# PCB 4 Layer StackUp 2 Questions

I read some of Electromagnetic Compatibilty Engineering book and it advices 4 layer stack as signal-ground-power-signal.

I allocated layer 2 as continuous ground nothing else. And since i will drive 4 motors i allocated layer 3 as lipo battery voltage's copper pour. Layer 1 and 4 are for signals and 5v, 3.3v connections that are necessary for mcu type components.

1) Since motors draw too much current 12 amps max each, would this affect ic components and can layer 3 handle this much current for 5-5cm board.Calculator says 50A with 0.7mil copper require around 340mm thickness (If i could use same pcb for power distrubiton to motors it would be nice)

2) Also i need to draw only one 0.5mm trace line for 1.8v from layer 3 is it okey?

• Airflow is important to get rid of $$I^2 R$$ losses. A good starting point for heavy copper is 1 A per mm track width. 12 mm tracks on the outside and if you run into trouble you can always add some solder to the outside tracks to increase the current. – skvery Feb 20 '17 at 18:45
• 1 oz copper is 1.37 mils. 2 oz usually does not cost any more so 2.74 mils is what you should use if external tracks. Internal tracks are usually 1 oz when the external are 2oz. So about a 1.3" (33mm) trace width. Internal tracks need to be much wider (2.6x) dependent on allowable temperate rise. – Misunderstood Feb 21 '17 at 1:15

Here are some general pointers since all we know at this point is that you are driving some high currents (12A) from a battery pack:

1. The ground and power planes are a good idea when there are going to be many chips / devices connected between them, distributed across the board. The planes set up a sort of distributed capacitance and low inductance which is effective at high frequencies. It is not a good idea to have the power for your motors flowing through the plane. The width of the trace and copper thickness determine the conductivity and total copper loss in the trace: outer layers can dissipate more due to convection.

2. Identify your high current paths. This will typically be from the battery +ve terminal to a MOSFET/switch, to the motor, and back. You should try to keep this path length as short as possible: perhaps even using wires instead of PCB traces.

3. You also mention that you need to draw only one 0.5mm trace line for 1.8v from layer 3 which I suspect is your control line. In this case you should lay it very carefully to ensure that the motor currents do not cause ground bounce.

In summary, it would be better if you were to post your circuit (some abstraction should be ok) in addition to how you plan to lay it out; and ask for feedback.

• Thanks for answer. I decided to create a power distrub. board seperately. Because that much current can cause heat and other problems so it is best to create a seperate board. Also if i add holes to heat dissipation of layer 3 then it ruins the design – snrIcmn Feb 20 '17 at 19:26

One square of copper foil is 0.0005 ohms (500 microOhms). Thus 50 amps thru that square produces 25 milliVolts drop.

simulate this circuit – Schematic created using CircuitLab