An overheating MCU (Microcontroller Unit) can lead to crashes, instability, or permanent damage. Here are the most common causes and how to fix them:

1. Excessive Current Draw (Most Common Cause)
Symptoms:

• MCU gets hot even in idle state.
• Voltage drops when MCU is active.

Possible Causes & Fixes:
Short Circuit

• Check for accidental solder bridges, miswired pins, or damaged PCB traces.
• Use a multimeter in continuity mode to test for shorts between VCC and GND.

Overloaded GPIO Pins

• Driving high-current devices (motors, LEDs, relays) directly from GPIO can exceed the MCU’s current limits (usually 20mA per pin, ~100mA total).
• Solution: Use a transistor (MOSFET/BJT) or driver IC (e.g., ULN2003) for high loads.

Power Supply Issues

• Incorrect voltage (e.g., feeding 5V to a 3.3V MCU).
• Unstable power (ripple/noise from a bad regulator).
• Solution: Use a stable LDO regulator (e.g., AMS1117) and check voltage with a multimeter.

2. Software Issues (Runaway Code)
Symptoms:

• MCU heats up only when running code.
• Core usage spikes to 100%.

Possible Causes & Fixes:
Infinite Loops or Blocking Code

• Example: A while(1) loop without delays or sleep modes.
• Solution: Add delays (delay()) or use low-power modes (SLEEP in AVR/STM32).

PWM or High-Frequency Signals

• Constantly toggling GPIO at high speeds (e.g., unoptimized bit-banged protocols) can cause heat.
• Solution: Use hardware timers/PWM instead of software loops.

Faulty Firmware (Clock Misconfiguration)

• Overclocking or incorrect clock settings (e.g., STM32 running at 72MHz without proper cooling).
• Solution: Verify clock settings in your IDE (e.g., STM32CubeMX, Arduino clock config).

3. Poor PCB Design or Layout
Symptoms:
Overheating persists even with correct code and wiring.

Possible Causes & Fixes:
Insufficient Decoupling Capacitors

• Missing 0.1µF ceramic capacitors near the MCU’s VCC pins can cause power noise.
• Solution: Add decoupling caps (100nF) close to each power pin.

Bad Grounding

• Long/weak GND traces increase resistance, causing heat.
• Solution: Use a solid ground plane and star grounding for high-current paths.

No Heat Dissipation

MCUs with high workloads (e.g., ESP32 running Wi-Fi) may need a heat sink or better airflow.

4. Defective MCU or Components
Symptoms:

• MCU overheats immediately on power-up (even without code).
• Burning smell or visible damage.

Possible Causes & Fixes:
Damaged MCU (ESD, reverse polarity, overvoltage).

Solution: Replace the MCU and check for input voltage spikes.

Faulty Voltage Regulator

• A bad LDO/DC-DC converter can deliver unstable voltage.
• Test: Measure VCC with a multimeter (should be stable at 3.3V/5V).

5. Environmental Factors
High Ambient Temperature (e.g., inside a sealed enclosure).

Solution: Add ventilation or a small fan.

Stacking Multiple Boards (trapped heat).

Troubleshooting Flowchart

1. Check for Shorts (VCC to GND).

2. Measure Current Draw (Idle vs. Active).

• Normal: <50mA (for most MCUs).
• High: >100mA indicates a problem.

3. Verify Code (remove all peripherals, test bare-minimum firmware).

4. Inspect PCB Layout (decoupling caps, grounding).

Quick Fixes Summary

Final Tip
If unsure, test the MCU on a breadboard with minimal circuitry to isolate the issue. Many overheating problems stem from shorts, bad power, or software bugs.