ESP32-S3-Mini-1 custom hardware-no signs of life from the processor

[AL_IgorO]

Working with a batch of newly build custom boards here and while most work, I have some population with no signs of processor life.
Checked so far:

• Supply rail looks ok

• No console output

• Nothing changes if I pulse RST

• Input current very low (700uA or so vs 13-16mA on working boards) and looks to be pulsing at 0.5s intervals to 1mA

• I see no leakage current into GPIOs that have pullups to rail

Looking for ideas what to check. If I assume that VIN pin is not soldered (hard to tell even on Xray), wouldn't I see the chip trying to boot from leakage into any GPIOs that have pullups connected?

[ESP_Sprite]

Smells like Vin not connected, I imagine the current through pullups is too little for the chip to properly boot up and that causes the input current spikes. Either that or some EN problem. (Or perhaps GND not connected?) If looking at the connections is hard, you could try to desolder or pry off the metals shield of the Mini and probe the pins of the ESP32 directly, perhaps that renders some insight.

[AL_IgorO]

Good point re shield! We tried Xray so far but with shield walls it's hard to tell.
As far as leakage into GPIO, can we assume they have typical ESD steering diodes to the rail? So ifI put supply voltage on one of them via small resistor I should see something change /micro boot?

[AL_IgorO]

Update, upon shield removal I could confirm power was present, but Enable pin was floating. Connected it to nearby power pin and the part booted.

[ESP_Sprite]

Nice I imagine you won't do it in production, but just for reference: the ESP32 does need a proper power-on reset (usually provided by something like an 10K/1U RC network) to properly boot up. Tying the EN to VCC will likely make it work most of the time, but we can't guarantee that functionality.

[AL_IgorO]

Absolutely- this was just a proof of concept to validate the failure mode. There is an RC on the board it's just not connected to this pin due to module soldering issue.
Which brings me to- any guidance on what the datasheet calls "proper amount of paste" if using center pads?
Stencil thickness, reduction percentages etc?
Our footprint is made per datasheet:

ESp32S3.PNG (35.62 KiB) Viewed 2723 times

[ESP_Sprite]

Sorry, I have no idea. If no one else answers, you could try mailing sales@espressif.com, they should have more details there I think.