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SN74AVC4T774: Issue with first production run

Jason Williams9
Prodigy 50 points
Part Number: SN74AVC4T774

Hi we have trailed 30 units successfully over 4 months of continuous operation using this part however in our first run of production boards we have found an issue with around 40% of the first 500 units. The SN74AVC4T774 has failed and this manifests itself in a verity of ways. First unit had an open circuit between the input to output direction (VCCA to VCCB) and the second looked at so far has a short pulling down the modems UART to ground (VCCB to VCCA). Below is out circuit in this section with 3V3on VCCA and 1V8 on VCCB. In the design VCCA is always powered on but VCCB gets cycled if there is a connection fault with the modem, say 100 times over three months from first units. This batch is different to the last the first batch used for the 30 boards has code ZVK73J the 500 has a code ZVK74J. the boards were loaded by a reputable supplier with the correct heat profiles and the chips came from Digikey. Have you ever seen this issue before please ?

 

  • 0
    TI__Guru* 97046 points
    Hi Jason,
    I have asked our translation expert to look into this and get back to you.
  • 0 in reply to Emrys Maier
    TI__Mastermind 46065 points
    Hi Jason,

    I'm happy to help with this issue but could I get some more details on this issue.

    The open and short issues, do they happen to all channels or are there certain channels that are affected? I see two devices in the schematic, both devices exhibit the same issues? When VCCB is cycled, are the inputs being driven? Lastly, would I be able to get scope shots of a working board and non-working board?

    I haven't seen an issue like this. The open you are experiencing could be attributed to the device being in a high impedance state. The short issue may be possible damage to the device, or a leakage path from the ESD protection. I'll go ahead and order some sample in case I also need to run some bench testing.
  • 0 in reply to Emrys Maier
    Prodigy 50 points

    Yes thanks

    Hi no only on certain channels

    No only one of the two IC’s so far

    Yes my belief is the on the VCCA side the inputs would be hi as the micro controller uart is idle hi

    Scope shots I can supply today thanks

    Is having OE tied to gnd  always a problem ?

    Cheers

    Jason

  • 0 in reply to Jason Williams9
    Prodigy 50 points

    Hi this a Ublox Schematic from their integrator s Manual

  • 0 in reply to Jason Williams9
    TI__Mastermind 46065 points
    Hi Jason,

    Could you specify which channel? Or is it a random channel for every failing device?

    Could you also specify which device is the one that is failing (U4 or U6)?

    I don't see anything wrong with the schematic. To your question, connecting the OE pin straight to GND is done quite often. We recommend to pull it up to VCC with a pull-up to keep the device in high impedance during power up and power down, but that would require an addition control pin to drive it low. Because of this, many people just tie it to ground to avoid using more I/O pins. It's possible like I said that the internal ESD structure could have been damaged causing the short.
  • 0 in reply to Dylan Hubbard
    Prodigy 50 points

    Hi sorry for the late response however I have just come off the production line where we had the issue.

    Both U6 and U4 were failing and the damaged channels seem random sometimes opens sometimes shorts.

    We received another batch this time from mouser of 100 IC's (batch 85 this time) and I personally supervised the rework of the IC on the failed boards.

    Out of 100 pieces this time we had zero failures of the reworked parts, that is 100 pass out of 100.

    The manufacturer did two runs initially, one of 100 pieces and one of 500 pieces.

    Out of the 100 off run they had 10% failures after testing

    Out of the 500 off run they had 40% failures after testing

    They were the same batch (74) from the same reel from the same supplier

    Thankyou for your help

     

  • 0 in reply to Jason Williams9
    TI__Mastermind 46065 points
    Hi Jason,

    Is it possible that the manufacture wasn't taking the necessary precautions when handling the devices when you weren't supervising? The randomness of the issue and the way it fails points strongly to ESD failure. You can start an FA with some of the failing parts in which i will be able to run some tests here.