An ESC rarely dies for no reason. It dies for a reason nobody saw happen. The good news is that a dead board usually leaves evidence, and once you know what to look for, you can tell a crash from a wiring problem or a bad setting in about a minute.

This matters for more than curiosity. If you don’t find the cause, the next ESC goes the same way.

Before you touch anything #

Take photos. Both sides of the board, good light, close enough to read the markings on the parts. Do it before you desolder anything, because desoldering heat can hide exactly the marks we need to see.

And please don’t plug it in again “just to check.” Despite Voltara having protections in place at startup that check the health of the MOSFETs and motors, it is safer to just not plug it in if obvious damage occurred.

Burnt MOSFETs and shoot-through #

Each motor phase is driven by two MOSFETs stacked between battery positive and battery negative. One connects the motor wire toward positive, while the other connects it toward negative. Under normal operation, they must never be fully on at the same time.

Shoot-through happens when both MOSFETs conduct at the same time. This creates an extremely low-resistance path directly across the battery and produces currents far beyond normal operating conditions. Damage can happen extremely quickly.

One advantage of the VOLTARA design is that each motor output has its own shunt resistor for current measurement. In severe failures, this shunt can also become the weakest point in the current path and open before the damage spreads further through the board.

Here’s the part that is often misunderstood. Shoot-through caused directly by incorrect firmware timing is uncommon on a properly functioning ESC. The gate driver inserts a small delay, called dead time, between switching one MOSFET off and switching the other on.

If this timing were fundamentally wrong, the problem would normally be repeatable across boards and operating conditions rather than appearing as an isolated failure after a crash, motor stall, or other extreme event.

What we usually find on damaged boards is a chain reaction:

1. One MOSFET fails first, commonly short-circuit.
2. Its partner later switches on normally into that short.
3. A very large current then flows through both devices, damaging the pair and sometimes surrounding components.

So the important question is usually: what caused the first MOSFET to fail?

Typical causes include:

• Voltage spikes. Missing or undersized battery capacitors, long battery leads, or poor connections increase voltage overshoot when the MOSFETs switch. Repeated spikes approaching or exceeding the MOSFET rating can eventually cause failure.
• Heat. Sustained high current, insufficient airflow, oversized propellers, or a tightly enclosed stack can push the power stage beyond its thermal limits.
• An abrupt motor stop from high RPM. A spinning motor stores considerable energy. A sudden stop or violent change in motor speed can produce very high phase currents and voltage transients inside the ESC.
• A shorted motor wire. Carbon fiber is conductive. Damaged insulation rubbing against an arm or another conductor can create phase-to-phase or phase-to-frame faults.
• A damaged motor. A partially or fully shorted winding can present an abnormal load to the ESC and may develop only after the motor is already running.
• Water, dirt, conductive debris, loose wire strands, or crash damage.

What it looks like: in the worst case, a pair of MOSFETs on one phase may appear cracked, discolored, or cratered. On VOLTARA ESCs, however, the phase shunt can sometimes open during a severe fault, leaving the MOSFETs looking relatively normal while the shunt itself is visibly damaged.

Often, one motor output will be dead while the other three continue to operate normally.

Another, less common way for both MOSFETs to briefly conduct is called parasitic turn-on.

This can happen during particularly violent electrical events involving:

• very fast voltage changes,
• hard braking or sudden motor stops,
• voltage ringing,
• and large switching spikes.

These disturbances can momentarily push the gate of a MOSFET high enough for it to partially switch on even though the gate driver is commanding it off.

The low-side MOSFET is the most common victim because fast movement of the phase voltage can couple through the MOSFET’s internal gate-drain capacitance and momentarily raise its gate voltage. The same basic effect can occur on the high side as well, although it is less common.

On a correctly functioning ESC, parasitic turn-on is generally associated with unusually severe electrical stress rather than normal operation.

Cracked and exploded capacitors #

“Capacitor failure” is really two separate problems with two separate causes.

The small ceramic capacitors on the board #

These are the tiny tan or grey rectangles scattered near the MOSFETs and the chips. They’re ceramic, and ceramic doesn’t bend. When the board flexes, they crack.

Boards flex more than people think. A carbon frame flexes in a crash, and a hard-mounted stack goes with it. Over-tightened stack screws bow the board before you’ve even flown. A screw head or nut touching the underside puts a pressure point right where you don’t want one. Even soldering thick battery leads with the iron parked on the pad for too long can thermally shock a cap sitting next to it.

The nasty thing about a cracked ceramic cap is the delay. The crack can be invisible, and the cap can keep working for days or weeks until it finally fails short. So “it died while I was just hovering” doesn’t mean hovering killed it. The damage often happened in a crash three packs earlier.

What it looks like: a small black crater or burnt spot next to a capacitor, sometimes a scorch mark on the board around it radiating and possibly damaging neighbouring parts. Occasionally just a hairline crack you can only see under magnification.

The big electrolytic capacitor on the battery leads #

This is the cylinder soldered at the battery pads. When it fails, the top bulges, it leaks, one leg is broken or melted, or it vents with a bang and leaves a mess.

Causes, in the order I see them:

• Voltage rating too low for the pack. A cap that was fine on 4S gets moved to a 6S build.
• Mounted too far away. A cap at the end of long legs does much less. Its job is to catch spikes right at the ESC.
• Reverse polarity. Soldered backwards, it fails the moment you plug in.
• Too small, or not low-ESR. It heats up absorbing ripple it can’t handle.

Braking makes the cap work harder too. When the ESC slows the motors quickly, some of that energy is pushed back onto the battery rail, and the capacitor has to absorb the fast transient before the battery and wiring can respond.

What to use: a low-ESR electrolytic, at least 35V for 6S and 50V for 8S, soldered as close to the battery pads as you can get it, with short legs.

Other things worth checking #

• One motor much hotter than the others, or smelling burnt. Suspect the motor before the ESC. Spin it by hand, feel for roughness, look for dark spots on the windings.
• Green or white crust on the board. Moisture. Dew counts. So does flying low over wet grass.
• Melted insulation on the phase wires near the pads. Look for excessive current or a poor/high-resistance solder joint.

Quick reference #

What you see	Most likely cause	Fix before the next board
FET pair burnt on one phase	Spike, heat, stall, or shorted motor wire	Add or upgrade the cap, shorten leads, check the motor and wire insulation
Small crater next to a small part	Cracked ceramic cap from board flex	Soft mounting, correct screw torque, nothing touching the underside
Big cap bulged or vented	Rating too low, too far away, reversed, or too small	Correct rating, low-ESR, mounted right at the pads
Dead output, no visible damage	Gate driver or MCU pin or one MOSFET shorted	Same as the first row
Corrosion	Moisture	Conformal coating, dry storage