[SI-LIST] Re: A short story
- From: Suresh Subramaniam <Suresh.Subramaniam@xxxxxxxxxx>
- To: weirsi@xxxxxxxxxx
- Date: Sat, 09 Apr 2005 20:35:39 -0700
Andy,Steve,
Thanks for your valuable insights.
On the issue of residual charge on the caps, I have never seen any board
(in the current lot) that has been powered up at least once return to
its "virgin" state on the 2.5V rail, even after having spent several
days in the unpowered state. So something that was 0.5 Ohms blows open
to ~100 Ohms. This is the point of concern.
-Suresh
steve weir wrote:
>Andy, I agree that an apparent half to one ohm load is not much of a board
>short, where a few to perhaps ten milliohms would be more typical of
>something in the etch. However, if the reading is persistent, it is not
>unthinkable that a component is responsible. Is it a defect? Well, lots
>of CMOS devices now have very low voltage thresholds. Prior to Virtex 4,
>FPGAs for one would exhibit sometimes huge current spikes as the CMOS
>biased into the linear region during supply ramp-up. If the supply
>impedance was not sufficiently low, or the voltage ramp fast enough, a
>lock-out condition could occur. With logic in the 1V range, this
>phenomenon is more and more common and can present headaches for power-on
>resets.
>
>I would just like to add that there are several steps that should be
>followed in board mfg. One of them is to condition any tantalum capacitors
>that may have been damaged during the solder process. This is
>particularly important with Mn02 tantalums where the failure mode is a
>short circuit and a fire. Kemet researchers have shown that proofing
>capacitors with a voltage limited current source 100% heals the damaged
>sites. John Prymak who will be on the capacitor inductance panel at
>DesignCon East just delivered a very nice paper on this subject at
>CARTS. I don't know if it is publically available or not.
>
>Regards,
>
>
>Steve.
>
>
>At 09:27 PM 4/9/2005 -0400, Andrew Ingraham wrote:
>
>
>>Most likely, there were and are no shorts on your boards. But there are
>>lots of capacitors.
>>
>>An ohmmeter measures the current. Depending on the charge of those
>>capacitors at the time you measure it, the current may be small, big,
>>positive, or negative. After powering off the board, they probabbly had
>>some charge they didn't have before, and so the current and the reading
>>differed.
>>
>>If you left the ohmmeter connected overnight, it might approach the same
>>reading regardless of whether the board was ever powered or not. (It might
>>take longer than overnight.)
>>
>>Try measuring the resistance of just an electrolytic capacitor. Reverse the
>>leads a few times and see what happens.
>>
>>Even without capacitors, 0.5 ohms isn't much of a short, if you are
>>measuring at the power planes. Your 14A regulator wouldn't blink. Expect a
>>real short to measure a lot less resistance.
>>
>>Regards,
>>Andy
>>
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>
>
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