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what's your recent fail?
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nctnico:
BTW: an interesting fail (not from my side) but I still think it is good to share: About a decade ago I have designed a PCB for a customer with a SoC and some DDR memory. For today's standards it is not a high density design. This board has always produced nicely without low numbers of failed boards but the customer changed to a different assembler and the number of failed boards increased massively. A lot of effort went into finding why the board failed and it turned out that when the DDR memory is cold a significant number of boards won't start. In the end the board itself remained the only suspect and it turned out the circuit board manufacturer had made the traces wider in order to produce it on a production process not quite suitable for the board. Probably some of the length tuned serpentine traces have a short between them creating a stub or timing error. The same components on a board made with a better etching process have no problems.
rsjsouza:
Many decades ago on an internship I worked on a prototype board (an ISA PC card) that had two voltage voltage regulators (7805, 7812) vertically mounted (not my design).

Along the process of debugging the board and its firmware, I had to remove and put the board back a few times a day. Due to space constraints, this board could only be placed beside another board that controlled the Data I/O 8051 EPROM programmer.

One day in a distraction I put our board on its slot but did not realize the two voltage regulators were touching the other board... A silent but deadly zap took the EPROM programmer down. Needless to say that the design was changed and the regulators were put horizontally...
exe:

--- Quote from: David Hess on May 02, 2021, 07:18:13 pm ---After several days of randomly changing various things with no positive result, I was inspired to leave it powered up, and after several more days it POSTed.  Now it reliably POSTs, so far.

--- End quote ---

That's weird but... When I worked in a datacenter I had similar issues with motherboards that at first appeared to be dead. Like, I remember after a power outage I had to repair a very old server, in a crudely-made enclosure (replace and hdd or something like that). I dropped a screw on the motherboard. The server shut down itself immediately and denied to start again. Worse yet, I didn't have spare parts for old piece of junk it was. Lucky me, somehow it started after 15 minutes after tens of attempts to boot it.
KE5FX:

--- Quote from: nctnico on May 02, 2021, 08:28:42 pm ---BTW: an interesting fail (not from my side) but I still think it is good to share: About a decade ago I have designed a PCB for a customer with a SoC and some DDR memory. For today's standards it is not a high density design. This board has always produced nicely without low numbers of failed boards but the customer changed to a different assembler and the number of failed boards increased massively. A lot of effort went into finding why the board failed and it turned out that when the DDR memory is cold a significant number of boards won't start. In the end the board itself remained the only suspect and it turned out the circuit board manufacturer had made the traces wider in order to produce it on a production process not quite suitable for the board. Probably some of the length tuned serpentine traces have a short between them creating a stub or timing error. The same components on a board made with a better etching process have no problems.

--- End quote ---

Did you specify the stackup, or go with the fab's default?  It may have changed.  Some of the Chinese fabs have been using a single layer of prepreg between the L1 and L2 copper, which is obviously like putting a capacitor in parallel with every node on those layers.  Makes for nice skinny 50-ohm traces, at least until they reach a component pad...
nctnico:

--- Quote from: KE5FX on May 03, 2021, 06:54:34 am ---
--- Quote from: nctnico on May 02, 2021, 08:28:42 pm ---BTW: an interesting fail (not from my side) but I still think it is good to share: About a decade ago I have designed a PCB for a customer with a SoC and some DDR memory. For today's standards it is not a high density design. This board has always produced nicely without low numbers of failed boards but the customer changed to a different assembler and the number of failed boards increased massively. A lot of effort went into finding why the board failed and it turned out that when the DDR memory is cold a significant number of boards won't start. In the end the board itself remained the only suspect and it turned out the circuit board manufacturer had made the traces wider in order to produce it on a production process not quite suitable for the board. Probably some of the length tuned serpentine traces have a short between them creating a stub or timing error. The same components on a board made with a better etching process have no problems.

--- End quote ---

Did you specify the stackup, or go with the fab's default?  It may have changed.  Some of the Chinese fabs have been using a single layer of prepreg between the L1 and L2 copper, which is obviously like putting a capacitor in parallel with every node on those layers.  Makes for nice skinny 50-ohm traces, at least until they reach a component pad...

--- End quote ---
The stackup is not very critical for this design but that wasn't the problem; the stackup was the same.
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