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AM3352ZCZ Differential Signals on USB Port0 are shorted

Mauro Cleris
Prodigy 170 points

Hello folks,

on 3 of 20 of our  PCBs (mounting AM3352ZCZ lot 26AGGNW) we found out that USB0_DP and USB0_DM traces are shorted.

We adopt 100% Electrical test and 100% BGA X-Ray test, so we do not believe that the concern is on the PCB or on the soldering.

Is it possible that the microprocessor itself is creating the short between the 2 traces?

Thanks for your help.

Mauro Cleris

 

 

 

 

  • 0
    TI__Guru**** 393215 points
    Hi Mauro,
     
    I think this is highly unlikely. However the differential impedance between USB0_DP and USB0_DM is 90Ohms. Is it possible that this is misleading the e-test in some way?
     
    Best Regards
    Biser
  • 0 in reply to Biser Gatchev-XID
    Prodigy 170 points

    Hi Biser,

    thx for your reply.

    At POWER OFF I measure a DC impedance of less tha 1Ohm between USB0_DP and USB0_DM.

    During the start-up phase both USB0_DP and USB0_DM move always together, at the end of the start-up both lines stay at 3.0V.

    At the moment, the only solution I see is to desolder the Microprocessor and check if the problem is on the board or directly

    on the BGA Balls.

    Best regards,

    Mauro

     

     

  • 0 in reply to Mauro Cleris
    TI__Guru**** 393215 points
    Mauro, how do your "normal" boards behave compared to what you said above?
     
    Best Regards
    Biser
  • 0 in reply to Biser Gatchev-XID
    Prodigy 170 points

    Hi Biser,

    on the 'normal' boards the processor put D+ to 3V3 (via 1K5 pull-up) and D- to GND (via 15K pull-down) (the USB port is set as DEVICE),

    on the 'not-working' board, as you can see, there is a resistive partition between D+ and D- producing the following voltage

    Vout=3V3*(15K/(15K+1K5))=3V.

    It seems the circuitry inside the processor is working properly, but the BGA balls are shorted, I have to go under X-Rays (unlucky I do not have a X-ray scope in the lab).

    Maybe is the internal bonding between the die and the package? The BGA pitch is 0.8mm, I do not expect soldering shorts with such a pitch.

    Is possible for you to verify if the lot 26AGGNW had some production problems?

    Regards and thx for the help,

    Mauro

     

     

  • 0 in reply to Mauro Cleris
    TI__Guru**** 393215 points
    Hi Mauro,
     
    I think it's highly unlikely that processors with such a defect will get out of the factory. Have you checked all external components for shorts, correct placement etc.? From what you tell me there seems to be a short somewhere, definitely. My suggestion is to check carefully everything on the board, and unsolder the processor as a last step.
     
    Best Regards
    Biser
  • 0 in reply to Mauro Cleris
    TI__Guru 62855 points

    We have not received reports of the USB signals being shorted in any of the AM335x devices.

    Since one USB signal is assigned to an outside ball and the other USB signal is assigned to the next inside ball you may be able to visually inspect the assembly for shorts.

    As a first step, I suggest you remove all external PCB components connected to these signals then check to see if the USB signals are still shorted.

    If so, I would try to use a small soldering iron and solder wick to remove the outside solder ball.  Once the outside ball is removed you should be able to see the next inside ball to determine if it was mis-formed which may indicate it was shorted to the outside ball.  After removing this ball check to see if the USB signals are still shorted.

    If so, the short is on the PCB not the AM335x device.  If not, the next step will be to remove the AM335x device.

    Regards,
    Paul

  • 0 in reply to peaves
    TI__Mastermind 27890 points

    To add to what Paul said...in every case where I have seen DP/DM shorted, the culprit was found to be an external IC. Typically, the external device (ESD) is defective or has been installed incorrectly.