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These are STM32F746NGH6 microcontroller chips in .8 mm pitch BGA package. I noticed that there are copper tracks on the bottom of this chip, some of which lead to the edge of the chip and are left exposed. What could be the purpose of these copper tracks? Would they be useful for factory programming/debugging?

bottom of bga chip side view

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  • \$\begingroup\$ They provide electrical continuity for the plating step of the PCB production process. These connections are subsequently broken when the edge of the PCB is routed to the final shape. \$\endgroup\$
    – Dave Tweed
    Feb 27 at 0:13

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"Hard gold" plating finish requires an electrical connection to the pad to be plated during the process. You'll find that with board edge connector too, although in those cases the board edge would usually be cut with a V-score. Not that it would necessarily matter, but V-score cut prevents the ends of the tracks you are seeing from being pushed against a surface and making contact.

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  • \$\begingroup\$ Thank you for your response! However, do you think hard gold plating would be necessary on this BGA chip that comes pre-balled from the factory? (This chip is fresh out of its packaging, those solder balls are not attached by me) Additionally, wouldn't stubs on every pad cause issues with SI/EMC issues, particularly with high speed digital signals? (This chip has clock speed over 200 MHz, and quite fast pins) \$\endgroup\$
    – crossroad
    Feb 27 at 15:41
  • \$\begingroup\$ Yes to both really. As you said it could be for debugging, such as testing the circuits in a panel, if the chips are bonded onto a panel, but I think that makes even less sense. Certainly a contact bed would be more standard and robust way to do it, unless they have some special reason that I cannot think of. Testing in panel with such routes does make sense at times, but not with a microcontroller chip for many reasons. So I'd say the same carrier has been used for a non-balled version. \$\endgroup\$
    – Ralph
    Feb 27 at 15:58

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