Short circuit inside CPU during operation

Question

I have received two boards from one of our customers that have the same defect. It is a short circuit between V_CC and GND. I was able to trace this short circuit down to the MCU (which is a LPC1114 from NXP) with a thermal camera. These boards were running quite some time at the customer site, until the short occurred.

In the pictures below you can see two different MCUs. One that is still soldered on the PCB and one that was removed from the PCB and supplied with V_CC/GND.

Edit1: In both pictures, Pin1 of the CPU is on top-right corner (TOP view of the MCU).

My questions to you:

• Has anyone ever experienced such an internal MCU short?
• What could be the cause of such an internal MCU short? (Guessing accepted :-)
• Is it possible that such a short could happen when there is too high temperatures involved?

Edit2: Pinout of the MCU

Answer 1

Typical cause would be some electrical transient from outside coupled to an I/O pin, which causes latch-up and destruction via the power supply. Proper design of protective circuitry will generally prevent that in situations short of a nearby lightning strike.

Power supply transients could also cause it, only a few volts might be enough, but usually that does not occur.

Momentary reversal of the supply voltage could also cause it.

Answer 2

I agree with other answers that an external overvoltage on I/O pin is a likely cause, though an external overvoltage on V_DD pin is also a possible cause.

To fix the circuit, it would be useful to know which I/O pin received the transient. Sometimes this can be determined by measuring the internal protection diodes of the chip using the multimeter diode mode:

1. Measure with the black lead on the V_DD pin and the red lead on each I/O pin. Take note of readings.
2. Measure with the red lead on the GND pin and the black lead on each I/O pin. Take note of readings.

For a fully functional chip, the expected values would be 0.6 V on each pin. Often a damaged I/O pin will show either an infinite or zero diode forward voltage, as the protection diode has burned out before other parts of the chip got damaged.

Answer 3

To my knowledge, yes, it belongs to a transient from outside. An I/O pin or V_DD with a scarce transient current and high I/O pin loads will short the CPUs.

• In the best of cases, sometimes, it's an internal diode or a (default) latched pin at the output inducing the short.
• It may remain shorted for undetermined time until the diode or MOSFET burns completely (depending on its maximum running current).
• If the power source to which the board is connected is a linear voltage source, or an SMPS with scarce current pumps (low frequency switching SMPS or defective switching capacitor or both), the effect of latched I/O pins may have extend to the CPU internals and never recover.
• If the CPU has low current consumption even if shorted it may be an external component wired to a interrupt pin halting the CPU from start in an indefinite looping state
• There may be several other temporary V_DD to ground running conditions to investigate.

Here are a few more ideas:

• Try using a precision voltage/current source to power sequence and check the oscilloscope for possible latched pins while it boots
• Try identifying I/O pins which may be open collectors and take measures. If it's fed high while latched low, you can temporarily reverse the pin voltage and the boot-sequence may jump over the current drain and actually boot