[SI-LIST] Re: A short story

  • From: "McCormick, Bob" <Bob.McCormick@xxxxxxxxxxx>
  • To: "'Suresh.Subramaniam@xxxxxxxxxx'" <Suresh.Subramaniam@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Sat, 9 Apr 2005 09:52:11 -0400

With regard to your question: "Also, is there a better/correct method to
measure pwr/gnd shorts ?"

One method is to use a thermal "sniffer" type probe.  I don't recall the
manufacturer or model since it has been years since I used this probe..but
there are probes out there for this problem. I had a similar situation where
we didn't have a short but a finite (low/unintentional impedance). We found
the problem in a matter of minutes with this probe. The probe had a narrow
light beam to show the area of the board being scanned...and I just moved
the probe up and down the board until I found the hot spot. One issue is you
could trigger the thermal sensor on a device that is supposed to be hot.. So
it depends how much power this "unintentional short" is dissipating heat
relative to other devices.

With regard to the measured impedances...do you have a "good board" to
compare? ..eg, Find out what the expected impedance 
to narrow down the problem (or pull out the schematics and start doing some
calculations to see what the impedances should be ...)
 

Good luck!

Bob McCormick
Hardware Engineer / SI
Stratus Technnologies
Maynard, MA





-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Suresh Subramaniam
Sent: Saturday, April 09, 2005 3:05 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] A short story


Hello All,

I am faced with the following situation: A fairly complex bare board 
shows no shorts between any of the power rails (1.2,2.5,3,3,5 and 12V) 
and GND. However, before power up, on a fully populated board, I am 
measuring about 10 Ohms on the 1.2V, 6 Ohms on 3.3V and 0.5 Ohms on 2.5V 
(measured with a standard Fluke multimeter) and the others are showing 
readings in the 1-2K Ohm range!  Since we had several boards on hand, I 
decided to power up one of the boards anyway. There was no smoke or 
hotspots on the board (the 1.2V and 2.5V switching regulators are rated 
for 14A).. And more importantly, majority of the interfaces tested are 
operating as advertised.  I remeasured the board after power off . Now 
the readings are ~100 Ohms on 2.5V and about the same as before on the 
other rails.

(Note: The pattern is the same on all boards.)

I  have also checked to make sure there are no symbol/footprint errors. 
Before I start to depopulate individual ICs to trace this short  on an 
unpowered board, I thought I would check in to find out if others have 
had this experience.

Also, is there a better/correct method to measure pwr/gnd shorts ?

Thanks
Suresh

-- 

--
 / 7\'7 Suresh Subramaniam        suresh.subramaniam@xxxxxxxxxx
 \ \    Xilinx                Telephone: 408-559-7778
 / /    2100 Logic Drive      Direct:    408-879-6172
 \_\/.\ San Jose, CA   95124  FAX:       408-377-9013



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