[SI-LIST] Re: How to solve short circuit issues in high dense pcbs
- From: "Istvan Nagy" <buenos@xxxxxxxxxxx>
- To: "rskiruban" <rskiruban@xxxxxxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 31 Oct 2011 21:14:16 -0000
Hi,
You might have known this already, but I think it's worth mentioning:
If you measure power planes with a multimeter, you would find that most low
voltage rails appear to be a short circuit, like 1...10 Ohms. For example a
1W+ processor's core and I/O rails normally look like this. That is not a
short circuit, but it's normal.
Sometimes there is really a short circuit. Then what I do is either use a
thermal camera, or use a logic analyzer on the power sequencing logic to
find out which power rail has the short. I normally use an FPGA for
sequencing and use the Actel-Identify on-chip debugger and some tricks in
the sequencer's VHDL code. The sequencer turns every rail on, and if a
powerOK signal doesn't get asserted on one rail (due to a short circuit
load), then the sequencer shuts down within milliseconds and latches all
signals (Power_OK and Power_Enable), then it displays them on my screen.
This way its easy to see which rail has really a short, without leaving it
on for seconds or minutes (risking a damage) to inspect by a human (me in
the lab). So its safe enough. If you dont have an FPGA on your board for
power sequencing, then you can still use an oscilloscope with lots of
channels.
You might end up lifting up inductors or MOSFETS for seeking the rail with a
short. Lots of power rails feed subordinate power rails, wherre each rail is
"generated" by a DC/DC converter or a powerswitch (Mosfet, especially for
ACPI motherboards). Any of the devices on any of those rails can be the
sinner, but you can isolate the rails by desoldering components (that would
connect them, like the inductors, mosfets).
regards,
Istvan Nagy
Bluechip Technology
UK
-----Original Message-----
From: rskiruban
Sent: Monday, October 31, 2011 8:01 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] How to solve short circuit issues in high dense pcbs
Hi All,
One of my board having a short circuit (Zero Ohms) across a power supply
(That supply connects to thousands of decoupling capacitors and 44 BGAs)
and Ground. Is there any method to identify the route cause for the short
without removing any components?
Note:
####
1. The board is not yet powered on.
2. It was verified that the PCB doesn't contains any short across supplies
before board assembly.
Thanks and regards
Kiruba Sankar
Project leader
Hardware Design & Development
Email: rskiruban@xxxxxxxxxxxxxxxxxx
web: www.datapatternsindia.com
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