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LMR60420: High failure rate in production using a reference design

Liam Kinne
Prodigy 10 points
Part Number: LMR60420

Tool/software:

I am having issues with the use of the LMR604205RAKR in my design. Almost half of a batch of 20 will exhibit a short on the input of the IC causing it to emit the magic smoke if you give it more than an amp or so. The design is very simple and basically follows one of the examples given in the datasheet. The only addition is an SMF28A for input protection which I have checked the polarity of.

Some units will fail like this immediately, others some time after they are assembled, which is making quality control very difficult. They are being tested at 24Vin with a good lab power supply current limited to 100mA. No load is on the output when they are tested.

My best guess is some kind of issue with the soldering, but I can't think of a bridged connection that would cause the chip itself to burn up.

Schematic for your consideration:

  • 0
    TI__Guru 72315 points

    Hello

    I agree that some sort of assembly issue may be causing the problem.

    A short on SW or BOOT might cause it.

    Also, check to be sure that the correct component values are on the failed boards.

    Can you send us images of your PCB layout.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 10 points

    The issue I have at the moment is I no longer have any working boards to compare with faulty ones. When I remove the IC from a board and probe resistance to PGND, I am getting 1.5Ohm or less when probing VIN, SW and BOOT.

    I just can't see how some boards work temporarily and then stop working later. If it was a solder bridge under the IC it should be a persistent issue.

  • 0 in reply to Liam Kinne
    TI__Guru 72315 points

    Hello

    If you still get a low Ohm reading with the device removed, then either the board

    or one of the components have shorted out.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 10 points

    Hi Frank. Sorry I should have clarified it is when I probe the removed component that I get the low readings. I have checked the board itself both populated and unpopulated and there don't seem to be any issues.