3 ms·

The article unfortunately was written by someone with no clue so we don't know why tombstoned components (shown in the picture) were not caught in inspection/te

The article unfortunately was written by someone with no clue so we don't know why tombstoned components (shown in the picture) were not caught in inspection/test. They imply the failures happened in the field, but that's not where tombstoning happens. Presumably what happened was that the supercap (looking like [1]) tombstoned in reflow. Then circuit test failed to test that it was installed so the unit was shipped. Subsequently in the field the unit suffered a sudden power loss with pending writes. Normally the supercap provides power for long enough to flush pending writes to NAND. But since it was open circuit, the power fail flush never finished, resulting in corrupted storage. Fixing the open circuit solder joint as you suggest does not remedy the problem for the user because their data is still gone. [1] https://www.digikey.com/en/products/detail/seiko-instruments/CPH3225A-2K/4747400 https://www.digikey.com/en/products/detail/seiko-instruments...

wkat4242 3y ago
> but that's not where tombstoning happens yeah I know, unless the board gets so hot it unsolders itself, which is very very doubtful (and definitely a fault of its own). I thought it was more of a stability problem though. Nothing a good backup should cover, and the device should be fine after soldering the component.
nurple 3y ago
One capacitor on a tank array would definitely reduce its total capacitance, but they are nearly always in parallel and would not cause a failure of the whole tank, and the device would be inoperative if the output of the array was shorted. I'm skeptical that losing one capacitor in the array would cause the failure mode you're describing. Especially if the age of the devices is considered, the array would have been designed with margin to withstand capacitance loss as the device ages.
- wkat4242 3y ago
  Perhaps tombstoning causes it to short the whole array? I could see that happening if it's positioned just wrong.
- lightedman 3y ago
  "I'm skeptical that losing one capacitor in the array would cause the failure mode you're describing." Depends on what the capacitor is being used for in the circuit. In many cases, having a cap fail open results in a higher current draw which kills the unit if left in operation for too long. This is the case on some of the off-road lighting I manufacture. If one cap is present and fails open at ground, the circuit overloads. If the cap is connected to ground but not the rest of the circuit, the circuit doesn't operate. Regardless, one component being off can cause a whole chain of maladies.

wkat4242 3y ago

> but that's not where tombstoning happens yeah I know, unless the board gets so hot it unsolders itself, which is very very doubtful (and definitely a fault of its own). I thought it was more of a stability problem though. Nothing a good backup should cover, and the device should be fine after soldering the component.

nurple 3y ago

One capacitor on a tank array would definitely reduce its total capacitance, but they are nearly always in parallel and would not cause a failure of the whole tank, and the device would be inoperative if the output of the array was shorted. I'm skeptical that losing one capacitor in the array would cause the failure mode you're describing. Especially if the age of the devices is considered, the array would have been designed with margin to withstand capacitance loss as the device ages.

wkat4242 3y ago
Perhaps tombstoning causes it to short the whole array? I could see that happening if it's positioned just wrong.
lightedman 3y ago
"I'm skeptical that losing one capacitor in the array would cause the failure mode you're describing." Depends on what the capacitor is being used for in the circuit. In many cases, having a cap fail open results in a higher current draw which kills the unit if left in operation for too long. This is the case on some of the off-road lighting I manufacture. If one cap is present and fails open at ground, the circuit overloads. If the cap is connected to ground but not the rest of the circuit, the circuit doesn't operate. Regardless, one component being off can cause a whole chain of maladies.