STM32F756VGT6 DFU USB not working
I have used DFU USB to program hundreds of STM32F7 devices over the past several years. It has worked reliably until the most recent batch of 200 boards using STM32F&56VGT6 devices. We had approximately a 10% failure of the DFU USB connection (USB not recognized). I have investigated all of the documented recommendation with no success on the failed boards. I have measured the external HSE crystal frequency which is within 32 HZ of 8.00 MHz, well within the +/-500ppm requirement for FS USB. I have double-checked voltages, PCB traces, connector soldering, USB cable, etc. to no avail. I have heated and cooled the PCB to see if there may be HSI drift.
My only conclusion is that the USB functionality inside the STM32F756VGT6 board is bad. However, I hate to rely on this as my "fix" for the problem. I have to way of concretely proving that the chip is indeed "bad".
Has anyone run into this situation with this particular STM32 variant (STM32F756VGT6)? Any other recommendations other than what is published in the Knowledge Base on this subject, or what I have tried?