around it.

2. Forgetting the "Return Path" for Vias

Imagine a signal traveling on the Top Layer (Layer 1). Its return current flows quietly underneath it on Layer 2 (GND).

Now, you drop a via and switch that signal to the Bottom Layer (Layer 4). The signal jumps down... but how does the return current get there? Layer 4 references Layer 3 (Power), not Ground. The return current has nowhere to go! It wanders around looking for the nearest capacitor to jump through, radiating noise the whole time.

The Fix: Place a Ground Via right next to your Signal Via. This gives the return current a bridge to jump layers with the signal.

3. Routing Sensitive Signals Under Noisy Power

On a standard stack-up (Signal - GND - Power - Signal), Layer 3 is usually your Power Plane (3.3V or 5V).

If you have a noisy switching regulator (like a buck converter) on your board, that power plane is going to be noisy. If you route sensitive analog signals (like ADC inputs) on Layer 4 directly underneath that noisy power section, your sensor readings will be garbage.

Keep sensitive traces away from the power switching loops, even if they are on different layers.

4. Not Calculating Impedance (The "Looks Good" Trap)

"I'll just make the USB traces 10 mils wide, that looks about right."

Please don't do this. For high-speed signals like USB, Ethernet, or SPI, the trace width matters immensely. The distance between Layer 1 (Signal) and Layer 2 (GND) determines the impedance.

If your fabricator uses a standard prepreg thickness (like 0.2mm), a 10-mil trace might result in 70 ohms impedance when you actually need 90 ohms for USB. Always use a calculator (like Saturn PCB Toolkit) or ask us at GSA Electronic for our stack-up details before you route.

5. Ignoring Thermal Reliefs on Multilayer Boards

This isn't an electrical issue; it's an assembly nightmare.

When you connect a component leg directly to a massive internal Power Plane on Layer 3, that plane acts like a giant heatsink. When you try to solder it, the heat gets sucked away instantly. The result? A "cold solder joint" that might break loose two weeks later.

Always enable Thermal Reliefs in your CAD software (Altium, KiCad, etc.) for plane connections. It makes hand soldering and reworking possible.

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